DEGENERATIVE DISCOGENIC SYNDROME
- The Road to Innovative Approaches -
International Published Data
GMCD Instructional Course Lectures
Dr. med. Guy M.C. Declerck MD (GMCD)
Medical FRCS-, FRCS Ed Orth-, M Ch Orth-, PhD-studies
Spinal Surgical and Research Fellow, Perth, Western Australia
Spinal Orthopaedic Surgeon and Surgical Instructor
Consultant R&D Innovative & Restorative Spinal Technologies
President International Association of Andullation Technology (IAAT)
Translation: Filip Vanhaecke PhD
Illustrative expertise and page layout: Lennart Benoot, Mincko, Halle, Flanders, Belgium
Review scientific literature: Medical Consulting Advice, Ostend, Flanders, Belgium
Support: International Association of Andullation Therapy (IAAT)
Legal advice: Anthony De Zutter, kornukopia.be
60. Fusion. Any alternatives?
Morbidity of fusion surgeries for the degenerative discogenic syndrome has emerged as an important problem over the years. As a consequence of lasting morbidity after fusion surgery, the search for alternative treatments widened.
While trying to maintain ‘physiologic’ motion in the intervertebral joint, dynamic stabilisation with halting and/or stabilizing the intradiscal degenerative processes aims at removing the pain (Burton). Hence, the emergence of non-fusion and more physiologic motion preserving surgical devices: artificial nucleus and disc (arthroplasty), dynamic stabilization systems and interspinous fixation systems (Abdullah; Bhatia; Cakir; Chow; Djurasovic; Esses; Goel; Kanayama; Karahalios; Morishita; Oxland; Rathonyi; Tsantrizos; Videbaek; Wang; Youssef).
61. Whatever - rigid or dynamic - fixation system cannot prevent degenerative processes in other IVDs.
The major biomechanical challenge of fusion surgery remains the creation of nonphysiologic rigidy at the operated intervertebral level which creates supranormal forces on other intervertebral disc levels above and below the fusion mass or at the level of the sacroiliac joints when the arthrodesis incorporates the sacrum. Indeed, spinal fusion surgery may trigger axial load redistribution, compelling other spinal and pelvic segments to bear additional forces that are sometimes beyond their natural capacity. Hence, the concept that these biomechanical consequences potentially initiate or accelerate the degenerative biochemical processes in the aging IVDs adjacent the fused area (Deyo; Lehman) eventually necessitating further surgical intervention (Gillet; Hambly).
In laboratory settings, biomechanical evaluations demonstrate that all kind of motion restricting fusion surgeries (anterior and/or posterior, long and/or short) produce compensatory abnormal motions and increased biomechanical stresses at the IVD levels adjacent to solid arthrodeses. However, in the clinical setting the impact of this increased adjacent biomechanical stress on the pre-existing, but silent and asymptomatic disc aging processes in the intervertebral spaces above and below the fusions is insignificant. In other words, whatever type of lumbar fusion will not induce nor accelerate earlier adjacent disc degeneration. Conferring to the natural evolution of discovertebral aging processes, disc degeneration develops progressively.
In commenting on gross IVD incompetence directly adjacent to the site of a lumbar arthrodesis, a large number of reports did not investigate the state of the unfused segments before surgery. Hence, it is unclear whether juxta-fusion degeneration is produced by increased biomechanical stress or is the result of a pre-existing condition. But those who preoperatively have disc degeneration at one level have a predisposition to degenerative discogenic changes at other levels. Furthermore, the majority of MRI degenerative changes occur over multiple levels not adjacent to the fusion. After posterior lumbar instrumented fusion, radiographic changes suggesting disc degeneration appear homogeneously at several levels cephalad to fusion and seem to be determined by individual characteristics (Pellisé). Moreover, the prevalence of similar degenerative changes above the fusion levels is similar to that occurring in normal and asymptomatic subjects.
Unless these irreversible degenerative changes (endplate damage, decreased nuclear volume, and annular tears) were already pre-existing before surgery, the increased biomechanical stresses at these adjacent levels following whatever type of lumbar fusion procedure do not result in clinically significant adjacent level degeneration.
These facts confirm the multifactorial origin of DDS: the essential interrelationship between the biochemical degenerative IVD processes and abnormal biomechanical factors in the predisposed individual by genetic and/or constitutional factors - to create the DDS. The same reflections are made following total disc arthroplasties (Bastian; Boden; Borenstein; Chow; Etebar; Frymoyer; Greenough; Ishihara; Inoue; Jensen; Kanamori; Kumar; Lee; Lehmann; Leong; Oda; Penta; Rahm; Videbaek; Wai).
62. Why lumbar intervertebral disc replacement surgery or arthroplasty?
Total disc (replacement) arthroplasty (TDA) is an invasive non-fusion technology which replaces an old and degenerated IVD with a prosthesis aiming at restoring the biomechanical functions of a previously normal functioning IVD (Zollner). TDA was thought off in the early 1980’s because of the high morbidity associated with an arthrodesis such as non-union, graft donor-site complications, and slow postoperative recovery.
63. Lumbar disc prosthesis. Routine malpositioning
No doubt, there are a lot of highly trained spinal surgeons who are able to implant something, which resembles an original young and healthy IVD, through the anterior anatomical environment of the lumbar spine. However, malpositioning of a lumbar disc prosthesis is rather routine than exceptional.
No inventor, no company, and no key opinion leader have till now been able to indicate how the essential mathematically calculated biomechanical indices for correct implantation can be seen during surgery. The centrode** related to the instantaneous axis of rotation (IAR)* and the center of rotation (COR)*** are extremely important for correct placement of the spinal motion-sparing devices (Panjabi; Sears).
For the existing types of mechanical prostheses in 2015, there simply exist no anatomical nor radiological landmarks which exactly reflect and precisely determine the essential biomechanical references and requirements. Then, it is an illusion to anticipate restoring or maintaining the normal (and essential) 6 kinematic motions (6 degrees of freedom) and the vertical translation/compression of the IVD at the operated level(s).
* The instantaneous axis of rotation (IAR) is the axis of rotation over a small part of the flexion/extension cycle. As the IAR cannot been seen, it is supposed to be located somewhere at the level of the anterior 2/3 to the posterior 1/3 of the vertebra.
** The centrode is the movement of the IAR within range during flexion/extension and can’t be localized either.
*** For locating the ‘center of rotation (COR)’, which transforms by rotation, the intersection of perpendicular bisectors of lines joining identical points on the vertebra between its original and final position of rotation needs to be graphically calculated. It can’t be seen!
64. Lumbar disc prosthesis. Major implanting problems and innovative developmental ideas
The author was given the opportunity to assist a lot of European key opinion leaders (KOL’s) and instruct a lot of spinal surgeons in the surgical procedure of TDA. For various reasons, major implanting problems remain because the pattern of disc and endplate shapes vary considerably from individual to individual or even in the same individual (Farfan). Then problems remain with
(a) sizing the ideal device,
(b) perfectly preparing the endplates,
(c) positioning the prosthesis correctly in the midline of the emptied intervertebral space,
(d) respecting the dynamic COR and IAR which differs at each lumbar level,
(e) providing an optimal shock absorption, and
(f) being able to maintain the movements at the implanted level.
At the same time the author was requested to review the complete related spinal literature with the aim to develop an already computer simulated innovative IVD prosthetic design to be implanted by a posterior or posterolateral approach. It concerned a novel implant with viscoelastic properties closer to the natural behavior of the normal IVD aiming at a more physiological loading, without interposing compartments, without non-physiological central load bearing point, and without non-physiological COR (Fig. 64).
Fig. 64. Examples of routine implanted TDA’s using interposing compartments.
65. Lumbar disc prosthesis. How it started?
The prosthetic IVD industry started in 1984. Following the poor clinical results of implantations of a simple metal ball into the IVD space in 1966 (Fernström), the first functional intervertebral endoprosthesis (SB1 Charité device) was implanted in East Germany by Kurt Schellnack and Karin Büttner-Janz, multiple Olympic Medal Winner in artistic gymnastics (Gold, Silver, and Bronze 1968/1972) and orthopaedic surgeon. The development was based on John Charnley’s low friction arthroplasty principle for peripheral joints. Following the early good short-term results, companies and engineers have since constructed a whole range of composite, mechanical, and elastic artificial IVD concepts to improve the implanting difficulties. Mobile bearing prostheses, which are based upon the same principles as total diarthrodial replacement in the extremities, are referred to from constrained* to unconstrained**, according to whether or not certain movements are restricted.
* The constrained (fixed axis of rotation) prostheses contain means of limiting segmental translation designed to reduce the anteroposterior shear forces on the facet joints.
