The contribution of anulus fibers to torque resistance

Tissue Engineering Strategies for Intervertebral Disc Treatment Using Functional Polymers

Intervertebral disc (IVD) is the fibrocartilage between the vertebrae, allowing the spine to move steadily by bearing multidirectional complex loads. Aging or injury usually causes degeneration of IVD, which is one of the main reasons for low back pain prevalent worldwide and reduced quality of life. While various treatment strategies for degenerative IVD have been studied using in vitro studies, animal experiments, and clinical trials, there are unsolved limitations for endogenous regeneration of degenerative IVD. In this respect, several tissue engineering strategies that are based on the cell and scaffolds have been extensively researched with positive outcomes for regeneration of IVD tissues. Scaffolds made of functional polymers and their diverse forms mimicking the macro- and micro-structure of native IVD enhance the biological and mechanical properties of the scaffolds for IVD regeneration. In this review, we discuss diverse morphological and functional polymers and tissue engineering strategies for endogenous regeneration of degenerative IVD. Tissue engineering strategies using functional polymers are promising therapeutics for fundamental and endogenous regeneration of degenerative IVD.

Biomechanical test protocols to detect minor injury effects in intervertebral discs

Intervertebral discs (IVDs) maintain flexibility of the spine and bear mechanical load. Annulus fibrosus (AF) defects are associated with IVD degeneration and herniation which disrupt biomechanical function and can cause pain. AF puncture injuries can induce IVD degeneration but are needed to inject therapies. Identifying small AF defects with biomechanical testing can be difficult because IVDs have a complex, composite structure and nonlinear biomechanical properties that are dependent on AF fiber tension. It remains unclear how choice of biomechanical testing protocols affect the sensitivity of biomechanical properties to AF injuries. This study determined whether axial preload or magnitude of cyclic axial or torsional testing affected the ability to detect minor AF defects in rat caudal motion segments using ex vivo biomechanical testing. Intact and injured motion segments were subjected to a repeated measures study design with multiple biomechanical testing protocols that varied axial tension-compression force amplitude (±1.6 N, ±8.0 N, ±16.0 N), axial preload (-1.6 N, -8.0 N, -16.0 N, corresponding to -0.1 MPa, -0.5 MPa, and -1.0 MPa, respectively), and torsional rotation angle (±10°, ±15°, and ±20°). Biomechanical properties obtained from the lowest force testing conditions for axial tension-compression (±1.6 N), axial preload (-1.6 N), and angular rotation (±10°) exhibited the largest differences in biomechanical properties between intact and injured conditions. Biomechanical properties determined under low axial force or torsion amplitudes involve less AF fiber tension and were most sensitive to injury. Low force testing protocols are recommended for detecting minor structural AF defects and may enable more precise assessments of IVD injuries, healing or repair.

Molecular Mechanisms of Intervertebral Disc Degeneration

Intervertebral disc degeneration is a well-known cause of disability, the result of which includes neck and back pain with associated mobility limitations. The purpose of this article is to provide an overview of the known molecular mechanisms through which intervertebral disc degeneration occurs as a result of complex interactions of exogenous and endogenous stressors. This review will focus on some of the identified molecular changes leading to the deterioration of the extracellular matrix of both the annulus fibrosus and nucleus pulposus. In addition, we will provide a summation of our current knowledge supporting the role of associated DNA and intracellular damage, cellular senescence's catabolic effects, oxidative stress, and the cell's inappropriate response to damage in contributing to intervertebral disc degeneration. Our current understanding of the molecular mechanisms through which intervertebral disc degeneration occurs provides us with abundant insight into how physical and chemical changes exacerbate the degenerative process of the entire spine. Furthermore, we will describe some of the related molecular targets and therapies that may contribute to intervertebral repair and regeneration.

Molecular mechanisms of biological aging in intervertebral discs

Advanced age is the greatest risk factor for the majority of human ailments, including spine-related chronic disability and back pain, which stem from age-associated intervertebral disc degeneration (IDD). Given the rapid global rise in the aging population, understanding the biology of intervertebral disc aging in order to develop effective therapeutic interventions to combat the adverse effects of aging on disc health is now imperative. Fortunately, recent advances in aging research have begun to shed light on the basic biological process of aging. Here we review some of these insights and organize the complex process of disc aging into three different phases to guide research efforts to understand the biology of disc aging. The objective of this review is to provide an overview of the current knowledge and the recent progress made to elucidate specific molecular mechanisms underlying disc aging. In particular, studies over the last few years have uncovered cellular senescence and genomic instability as important drivers of disc aging. Supporting evidence comes from DNA repair-deficient animal models that show increased disc cellular senescence and accelerated disc aging. Additionally, stress-induced senescent cells have now been well documented to secrete catabolic factors, which can negatively impact the physiology of neighboring cells and ECM. These along with other molecular drivers of aging are reviewed in depth to shed crucial insights into the underlying mechanisms of age-related disc degeneration. We also highlight molecular targets for novel therapies and emerging candidate therapeutics that may mitigate age-associated IDD. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1289-1306, 2016.

Course Setting as a Prevention Measure for Overuse Injuries of the Back in Alpine Ski Racing: A Kinematic and Kinetic Study of Giant Slalom and Slalom

Background:

A combination of frontal bending, lateral bending, and torsion in the loaded trunk has been suggested to be a mechanism leading to overuse injuries of the back in Alpine ski racing. However, there is limited knowledge about the effect of course setting on the aforementioned back-loading patterns.

Purpose:

To investigate the effect of increased gate offset on the skier’s overall trunk kinematics and the occurring ground-reaction forces and to compare these variables between the competition disciplines giant slalom (GS) and slalom (SL).

Study Design:

Controlled laboratory study.

Methods:

Ten top-level athletes were divided into GS and SL groups. Both groups performed a total of 240 GS and 240 SL turns at 2 different course settings. The overall trunk movement components (frontal bending, lateral bending, and torsion angle) were measured using 2 inertial measurement units fixed on the sacrum and sternum. Total ground-reaction forces were measured by pressure insoles.

Results:

In SL, ground-reaction force peaks were significantly lower when the gate offset was increased, while in GS, no differences between course settings were observed. During the turn phase in which the highest spinal disc loading is expected to occur, the back-loading patterns in both GS and SL included a combination of frontal bending, lateral bending, and torsion in the loaded trunk. SL was characterized by shorter turns, lower frontal and lateral bending angles after gate passage, and a trend toward greater total ground-reaction force peaks compared with GS.

Conclusion:

Course setting is a reasonable measure to reduce the skier’s overall back loading in SL but not in GS. The distinct differences observed between GS and SL should be taken into account when defining discipline-specific prevention measures for back overuse injuries.

Clinical Relevance:

To reduce the magnitude of the overall back loading, in SL, minimal gate offsets should be avoided. Prevention measures in GS might particularly need to control and/or reduce the magnitude of frontal and lateral bending in the loaded trunk, whereas prevention measures in SL might especially need to mitigate the short and high total ground-reaction force peaks.

Fundamental biomechanics of the spine--What we have learned in the past 25 years and future directions

Since the publication of the 2nd edition of White and Panjabi׳s textbook, Clinical Biomechanics of the Spine in 1990, there has been considerable research on the biomechanics of the spine. The focus of this manuscript will be to review what we have learned in regards to the fundamentals of spine biomechanics. Topics addressed include the whole spine, the functional spinal unit, and the individual components of the spine (e.g. vertebra, intervertebral disc, spinal ligaments). In these broad categories, our understanding in 1990 is reviewed and the important knowledge or understanding gained through the subsequent 25 years of research is highlighted. Areas where our knowledge is lacking helps to identify promising topics for future research. In this manuscript, as in the White and Panjabi textbook, the emphasis is on experimental research using human material, either in vivo or in vitro. The insights gained from mathematical models and animal experimentation are included where other data are not available. This review is intended to celebrate the substantial gains that have been made in the field over these past 25 years and also to identify future research directions.

