The effects of bone density and disc degeneration on the structural property distributions in the lower lumbar vertebral endplates

Intervertebral disc degeneration in the dog. Part 1: Anatomy and physiology of the intervertebral disc and characteristics of intervertebral disc degeneration

Intervertebral disc (IVD) degeneration is common in dogs and can give rise to a number of diseases, such as IVD herniation, cervical spondylomyelopathy, and degenerative lumbosacral stenosis. Although there have been many reports and reviews on the clinical aspects of canine IVD disease, few reports have discussed and reviewed the process of IVD degeneration. In this first part of a two-part review, the anatomy, physiology, histopathology, and biochemical and biomechanical characteristics of the healthy and degenerated IVD are described. In Part 2, the aspects of IVD degeneration in chondrodystrophic and non-chondrodystrophic dog breeds are discussed in depth.

Anterior cervical interbody constructs: effect of a repetitive compressive force on the endplate

Graft subsidence following anterior cervical reconstruction can result in the loss of sagittal balance and recurring foraminal stenosis. This study examined the implant-endplate interface using a cyclic fatigue loading protocol in an attempt to model the subsidence seen in vivo. The superior endplate from 30 cervical vertebrae (C3 to T1) were harvested and biomechanically tested in axial compression with one of three implants: Fibular allograft; titanium mesh cage packed with cancellous chips; and trabecular metal. Each construct was cyclically loaded from 50 to 250 N for 10,000 cycles. Nondestructive cyclic loading of the cervical endplate-implant construct resulted in a stiffer construct independent of the type of the interbody implant tested. The trabecular metal construct demonstrated significantly more axial stability and significantly less subsidence in comparison to the titanium mesh construct. Although the allograft construct resulted in more subsidence than the trabecular metal construct, the difference was not significant and no difference was found when comparing axial stability. For all constructs, the majority of the subsidence during the cyclic testing occurred during the first 500 cycles and was followed by a more gradual settling in the remaining 9,500 cycles.

Are stand-alone cages sufficient for anterior lumbar interbody fusion?

Anterior lumbar interbody fusion (ALIF) has increased in popularity because it has advantages over posterior fusion. Because there is disagreement about the stability of stand-alone cage ALIF, some surgeons use various types of supplementary fixation, including anterior plates, pedicle screw systems and translaminar screws, to increase segmental stability. Many factors associated with both the cages and endplates influence the time of onset and extent of subsidence after use of stand-alone cage ALIF. A large round cage with an adequate central opening is recommended to facilitate maximum contact with the periphery of the endplate. With regard to the relationship between radiographic fusion and recurrence of symptoms with the development of subsidence, most researchers have reported finding no correlation. Subsidence may be due to a process of bone incorporation between cages and endplates. Does subsidence or nonfusion really matter clinically? Further prospective, randomized controlled trials are very much needed to answer these questions.

Morphology of the human vertebral endplate

It is presumed that poor intervertebral disc cell nutrition is a contributing factor in degeneration, and is exacerbated by vertebral endplate sclerosis. Yet, quantitative relationships between endplate morphology and degeneration are unavailable. We investigated how endplate bone microstructure relates to indices of disc degeneration, such as morphologic grade, proteoglycan content, and cell density. Intervertebral core samples [n = 96, 14 subjects, L1-L5 level, ages 35-85 (64 ± 16 years), degeneration grade 1 (n = 4), grade 2 (n = 32), grade 3 (n = 44), grade 4 (n = 10), grade 5 (n = 6)] that included subchondral bone, cartilage endplate, and adjacent nucleus were harvested from human cadaveric lumbar spines. The morphology of the vertebral endplate was analyzed using µCT and the adjacent nucleus tissue was collected for biochemical and cellular analyses. Relationships between vertebral endplate morphology and adjacent disc degeneration were analyzed. Contrary to the prevailing notion, vertebral endplate porosity increased between 50% and 130% and trabecular thickness decreased by between 20% and 50% with advancing disc degeneration (p < 0.05). We also observed that nucleus cell density increased (R(2)  = 0.33, p < 0.05) and proteoglycan content decreased (R(2)  = 0.47, p < 0.05) as the endplate became more porous. Our data suggest that endplate sclerosis is not a fundamental factor contributing to disc degeneration. Rather, the opposite was observed in our samples, as the endplate became progressively more porous with age and degeneration. Since ischemic disc cell behavior is commonly associated with degenerative change, this may be related to other factors such as the quality of vertebral capillaries, as opposed to decreased permeability of intervening tissues.