** The unconstrained (mobile bearing core) prostheses permit six degree-of-freedom segmental motion around and along three orthogonal axes of rotation. It uses a free-floating polyethylene middle core to mimic the movement of the center of the natural nucleus posteriorly in lumbar flexion and anteriorly in extension.
66. Lumbar disc prosthesis. An alternative to fusion?
Theoretically, completely removing the degenerated IVD and replacing it by an artificial implant is an alternative to fusion. But in contrary to surgically fusing one vertebra to another and in the hope to settle the distal lumbar pain, TDA technologies are designed to
(1) restore the disc height,
(2) maintain non-painful mobility, and
(3) obtain greater stiffness.
As such it is hoped that the spine could spontaneously restore its lumbar lordosis and proper asymptomatic natural sagittal balance in their suspected original young/adolescent status.
Unfortunately, TDA’s are not able to address compressive forces. TDA’s have no influence on adjacent IVD biochemical degenerative processes as this phenomenon is rather caused by genetic predisposition and normal aging processes.
Moreover, how can TDA’s replace painful IVDs if it is still completely impossible to see pain during radiography, discography, and magnetic resonance imaging. The techniques are quite imprecise means of identifying a IVD as the isolated source of low back pain. Abnormalities and all kind of artefacts at the level of the IVD and the zygapophyseal facetal joints on imaging studies are not synonymous with pain!
67. Lumbar disc prosthesis. Initial enthusiasm
The initial increased enthusiasm for implanting disc prostheses is based on published results of a randomised and prospective FDA study comparing 2-year follow-up results of TDA’s to those of a ‘control group’ consisting of BAK cage anterior interbody fusions (Hochschuler 2003; McAfee 2003).
Although no failures were recorded with the prosthetic device, patient related clinical outcome results showed no statistical significant difference with patients who were operated on with a BAK cage.
However, in another study of Nottingham (Lam), BAK ALIF cage surgery already had proven to give clinically 65 % unsatisfactory results. Indeed, and as a result of further evaluation, BAK cages have been shown not to work and therefore no longer are implanted. Strangely enough, by 2006 more than 15000 artificial total disc arthroplasty surgeries had been performed.
68. Lumbar disc prosthesis. Still experimental
If, in comparison to total hip and/or total knee replacement procedures, success means an (almost) complete relief of pain, return to full function of the joint, return to full previous professional and social activities, then invasive spinal non-fusion intervertebral disc replacements are still in experimental phase and far from a success.
Related to the above mentioned biomechanical prerequisites to implant a TDA, excellent results only would be obtained if a TDA is inserted correctly. Because the IAR cannot be taken into account during the surgical arthroplasty procedures, normal motions will not be restored. Abnormal forces through a non-physiological axis will be one of the causes of failures.
Even in those with a - by chance - successful biomechanical implantation result, the long-term motion achieved is likely to be less than physiologic. Spontaneous fusion after disc replacement eventually occurs very regularly. Indeed, TDA still has to prove to uniformly preserve motion at the operative level.
69. Lumbar disc prosthesis. Superior to fusion?
To date, and in terms of clinical outcome, the author is not aware of any evidence in the published randomized controlled trials that TDA is superior to spinal fusion.
Although the original expectations were perfect, because of the unpredictable evolution of the intervertebral movements, the published long-term clinical outcomes of TDA remain unsatisfactory and simply equal those of rigid fusion surgeries: disappointing and sometimes mediocre.
Indeed, numerous chronic LBP sufferers are undergoing some kind of arthroplasty without any benefit whatsoever.
70. Lumbar disc prosthesis. What will happen in the long term?
Till now, nobody knows what will happen to an even perfectly implanted IVD prosthesis once the supporting subchondral bone of the vertebrae decreases in quality due to the irreversible effects of stress-riser processes as the patient ages and becomes osteopenic and osteoporotic (extrusion? subsidence? stenosis?).
On the other hand, repeated replacements, prosthesis removal and revision procedures always will remain something of an experimental procedure due to an already distorted anterior anatomical environment by the previous interventions.
71. Lumbar disc prosthesis. Why perfect results are awaiting.
Although the idea is perfect from biomechanical point of view, perfect results are still awaiting.
The surgical technique of a TDA is by no means comparable to that of the much simpler and nearly perfectly designed surgical arthroplasty techniques for replacing a hip, knee or shoulder joint.
The longevity of the actual TDA’s is unknown, but inherently limited due to their inability to biointegrate and subsequently remodel.
Furthermore, all implants generate wear debris that can cause cellular reactions that lead to osteolysis and other potentially deleterious effects (Hallab; von Knoch).
There are the anticipated non-reparable approach related complications (vessels, nerve root stretching, ureters, retrograde ejaculation with associated sterility), thrombosis with potentially fatal embolism, vertebral body fractures, malpositioning and subsidence of the implanted device, implant failures, zygapophyseal facet joint encroachment, calcifications, and spontaneous fusion.
72. Lumbar disc prosthesis. Why implanting something that only resembles the natural IVD?
It is not the fault of surgeons that companies offer to implant something to be implanted that is still not compatible with the biologic nor the mechanical functional capacities of the normal adolescent IVD. Indeed, the biochemical and biomechanical behavior of the intervertebral joint in the spine is very complex (Adams; Gracovetsky; Sears; White & Panjabi).
However, nothing will halt the search for optimal solutions for chronic LBP related to degenerative discogenic syndrome. It is very probable that one day someone somewhere in a laboratory will be able to develop something that is biologically compatible with the viscoelastic but not necessarily with the anatomical characteristics of the IVD.
73. Lumbar disc prosthesis. What about its future?
Of course there is a future for ideally designed total IVD replacement technology.
However, the author does not fall into faulty logic that whatever type of actually existing total arthroplastic disc, which mechanically differ considerably,
(1) will be better than arthrodesis,
(2) will decrease adjacent segment IVD degeneration, and
(3) will cause no further problems at all.
To date, the option chosen for the principal models on the world market is the preservation of mobility at the cost of physiologic damping and load distribution provided by healthy IVDs.
In contrast with replacements of degenerated synovial joints like the shoulder, hip, knee, replacement of a degenerated IVD will not change the natural evolution in the remaining IVDs which seek to stiffen up when aging.
It is well known that clinical improvement of symptoms in CLBP parallels the decrease in motion of the lumbar spine while aging. Consequently, it is not at all essential that a prosthesis has to equal the quality and quantity of motion associated with the natural functions of the nucleus pulposus (i.e. axial compression and translation) and of the annulus (i.e. resisting motion).
74. Lumbar disc prosthesis. Why to replicate the nucleus pulposus?
In the author’s opinion, there is no reason why the NP needs to be replicated in the innovative researched designs.
Competitive search and research (in-vitro, FEA, and in-vivo studies) goes on for the development of an ideal, ingenious, safe and biomechanical effective lumbar spinal IVD concept in the hope of constructing an innovative IVD design that mimics the intact biomechanical viscoelastic behavior for allowing correct load distribution comparable to that of a normal spinal segment.
One day, a bioengineered IVD will replace the currently implanted metal and polymer TDA’s which do not biointegrate, release exogenous wear particles and initiate wear-induced osteolysis . Unfortunately, the medical research world is still far off such a solution as it is not a ‘piece of cake’!
75. Soft dynamic pedicular stabilisation system. Concept of motion preservation
The value of simply implanting a posterior pedicle screw-based system without plates but with ligaments (thus impossible at L5-S1) as a dynamic stabilisation procedure is not resolved and not completed as yet.
The concept of motion preservation in the spine and particularly at the level of the IVDs remains an intriguing one because motion never has been scientifically proven to be painful. Moreover, there is no such a thing as absolute motion in every joint for a whole human life. As humans get older, joint mobility gradually decreases.
As early as the 1960’s, in the other realms of orthopaedic surgery, everything possible was being attempted just to avoid fusion. The vertebral column remained and still is the last bastion where fusion is considered the unique solution for surgical stabilisation.
Fig. 75. In a so-called dynamic or flexible pedicle screw-based system, the implantation of pedicle screws is continued but the heads of the screws are no longer spanned by plates but replaced by elastic or nonelastic ligaments. Sometimes plastic cylindrical spacers or helical springs have been used (not illustrated). On the left an intact decorated ‘Christmas tree’ is still visible 10 years postoperatively. This patient needed revision surgery because of recurring LBP due to the natural progress of the degenerative discogenic processes which cannot be neutralised by this technique.
76. Dynamic ligamentous pedicular stabilisation system. The problem of the connecting ligaments.
The author was a strong believer of such a dynamic ligamentous approach. He implanted and supervised 1.398 of these soft dynamic stabilisation systems in and outside Belgium. His long-term experience indicated two major problems which compromised the outcome results in the long term: the weaknesses of the ligaments (= similar to the natural decay of a decorated Christmas tree – Fig. 75) and the fact that initially nothing could be done to neutralise the degenerative processes in the IVD. His straight fort conclusion is that it is a mistake to design an implant with infinite physiological motion potentials as no functional joint segment remains normal during a lifetime.