Potential Mechanisms Leading to Overuse Injuries of the Back in Alpine Ski Racing: A Descriptive Biomechanical Study

BACKGROUND

Overuse injuries of the back are a common complaint among top athletes and of competitive alpine skiers in particular. However, there is limited understanding about the sport-specific causes of these injuries that is essential for their prevention.

PURPOSE/HYPOTHESIS

This study was undertaken to describe the sport-specific, overall trunk kinematics and skiers' loading during giant slalom turns and to assess the plausibility of the hypothesis that a combination of frontal bending, lateral bending, and/or torsion in the loaded trunk might be a potential mechanism leading to overuse injuries of the back in alpine ski racing.

STUDY DESIGN

Descriptive laboratory study.

METHODS

Eight European Cup-level athletes performed giant slalom runs with 2 different pairs of skis (varying in length, width, and sidecut). They were analyzed with respect to selected kinematic variables related to spinal disc loading. The overall trunk movement components (frontal bending, lateral bending, and torsion) were measured using 2 inertial measurement units fixed on the sacrum and sternum. Total ground-reaction forces were measured by pressure insoles.

RESULTS

During the turn phase in which the total ground-reaction forces were the greatest (up to 2.89 times the body weight), the highest average values of frontal bending (38.7°), lateral bending (14.7°), and torsion (7.7°) in the trunk occurred. Similar magnitudes were observed when skiing on longer, giant slalom skis with less width and sidecut.

CONCLUSION

The typical loading patterns of the back in alpine ski racing include a combined occurrence of frontal bending, lateral bending, and torsion in the loaded trunk. Because these factors are known to be related to high spinal disc loading, they may be considered important components of mechanisms leading to overuse injuries of the back in alpine ski racing.

CLINICAL RELEVANCE

Prevention measures should aim to control and/or reduce the magnitude of frontal bending, lateral bending, and torsion in the trunk, as well as the peak loads, while skiing.

Characteristics of lumbar disc herniation with exacerbation of presentation due to spinal manipulative therapy

The aim of this article was to delineate the characteristics of lumbar disc herniation (LDH) in patients with exacerbation of symptoms caused by spinal manipulative therapy (SMT). The main emphasis should be on the prevention of this condition by identifying relevant risk factors. Detailed clinico-radiological profiles of a total number of 10 LDH patients with exacerbation of presentation after SMT were reviewed. All the patients underwent neurological and magnetic resonance imaging examinations. Laminectomy and discectomy were performed, and follow-up was carried out in all patients. The duration of symptoms in the patients before SMT was 4-15 years. After the therapy, an acute exacerbation of back and radicular pain was observed within 24 h. Magnetic resonance imaging showed that L4-L5 was the most frequently affected level observed (7 patients), and each patient had a large disc fragment in the spinal canal. The disc fragments were classified into 3 types according to their localizations. The time internal between the exacerbation of presentation and surgery was 23.1 days. No perioperative complications were noted. All the patients were relieved of radicular pain a few days after surgery. During postoperative follow-up, all patients regained the ability to walk; one patient received catheterization for 1 month and another for 6 months. Eight patients reported a complete resolution of presentation and the rest 2 patients were significantly improved. SMT should be prohibited in some LDH patients to prevent neurological damages, in whom there are 5 possible risk factors. Surgical results for these patients are encouraging.

The relationship between pelvis-trunk coordination and low back pain in individuals with transfemoral amputations

Low back pain (LBP) is common in individuals with transfemoral amputation and may result from altered gait mechanics associated with prosthetic use. Inter-segmental coordination, assessed through continuous relative phase (CRP), has been used to identify specific patterns as risk factors. The purpose of this study was to explore pelvis and trunk inter-segmental coordination across three walking speeds in individuals with transfemoral amputations with and without LBP. Nine individuals with transfemoral amputations with LBP and seven without pain were compared to twelve able-bodied subjects. Subjects underwent a gait analysis while walking at slow, moderate, and fast speeds. CRP and CRP variability were calculated from three-dimensional pelvis and trunk segment angles. A two-way ANOVA and post hoc tests assessed statistical significance. Individuals with transfemoral amputation demonstrated some coordination patterns that were different from able-bodied individuals, but consistent with previous reports on persons with LBP. The patient groups maintained transverse plane CRP consistent with able-bodied participants (p = 0.966), but not sagittal (p < 0.001) and frontal plane CRP (p = 0.001). Sagittal and frontal CRP may have been re-optimized based on new sets of constraints, such as protective rigidity of the segments, muscular strength limitations, or prosthesis limitations. Patients with amputations and without LBP exhibited few differences. Only frontal and transverse CRP shifted toward out-of-phase as speed increased in the patient group with LBP. Although a cause and effect relationship between CRP and future development of back pain has yet to be determined, these results add to the literature characterizing biomechanical parameters of back pain in high-risk populations.

Comparison of animal discs used in disc research to human lumbar disc: torsion mechanics and collagen content

STUDY DESIGN

Experimental measurement and normalization of in vitro disc torsion mechanics and collagen content for several animal species used in intervertebral disc research and comparing these with the human disc.

OBJECTIVE

To aid in the selection of appropriate animal models for disc research by measuring torsional mechanical properties and collagen content.

SUMMARY OF BACKGROUND DATA

There is lack of data and variability in testing protocols for comparing animal and human disc torsion mechanics and collagen content.

METHODS

Intervertebral disc torsion mechanics were measured and normalized by disc height and polar moment of inertia for 11 disc types in 8 mammalian species: the calf, pig, baboon, goat, sheep, rabbit, rat, and mouse lumbar discs, and cow, rat, and mouse caudal discs. Collagen content was measured and normalized by dry weight for the same discs except the rat and the mouse. Collagen fiber stretch in torsion was calculated using an analytical model.

RESULTS

Measured torsion parameters varied by several orders of magnitude across the different species. After geometric normalization, only the sheep and pig discs were statistically different from human discs. Fiber stretch was found to be highly dependent on the assumed initial fiber angle. The collagen content of the discs was similar, especially in the outer annulus where only the calf and goat discs were statistically different from human. Disc collagen content did not correlate with torsion mechanics.

CONCLUSION

Disc torsion mechanics are comparable with human lumbar discs in 9 of 11 disc types after normalization by geometry. The normalized torsion mechanics and collagen content of the multiple animal discs presented are useful for selecting and interpreting results for animal disc models. Structural organization of the fiber angle may explain the differences that were noted between species after geometric normalization.

Live free or die: stretch-induced apoptosis occurs when adaptive reorientation of annulus fibrosus cells is restricted

High matrix strains in the intervertebral disc occur during physiological motions and are amplified around structural defects in the annulus fibrosus (AF). It remains unknown if large matrix strains in the human AF result in localized cell death. This study investigated strain amplitudes and substrate conditions where AF cells were vulnerable to stretch-induced apoptosis. Human degenerated AF cells were subjected to 1 Hz-cyclic tensile strains for 24h on uniformly collagen coated substrates and on substrates with 40 μm stripes of collagen that restricted cellular reorientation. AF cells were capable of responding to stretch (stress fibers and focal adhesions aligned perpendicular to the direction of stretch), but were vulnerable to stretch-induced apoptosis when cytoskeletal reorientation was restricted, as could occur in degenerated states due to fibrosis and crosslink accumulation and at areas where high strains occur (around structural defects, delaminations, and herniations).

Percutaneous endoscopic lumbar discectomy and interbody fusion with B-Twin expandable spinal spacer

BACKGROUND

Posterior lumbar interbody fusion (PLIF) is biomechanically sound as it ablates the degenerated disc, restores the intervertebral height, relieves foraminal stenosis, and positions the bone graft along the weight-bearing axis. But this conventional procedure also results in significant traction on the dural sac and the cauda equina and is thereby a potential source of neurologic damage. Therefore, we performed a minimally invasive technique: percutaneous endoscopic discectomy and interbody fusion (PEDIF) with B-Twin expandable spinal spacer (B-twin ESS) to treat symptomatic lumbar degenerative disc disease and explored the clinical outcome.