Pathophysiology and biomechanics of the aging spine

AGING OF THE SPINE IS CHARACTERIZED BY TWO PARALLEL BUT INDEPENDENT PROCESSES: the reduction of bone mineral density and the development of degenerative changes. The combination of degeneration and bone mass reduction contribute, to a different degree, to the development of a variety of lesions. This results in a number of painful and often debilitating disorders. The present review constitutes a synopsis of the pathophysiological processes that take place in the aging spine as well as of the consequences these changes have on the biomechanics of the spine. The authors hope to present a thorough yet brief overview of the process of aging of the human spine.

A new bone surrogate model for testing interbody device subsidence

STUDY DESIGN

An in vitro biomechanical study investigating interbody device subsidence measures in synthetic vertebrae, polyurethane foam blocks, and human cadaveric vertebrae.

OBJECTIVE

To compare subsidence measures of bone surrogates with human vertebrae for interbody devices varying in size/placement.

SUMMARY OF BACKGROUND DATA

Bone surrogates are alternatives when human cadaveric vertebrae are unavailable. Synthetic vertebrae modeling cortices, endplates, and cancellous bone have been developed as an alternative to polyurethane foam blocks for testing interbody device subsidence.

METHODS

Indentors placed on the endplates of synthetic vertebrae, foam blocks, and human vertebrae were subjected to uniaxial compression. Subsidence, measured with custom-made extensometers, was evaluated for an indentor seated either centrally or peripherally on the endplate. Failure force and indentation stiffness were determined from force-displacement curves.

RESULTS

Subsidence measures in human vertebrae varied with indentor placement: failure forces were higher and indentors subsided less with peripheral placement. Subsidence measures in foam blocks were insensitive to indentor size/placement; they were similar to human vertebrae for centrally placed but not for peripherally placed indentors. Although subsidence measures in synthetic vertebrae were sensitive to indentor size/placement, failure force and indentation stiffness were overestimated, and subsidence underestimated, for both centrally placed and peripherally placed indentors.

CONCLUSION

The synthetic endplate correctly represented the human endplate geometry, and thus, failure force, stiffness, and subsidence in synthetic vertebrae were sensitive to indentor size/placement. However, the endplate was overly strong and thus synthetic vertebrae did not accurately model indentor subsidence in human cadaveric vertebrae. Foam blocks captured subsidence measures more accurately than synthetic vertebrae for centrally placed indentors, but because of their uniform density were not sufficiently robust to capture changes generated from different indentor sizes/placements. The current bone surrogates are not accurate enough in terms of material property distribution to completely model subsidence in human cadaveric vertebrae.

Replicating interbody device subsidence with lumbar vertebrae surrogates

Bone surrogates are proposed alternatives to human cadaveric vertebrae for assessing interbody device subsidence. A synthetic vertebra with representations of cortices, endplates and cancellous bone was recently developed as an alternative surrogate to polyurethane foam blocks. The ability of the two surrogates to replicate subsidence has not been fully assessed, and was evaluated by indenting them with ring-shaped indenters and comparing their performance with human cadaveric vertebrae using qualitative characteristics and indentation metrics. The sensitivity of each surrogate to a centrally or peripherally placed indenter was of particular interest. Many indentation characteristics of the foam blocks were similar to those of human cadaveric vertebrae, except their insensitivity to centrally and peripherally placed indenters, owing to their homogeneous mechanical properties. This is distinctly different from the cadaveric vertebrae, where a peripherally placed indenter indented significantly less than a centrally placed indenter, because of endplates. By contrast, the synthetic vertebra was sensitive to peripherally placed indenters owing to its bi-material composition, including a thickened peripheral endplate. However, an overly strong synthetic endplate resulted in unrepresentative indentation shape and depth. Both surrogates produced similar results to human cadaveric vertebrae in certain respects, but neither is accurate enough in terms of material property distribution to model subsidence completely in human cadaveric vertebrae.