In the meantime, scientists around the world have been searching for ways to mass-produce extraordinary tough silk produced by the arthropods in the wild, spiders and silkworms. Scientists try to understand how spiders form their architecture with silk proteins. Developing such structural proteins may be the next material to spark a revolution in industry (Andersson; Bourzac; Xu). The long-term ruptures of the actually machined ligaments for the soft ligamentous stabilisation procedures, could then probably be avoided. Then, the posterior dynamic ligamentous stabilisation system probably would be able to
(1) rigidify an intervertebral segment that lost its initial rigidity,
(2) maintain the surgically obtained stiffness,
(3) allow a certain ‘controlled’ segmental mobility, and
(4) provide a less stressful load transmission through the posterior part of the spinal motion segment.
77. Dynamic ligamentous pedicular stabilisation system. What about the degenerated IVD?
A dynamic ligamentous ‘load control’ procedure could have resolved the impact of abnormal loading influences on the progressing biochemical (and inflammatory) degenerative intradiscal changes … if at the same time ‘something’ could have been done to the degenerative IVD as well.
Rigid fusions and arthroplasties concentrate on replacing a degenerated IVD by bone or an artificial disc by removing the degenerated disc entirely. Soft dynamic stabilisations leave the degenerative IVD alone or even destroy it further by discectomy.
However, it became evident that leaving a degenerated IVD alone has no advantages. Although the early patient outcomes indicated improvement in functional results and pain expression, the long-term results were becoming unsatisfactory and expressed similar disappointing figures, the rigid procedures alike (Askar; Brechbühler; Gardner; Grevitt; Grob; Hadlow; Kanayama; Madan; Onda; Rigby).
78. Dynamic ligamentous pedicular stabilisation system. A new biomechanical intradiscal environment
Flexible and dynamic stabilisation systems create a new biomechanical intradiscal environment that influences the disc biology with the aim of favoring cicatrisation the degenerating IVD (Sénegas).
In vitro studies and finite element analyses of some of these posterior dynamic implants indicate that the intradiscal pressure in the spanned degenerated IVD decreases and that abnormal motion (flexion, extension, lateroflexion) and abnormal loads are reduced.
Since a degenerated IVD cannot restore itself, it remains essential to generate a suitable and biological IVD, i.e. by implanting mesenchymal stem cells. Indeed, neutralisation of the degenerative cascade in the affected spinal segment always will need a combination of two therapies: a biological intradiscal therapy combined with a mechanical dynamic restabilisation either using pedicular screws or an interspinous device (Mulholland; Sengupta).
79. Dynamic ligamentous pedicular stabilisation system. The author’s suggestion for further investigation
The author suggested the company to investigate the possibility of developing an additional perpedicular instrument. To no avail!
Not at all being interested to be compared to a mechanic who is compelled to assemble the engineered pieces from the industry, the author discontinued his function in solely acting as a surgical technical performer instructed by commercial companies, and continued his search.
Years later, he utilised another design of a cementoplasty needle. Before implanting the original soft ligamentous system, autologous bone marrow aspirations centrifuged in the operating room were injected in the degenerated IVD in 19 patients suffering the degenerative discogenic syndrome.
Simple subjective questioning 3 years postoperatively indicated much better pain-relieving results (18 out of 19) than in all previous 1.379 cases. At that time, none of the implanted ligaments was teared. Five years postoperatively, when all ligaments were ruptured (= decayed Christmas tree!), identical subjective questionnaires still revealed excellent pain relief in 16 out of the 19 operated patients. Two patients underwent a rigid fixation but without improvement. One patient had died in a car accident.
Unfortunately, and because of the high personal costs, no MRI evaluation was performed. However, Robert Fraser from the Adelaide Centre for Spinal Research, Adelaide, Australia suggested that the potential ‘regenerative images’ on MRI image only could reflect a reparative and healthier fibrocartilaginous healing tissue related to ingrowth of bloodvessels (Fraser). Understanding that complete in-to-out posterior annular tears in degenerative IVDs spontaneously can heal up by out-to-ingrowing fibrotic tissue, it is not impossible that tears in the endplate and in the annulus could have healed up as well by in-to-outgrowing fibrotic tissue.
80. Interspinous devices
Surgical techniques that allow implantation of devices that redistribute load ‘more or less normally’ and allow some kind of ‘remaining’ motion at the level of a degenerated lumbar IVD have been developed as an alternative to interbody fusion and total disc replacement.
Interspinous devices are another type of surgical treatment in the hope to relieve the pain associated to the degenerative discogenic syndrome.
Because of our orthograde posture, the daily biomechanical compression and shear loads (walking, sitting, standing, rotation, bending, extending, etc …) may be detrimental to the human IVDs. But in quadrupedal animals it is shown that distraction following compression may restore the IVD.
Devices positioned between the spinous processes act as spacers aiming at reducing stresses and pressures in the IVD and limiting lumbar extension.
Experimental studies in vitro and in animal models demonstrate that compression or external loads
(a) decrease the IVD height,
(b) increase intradiscal pressure,
(c) induce IVD degeneration,
(d) disorganise the annulus architecture, and
(e) induce increased apoptosis in the annulus and in the cartilage endplate.
Distraction reverses these changes and
(1) induces IVD regeneration,
(2) increases IVD height,
(3) decreases the number of apoptotic cells,
(4) restores the lamellar morphology of the annulus,
(5) decreases but does not normalise the intradiscal pressure,
(6) leads to significant higher signal intensity in the NP on MRI, and
(7) leads to substantial ‘regeneration at a cellular level’ in the NP meaning an upregulation of the extracellular matrix genes COL 1 and COL 2, decorin and biglycan (Christie; Flynn; Goyal; Guehring; Kabir; Kroeber; Minns; Ranu; Sénégas; Siddiqui; Swanson; Unglaub; Wilke; Wiseman).
The interspinous distraction devices, consisting of metal or flexible spacers and simply inserted between or attached to the spinous processes, no longer are rigidly connected to the vertebrae. Pedicle screws are no longer needed. These floating devices, some of which can even be implanted under local anesthesia, distract the spinous processes and aim at off-loading the posterior annulus (and facet joints).
Since the idea first was developed in Poland during the late 1930s (Fig. 80), a huge variety of such implants have continuously been refined by the engineers of the different biomechanical industries. From biomechanical point of view, the implantation of these devices effectively restricts motion at the operated segment and reduces the intradiscal pressure. The measured pressures are compatible with the well-known experimentally recorded pressures (Adams; Andersson).
Fig. 80a. Different types of interspinous devices. (A) The earliest known interspinous device developed in Poland in the late 1930s. (B) An interspinous implant that can be implanted percutaneously.
Fig. 80b. Simple set of instruments for percutaneously implanting an interspinous device.
The issue of simply placing an interspinous implant as dynamic stabilisation procedure is not resolved as yet. This is basically a biomechanical discussion as it is impossible to visualise the IAR and the position of the centrode in the spine in the lumbar spine in the living. Satisfactory long-term results (5 to 10-years) are still awaited. Problems like fractures of the spinous processes, subsidence of the implant into the bone or dislocation may be expected.
81. Gene therapy. An innovative way of treatment?
A human individual is made up of a population of cells (an average of 40.000 billion), each with its own ‘personal’ genome. Varying levels of chromosomal mosaicism are present in a variety of healthy somatic human tissues and cell types (Abyzov; Baillie; Evrony; O’Huallachain; Macosko).
The somatic mosaicism has been recognised for decades to be important in the disease mechanisms (Lupski). Changes in gene expression orchestrate cell-fate decisions which may cause irreversible dysfunction of organs and cause all kind of pathologies. Therefore, gene therapy probably will be the definite innovative way for treating all kind of diseases and conditions causing signs and symptoms in humans.
82. Gene therapy. Definition
Gene therapy shuttles and incorporates a new or modified genetic sequence into the host genome of target cells. I.e. gene therapy succeeds in transferring genes to mesenchymal stem cells (MSCs) to drive them toward other specific phenotypes (see topics on stem cells).
83. Gene therapy. Viral carriers
Gene therapy techniques may accomplish transfers of genetic sequences with use of naturally occurring viruses, such as adenoviruses, lentiviruses, and retroviruses.
Having removed their harmful genetic sequences, the viruses are engineered to contain a promotor gene of a specific and/or therapeutic cDNA sequence (= a DNA copy synthesized from mRNA). When the virus encounters the human target cells, it inserts its genetic material into their genomes to fix DNA errors and obtain the desired biologic effect.