METHODS

From June 2004 to December 2006, 43 consecutive patients with symptomatic lumbar degenerative disc disease were included in this study. There were 28 males and 15 females, with average age of 47 years (range 26-63). Following a routine micro-endoscopic discectomy or percutaneous nucleotomy, endplate curettage was meticulously carried out. The intervertebral space was packed with autograft cancellous bone cut from lumbar delivered through a 5-mm diameter funnel. The B-twin ESS was introduced into the intervertebral space and then expanded. Both stages were monitored by C-arm fluoroscopy.

RESULTS

The mean operative time was 110 ± 36 min (80-150). The mean blood loss was 350 ± 68 ml (210-700). The mean length of hospitalization was 6.6 ± 2.9 days (3-10). All patients were evaluated in follow-up of 12-30 months (mean 18). According to radiological evidence of fusion on dynamic X-ray plain film, 31 cases (72.1%) achieved bone graft fusion after 6 months postoperatively; at the final follow-up, union of the bone graft has been established in all but one patient (97.7%). The subsidence degree of the B-twin ESS was 20-30% in 2 cases and in rest of the cases less than 10%. According to the modified criteria of Chinese Orthopaedic Association, excellent in 31 cases, good in 8 cases, fair in 3 cases, poor in 1 case, and the rate of excellent and good cases was 91%.

CONCLUSION

The PEDIF technique provided an option for percutaneous interbody fusion similar to that in open surgery while minimizing destruction to adjacent tissues. This technique was safe and exhibited a trend toward decreased intraoperative blood loss, length of stay and the risk of neurological complications.

Genipin-crosslinked fibrin hydrogels as a potential adhesive to augment intervertebral disc annulus repair

Treatment of damaged intervertebral discs is a significant clinical problem and, despite advances in the repair and replacement of the nucleus pulposus, there are few effective strategies to restore defects in the annulus fibrosus. An annular repair material should meet three specifications: have a modulus similar to the native annulus tissue, support the growth of disc cells, and maintain adhesion to tissue under physiological strain levels. We hypothesized that a genipin crosslinked fibrin gel could meet these requirements. Our mechanical results showed that genipin crosslinked fibrin gels could be created with a modulus in the range of native annular tissue. We also demonstrated that this material is compatible with the in vitro growth of human disc cells, when genipin:fibrin ratios were 0.25:1 or less, although cell proliferation was slower and cell morphology more rounded than for fibrin alone. Finally, lap tests were performed to evaluate adhesion between fibrin gels and pieces of annular tissue. Specimens created without genipin had poor handling properties and readily delaminated, while genipin crosslinked fibrin gels remained adhered to the tissue pieces at strains exceeding physiological levels and failed at 15-30%. This study demonstrated that genipin crosslinked fibrin gels show promise as a gap-filling adhesive biomaterial with tunable material properties, yet the slow cell proliferation suggests this biomaterial may be best suited as a sealant for small annulus fibrosus defects or as an adhesive to augment large annulus repairs. Future studies will evaluate degradation rate, fatigue behaviors, and long-term biocompatibility.

Facet joint biomechanics at the treated and adjacent levels after total disc replacement

STUDY DESIGN

Biomechanical study using human cadaveric lumbar spines.

OBJECTIVE

To evaluate effects of total disc replacement (TDR) on spine biomechanics at the treated and adjacent levels.

SUMMARY OF BACKGROUND DATA

Previous studies on spine biomechanics after TDR were focused on facet forces and range of motion and report contradictory results. Characterization of contact pressure, peak contact pressure, force, and peak force before and after TDR may lead to a better understanding of facet joint function and may aid in prediction of long-term outcomes after TDR.

METHODS

Seven fresh-frozen human cadaveric lumbar spines were potted at T12 and L5 and installed in a 6 degrees of freedom displacement-controlled testing system. Displacements of 15° flexion/extension, 10° right/left bending, and 10° right/left axial rotation were applied. Contact pressure, peak contact pressure, force, peak force, and contact area for each facet joint were recorded at L2-L3 and L3-L4 both before and after TDR at L3-L4. The data were analyzed with analysis of variance and t tests.

RESULTS

Axial rotation had the most impact on contact pressure, peak contact pressure, force, peak force, and contact area in intact spines. During lateral bending and axial rotation, TDR resulted in a significant increase in facet forces at the level of treatment and a decrease in contact pressure, peak contact pressure, and peak force at the level superior to the TDR. With flexion/extension, there was a decrease in peak contact pressure and peak contact force at the superior level.

CONCLUSION

Our study demonstrates that rotation is the most demanding motion for the spine. We also found an increase in facet forces at the treated level after TDR. We are the first to show a decrease in several biomechanical parameters after TDR at the adjacent superior level. In general, our findings suggest there is an increase in loading of the facet joints at the level of disc implantation and an overall unloading effect at the level above.

The effects of needle puncture injury on microscale shear strain in the intervertebral disc annulus fibrosus

BACKGROUND CONTEXT

Needle puncture of the intervertebral disc (IVD) is required for delivery of therapeutic agents to the nucleus pulposus and for some diagnostic procedures. Needle puncture has also been implicated as an initiator of disc degeneration. It is hypothesized that needle puncture may initiate IVD degeneration by altering microscale mechanical behavior in the annulus fibrosus (AF).

PURPOSE

Quantify the changes in AF microscale strain behavior resulting from puncture with a hypodermic needle.

STUDY DESIGN

Cadaveric IVD tissue explant study.

METHODS

Annulus fibrosus explants from bovine caudal IVDs that had been punctured radially with hypodermic needles were loaded in dynamic sinusoidal shear while being imaged with a confocal microscope. Digital image analysis was used to quantify local tissue strain and damage propagation with repeated shearing.

RESULTS

Needle puncture changed the distribution of microscale shear strains in the AF under load from homogenous (equal to far field) to a distinct pattern of high (4× far field) and low (0.25× far field) strain areas. Repeated loading did not cause further growth of the disruption beyond the second cycle.

CONCLUSIONS

Needle puncture results in a drastic alteration of microscale strain behavior in the AF under load. This alteration may directly initiate disc degeneration by being detrimental to tissue-cell mechanotransduction.

Healing of a painful intervertebral disc should not be confused with reversing disc degeneration: implications for physical therapies for discogenic back pain

BACKGROUND

Much is known about intervertebral disc degeneration, but little effort has been made to relate this information to the clinical problem of discogenic back pain, and how it might be treated.

METHODS

We re-interpret the scientific literature in order to provide a rationale for physical therapy treatments for discogenic back pain.

INTERPRETATION

Intervertebral discs deteriorate over many years, from the nucleus outwards, to an extent that is influenced by genetic inheritance and metabolite transport. Age-related deterioration can be accelerated by physical disruption, which leads to disc "degeneration" or prolapse. Degeneration most often affects the lower lumbar discs, which are loaded most severely, and it is often painful because nerves in the peripheral anulus or vertebral endplate can be sensitised by inflammatory-like changes arising from contact with blood or displaced nucleus pulposus. Surgically-removed human discs show an active inflammatory process proceeding from the outside-in, and animal studies confirm that effective healing occurs only in the outer anulus and endplate, where cell density and metabolite transport are greatest. Healing of the disc periphery has the potential to relieve discogenic pain, by re-establishing a physical barrier between nucleus pulposus and nerves, and reducing inflammation.

CONCLUSION

Physical therapies should aim to promote healing in the disc periphery, by stimulating cells, boosting metabolite transport, and preventing adhesions and re-injury. Such an approach has the potential to accelerate pain relief in the disc periphery, even if it fails to reverse age-related degenerative changes in the nucleus.

The relationship between lumbar spine kinematics during gait and low-back pain in transfemoral amputees

OBJECTIVE

Low-back pain is an important cause of secondary disability in transfemoral amputees. The primary aim of our study is to assess the differences in lumbar spine kinematics during gait between transfemoral amputees with and without low-back pain.