The prevalence of MRI-defined spinal pathoanatomies and their association with modic changes in individuals seeking care for low back pain

Modic changes are of increasing interest, however their age and gender prevalence are not well described. To date, the associations between Modic changes and other common vertebral pathologies have only been described in small samples (n < 100). Our aim was, in a large dataset of people with low back pain, to (1) describe the prevalence of a range of spinal pathoanatomies, and (2) examine the association between Modic changes and stages of intervertebral disc (IVD) pathology. Common pathologies were coded from the lumbar spine MRIs from 4,233 consecutive people imaged while attending a publicly-funded secondary care outpatient facility in Denmark. Prevalence data were calculated by pathology and by vertebral level. Prevalence was also calculated by age and gender categories for Modic changes. The association between stages of IVD pathology (degeneration, bulge, herniation) and Modic changes at L4/5 and L5/S1 was expressed using prevalence ratios (PR) and 95% confidence intervals. The prevalence of Modic changes and IVD pathology were greater in L4/5 and L5/S1, compared with the upper lumbar spine. There was no significant gender difference in prevalence of Modic changes (p = 0.11). The prevalence of IVD disc pathology occurring concurrently with Modic changes ranged from 11.5 to 17.5% (Type 1), 8.5 to 12.7% (Type 2) and 17.1 to 25.6% (Type 1 and/or 2) while the prevalence occurring in the absence of Modic changes ranged from 0.5 to 6.3% (Type 1), 0.3 to 4.9 (Type 2), 0.8 to 9.7% (Type 1 and/or 2). The associated PR for IVD pathology occurring concurrently with Modic changes ranged from 1.8 to 29.2 (p < 0.05). The highest PR (29.2) was between degeneration and Modic changes, indicating that it is rare for Modic changes to occur without disc degeneration. Spinal pathoanatomy was common in this population, particularly IVD pathologies, and a consistent trend of a relatively greater prevalence in the lower lumbar spine was identified. Modic changes were more likely to be present among individuals with IVD pathology than without, which may implicate mechanical factors as being one aetiological pathway for Modic changes, although other hypotheses may equally explain this association.

The strength profile of the thoracolumbar endplate reflects the sagittal contours of the spine

STUDY DESIGN

human cadaveric biomechanical study of indentation tests on the thoracolumbar vertebral endplates.

OBJECTIVE

to map the strength profile of the thoracolumbar endplates using indentation testing, and to document any changes in this profile with vertebral level.

SUMMARY OF BACKGROUND DATA

the strength profile of the lumbosacral endplate was described previously. Based on these data, we hypothesized that the periphery of the endplate would be stronger than the center and that the strength profile would vary with the sagittal contour and level of the spine.

METHODS

indentation testing was performed on the T9, T12, and L2 endplates of fresh-frozen human cadaver spines, using a materials testing machine. A 3-mm hemispherical indenter was lowered at 0.2 mm/s to a depth of 3 mm to produce local endplate failure. A minimum of 25 indentations were performed in a rectangular grid (rows: lateral, left to right; columns: A-P, anterior to posterior). Three-way analysis of variance was used to address changing strength profile patterns.

RESULTS

there were highly significant variations of indentation strength across the endplates in both the lateral and anterior to posterior directions. Each row of indentations was significantly stronger than the rows anterior to it (P < 0.04), except for the most anterior row. The most lateral columns were stronger than the central columns (P < 0.05). The ratio of the mean strength of the posterior row compared to that of the anterior row was significantly different across levels (P = 0.026).