Therefore, gene therapy allows the patient's own cells to serve as a bioreactor to synthesize appropriate and biologically active molecules.
84. Gene therapy. Viral carriers can target the few remaining cells in the degenerating IVD.
Direct adeno-virus mediated approaches in vitro and in vivo already can successfully and efficiently transfer therapeutic genes to the target cells in a degenerating IVD (An; Kawakami; Larson; Nishida; Paul; Risbud; Shimer; Wallach; Yoon).
85. Gene therapy. Non-viral carriers do not trigger immune reactions.
Viral vectors carrying the cDNA may trigger massive immune reactions that can shut down somebody’s organs.
Because of ethical problems with gene transfer (Anderson), non-viral carrier systems are being developed (liposomes, naked DNA, and gene-activated matrices) that would insert themselves into the genome more precisely avoiding at random disruption of other important genes.
A particle-mediated gene transfer has been developed where e.g. gold particles coated with a particular gene are transferred to produce transgenic plants and animals. This so-called ‘gene gun-mediated system’ has been used in vitro to transfer growth factor genes to nuclear and annular cells of the IVD (Klein; Le Maitre; Matsumoto; Moon; Nishida; Tang; Wang).
86. Gene therapy influences the metabolic processes in the degenerating IVD cells.
Whatever the genetic modification technique, the transfers of exogenous genes aim at enhancing the anabolic processes (e.g. upregulating the matrix production of proteoglycans and collagens) and inhibiting the matrix catabolic processes (e.g. MMP expression) for prolonged periods of time.
Introducing anabolic cDNA for encoding intracellular regulators (SOX9), for a variety of anabolic and anti-catabolic cytokines (i.e. growth factors TGF beta1 and BMP2), and cDNA transfers for anti-catabolics (TIMP1) has been shown to increase proteoglycan and collagen synthesis and preserve the architecture of IVD tissue (Franceschi; Levicoff; Nishida; Parikh; Prockop).
This all suggests that a cocktail of anabolic and anti-catabolic factors may be required for optimal stimulation of the IVD cells.
87. Molecular therapy strategies. ‘Nil novi sub sole’: Prometheus’s liver
‘Nil novi sub sole’. The scientific dream of regenerating human tissues already is illustrated in Greek mythology where an eagle feeds on Prometheus’ liver each day. The amazing regenerative properties of the liver are since long known by surgeons who need to excise parts of the liver. Indeed, the liver is the ‘champion’ of mammalian tissue regeneration. Moreover, in the amphibian species, a physiological property during limb regeneration occurs as well.
88. Molecular therapy strategies. Direct injection of molecules in IVDs of small animals
In the intervertebral discs the balance between synthesis and degradation of the large proteoglycan and collagen molecules in the extracellular matrix (ECM) always will determine its biochemical composition and structure dictated by the laws of the normal aging processes and genetic inheritance. Additionally, the livelong impact of chronic effects of decreasing IVD nutrition (= metabolite transport) and the variety of daily loading patterns progressively but continuously interfere and degrade the ECM of the IVDs over many years.
Because the IVD has no intrinsic capacities for regeneration, molecular therapy strategies are explored to retard, stabilise, or even to reverse the degenerative discogenic processes. Different classes of biologically active molecules, knowing to increase the accumulation of ECM, have been injected directly into the IVD of small animals but rapidly cleared biologically. To replace this simple molecular injection therapy and in order to provide a longer period or a continuity in synthesizing the ECM molecules, NP cells genetically modified by nonviral or viral vectors are researched to sustain the synthetic activities.
89. Molecular therapy strategies. Viral carriers transfer molecules to the cells in the IVD
The strategy of treating IVD cells with bioactive molecules for regenerative purposes is extensively investigated in in vitro animal studies. Intracellular regulators, growth factors, and anti-catabolic agents, delivered in the NP cells by adenoviruses (gene therapy) to biologically enhance IVD cell metabolism and phenotype show evidence of inhibiting the progression of IVD degeneration in one way or the other.
90. Molecular therapy strategies. Transfer of intracellular regulators
Intracellular regulators (SMADs, LMP-1, SOX 9, Link N) can control mechanisms in the cells resulting in an increased synthesis of the proteoglycans and collagens II in the nucleus pulposus (Table 90).
Types of molecules
Protection from apoptosis
+ BMP-2 & -7
Table 90. Upregulation (+) of cell metabolism and phenotypes in the nucleus pulposus by different classes of molecules. AF stands for the effect in the annulus fibrosis.
91. Molecular therapy strategies. Transfer of growth factors
Growth factors (insulin-like growth factor-1 / IGF; basic fibroblast growth factor-2 / FGF; platelet derived growth factor / PDGF; epidermal growth factor / EGF; transforming growth factor-β / TGF-β; bone morphogenetic proteins / BMP; growth and differentiation factor-5 / GDF) are typical mitogenic cytokines.
These proteins bind to specific transmembrane receptors, lead to an activation of intracellular signaling transcription factors and stimulate the nuclear gene expression. This cascade results in chondrocytic-like phenotypic cell proliferation, differentiation and enhancement of their ECM synthesis (An; Chujo; Gruber; Masuda; Nishida; Osada; Thompson; Tsai; Walsh; Yoon).
Some growth factors (TGFs-β, BMPs and GDF-5) normally reside in the ECM and protect the chondrocytic-like cells in the NP to phenotypically change into fibrocytes (Table 90).
Oxygen is a limiting factor in the survival of transplanted stem cells and connective tissue progenitors (Kunz-Schughart) . However, cells in cartilage are exceptional for maintaining viability in avascular and hypoxic conditions (Garcia). Transforming growth factor beta (TGF-β1), in conditions of hypoxia - which is an environmental condition that exists in the degenerating IVD - activates the expression of genes of the NP cells for the synthesis of all proteoglycans, Sox-9, and collagen types II and type X in the ECM. Exposing and culturing mesenchymal stem cells (MSCs) to TGF-β1, to low oxygen tensions, and alongside cells from the NP, can activate genes in the MSCs to differentiate into a phenotype consistent to the chondrocytic-like cells in the NP and to increase the aggrecan and type II collagen synthesis (Bartels; Le Visage; Richardson; Risbud; Sakai; Steck; Tuli).
In vitro studies indicate that platelet-rich plasma (PRP) - a plasma fraction that can be produced by centrifugal separation of whole blood of humans in the operating room - contains multiple growth factors concentrated at high levels. PRP is an effective stimulator of chondrocytic-like NP cell proliferation and proteoglycan and collagen synthesis by porcine NP and AF cells cultured in alginate beads (Akeda; Dugrillon; Landesberg; Okuda; Weibrich).
92. Molecular therapy strategies. Transfer of anti-catabolics
Although promising during animal studies, anti-catabolics (Table 90), such as tumor necrosis factor alpha inhibitor (anti-TNF-α), do not seem to have effective therapeutic effects when injected intradiscally in patients with chronic degenerative discogenic low back pain (Cohen).
Matrix metalloproteinases (MMPs), which are very potent degrading collagenases in the NP, destroy collagen type II and cause the IVD to lose its mechanical strength (Mitchell; Reboul). Tissue inhibitors of matrix metalloproteinases (TIMPs) are endogenous inhibitors of MMPs and regulate the matrix degradation by increasing the proteoglycan synthesis (Hsieh). Unfortunately, the exogenous TIMPs, whose genes can be transferred by viruses to the NP, only affect the IVD in a limited amount (Franceschi; Handa; Risbud). Anti-MMPs, such as the interleukin-1 receptor antagonist (IL-1RA), are able to block the catabolic and inflammatory cytokine interleukin-1 (IL-1) released by regulated cell apoptosis and prevent the breakdown and loss of the proteoglycans by inhibiting the degradative MMP’s within the IVD.
93. Molecular therapy strategies. Is there a possibility of exogenous growth factor therapy in humans?
All of the growth factors present some in vitro animal data showing that stimulation of ECM synthesis by IVD cells alters the balance of homeostasis by shifting cellular metabolism to the anabolic state.
All these in vitro obtained data support the possibility of an exogenous growth factor therapy as a potential additional therapeutic intradiscal strategy in humans which would intend
(1) to stimulate the nuclear and annular cells as well as implanted MSC cells in the human IVD to divide,
(2) to stimulate both aggrecan and collagen synthesis,
(3) to restore IVD height and as such
(4) to prevent, arrest or reverse the degenerative changes in the ECM of the IVD (An; Maerz; Thompson).
However, the in vivo animal experiments only indicate delayed and temporary responses and no evidence on their efficacy in treating IVD degeneration. The principal reason seems to be related to the IVD cells. At all times it will remain essential that the few but always decreasing number residing cells in the human degenerating IVD are able to respond to the applied growth factor(s) in order to result in biological effect(s).