DESIGN

Lumbar spine kinematics in three planes were measured when the subjects walked in a motion analysis laboratory. Nine transfemoral amputees with low-back pain, eight transfemoral amputees without low-back pain, and six healthy, nonamputee subjects participated.

RESULTS

The Amputee Pain and Amputee No Pain groups were essentially the same in terms of all demographic and potentially confounding variable measures. Transfemoral amputees with low-back pain showed greater transverse plane rotational excursion in their lumbar spine during walking when compared with transfemoral amputees without low-back pain (P = 0.029; effect size = 1.03). There were no significant differences in sagittal or coronal plane lumbar spine excursions during walking between these two groups.

CONCLUSIONS

Although our study design does not allow for proving causation, increased transverse plane rotation has been associated with intervertebral disc degeneration, suggesting that increased transverse plane rotation secondary to walking with a prosthetic limb may be a causative factor in the etiology of low-back pain in transfemoral amputees. Identifying differences in lumbar motion can lead to potential preventative and therapeutic intervention strategies.

Needle puncture injury of the rat intervertebral disc affects torsional and compressive biomechanics differently

Needle puncture is a common method of inducing intervertebral disc (IVD) degeneration in small animal models and may have some similarities to IVD injury conditions such as herniation. Yet, the influence of puncture injuries on IVD biomechanics is not well understood. This study quantified the acute effects of anular injury on the biomechanics of rat caudal IVDs in compression and torsion following puncture with 30, 25 and 21 G needles. In compression, puncture injury reduced elastic stiffness by 20% for all needle sizes, but differences between control and punctured discs did not remain after compressive overload. In contrast, torsional parameters associated with anular fiber tension were affected proportionally with needle size. We conclude that IVD injuries that penetrate through the thickness of the annulus affect IVD biomechanics through different mechanisms for compression and torsion. Anular injuries affect torsional properties in a manner directly related to the amount of fiber disruption and compressive properties in a manner that affects pressurization.

Percutaneous posterior-lateral lumbar interbody fusion for degenerative disc disease using a B-Twin expandable spinal spacer

Degenerative disc disease (DDD) causes gradual intervertebral space collapse, concurrent discogenic or facet-induced pain, and possible compression radiculopathy. A new minimal invasion procedure of percutaneous posterior-lateral lumbar interbody fusion (PPLIF) using a B-Twin stand-alone expandable spinal spacer (ESS) was designed to treat this disease and evaluated by follow-up more than 1 year. 12 cases with chronic low back pain and compressive radiculopathy due to DDD refractory were selected to conservative treatment. Under fluoroscopy in the posterior-lateral position, a K-wire was advanced into the intervertebral space and a dilator and working cannula were introduced into the disc space step by step. Discectomy and endplate scratching were performed through the cannula using pituitary forceps and endplate curettage. An ESS was inserted into the intervertebral space by a B-Twin expandable spinal delivery system after some bone graft chips implanted into the disc space. The ongoing study includes intraoperative difficulties, complications, radiologic evidence of fusion and clinical outcome as scored by pre- and postoperative questionnaires pertaining to pain intensity and degree of disability. The 12 procedures of lumbar interbody fusion using stand-alone expandable spinal system through percutaneous approach were successful. Radiologic study demonstrated fusion in a total of 11 cases and only 1 exception after more than 1 year visiting. The values of Visual Analog Scale (VAS) on movement and Oswestry Disability Index (ODI) dropped by more than 80 and 67.4%, respectively. Disk space heights averaging 9.0 mm before procedure were increased to 11.5 mm 1 month (a significant difference compared with preprocedure, P < 0.01) after surgery and stabilized at 10.8 mm upon final follow-up (a significant difference compared with preprocedure, P < 0.01). The results demonstrated that the percutaneous approach for posterior-lateral lumbar interbody fusion using expandable spinal system is a valuable micro-invasion method for the DDD patients and can achieve the same outcome as with other methods.

In situ contact analysis of the prosthesis components of Prodisc-L in lumbar spine following total disc replacement

STUDY DESIGN

A three-dimensional, nonlinear finite element analysis was performed to predict the in situ contact interaction of prosthesis components of the Prodisc-L in a multisegmental lumbar model following total disc replacement (TDR).

OBJECTIVE

Efforts were made to investigate how the TDR implant contact characteristics could affect the 3-dimensional kinematics, facet loads of the lumbar spine following TDR.

SUMMARY OF BACKGROUND DATA

Although spinal motion analyses of human lumbar cadaveric models after Prodisc TDR have been widely studied, the interaction of the disc prosthesis, particularly its in situ contact mechanics, is never known.

METHODS

A validated intact multisegmental lumbar finite element model L2-L4 was altered to accommodate the TDR prosthesis through anterior approach. At L3-L4 disc space, the Prodisc-L of 6 degrees lordosis angle was implanted centrally. The model was subjected to compressive preload and pure moments to create flexion, extension, lateral bending, and axial rotation motion in physiologic range. The contact interaction between the superior component of Prodisc-L and the UHMWPE inlay were assessed in terms of contact region (CR), contact area (CA), and contact pressure (CP). Parameters of range of motion (ROM) and facet loading transfer were simultaneously analyzed and compared with those of the intact model.

RESULTS

The predicted contact area was 3.5 times larger in flexion than that observed in extension, whereas the maximum contact pressure in the disc articulation was very similar with 15.1 MPa for flexion and 14.5 MPa for extension. Joint surface incongruence was developed in extension motion. The implanted model exhibited a 91.4% increase in ROM accompanied by a 150.6% rising in facet force during extension, while the flexion motion showed the least effects of TDR. In lateral bending and axial rotation, the abnormal joint "lift off" was not seen.

CONCLUSION

The in situ function of the TDR prosthesis was highly dependent on how well the device could incorporate itself into the mechanical environment in the disc space, which has been determined by the rest of the spinal structures, including the retained disc anulus, articular facets, ligaments, vertebrae, and muscular stabilizers. The different contact interaction of the artificial disc components revealed here could be attributed to the violation of this mechanical environment which, in turn, may bring adverse effects to those spinal elements.

Measurement of local strains in intervertebral disc anulus fibrosus tissue under dynamic shear: contributions of matrix fiber orientation and elastin content

Shear strain has been implicated as an initiator of intervertebral disc anulus failure, however a clear, multi-scale picture of how shear strain affects the tissue microstructure has been lacking. The purposes of this study were to measure microscale deformations in anulus tissue under dynamic shear in two orientations, and to determine the role of elastin in regulating these deformations. Bovine AF tissue was simultaneously shear loaded and imaged using confocal microscopy following either a buffer or elastase treatment. Digital image analysis was used to track through time local shear strains in specimens sheared transversely, and stretch and rotation of collagen fiber bundles in specimens sheared circumferentially. The results of this study suggest that sliding does not occur between AF plies under shear, and that interlamellar connections are governed by collagen and fibrilin rather than elastin. The transverse shear modulus was found to be approximately 1.6 times as high in plies the direction of the collagen fibers as in plies across them. Under physiological levels of in-plane shear, fiber bundles stretched and re-oriented linearly. Elastin was found to primarily stiffen plies transversely. We conclude that alterations in the elastic fiber network, as found with IVD herniation and degeneration, can therefore be expected to significantly influence the AF response to shear making it more susceptible to micro failure under bending or torsion loading.