CONCLUSION

the periphery of the thoracolumbar endplate was stronger than the center. The difference in posterior to anterior ndplate strength ratio between vertebral levels suggests a relative strength increase in the anterior aspect of the endplate with rostral ascent into the thoracic spine.

Design and finite-element evaluation of a versatile assembled lumbar interbody fusion cage

INTRODUCTION

When an interbody cage is inserted into a human being's lumbar spine, not only the design, but also the material used is considerably crucial, particularly when minimally invasive lumbar fusion (MILIF) approaches are considered. The purpose of this study was to design a multi-function cage (either for MILIF or open lumbar interbody fusion) and also to evaluate the strength of the design based on a finite-element model analysis.

METHOD

Three-dimensional finite-element models that were instrumental in the reproduction of post-operative conditions under which different cages, such as assembled lumbar interbody fusion cages (ALIFC) and the separated ones, could be examined and traced after implantation were developed. Simulations were run to realize various loading conditions including axial compression, flexion, extension, lateral bending and rotation under a constant compressive preload. Meanwhile, the evaluation results derived from FEMs data focused on endplate stress distribution, peak stress of von Mises and stress of cage. Stress distributions on the bone surface were evaluated and discussed as well.

RESULTS

The consequences of cage insertion, high strains and stresses, were concentrated in the areas where the cage and endplate were in contact with each other. Simultaneously, contact stresses around the implants seemed to be concentrated around the periphery of the device. After implantation of ALIFC, the stiffness of the new cages was similar to that of traditional cages in an assemble condition, according to the biomechanical data dealing with FEM. Once a separated cage was in the place of an assembled cage, the stresses would get symmetrically distributed in the lateral areas of the endplate and decrease significantly at the center where the separated cage was not in contact with the endplate. The stress of the cage was going to be high once being rotating; most significant difference of stresses distribution due to the alternative choice has been found in the state of rotation. On comparison of peak von Mises stresses on the endplates in the new cage, the stresses were symmetrically distributed in the lateral areas of the endplate when a separated cage was used in place of an assembled cage.

CONCLUSION

The new cage was more advantages with regard to endplate stress distribution, peak stress of von Mises and stress of cage than the assembled state. ALIFC can provide sufficient primary stability for lumbar intervertebral fusion and the new cage may be regarded as a suitable device for load-bearing implantation.

Use of co-registered high-resolution computed tomography scans before and after screw insertion as a novel technique for bone mineral density determination along screw trajectory

INTRODUCTION

Bone mineral density (BMD) is an important factor in the examination of the performance of bone instrumentation both in and ex vivo, and until now, there has not existed a reliable technique for determining BMD at the precise location of such hardware. This paper describes such a technique, using cadaveric human sacra as a model.

METHODS

Nine fresh-frozen sacra had solid and hollow titanium screws placed into the S1 pedicles from a posterior approach. High-resolution micro-computed tomography (CT) was performed on each specimen before and after screw placement. All images were reconstructed with an isotropic spatial resolution of 308 mum, reoriented, and the pre-screw and post-screw scans were registered and transformed using a six-degree rigid-body transformation matrix. Once registered, two points, corresponding to the center of the screw at the cortex and at the screw tip, were determined in each scan. These points were used to generate cylindrical regions of interest (ROI) with the same trajectory and dimensions as the screw. BMD measurements were obtained within each of the ROI in the pre-screw scan. To examine the effect of artefact on BMD measurements around the titanium screws, annular ROI of 1 mm thickness were created expanding from the surface of the screws, and BMD was measured within each in both the pre- and post-screw scans.