The idea of using the experimentally innovative molecular treatments, developed in animals but for dealing directly with the causative agents for IVD degeneration in humans, remains very challenging. The techniques for potential regeneration of human IVD tissue will depend on improvement of the
The question remains whether the metabolically impaired cells in a human degenerated IVD are able to respond to these biologic molecules. Therefore, the potential of molecules as sole therapy may be insufficient in the prevention and treatment of degenerative IVD conditions. Indeed, too much independent factors are involved. The structural failure of an intervertebral disc is irreversible and always progresses. Genetic inheritance, age, progressing inadequate metabolite transport, and loading histories gradually weaken the human IVDs to such an extent that structural failure continuously occurs during all activities of daily living.
94. Molecular therapy strategies. Can they settle signs and symptoms of degenerative discogenic syndrome?
The author shares the opinion that molecular treatment strategy to repair or regenerate biologically the NP and AF in a degenerating IVD only can be successful when the NP and its cells are still present and thus in the relatively early stages rather than in the late stages of IVD degeneration (but how to quantify?).
In the degenerating and degenerative human IVDs, cells are sparse (or already absent), age as well, become metabolically more inactive, and undergo apoptosis which is associated with higher production of pro-inflammatory cytokines causing inflammation (Kaczmarek; Pasparakis; Vandenabeele; Wallach). Since most of patients suffering the degenerative discogenic syndrome already are in the fourth decade or older and present a longstanding degradative IVD involution, the molecular treatment concept aiming for disc regeneration may fail due to the already longstanding, important but irreversible structural alterations (Urban).
Importantly! Even if a molecular therapy may slow down or reverse the histological and morphological features (~ regeneration) of human IVD degeneration, till now nobody knows if an innovative biochemical molecular treatment alone will be able to resolve signs and symptoms associated with the degenerative IVD changes which cause the degenerative discogenic syndrome (DDS) in humans. In osteoarthritic conditions - patients with osteoarthritis in their shoulders, hips, knees - use of engineered mesenchymal stem cells to repair cartilage demonstrated new cartilage growth but without clinical improvement (Matsumoto; Wakitani).
95. Cellular therapy strategies. Research in small animals
Because molecular therapies alone may not succeed in restoring the normal surrounding anatomy, the normal biomechanics and motion of the IVD, cell-based therapeutic strategies which could stimulate the EC matrix synthesis as well are fully evaluated.
In small animals, cell-based IVD therapeutic studies give promising results. Implanting, reinserting or injecting several types of cells, such as a whole disc, cultured (and genetically engineered) NP cells, and mesenchymal stem cells inside an induced degenerative IVD, indicate that these cells may remain viable.
The cells may preserve the IVD or slow the degenerative processes by restoring the production of proteoglycans and collagens type II (Acosta; An; Crevenstein; Fassett; Ganey; Gruber; Henriksson; Hiyama; Li; Masuda; Nishimura; Nohaus; Nomura; Okuma; Thompson; Walsh; Wei; Yoon).
But the reasons why many cell transplantation methods work well in small animals - like rats and rabbits - very probably are related to
(1) the presence of large numbers of notochordal cells (but absent in human IVD) which are known to have a much higher metabolic proteoglycan synthetic activity, and to
(2) a shorter passive diffusion distance of 500 to 1000 µm for metabolites.
96. Cellular therapy strategies. Very challenging because the human IVD is hypoxic and acid.
When the human IVD starts aging and degenerating,
(a) the supply of nutrients (glucose, amino acids, oxygen) and the removal of accumulated waste (lactic acids and free radicals) decrease because the endplates calcify and sclerose occluding their bloodvessels,
(b) the cells change, alter their metabolism, induce degrading matrix metalloproteinases (MMP’s), and produce more degraded products,
(c) the daily loading patterns and shear stresses may lead to hypermobility, and
(d) the number of cells decreases because of apoptosis increasing the pro-inflammatory cytokine production.
The result is a human intradiscal environment becoming acid and hypoxic. Then, increasing the number of cells may induce an opposite effect because of an increasing demand for nutrients. More cells may die. Unregulated cell death or accidental cell necrosis will lead to an increased release of products of cell lysis and adding debris to the site. Regulated cell death or apoptosis, by releasing factors collectively described as damage-associated molecular patterns (DAMPs) from the disintegrating cell (Kaczmarek; Pasparakis; Vandenabeele; Wallach), will accentuate local inflammation following implantation itself.
Indeed, reproducing the above mentioned promising animal data to ‘regenerate’ or ‘repair’ inhospitable degenerative and/or surgically damaged human IVD tissue remains very challenging.
97. For cellular therapy strategies to succeed in humans, ‘something’ more is needed!
In larger animals and humans, the innovative cell-based intradiscal therapies could fail because of
(a) the absence of notochordal cells,
(b) the metabolically and biomechanically distinct cell populations and different organisations of the nuclear and annular matrices,
(c) the decreasing or absent nutrition,
(d) the passive diffusion distance of more than 500 to 1000 µm,
(e) the extreme dependence of IVD cells on mechanical conditions,
(f) the presence of degrading enzymes,
(g) the presence of pro-inflammatory cytokines, and
(h) aging and apoptotic processes of the implanted cells as well.
Because of the undisputable importance of the combination of biology and biomechanics in the IVD - the so-called mechanobiology - cellular treatment strategies only will succeed if the daily loading conditions onto the IVD can be restored between normal limits.
Moreover, because the supplemented cells may leak through the in-to-out but invisible degenerative annular tears and, of course, through the iatrogenic ‘limited’ annular incision or puncture wounds, another implantation technique needs to be developed without further damaging the annulus fibrosus.
98. Mesenchymal stem cells and their progenitor cells. Essentials
All individual cells that construct the organs and tissues in our body are dying and being replaced continuously. All cells of our skin are replaced in two to three weeks. It takes a few weeks to replace the cell lining of the gut, and the replacement of the blood clotting platelets requires 10 days.
Nature has set up a mechanism that maintains a constant population of working cells in all tissues and is consistent throughout the body. All cells found in a particular organ arise from a single common parent known as mesenchymal stem cell or MSC (Caplan) which gives rise to a larger population of progenitor cells. In the body, niches of MSCs are present as enclaves surrounded by specific cells, i.e. as stromal cells that form the connective tissue in bone marrow (Schofield).
The MSC is an unspecialised cell capable of long-term self-renewal. A MSC divides in two daughter cells. One daughter cell is exactly like the mother cell. The cell retains the long-term ‘mother’ stem cell identity and never differentiates into another cell type. Its biological function remains totally different from the second daughter cell, the progenitor cell. This second cell is short-lived and rapidly proliferates. The cell progresses until it differentiates into specialised somatic cells of different connective tissue lineages depending or their location: osteoblasts (bone), chondrocytes (cartilage), myoblasts (muscle), adipocytes (fat), fibroblasts (fibrous tissue), etc. In addition to generating cells of several tissue lineages, MSCs also give rise to supportive connective stromal cells (Bruder; Friedenstein; Gerson; Jiang; Muschler; Pittenger; Poiraudeau; Prockop; Risbud; Sakai; Sive; Yoo; Yoshimura; Young; Zuk). Otherwise, progenitor cells may die by apoptosis.
Signals from multiple sources to the MSC niches promote the maintenance of the MSCs in their undifferentiated state and keep them quiet until they are called upon to produce new cells (Hsu; Schofield). Differentiated daughter cells are known to provide feedback signals to regulate MSC replication and differentiation (Hsu). Finally, MSCs do regulate themselves as well using an autocrine mechanism (Lim).
Multipotent MSCs have been identified in many adult tissues such as blood, bone marrow, skeletal muscles, skin, synovial tissue in the joints, heart tissue, adipose tissues, etc. The ability of MSCs to self-renew and differentiate enables them to replace cells that die by apoptosis and, therefore, are critical to the maintenance of a tissue’s health. The widespread presence of progenitors in all adult tissues is essential for their repair and therapeutic effects. I.e. they play important roles in the body’s ability to heal wounds after an injury or to repair a cardiac infarct (Caplan; Fraser; Friedenstein; Gimble; Greenberg; Kashofer; Mizumo; Moore; Muschler; Pittenger; Rozenzweig; Steinert).
Human adult MSCs are multipotent and ‘plastic’. They are guided by appropriate cytokine growth factors, the presence of other cells, or local physical signals in their microenvironment that control their activation, proliferation, migration, differentiation and survival.