Localized Intervertebral Disc Injury Leads to Organ Level Changes in Structure, Cellularity, and Biosynthesis

A literature review and new data are presented to evaluate the influence of intervertebral disc (IVD) injury on biomechanics, cellularity, inflammation, and biosynthesis. Literature and new experimental evidence support the hypothesis that localized injury in the disc can lead to immediate and long-term organ level changes in biomechanics and biology of the IVD. Biomechanical properties defining motion segment bending behaviors sensitive to injuries that affect anulus fibrosus (AF) integrity and nucleus pulposus (NP) pressurization. Axial mechanics and IVD height measurements show sensitivity to puncture and other injuries that reduce NP pressurization. Torsional biomechanics are strongly affected by the extent and location of AF lesions but are less sensitive to reduced NP pressurization. IVD injuries such as puncture and stab incisions may also lead to a cascade of biological changes consistent with degeneration, including loss of cellularity, altered biosynthesis and inflammation. New results on effects of 25G needle injection of saline into a bovine IVD organ culture model demonstrated a loss of cellularity and down-regulation of matrix gene expression, providing a specific example of how a minor injury affects the IVD organ response. We conclude that localized injuries in the IVD can induce an organ level degenerative cascade through biomechanical and biological mechanisms, and their interactions. Attempts at IVD repair should target the dual biomechanical roles of the anulus of maintaining nucleus pressurization and transmitting loads across the vertebrae. Biologically, it remains important to maintain IVD cellularity and biosynthesis rates following injury to prevent downstream degenerative changes.

The pathophysiology of disc degeneration: a critical review

The pathophysiology of intervertebral disc degeneration has been extensively studied. Various factors have been suggested as influencing its aetiology, including mechanical factors, such as compressive loading, shear stress and vibration, as well as ageing, genetic, systemic and toxic factors, which can lead to degeneration of the disc through biochemical reactions. How are these factors linked? What is their individual importance? There is no clear evidence indicating whether ageing in the presence of repetitive injury or repetitive injury in the absence of ageing plays a greater role in the degenerative process. Mechanical factors can trigger biochemical reactions which, in turn, may promote the normal biological changes of ageing, which can also be accelerated by genetic factors. Degradation of the molecular structure of the disc during ageing renders it more susceptible to superimposed mechanical injuries. This review supports the theory that degeneration of the disc has a complex multifactorial aetiology. Which factors initiate the events in the degenerative cascade is a question that remains unanswered, but most evidence points to an age-related process influenced primarily by mechanical and genetic factors.

A repeatable ex vivo model of spondylolysis and spondylolisthesis

STUDY DESIGN

An ex vivo biomechanical study using porcine spinal segments.

OBJECTIVE

To produce a biomechanical model of both spondylolysis and spondylolisthesis using an accelerated cyclic loading model with intermittent impulse loads.

SUMMARY OF BACKGROUND DATA

Only a few models of spondylolisthesis appropriate for biomechanical testing have been presented previously. Past modeling attempts have largely required nonphysiologic gross fracture of the pars before testing and have resulted in nonphysiologic endplate fracture. In these tests no clinically relevant spondylolisthesis was seen at the end of testing. A reproducible, clinically relevant model of both spondylolysis and spondylolisthesis would allow study of these disease processes, and facilitate the development and evaluation of advanced spinal implants optimized specifically for these pathologies.

METHODS

Five porcine lumbar functional spinal units were tested (2 L4-L5, 3 L6-S1) after small notches had been created in the pars and after the disc had specific collagen fibers in the anterior anulus sectioned. Specimens were loaded with a constant cranial-caudal compressive force of 300 N and the application of cyclic anterior shear loads between 300 and 600 N with intermittent impulse loads to 1500 N until pars fracture occurred. Elevated cyclic loading then continued between 500 and 800 N.

RESULTS

All specimens displayed bilateral pars fracture with the fractures passing through the points of notching and no damage to endplates or facet joints. Clinically-relevant Grade II spondylolisthesis was achieved in all 5 specimens. The mean slip at the conclusion of testing was 33%.

CONCLUSION

Cyclic shear loading with intermittent impulse loads can reliably create fracture in the pars interarticularis in ex vivo porcine spine segments. Subsequent cyclic anterior motion of the superior vertebra results in clinically-relevant spondylolysis and spondylolisthesis.

The influence of posture and loading on interfacet spacing: an investigation using magnetic resonance imaging on porcine spinal units

STUDY DESIGN

Basic scientific investigation using porcine spine segments and magnetic resonance imaging.

OBJECTIVE

To quantify the effects of flexion-extension postures and loading history on the distance between the facet articular surfaces.

SUMMARY OF BACKGROUND DATA

Increased axial twist motion is used clinically to indicate instability and has been implicated as a potential cause of low back pain. Recently, it has been demonstrated that larger twist angles can be achieved when coupled with forward flexion in vivo. These findings suggest a postural mechanism may be responsible for modulating how the facet joints articulate, thereby affecting the moment resisting capability of the facets and altering the load distribution between the facet joints and the disc.

METHODS

Four porcine cervical spine motion segments (C3-C4) were exposed to a compressive preload. Two of these specimens were also exposed to 5000 repeats of flexion-extension motions. The interfacet spacing was measured from magnetic resonance images of 6 postures: neutral, maximum flexed, maximum extended, neutral-twisted, maximum flexed-twisted, and maximum extended-twisted. The range of axial twist angle was quantified in the neutral, flexed, and extended postures.

RESULTS

Flexion-extension postures and loading history caused a difference in the interfacet spacing and twist angle measured. Repetitive loading and flexed postures independently increased the spacing and twist angle, whereas the preload condition and extended postures independently decreased the measures. The 2 specimens that underwent the preload only condition suffered no damage to the disc or vertebrae. Of the repetitively loaded specimens, 1 had a vertebral fracture with initiation of herniation, and the second had a complete herniation.

CONCLUSION

The findings support a posture-dependent injury mechanism and may account for the previously reported in vivo posture-dependent passive rotational differences quantified for combined postures. The changes in spine mechanics and resulting load distribution due to coupled postures may be a key to understanding the formation of low back injuries and eventually clinical spine instability.

A new laser scanning technique for imaging intervertebral disc displacement and its application to modeling nucleotomy

BACKGROUND

Nucleotomy is a standard procedure for treating disc prolapse. It can reduce intervertebral disc height, flattening and displacing the disc, which could lead to a painful narrowing of the foramina due to nerve root compression. The purpose of this study was to investigate the disc displacement of a complete spinal segment with and without nucleotomy. We hypothesized that a nucleotomy under a certain load combination might amplify disc displacement.

METHODS

A laser scanner was developed for recording three-dimensional disc displacement of six loaded L4-5 specimens for three conditions: intact, disc with vertebral bodies and subsequent nucleotomy. Specimens were exposed to pure moments of 7.5 N m in the three principal anatomical directions. Disc displacement was obtained at maximal deflection. A finite element model was validated and subsequently utilized to determine disc displacement. The task of the finite element model was to provide supplemental data for the posterolateral region, which could not be measured from intact specimens.

FINDINGS

Disc displacement measurements of intact specimens were limited to the anterior part of discs, whereas the finite element model was able to provide the missing data of the dorsal disc region. The simulation of load combinations showed that the highest disc displacement was 1.9 mm at the lateral or posterolateral region. The nucleotomy increased the disc displacement up to 2.1mm, whereas the displacement zenith migrated posterolaterally.

INTERPRETATION

These results could be a possible explanation for disadvantages of nucleotomy as a treatment. With the methodology presented here, we would be able to assess the performance of nucleus implants by determining the disc displacement map. This could also give us appropriate information of the annular deformation, which is needed for the development of motion preserving implants.

The natural history of age-related disc degeneration: the pathology and sequelae of tears

STUDY DESIGN

A quasi 3-dimensional pathologic survey of tears in the L4-L5 disc.

OBJECTIVE

To seek accurate information on the pathogenesis and outcomes of tears to facilitate correlation with radiologic imaging and biomechanical testing; and to improve laboratory models for testing hypotheses of disc function and failure.

SUMMARY OF BACKGROUND DATA

Tears are evidence of structural failure involving the anulus. There are substantial differences in the structure and function of the anterior and posterior anulus and the nonlamellar "nucleus" is much smaller than generally conceptualized and modeled.

METHOD

Microscopy of sections prepared from 5-mm-thick parallel sagittal slices of 70 L4-L5 discs was used to construct maps of tears in each slice and record other features of interest. A template was used to classify data for analysis.