RESULTS

The registration process was accurate to 190 mum, with a precision of 189 mum and error in BMD measurement of +/-2% in repeated scans. BMD values in the cylindrical ROI corresponding to screw trajectories were not statistically different from side to side of each specimen (p=0.23). Metal artefact created significant differences in BMD values (p=0.001) and followed an exponential decay curve as distance from the screws increased, approaching a low value of approximately 20 mg HA cm(-3), but not disappearing completely.

SUMMARY

CT in the presence of metal creates artefact, making measured BMD values near implants unreliable. This technique is accurate for determination of BMD, non-destructive, and eliminates the problem of this metal artefact through the use of co-registered scans. This technique has applications both in vitro and in vivo.

Vertebral fractures usually affect the cranial endplate because it is thinner and supported by less-dense trabecular bone

INTRODUCTION

Cranial endplates of human vertebrae are injured more often than caudal, in both young and elderly spines. We hypothesise that cranial endplates are inherently vulnerable to compressive loading because of structural asymmetries in the vertebrae.

METHODS

Sixty-two "motion segments" (two vertebrae and the intervening disc and ligaments) were obtained post-mortem from thirty-five human spines (17F/18M, age 48-92 yrs, all spinal levels from T8-9 to L4-5). Specimens were compressed to failure while positioned in 2-6 degrees of flexion, and the resulting damage characterised from radiographs and at dissection. 2 mm-thick slices of 94 vertebral bodies (at least one from each motion segment) were cut in the mid-sagittal plane, and in a para-sagittal plane through the pedicles. Microradiographs of the slices were subjected to image analysis to determine the thickness of each endplate at 10 locations. Optical density of the endplates and adjacent trabecular bone was also measured. Measurements obtained in cranial and caudal regions, and in mid-sagittal and pedicle slices, were compared using repeated measures ANOVA with age, level and gender included as between-subject factors. Linear regression was used to determine significant predictors of compressive strength (failure stress).

RESULTS

Fracture affected the cranial endplate in 55/62 specimens. Cranial endplates were thinner than caudal (p=0.003) by 14% and 11% on average, in mid-sagittal and pedicle slices respectively. Caudal but not cranial endplates were thicker at lower spinal levels (p=0.01). Optical density of trabecular bone adjacent to the endplates was 6% lower cranially than caudally (p=0.004), and the average optical density of trabecular bone in mid-sagittal slices was 10% lower in women than in men (p=0.025). Vertebral yield stress (mean 2.22 MPa, SD 0.77 MPa) was best predicted by the density of trabecular bone underlying the cranial endplate of the mid-sagittal slice of the fractured vertebra (r(2)=0.67, p=0.0006).

CONCLUSIONS

When vertebrae are compressed naturally by adjacent intervertebral discs, cranial endplates usually fail before caudal endplates because they are thinner and supported by less dense trabecular bone.

The role of spinal concave-convex biases in the progression of idiopathic scoliosis

Inadequate understanding of risk factors involved in the progression of idiopathic scoliosis restrains initial treatment to observation until the deformity shows signs of significant aggravation. The purpose of this analysis is to explore whether the concave-convex biases associated with scoliosis (local degeneration of the intervertebral discs, nucleus migration, and local increase in trabecular bone-mineral density of vertebral bodies) may be identified as progressive risk factors. Finite element models of a 26 degrees right thoracic scoliotic spine were constructed based on experimental and clinical observations that included growth dynamics governed by mechanical stimulus. Stress distribution over the vertebral growth plates, progression of Cobb angles, and vertebral wedging were explored in models with and without the biases of concave-convex properties. The inclusion of the bias of concave-convex properties within the model both augmented the asymmetrical loading of the vertebral growth plates by up to 37% and further amplified the progression of Cobb angles and vertebral wedging by as much as 5.9 degrees and 0.8 degrees, respectively. Concave-convex biases are factors that influence the progression of scoliotic curves. Quantifying these parameters in a patient with scoliosis may further provide a better clinical assessment of the risk of progression.