MSCs always maintain their multidifferentiation potential. When exposed to inductive extracellular cues, MSCs from vertebrates may genetically be programmed to differentiate into cell types that are not normally present in their residing tissues/organs. This means that MSCs derived from one differentiated tissue can trans-differentiate in vitro and in vivo into multiple mesenchyme-derived cell types of a tissue of another phenotypic type, including into osteoblasts (bone), chondroblasts (cartilage), adipocytes (fat), fibroblasts (fibrous tissue), myoblasts (muscle), heart, inner ear, kidney, skin, etc (Bennett; Caplan; Fuchs; Grigoriadis; Kashofer; Kørbling; Ma; Moore; Nöth; Pittenger; Rafii; Risbud; Ryan; Sakai; Song; Terada; Tsonis; Weissman; Yamamoto; Ying; Zuk). Unfortunately, MSCs do age as well and their potency diminishes accordingly (Sethe; Stolzing).
Adipose tissues probably contain the most powerful MSCs. Liposuction aspirates from human abdominal fat contain a significantly higher concentration of pluripotent progenitor cells than bone marrow cells. And all adipose-derived MSCs have the ability to differentiate into cells of osteogenic, chondrogenic , and fibrogenic lineage (Fraser; Hsu; Lin; Miyazaki; Peterson; Zuk).
99. Mesenchymal stem cells. Applications in fracture treatment
During his nearly 3-year stay in a Third World country (in the early 1980s), the author was confronted with 58 non-healing tibial diaphyseal fractures. Because standard high-tech open-grafting procedures were not possible and intramedullary nails non-available, and because he had studied the huge literature regarding the healing potential of bone marrow during his orthopaedic residency (Friedenstein), he had no choice but to try an innovative an experimental procedure of which he had no clue if it could or would work.
Using a 10 cc syringe fitted with an 18 G needle, 80 to 90 cc bone marrow was aspirated from the iliac crests and processed with a simple centrifuge at 1000rpm for 5min. The concentrated buffy coat was then aspirated in smaller syringes and percutaneously injected inside the non-united area (+/- 20 to 30 cm³). The fractures were further immobilized with bamboo reinforced POP to allow full weight bearing. To his surprise 52 out of the 58 fractures (except those in the elderly patients) healed over a period of 3 to 4 months by formation of a large callus formation. The procedure very rarely presented a problem except for a haematoma at the aspiration site.
The physiological explanation of what exactly had happened only was fully understood years later (Akeda; Caplan; Cassiede; Connolly; Dugrillon; Gangji; Grigoriadis; Healey; Hernigou; Landesberg; Lindholm; Majors; Muschler; Okuda; Pittenger; Tiedeman; Weibrich):
1) bone marrow not only contains multiple growth factors (platelet-derived growth factor PDGF, transforming growth factor-beta TGF-beta and vascular endothelial growth factor VEGF) but pluripotent mesenchymal stem cells (MSCs) and their progenitors as well,
2) aspirated bone marrow contains a mean of proximately forty million nucleated cells and approximately 2.000 connective tissue progenitor cells per milliliter or about one connective tissue progenitor cell per 20.000 cells (Muschler),
3) humans differ significantly from one another with respect to the cellularity of bone marrow and the prevalence of progenitor cells (Muschler),
4) a decline in progenitor cells occurs by aging (Muschler),
5) if the number of progenitor cells is <70,000/cm³, an adverse outcome is likely (Muschler),
6) red blood cells should be excluded as peripheral blood causing contamination,
7) MSCs provide significant osteoinductive capabilities,
8) MSCs are capable of developing into mature osteoblasts when exposed to the appropriate growth factors (Cassiede; Grigoriadis).
100. Mesenchymal stem cells. Basis research data: discogenic degeneration in animals
Discogenic degeneration in quadrupedal animals needs to be induced. It usually is performed by simply puncturing the annulus fibrosus (Osti) or sometimes by discectomy. However, the resulting degenerative IVD changes will never exactly mimic those of the human age-dependent IVD degenerative cascade. Moreover, nearly all animals have remaining notochordal cells which are well known to prevent IVD degeneration by producing large amounts of proteoglycans responsible for attracting water into the IVD. Nevertheless, studying the fate of mesenchymal stem cells (MSCs) in such ‘induced’ animals is particularly promising for the development of innovative MSC-based therapies. Translating the data obtained in animals to the human degenerating and degenerated IVD may one day result in neutralising the degenerative processes causing low back pain (LBP) related to the degenerative discogenic syndrome (DDS).
In all settings in which cells are transplanted, access to substrate molecules (oxygen, glucose, and amino acids) and clearance of products of metabolism (CO2, lactate, and urea) are critical to cell survival (Muschler). In all vertebrates, oxygen is the major limiting factor in the survival of transplanted MSCs and their connective tissue progenitors. However, these cells can be preadapted to hypoxic conditions prior to transplantation.
This is important because the NP cells in cartilage are exceptional for maintaining viability in avascular and hypoxic conditions (Garcia). Exposing and culturing MSCs to TGF-β1, to low oxygen tensions, and alongside cells from the NP, will activate genes in the MSCs to differentiate into a phenotype consistent to the chondrocytic-like cells in the NP able to survive in harsh conditions. Such transdifferentiated MSCs are even stimulated by prolonged hypoxia to increase their aggrecan and type II collagen synthesis (Abdel-Aleem; Bartels; Caplan; Jargiello; Le Visage; Richardson; Risbud; Sakai; Steck; Tuli). When such MSCs are not exposed to growth factors (TGFs-β, BMPs and GDF-5), their culturing rather favours the formation of fibrocytes (Caplan; Jargiello).
Many MSCs and progenitor cells in bone and bone marrow have a high capacity to survive in hypoxic conditions (Burwell) and are stimulated as well by hypoxia (Ivanovic; Lennon; Morrison; Ramirez-Bergeron; Studer). The capacity to convert to glycolysis - a 10-enzyme catalysed reaction to break down glucose - in response to hypoxia is the major adaptive mechanism. Prolonged hypoxia favours their formation of cartilage or fibrous tissue (Caplan; Connolly). In quadrupedal animal models of IVD degeneration, bone marrow MSCs injected in the void of extracted NP survive for months in dogs (Hiyama; Hohaus), pigs (Henriksson), rabbits (Sakai), and sheep (non-publicized data). Some of these MSCs differentiate into chondrocyte-like NP cells that are phenotypically similar to those in the NP (Bi; Risbud; Steinert).
The transplanted MSCs induce the synthesis of the extracellular matrix proteins, including aggrecan and other proteoglycans, and types I and II collagens. They also inhibit expressions of degradative enzymes and inflammatory cytokines. Finally, the injection of such MSCs results in better preservation of height and water content of the IVD (Henriksson; Hiyama; Hohaus; Miyamoto; Sakai; Verzijl).
These biological features temporarily delay further progression of NP degeneration by maintaining the structural morphology of the IVD (Masuda; Walsh; Yoshimura). Indeed, this researched method of only implanting autologous but transdifferentiated MSCs can halt the degenerative progression to some extent but there is no evidence of fully arresting the IVD degenerative cascade (Hoogenboom; Huang; Leung; Nerurkar). Sometimes, and depending on the ‘severity’ of IVD degeneration, partial regeneration is observed.
101. Mesenchymal stem cells. Arguments for their injection in the degenerated human intravertebral disc
To the author, it gradually became clear that aspirating autologous MSCs from bone marrow and from abdominal fat and implanting these into a degenerated IVD seemed to be well suited as a more pathophysiological approach in arresting (or potentially reversing) the degenerative intradiscal processes in bipedal humans. It is hoped that one day ‘MSC-based tissue engineering’ approaches will be developed to replace the destructive discogenic procedures associated with rigid fixations (arthrodesis) and artificial implants (nucleoplasty and disc arthroplasty) in the treatment of the degenerative discogenic syndrome.
What are the arguments to try to inject MSCs directly into the IVD?
1) The NP contains MSCs that are similar to the MSCs recovered from bone marrow (Blanco),
2) MSCs in the degenerated cartilaginous endplate have a similar morphology, proliferation rate, cell cycle, and stem cell gene expression to bone-marrow MSCs. In addition, these endplate MSCs can be induced into osteoblasts, chondroblasts or fibroblasts, and are superior to the bone marrow derived MSCs in terms of osteogenesis, chondrogenesis, or fibrogenesis (depending or their induction) (Liu),
3) progenitor cells can easily be harvested in great numbers from the iliac crest marrow and from abdominal fat (Hoogenboom; Majors; McLain; Muschler),
4) the prevalence of progenitors in fat is one per 4.000 cells (Muschler),
5) aspirated bone marrow contains growth factors and one progenitor cell per 20.000 cells (Muschler),
6) bone marrow MScs contain cell populations which stain positively for mesenchymal osteoblastic phenotypes (Cbfa1 and Msx-2) and for chondrocytic phenotypes (Sox-9),
7) directly processing bone marrow by a centrifuge before transplantation removes components of bone marrow that may inhibit the fibroblastic response and increases the number of fibroblast progenitor cells (Muschler),
8) autologous MSCs possess extensive self-renewal capability (Caplan),
9) human MSCs have been injected into porcine IVDs. They were found to have survived, differentiated toward IVD cells, and stimulated the synthesis of EC matrix components (Crevenstein; Henriksson; Hiyama; Le Visage; Mackay; Serigano; Vadalà; Wei; Zhang; Wei),
10) autologous MSCs escape alloantigen rejection because the IVD is an avascular structure and as such immune-privileged (Leung; Liu; Nomura; Ryan; Zhang).