RESULTS

Multiple-level analysis detected 20% more tears than in a single disc section. Concentric, perinuclear, and radiating tears often appeared first in the posterior disc and were numerous throughout life. However, rim lesions, transdiscal tears, endplate separations, and Schmorl's nodes were infrequent in young discs. Rim lesions and transdiscal tears markedly increased in the older discs while the other tears showed modest growth. In elderly discs, many tears acquired blood vessels accompanied by nerves capable of transmitting pain. Apart from about 15% of rim lesions, healing of tears by scar tissue was absent. Links between various types of tears result in complex discographic images from older discs and the cavitation of transdiscal tears lead to segmental instability.

CONCLUSION

Tears in the L4-L5 disc show different patterns of incidence with aging, which can be explained by current biomechanical concepts. Tears may not only perturb disc function and cause segmental instability, but the frequency of neovascularization accompanied by neoinnervation indicates that pain originating within the degenerate disc should not be dismissed as the frequent evidence of bleeding into the tear lumen indicates the susceptibility of the vessels to trauma.

In vitro study of biomechanical behavior of anterior and transforaminal lumbar interbody instrumentation techniques

OBJECTIVE

To study the biomechanical behavior of lumbar interbody instrumentation techniques using titanium cages as either transforaminal lumbar interbody fusion (TLIF) or anterior lumbar interbody fusion (ALIF), with and without posterior pedicle fixation.

METHODS

Six fresh-frozen lumbar spines (L1-L5) were loaded with pure moments of +/-7.5 Nm in unconstrained flexion-extension, lateral bending, and axial rotation. Specimen were tested intact, after implantation of an ALIF or TLIF cage "stand-alone" in L2-L3 or L3-L4, and after additional posterior pedicle screw fixation.

RESULTS

In all loading directions, the range of motion (ROM) of the segments instrumented with cage and pedicle screw fixation was below the ROM of the intact lumbar specimen for both instrumentation techniques. A significant difference was found between the TLIF cage and the ALIF cage with posterior pedicle screw fixation for the ROM in flexion-extension and axial rotation (P < 0.05). Without pedicle screw fixation, the TLIF cage showed a significantly increased ROM and neutral zone compared with an ALIF cage "stand-alone" in two of the three loading directions (P < 0.05).

CONCLUSION

With pedicle screw fixation, the ALIF cage provides a higher segmental stability than the TLIF cage in flexion-extension and axial rotation, but the absolute biomechanical differences are minor. The different cage design and approach show only minor differences of segmental stability when combined with posterior pedicle screw fixation.

High pressures and asymmetrical stresses in the scoliotic disc in the absence of muscle loading

BACKGROUND

Loads acting on scoliotic spines are thought to be asymmetric and involved in progression of the scoliotic deformity; abnormal loading patterns lead to changes in bone and disc cell activity and hence to vertebral body and disc wedging. At present however there are no direct measurements of intradiscal stresses or pressures in scoliotic spines. The aim of this study was to obtain quantitative measurements of the intradiscal stress environment in scoliotic intervertebral discs and to determine if loads acting across the scoliotic spine are asymmetric. We performed in vivo measurements of stresses across the intervertebral disc in patients with scoliosis, both parallel (termed horizontal) and perpendicular (termed vertical) to the end plate, using a side mounted pressure transducer (stress profilometry)

METHODS

Stress profilometry was used to measure horizontal and vertical stresses at 5 mm intervals across 25 intervertebral discs of 7 scoliotic patients during anterior reconstructive surgery. A state of hydrostatic pressure was defined by identical horizontal and vertical stresses for at least two consecutive readings. Results were compared with similar stress profiles measured during surgery across 10 discs of 4 spines with no lateral curvature and with data from the literature.

RESULTS

Profiles across scoliotic discs were very different from those of normal, young, healthy discs of equivalent age previously presented in the literature. Hydrostatic pressure regions were only seen in 14/25 discs, extended only over a short distance. Non-scoliotic discs of equivalent age would be expected to show large centrally placed hydrostatic nuclear regions in all discs. Mean pressures were significantly greater (0.25 MPa) than those measured in other anaesthetised patients (<0.07 MPa). A stress peak was seen in the concave annulus in 13/25 discs. Stresses in the concave annulus were greater than in the convex annulus indicating asymmetric loading in these anaesthetised, recumbent patients.

CONCLUSION

Intradiscal pressures and stresses in scoliotic discs are abnormal, asymmetrical and high in magnitude even in the absence of significant applied muscle loading. The origin of these abnormal stresses is unclear.

Application of a new calibration method for a three-dimensional finite element model of a human lumbar annulus fibrosus

BACKGROUND

Major deficits of many finite element models of the lumbar spine are the oversimplification, assumed constellation of the material properties or the insufficiently performed calibration using experimental in vitro data. The aim of this study was, to develop a method for calibrating the two-composite structure of the annulus fibrosus, the ground substance and collagen fibers.

METHODS

For that purpose, a three-dimensional, non-linear finite element model of a denucleated intervertebral disc with the adjacent vertebral bodies (L4-L5) was created. Previously performed in vitro experiments provided experimental data for the range of motion in each load direction, needed for calibration. A method was developed to determine the individual contribution of the fibers and the ground substance for bending moments with four different magnitudes (2.5, 5.0, 7.5 and 10 Nm). For each bending moment, the stiffness of fibers was varied to approximate the Young's modulus of the ground substance in order to fulfil the required range of motion obtained from in vitro results within an accuracy of 99%.

RESULTS

Infinite material parameter combinations of collagen fibers and ground substance led to the same range of motion, which were different for each bending moment. However, there was only one combination, which was valid for all applied bending moments; and in all load direction.

INTERPRETATION

This calibration method was performed on range of motion data; however, the procedure could also be applied to other loading scenarios and measurement parameters like disc bulge, translation and intradiscal pressure.

Stepwise reduction of functional spinal structures increase range of motion and change lordosis angle

Many investigators have performed studies on specific defect situations or determined the contribution on isolated structures. Investigating the contribution of functional structures requires obtaining the kinematic response directly on spinal segments. The purpose of this study was to quantify the function of anatomical components on lumbar segments for different loading magnitudes. Eight spinal segments (L4-5) with a median age of 52 years (ranging from 38 to 59 years) and a low degree of disc degeneration were utilized for the in vitro testing. Specimens were mounted in a custom-built spine tester and loaded with pure moments (1-10 N m) to move within three anatomical planes at a loading rate of 1.0 degrees /s. Anatomy was successively reduced by: ligaments, facet capsules, joints and nucleus. Data were evaluated for range of motion, neutral zone and lordosis angle. Transection of posterior ligaments predominantly increased specimen flexion for all bending moments applied. Supraspinous ligament also indicated to resist in extension slightly, whereas the facet capsules did not. Facet joints contributed to axial rotation, but not in lateral bending. The anterior longitudinal ligament was found to slightly resist in axial rotation, but strongly in extension. Nucleotomy caused largest increase of all movements. The unloaded posture of the specimens changed after ligament dissection, indicating ligament pretension. The region of lumbar spine is interesting for finite element (FE) simulation due to the high evidence of disc degeneration and injuries. This study may help to understand the function of specific anatomical structures and assists in FE model calibration. We suggest to start a calibration procedure for such models with the smallest functional structure (annulus) and to cumulatively add further structures.

The influence of static axial torque in combined loading on intervertebral joint failure mechanics using a porcine model

BACKGROUND

The spine is routinely subjected to repetitive combined loading, including axial torque. Repetitive flexion-extension motions with low magnitude compressive forces have been shown to be an effective mechanism for causing disc herniations. The addition of axial torque to the efficacy of failure mechanisms, such as disc herniation, need to be quantified. The purpose of this study was to determine the role of static axial torque on the failure mechanics of the intervertebral joint under repetitive combined loading.