Footprint mismatch in lumbar total disc arthroplasty

Lumbar disc arthroplasty has become a popular modality for the treatment of degenerative disc disease. The dimensions of the implants are based on early published geometrical measurements of vertebrae; the majority of these were cadaver studies. The fit of the prosthesis in the intervertebral space is of utmost importance. An undersized implant may lead to subsidence, loosening and biomechanical failure due to an incorrect center of rotation. The aim of the present study was to measure the dimensions of lumbar vertebrae based on CT scans and assess the accuracy of match in currently available lumbar disc prostheses. A total of 240 endplates of 120 vertebrae were included in the study. The sagittal and mediolateral diameter of the upper and lower endplates were measured using a digital measuring system. For the levels L4/L5 and L5/S1, an inappropriate size match was noted in 98.8% (Prodisc L) and 97.6% (Charite) with regard to the anteroposterior diameter. Mismatch in the anterior mediolateral diameter was noted in 79.3% (Prodisc L) and 51.2% (Charite) while mismatch in the posterior mediolateral diameter was observed in 91.5% (Prodisc L) and 78% (Charite) of the endplates. Surgeons and manufacturers should be aware of the size mismatch of currently available lumbar disc prostheses, which may endanger the safety and efficacy of the procedure. Larger footprints of currently available total disc arthroplasties are required.

Posterior lumbar interbody fusion

Posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) create intervertebral fusion by means of a posterior approach. Both techniques are useful in managing degenerative disk disease, severe instability, spondylolisthesis, deformity, and pseudarthrosis. Successful results have been reported with allograft, various cages (for interbody support), autograft, and recombinant human bone morphogenetic protein-2. Interbody fusion techniques may facilitate reduction and enhance fusion. The rationale for PLIF and TLIF is biomechanically sound. However, clinical outcomes of different anterior and posterior spinal fusion techniques tend to be similar. PLIF has a high complication rate (dural tear, 5.4% to 10%; neurologic injury, 9% to 16%). These findings, coupled with the versatility of TLIF throughout the entire lumbar spine, may make TLIF the ideal choice for an all-posterior interbody fusion.

The distribution of mineral density in the cervical vertebral endplates

Subsidence of various constructs into the vertebral body is a well-known complication in anterior fusion. Information on bone structure is needed, as a basis for improving these procedures. There are, however, no data available on the distribution of mineral density within vertebral endplates. In this study the regional distribution of mineralization within the cervical endplates with respect to endplate orientation (inferior and superior endplate) and level distribution (C3-C7) was examined by means of computed tomographic osteoabsorptiometry (CT-OAM). The distribution of mineralization in 80 cervical endplates of 8 spinal columns (4 male, 4 female, age range 38-62 years) in vertebrae C3-C7 was investigated by CT osteoabsorptiometry (CT-OAM). The subchondral mineralization distribution revealed considerable topographic differences within each endplate, whereby the areas of greatest density were found in the peripheral marginal zones with maxima in the posterolateral surface, whereas mineralization density was much lower in the central areas. The superior endplates showed an additional posteromedial maximum, whereas the inferior endplates showed an additional anterior mineralization maximum. Comparison of the distribution patters of inferior and superior endplates at different levels from C3 to C7 reveals a uniform increase of mineralization in the anterior portions from cranial to caudal. The mineralization distribution showed characteristic reproducible patterns. The maximal values occurred in the posterolateral parts, and can thus be considered a morphological substrate of high long-term loading. This can serve as a basis for improved prosthesis design and the anchorage point for various fusion techniques.