102. Mesenchymal stem cells. Some unresolved problems for human intradiscal application
Some major problems remain to be able to neutralise the intradiscal degenerative processes.
Firstly. A mesenchymal cell density of 100 million progenitor cells per ml is required (Zhang).
Secondly. The actually existing MSC delivery systems are prepared in room air so that an enriched cellular graft of human fat and concentrated bone marrow MSCs can be saturated with oxygen.
103. Scaffold tissue engineering. Meaning
Tissue engineering, a method for harvesting and transplanting tissue-forming cells, is exiting. For decades scientists believe that, by seeding cells into a degradable biomaterial and architectural scaffold ex vivo, they can fabricate functional biological substitutes for the replacement of lost or damaged tissues in vivo (Alini; Andersen; Badylak; Baiguera; Bertram; Butler; Caterson; Gonfiotti; Griffith; Hubbell; Langer; Lee; Li; Muschler; Pfeiffer; Nesti; Russell; Sakai; Sato; Vogel).
But whole organ tissue engineering is akin to converting a Ford into a Ferrari while driving at top speed. Newly synthesized scaffolds should be comparable to and integrated within native tissues as to assure proper long-term biomechanics and minimize risk migration of the scaffold.
104. Scaffold tissue engineering related to the degenerated IVD
Signs and symptoms related to the degenerative processes in the IVD are the clinical expression of an IVD failing as a functionally integrated unit.
The ideal biological and cell-based tissue-engineered solution is to repair, replace, or regenerate the main components of the IVD, the annulus fibrosis and the nucleus pulposus, which are injured and resected during surgery as well.
To achieve such a biological engineered IVD, fundamental knowledge in physics, chemistry, and biology of the IVD needs to be translated into practical and effective therapeutic strategies (Chen; Ganey; Huang; Imai; Leung; Le Visage; Masuda; Meisel; Muschler; Nerurkar; Nerurkar; Nesti; Paesold; Sakai; Tsai; Wilke).
105. Scaffold tissue engineering for the degenerated IVD aims at replacing actual existing surgeries.
If the aim is to restore height and physiological mobility at the level of a degenerated IVD, tissue engineering would require a replacement of the morbid NP by an artificial compound such as a collagen scaffold.
The aim of a bioengineered IVD is to replace the currently implanted plate and screw devices (arthrodesis) as well as the metal and polymer total disc replacement (arthroplasties) devices which do not biointegrate, release exogenous wear particles with unknown long-term consequences, and initiate wear-induced osteolysis. Bioengineered IVDs do biointegrate and as such may potentially maintain and even improve their functions with time (Hallab; von Knoch).
Till now (2015), none of the engineered artificial implants fulfills the clinical and biomechanical expectations.
106. Scaffold tissue engineering hopes to invent an ideal matrix for the IVD
It would be a marvelous invention if an in vivo implantation of a single population of MSCs into a suitable scaffold could lead to the in vivo production of a cartilage, osseous or fibrous composite. Scaffold matrices
(1) serve as space-holders to prevent encroachment of surrounding tissues into a graft,
(2) are the ideal vehicle to provide for directing cells into a graft site,
(3) provide a void volume in which vascularisation, new tissue formation and remodelling can occur,
(4) have surfaces that facilitate the attachment, survival, migration, proliferation, and differentiation of stem cells and progenitors (Badylak; Muschler).
107. Scaffold tissue engineering for an IVD: very challenging … and how to implant the engineered scaffold?
The development of a tissue engineered scaffold for guiding engineered MSCs in the intervertebral disc is challenging (Alini).
Major problems remain:
(1) the impenetrability of the endplates,
(2) the biomolecular construction of the scaffold,
(3) a method to guide MSCs through the scaffold, and
(4) the implantation method of the scaffold.
The existing technique of implantation a scaffold (with cells) through a hole in the annulus fibrosus remains one of the major reasons for the surgical, biomechanical and clinical failures (Allen; Husson; Ray; Wilke).
108. Scaffold tissue engineering for an IVD. Cell growth and differentiation
Understanding the role of biomechanics in controlling cell growth and differentiation is essential to delineate how the mechanical characteristics of a tissue can be matched.
For engineering synthetic matrices to promote tissue regeneration, it remains essential to consider that living cells continuously measure, process, and store cellular and environmental information in response to specific signals. Cells have an arsenal of potential mechanisms for motility at their disposal and use it to adapt to the complex physical and chemical environments that they encounter in vivo (DeSimone). I.e. it has been shown that the plasticity of an individual fibroblast depends on differences in ECM architecture (Petrie).
Therefore, bioengineers aim at constructing ‘computerized’ living cells to control targeted biological processes (Weber) such as for guiding them correctly in a scaffold. Introducing precise mutations into the genomic DNA, which stores information in living cells, converts transient information into permanent genomically encoded memory for long-lasting responses (Farzadfard).
109. Scaffold tissue engineering for an IVD. Cells need a matrix.
Cells alone are not sufficient for ‘reconstructing’ an organ. A scaffold or matrix is almost certainly required for fabrication of tissue-engineered constructs.
But there is still a long way to go for machining or engineering an ideal scaffold that structurally, biochemically, and biomechanically is biocompatible with the seeded cells (Kandel). The bulk material of scaffolds should be made of biological and resorbable materials, have a three-dimensional architecture and porosity, and have sufficient strength to bear or share substantial load after implantation. The scaffolds need to be precoated with bioactive molecules (i.e. growth factors) to elicit a receptor-mediated signal to the targeted MSCs enhancing new tissue formation, provide and maintain an environment of tissue osmolarity and pH, and finally, be able to clear the degradation products of the degradable matrices (Abukawa; Bouxsein; Langer; Lee; Lutolf; Muschler; Pachence; Sales; Venugopal).
110. Scaffold tissue engineering for an IVD. Nanotechnological scaffold?
None of the broad range of scaffolds, already available for clinical use, fulfils all of these prerequisites. Unlike metal, ceramic, and synthetic polymer devices, a tissue-engineered scaffold will not release exogenous wear particles over the life of the implant.
Cancellous bone, allograft bone matrix, spongelike polymers made of collagens or hyaluronan, mineral based matrices (tricalcium phosphate) cannot be used as they are not sturdy enough scaffolds to maintain potentially restored IVD disc height (Ponticiello; Vernengo).
Implantation in the IVD of in vitro bioengineered disc tissue has been tried (Alini; Ganey; Meisel).
The development of customised nanotechnological 3D-printed scaffolds (10-9 m) probably will revolutionize IVD surgery (Birchall; Christensen; de Mel).
111. Scaffold tissue engineering. What about the sclerotic endplates in the degenerated IVD?
If the scaffold needs to repair the degenerated IVD and be capable of restoring and maintaining the IVD height and the normal function of the IVD, it will not be restorative when the endplates are sclerotic and thickened because fluid and nutrient diffusion is minimal or non-existent. In order for this technology to succeed, it would be essential to pre-treat the vertebral endplate by decalcifying the impermeable and sclerotic cartilaginous endplate and/or to stimulate the vascularity of the osseous part of the endplate.
112. Why not using a scaffold of fat graft tissue seeded with engineered MSCs?
In the author’s opinion and surgical experience, it is not always essential to restore the normal IVD height and its physiological mobility. It may be sufficient to ‘transform’ the degenerated IVD into a painless fibrous structure. What then remains important is to be sure the nerve roots are not encroached by further narrowing of the spinal canal and the spinale neuroforamina.
If fibrous cicatrisation of the degenerated IVD could be the main goal, the biodegradable matrix-rich scaffold does not need to have a special microscopic architecture to provide ‘room’ for cells. It only needs to allow replacement of the void left by the degenerative processes with a fibrous tissue.
Intradiscal implantation of a scaffold of fat graft tissue seeded with engineered MSCs and their progenitor cells may stimulate fibrogenesis and replace the NP by fibrous tissue. Propping up the empty NP with fat, engineered MSCs and GFs will not regenerate the NP. Further biomechanical load might transform this autologous fat grafts into fibrotic scar tissue.
Such a scaffold will not elicit immunological reactions nor inflammatory responses, but may stabilize the inflammatory and pain producing mechanisms by formation of fibrotic scar tissue.