METHODS

Repetitive flexion-extension motions combined with 1472 N of compression were applied to two groups of nine porcine motion segments. Five Nm of axial torque was applied to one group. Load-displacement behaviour was quantified, and planar radiography was used to document tracking of the nucleus pulposus and to identify fractures.

FINDINGS

The occurrence of facet fractures was found to be higher (P=0.028) in the axial torque group (7/9), compared to the no axial torque group (2/9). More hysteresis energy was lost up to 3000 cycles of loading in the axial torque group (P<0.014). The flexion-extension cycle stiffness was not different between the two groups until 4000 cycles of loading, after which the axial torque group stiffness increased (P=0.016). The percentage of specimens that herniated after 3000 cycles of loading was significantly larger (P=0.049) for the axial torque group (71%) compared to the no axial torque group (29%).

INTERPRETATION

Small magnitudes of static axial torque alter the failure mechanics of the intervertebral disc and vertebrae in combined loading situations. Axial torque appears to accelerate the susceptibility for injury to the intervertebral joint complex. This suggests tasks involving axial torque with other types of loading, apart from axial twist motion, should be monitored to assess exposure and injury risk.

Anterior thoracic posture increases thoracolumbar disc loading

In the absence of external forces, the largest contributor to intervertebral disc (IVD) loads and stresses is trunk muscular activity. The relationship between trunk posture, spine geometry, extensor muscle activity, and the loads and stresses acting on the IVD is not well understood. The objective of this study was to characterize changes in thoracolumbar disc loads and extensor muscle forces following anterior translation of the thoracic spine in the upright posture. Vertebral body geometries (C2 to S1) and the location of the femoral head and acetabulum centroids were obtained by digitizing lateral, full-spine radiographs of 13 men and five women volunteers without previous history of back pain. Two standing, lateral, full-spine radiographic views were obtained for each subject: a neutral-posture lateral radiograph and a radiograph during anterior translation of the thorax relative to the pelvis (while keeping T1 aligned over T12). Extensor muscle loads, and compression and shear stresses acting on the IVDs, were calculated for each posture using a previously validated biomechanical model. Comparing vertebral centroids for the neutral posture to the anterior posture, subjects were able to anterior translate +101.5 mm+/-33.0 mm (C7-hip axis), +81.5 mm+/-39.2 mm (C7-S1) (vertebral centroid of C7 compared with a vertical line through the vertebral centroid of S1), and +58.9 mm+/-19.1 mm (T12-S1). In the anterior translated posture, disc loads and stresses were significantly increased for all levels below T9. Increases in IVD compressive loads and shear loads, and the corresponding stresses, were most marked at the L5-S1 level and L3-L4 level, respectively. The extensor muscle loads required to maintain static equilibrium in the upright posture increased from 147.2 N (mean, neutral posture) to 667.1 N (mean, translated posture) at L5-S1. Compressive loads on the anterior and posterior L5-S1 disc nearly doubled in the anterior translated posture. Anterior translation of the thorax resulted in significantly increased loads and stresses acting on the thoracolumbar spine. This posture is common in lumbar spinal disorders and could contribute to lumbar disc pathologies, progression of L5-S1 spondylolisthesis deformities, and poor outcomes after lumbar spine surgery. In conclusion, anterior trunk translation in the standing subject increases extensor muscle activity and loads and stresses acting on the intervertebral disc in the lower thoracic and lumbar regions.

Aplastic Articular Facets in a Dog With Intervertebral Disk Rupture of the 12th to 13th Thoracic Vertebral Space

A 6-year-old, female spayed Pomeranian was presented with acute hind-limb paraplegia with the presence of deep pain perception and urinary incontinence. Myelography showed a Hansen type I herniation of the12th to 13th thoracic intervertebral space (T12–13). Articular facets of the T12–13 and T13 to first lumbar vertebra (L1) were absent. The spinal cord was decompressed using a bilateral T12–13 modified lateral hemilaminectomy (pediculectomy). The aplastic sites were associated with minimal instability of the vertebral column, and stabilization of the vertebral column was not required. Familiarity with this condition is important, because articular facet aplasia may cause vertebral instability and may require an adjusted surgical approach or vertebral reduction and fusion following decompression.

Kinematic response of lumbar functional spinal units to axial torsion with and without superimposed compression and flexion/extension

Experimental data suggest that lumbar torsion contributes to lumbar disc degenerative changes, such as instability, spondylolisthesis and spinal canal stenosis. However, some basic mechanical characteristics of the lumbar spine under torsional loading have not yet been reported in detail. For example, the function of the facet joints under combined mechanical loads such as torsion with superimposed flexion or extension postures is an area of interest about which little biomechanical data have been reported. In this study, the kinematic response to axial torsion with superimposed axial compression (200 N), compression-flexion (3 and 6 Nm) and compression-extension (3 and 6 Nm) was investigated in 10 cadaveric lumbar functional spinal units. Range of motion (ROM), and helical axes of motion (HAM), were analyzed. There was no difference in ROM between no preload, pure compressive and flexion-compression preload conditions. The ROM was significantly reduced by both extension-compression preload conditions (11% reduction for 3 Nm and 19% reduction for 6 Nm of extension) compared to the pure compressive preload. For no preload, the average HAM position in the transverse plane of the intervertebral disc was near the posteriormost part of the disc and located laterally on the side contralateral to the applied torsional moment. In the transverse plane, the HAM position showed a discrete trend towards the posterior part of the specimens during extension. Kinematic data were visualized using computer animation techniques and CT-based reconstructions of the respective specimens. This information may be used for identifying and characterizing physiologic and pathologic motion and for specifying conservative and surgical treatment concepts and, thus, may find application to identifying indications for spinal fusion or in evaluating the effect of future semi-flexible instrumentation.

Safety of spinal manipulation in the treatment of lumbar disk herniations: a systematic review and risk assessment

OBJECTIVE

To provide a qualitative systematic review of the risk of spinal manipulation in the treatment of lumbar disk herniations (LDH) and to estimate the risk of spinal manipulation causing a severe adverse reaction in a patient presenting with LDH.

DATA SOURCES

Relevant case reports, review articles, surveys, and investigations regarding treatment of lumbar disk herniations with spinal manipulation and adverse effects and associated risks were found with a search of the literature.

DATA SYNTHESIS

Prospective/retrospective studies and review papers were graded according to quality, and results and conclusions were tabulated. From the data published, an estimate of the risk of spinal manipulation causing a clinically worsened disk herniation or cauda equina syndrome (CES) in patients presenting with LDH was calculated. This was compared with estimates of the safety of nonsteroidal anti-inflammatory drugs (NSAIDs) and surgery in the treatment of LDH.

RESULTS

An estimate of the risk of spinal manipulation causing a clinically worsened disk herniation or CES in a patient presenting with LDH is calculated from published data to be less than 1 in 3.7 million.

CONCLUSION

The apparent safety of spinal manipulation, especially when compared with other "medically accepted" treatments for LDH, should stimulate its use in the conservative treatment plan of LDH.

Altered disc mechanics in mice genetically engineered for reduced type I collagen

STUDY DESIGN

Mechanically test lumbar discs of transgenic mice in compression-tension and torsion.

OBJECTIVES

Determine if a reduction in type I collagen results in decreased disc mechanics.

SUMMARY OF BACKGROUND DATA

Quantitative relationships between disc structure and function would improve the understanding of disc generation and are essential relationships for functional tissue engineering. The reduced type I collagen transgenic mouse has been used in structure-function studies of bone and tendon, but not intervertebral discs. Methods for testing mouse discs have recently been developed, making disc structure-function studies possible.

METHODS

Microradiographed and mechanically tested lumbar discs from control and collagen-reduced mice in both compression-tension and torsion were used. Disc area and polar moment of inertia were determined from radiographic data, stiffness from mechanical data, and apparent modulus from geometric and mechanical data.

RESULTS

Collagen-reduced discs had a larger area and polar moment of inertia compared to controls. The linear and torsional stiffness of collagen-reduced and control discs were not significantly different. Finally, the apparent modulus of collagen-reduced discs was significantly less than controls in compression (73% of control) and torsion (50%).

CONCLUSIONS

Compared to controls, collagen-reduced discs had reduced apparent modulus in both loading directions, suggesting that the transgenic disc tissue was mechanically inferior to controls. These results are consistent with the widely accepted functional role of type I collagen in disc mechanics, and therefore supports the use of transgenic mice to study structure-function relationships of the disc. Future work will focus on quantifying structure-function relationships related to degeneration, as well as those relevant to the design of tissue-engineered disc replacements.

Adapting innovative motion-preserving technology to spinal surgical practice: what should we expect to happen?

STUDY DESIGN

A literature-based review of approach-related morbidity and a conjectural analysis of potential complications of disc arthroplasty based on experience with total joint arthroplasty.

OBJECTIVE

To describe predictable complications of disc arthroplasty and possible strategies for minimizing or treating these complications.

SUMMARY OF BACKGROUND DATA

There is a significant experience with anterior approach-related morbidity in spinal surgery. There is also extensive experience with extremity total joint arthroplasty. The combination of these experiences should predict certain occurrences that will occur with the advent of disc arthroplasty in the spine.

METHODS

Review of the medical literature associated with anterior approach to the lumbar spine for spinal fusion was done. Sequential steps for performance of disc arthroplasty and possible problems with each step were evaluated and possible complications identified. Parallel experience in total joint arthroplasty was reviewed for possible predictive experience.

RESULTS

There are definable approach-related morbidities that will occur, regardless of prosthesis design and implantation technique. Prosthesis design involves a series of tradeoffs for risks and benefits. Revisions are inevitable; rate of revision and time to revision remain to be determined.

CONCLUSIONS

Disc arthroplasty will offer benefits over current fusion techniques. It will come at a cost and certain complications are entirely predictable. There will be deaths from the procedure, due to thromboembolic phenomenon or due to uncontrollable hemorrhage from irreparable vascular injury, especially on repeat operations. There will be prostheses that dislodge. There will be infections that require device removal, a very high-risk procedure. There will be a deterioration of results in the hands of the general medical community as opposed to the hands of the initial investigators, a learning curve if you will. The access surgeon will be critical to minimizing morbidity. Design considerations compete with anatomic constraints. Material choices all have pros and cons. Spine surgeons as a whole are excited about this opportunity, but we must be diligent to minimize these predictable adverse events to make the risk benefit profile the best that it can be for our patients.

Posterior Lumbar Interbody Fusion for Degenerative Disc Disease Using a Minimally Invasive B-Twin Expandable Spinal Spacer

Acquired degenerative disc disease causes gradual disc space collapse, concurrent discogenic or facet-induced pain, and possible compression radiculopathy. Surgical treatment aims to re-expand the intervertebral space and stabilize the involved segment in balanced alignment until fusion is complete. The prevailing methods make use of a twin cage device of predetermined size. Their implantation requires extensive exposure, entailing the sacrifice of posterior stabilizing structures. The procedure also results in significant traction on the dural sac and the cauda equina and is thereby a potential source of neurologic damage. The new expandable spinal spacer (ESS) was designed to mitigate all the shortcomings alluded to above. A prospective multicenter clinical study was conducted of 87 patients with chronic low back pain due to degenerative disc disease, treated by posterior lumbar interbody fusion (PLIF) using a newly designed ESS. The study protocol was approved by the ethics committees of all the participating institutions. The objective was to test the safety and efficacy of the device. Each participant was followed periodically for >1 postoperative year. The ongoing record included intraoperative difficulties and complications, if any, radiologic evidence of fusion and clinical outcome as scored by pre- and postoperative questionnaires pertaining to pain intensity and degree of disability. No dural lacerations or neurologic deficit occurred. There were no mechanical failures of the spacer. Radiologic study demonstrated fusion in all but one patient. Disc space height that averaged 7.53 +/- 2.42 mm before surgery increased to 10.03 +/- 2.00 mm at the time of surgery and stabilized at 9.47 +/- 2.10 mm upon final follow-up. Visual Analog Scale and Oswestry Index decreased by 60% and 58%, respectively. PLIF using the ESS achieves the same ultimate outcome as do other methods currently in use but does not share the handicaps and hazards and is more user-friendly to the surgeon.

Immunohistochemical analysis of the extracellular matrix in the posterior capsule of the zygapophysial joints in patients with degenerative L4-5 motion segment instability

OBJECT

Although the hypertrophied shape of the zygapophysial joints in degenerative instability of the lumbar spine is well known, its underlying pathophysiological mechanism is unclear. The authors sought to provide evidence that there is increased fibrocartilaginous metaplasia in the posterior joint capsule resulting from greater mechanical loading; the authors suggest that these capsular changes are central to understanding the altered joint shape.

METHODS

The LA-5 posterior articular complex was removed in 14 patients undergoing fusion for degenerative instability. After methanol-assisted fixation, cryosections were immunolabeled for a wide range of extracellular matrix molecules. These were collagens (Types I, II, III, V, and VI), glycosaminoglycans (chondroitin 4 and 6 sulfates; dermatan- and keratan-sulfate), and proteoglycans (versican, tenascin, aggrecan, and its associated link protein). The grade of degeneration of the articular complexes was assessed radiologically and histologically.

CONCLUSIONS

The results of this study provide molecular evidence for an altered loading history on the joint capsule. The pronounced loss of intervertebral disc height that occurred in all patients with severe degeneration of the lumbar motion segment promotes an increased range of axial rotation that places the posterior capsule under greater mechanical load. Compared with normal joints studied previously, the posterior capsules involved in these degenerative joint complexes were hypertrophied and fibrocartilaginous throughout. Cartilaginous metaplasia was especially pronounced at the attachment sites (entheses) where the fibrocartilage now extended beyond the original level of the joint space, and capped the osseous spurs arising from these attachment sites.

Recent advances in lumbar spinal mechanics and their significance for modelling

Mathematical models are often used to quantify the overall forces and moments acting on the lumbar spine. However, if the purpose of the research is to explain how spinal tissues can be injured, it is necessary to distribute the overall forces and moments between (and within) different spinal structures, because it is the concentration of force which causes injury, and elicits pain. This paper reviews recent experimental evidence concerning the distribution of forces and moments acting on the lumbar spine. Lordotic postures increase loading of the posterior annulus and apophyseal joints, whereas moderately flexed postures tend to equalise compressive stress across the disc, and unload the apophyseal joints. Sustained compression reduces the volume and pressure of the nucleus pulposus, while increasing compressive stresses in the annulus and neural arch. Sustained compression also reduces disc height, giving some slack to collagen fibres in the intervertebral disc and ligaments, and causing them to resist bending less. Disc degeneration has a similar effect on disc height, and stress distributions. On the other hand, discs and ligaments can be subjected to greater bending moments following a period of sustained or repetitive bending, because sustained bending impairs the normal protective reflex from the back muscles, and repetitive bending fatigues the back muscles, reducing their ability to protect the spine. Incorporating this information into mathematical models will make them better able to identify which activities are most likely to injure the lumbar spine in life.

Pain Generators of the Lumbar Spine

Different anatomical structures and pathophysiological functions can be responsible for lumbar pain, each producing a distinctive clinical profile. Pain can arise from the intervertebral disc, either acutely as a primary disc related disorder, or as result of the degradation associated with chronic internal disc disruption. In either case, greatest pain provocation will be associated with movements and functions in the sagittal plane. Lumbar pain can also arise from afflictions within the zygapophyseal joint mechanism, as result of synovitis or chondropathy. Either of these conditions will produce the greatest pain provocation during three-dimensional movements, due to maximal stress to either the synovium or joint cartilage. Finally, patients can experience different symptoms associated with irritation to the dural sleeve, dorsal root ganglion, or chemically irritated lumbar nerve root. Differential diagnosis of these conditions requires a thorough examination and provides information that can assist the clinician in selecting appropriate management strategies.