Such a scaffold may have the consistency for needle-guided graft delivery. The aim is to provide the degenerated segment the same ‘biomechanical stiffness ’ as the fibrous tissue seen in very degenerated but painless human IVDs.
113. Conclusions by the author
In his famous ‘The Logical Song’, the Voice of Supertramp Roger Hodgson proclaims that ‘Sometimes, it’s essential to be sensible, logical, responsible and practical’.
The author believes that the public needs to better understand the uncertain practice of spinal directed medicine and especially spinal surgery. “What spinal medicine is doing, is intrinsically uncertain and sometimes dangerous, even though much of it is based on ‘science’. It is absurd to talk of zero harm in spinal medicine, especially in spinal surgery. I’ve never been complacent about my bad results. In truth, there have been very few real advances in spinal surgery during my time as a spinal orthopaedic surgeon. Continuous uncertainty remains inherent in my work and I still get nervous before an operation. Most of my clinical spinal life, I spent in a state of mild anxiety because the decision to prescribe a particular treatment could never be done with an absolute statistical certainty. Unfortunately, I witnessed the corrupting effects of profit-seeking in the spinal biotechnical companies interfering in promising innovative spinal treatments which could disrupt their businesses (Phishing for Phools by Akerlof & Shiller). Because of the change in working patterns brought about by the European Working Time Directive, it is just a fact of life that today’s spinal surgeons, having finished their clinical and surgical fellowships, do not gain sufficient expertise in basic sciences related to the pathologies of the spine. However, there is an absolute need for teaching and supporting the spinal surgeons in basic spinal sciences”.
Percutaneous engineered mesenchymal stem cell (MSC) therapy, mediating the neutralisation of the IVD degeneration, may one day bridge the gap between the two current alternatives for patients with low back pain (LBP) related to the degenerative discogenic syndrome: inadequate pain management at one end and inadequate invasive surgery at the other (Brox; Deyo; Fairbank; Fritzell; Mulholland; Yuan).
Indeed, in comparison to the treatment results of very complex limb fractures as well as in comparison to diabetes type I and II where the combination of insulin and nutritional changes seem to interfere positively with the normal biochemistry pathways of the human body, none of all the varying, long-time existing and continuous new developed alternative, conservative, non-invasive, and surgical spinal procedures - whatever they might have been and be till now - has an universally accepted and proven long-lasting beneficial and painless effect. Simply because none of these hundreds of treatments interferes directly with the degenerative processes in the aging IVD. But it’ right. Whenever there’s a new technology, it goes from a peak of inflated expectations, to a trough of disillusionment, to a slope of enlightenment, and a plateau of productivity.
LBP due to the degenerative discogenic syndrome is the most prominent pathological condition in the world causing most years lived with disability (Vos). Permanently treating the recurring symptoms of the underlying but irrevocably progressing and (as yet) non-curable degenerative processes in the IVD (whatever the therapy), will continue to create huge financial costs both to the sufferers and to society. No doubt that societies and LBP-patients are still awaiting a ‘miracle cure’. Hopefully, in the not too distant future, the emphasis of treating degenerative discogenic conditions will shift from annihilating to preserving the biomechanical functions of the IVD.
Then, what characterises the degeneration of an intervertebral mobile segment? And why is it so difficult to provide a simple but well working treatment for the majority of these LBP sufferers?
Degeneration, involving the IVD and the zygapophyseal facetal joints, is influenced by a complex interaction among genetics, age, both local and systemic biological stimuli, and mechanical factors.
In the bipedal human beings, the early IVD degrading stages are characterized by a decreased extracellular matrix synthesis (proteoglycans and collagens), a decreased capacity of hydrating the IVD, and an increasing abnormal pro-inflammatory cytokine and matrix degrading metalloproteinase (MMPs) environment. All cause substantial alterations of intradiscal pressure, of mechanical load distribution, of functional motion patterns, and result in a loss of IVD height. But because the biology of the IVD is continuously dominated by the presence or absence of mechanical loads and stresses onto the intervertebral segment, the therapeutic focus will need to move from present day simple anatomic and radiological considerations to emphasis on maintaining the essential mechanobiology of the IVD. And that seems to be more difficult than going to the moon or other planets.
Human degenerated IVDs have an inexistent healing potential as they are very large avascular structures, do not contain (healing) notochordal cells, have a very low cell density with an increasing number of dying cells by apoptosis, present a disturbed or non-existent nutrition of the nucleus pulposus, contain senescent cells with low metabolic activity, and develop a progressing fibrous transition. All these elements result in a NP becoming acidic, hypoxic, ischemic, inflammatory, and poor in nutrients (Brisby; Horner; Nguyen-minh; Urban; Travascio; Wang).
For these reasons it will be extremely challenging for whatever biologically based therapy to reverse and/or to regenerate the aging and degenerative IVD in humans if only the researched innovative gene therapies, cellular (i.e. MSC/chondrocytes) and molecular (i.e. growth factor therapy) approaches are applied into the damaged and unsupported IVD (Kobayashi; Snake). If the major aim is to influence the cellular and molecular components of the IVD to arrest and to neutralise or even to fully regenerate the degenerating or degenerated IVD, then this biological environment needs to be mechanically protected and supported to equalise the daily sustained loads (sitting, standing, bending for over, etc).
On the other hand, it is the author’s conviction that it is physiologically inappropriate to fully regenerate a degenerating IVD in a middle-aged individual. The normal age-related degenerative processes start in all IVDs from the age of 25 onwards. The majority of the world’s population at the age of 40 already presents major age-related molecular and degenerative changes all over their bodies and even in their painless degenerating spines.
Why then is it really essential to entirely reconstruct the normal biochemical disc architecture (Panjabi’s center of rotation, neutral zone, nucleode)? Is it really imperative to restore the localised limited intervertebral motions as the other remaining spinal segments, and of course the hip joints, largely compensate for the loss of IVD motion? Is it really required to rejuvenate aging, senescent, and apoptotic cells in the NP? Is it of any interest to provide new and regenerated tissues into an aging body?
Asking for an aging structure to become ‘young and functionally attractive’ again, probably is the ultimate human dream! But simply stated, as long as there exists no sensible, no logical, no responsible, and no practical therapeutic biological approach, taking into account all the known elements of the degenerative IVD, the underlying degenerative pathology in the IVD will not and cannot be addressed directly. Nature goes her way.
All actually researched innovative biological therapies aim at stimulating the remaining viable, still dividing, but metabolically impaired cells in the NP and at inhibiting the degradation and/or enhancing the synthesis of the ECM. Indeed, in the early stages of the aging IVD the degradative processes still do not overwhelm the anabolic capabilities of the disc. By arresting the intradiscal degenerative processes, the biological treatments hope at least to annihilate the painful loadbearing functions and motions to make aggressive surgery obsolete in patients in the fourth decade or older.
Some major problems remain before the new generation of biological therapies will be able to prove their efficacy and efficiency.
It is still totally unknown how to define ‘early degenerative changes’. Radiological techniques arbitrarily describe different degenerative stages but they are not necessarily associated with pain and have no therapeutic value. And how can pain be objectively evaluated? In the future innovative imaging techniques, comparable to the CLARITY scanning in mice, need to be developed to show cellular and molecular changes related to pain. At the same time a quantitative method to detect early stages of disc degeneration is needed for the emerging innovative gene, cell and molecular therapies to be efficient.
The technique to inject a product intradiscally is not too difficult. But until now it is only performed by creating a puncture or incisional wound into the annulus fibrosus. However, it is well known that inducing microdamages by the used needles or blades (Osti) cause intradiscal inflammatory response and further increase the degenerative processes and fibrous scar formation in the IVD. It is not recommended to further worsen the function of the IVD by increasing the outer annulus to bulge outwards and the inner annulus to bulge inwards when under axial load. Both annular bulgings provoke the circumferential annular tears to progress and to cause further IVD joint insufficiency. Although it has been demonstrated that the annulus has some potential to repair itself (Sélard), it will be essential to avoid an annular approach for whatever clinical application to become successful in neutralising the degenerative processes. In the author’s opinion another needle-guided approach needs to be thought of, taught, and commercialized.
It is well known that transplanted thin corneal grafts (as the author underwent himself) and thin cartilaginous grafts are not rejected while the vascularization in the receiving to-be-grafted tissue is non-existent. However, implanting cells in an acidic, hypoxic, ischemic, inflammatory environment nearly without nutrients, is not a situation where successful treatment can be anticipated. There exists a saying that indicates that ‘you can inject all the best cells, but if you don’t have the right combination of healing goodies around them, it’s useless’. Even if it is already possible to engineer MSCs to function in hypoxic conditions, at least, the degenerated nucleus pulposus (with all the debris) should be aspirated.
I refer to the separate chapter