Correlation of cervical endplate strength with CT measured subchondral bone density

Cervical interbody device subsidence can result in screw breakage, plate dislodgement, and/or kyphosis. Preoperative bone density measurement may be helpful in predicting the complications associated with anterior cervical surgery. This is especially important when a motion preserving device is implanted given the detrimental effect of subsidence on the postoperative segmental motion following disc replacement. To evaluate the structural properties of the cervical endplate and examine the correlation with CT measured trabecular bone density. Eight fresh human cadaver cervical spines (C2-T1) were CT scanned and the average trabecular bone densities of the vertebral bodies (C3-C7) were measured. Each endplate surface was biomechanically tested for regional yield load and stiffness using an indentation test method. Overall average density of the cervical vertebral body trabecular bone was 270 +/- 74 mg/cm3. There was no significant difference between levels. The yield load and stiffness from the indentation test of the endplate averaged 139 +/- 99 N and 156 +/- 52 N/mm across all cervical levels, endplate surfaces, and regional locations. The posterior aspect of the endplate had significantly higher yield load and stiffness in comparison to the anterior aspect and the lateral aspect had significantly higher yield load in comparison to the midline aspect. There was a significant correlation between the average yield load and stiffness of the cervical endplate and the trabecular bone density on regression analysis. Although there are significant regional variations in the endplate structural properties, the average of the endplate yield loads and stiffnesses correlated with the trabecular bone density. Given the morbidity associated with subsidence of interbody devices, a reliable and predictive method of measuring endplate strength in the cervical spine is required. Bone density measures may be used preoperatively to assist in the prediction of the strength of the vertebral endplate. A threshold density measure has yet to be established where the probability of endplate fracture outweighs the benefit of anterior cervical procedure.

Loss of cervical endplate integrity following minimal surface preparation

STUDY DESIGN

In vitro biomechanical study.

OBJECTIVES

This biomechanical study was designed to evaluate the loss of endplate integrity with incremental removal of the endplate.

SUMMARY OF BACKGROUND DATA

The position of the anterior cervical motion preserving prosthesis is very important. Unlike interbody bone graft, where a certain amount of settling is tolerable and potentially advantageous with respect to the fusion rate, a settled total disc replacement will not function properly and may dislodge. Partial or aggressive endplate removal may be a factor resulting in subsidence of an interbody device. This study was designed to precisely examine the change of endplate strength following precise burring of the surface.

METHODS

Eight human cadaver cervical spines (C3-C7) were dissected and 6 locations on the endplates from each vertebra were biomechanically tested using an indentation test protocol. Pairs of locations were randomly assigned to be burred to the depth of 0 mm (intact), 1 mm, or 2 mm before the testing using a flat 3-mm end mill. Strength of the endplate was statistically analyzed to examine the effect of the depth of the burr and any regional variations.

RESULTS

Significant differences (P < 0.0001) in endplate strength was noted between the intact endplate (106 +/- 86 N) and burred endplates (1 mm depth, 59 +/- 49 N; 2 mm depth, 51 +/- 46 N). No significant differences existed between the burr depths of 1 and 2 mm (P = 0.21). The posterior endplate was significantly stronger than the anterior endplate irrespective of depth of burr.

CONCLUSION

There is a significant loss of endplate integrity when 1 mm of endplate (44% loss) or 2 mm of endplate (52% loss) is removed. Although the implant interface plays an important role in the magnitude of the subsidence of a device, this study in general shows that the endplate is important in terms of maximizing the strength of a construct.

Can periods of static loading be used to enhance the resistance of the spine to cumulative compression?

Results of in vitro studies conducted on isolated bone specimens have indicated a higher tolerance to static load than exists when exposed to cyclic loading, when controlled for creep rate. If this difference in load tolerance exists, it may be exploited to extend the life of vertebral bone exposed to repetitive compression, and potentially alter the development of spinal injury. However, little work has been conducted on functional spinal units to determine if bone displays this characteristic within an intact joint. Additionally, static loading may result in load redistribution within the intervertebral disc forcing more of the compressive load towards the periphery of the endplate away from the nucleus. In order to examine these potential mechanisms, 218 osteoligamentous porcine functional spinal units were assigned to one of 15 loading scenarios. This involved one of three normalized peak load magnitudes (50%, 70% and 90% of estimated compressive tolerance) and one of five normalized static load applications (0%, 50%, 100%, 200% and 1000% of the total dynamic work duration). Load magnitude significantly altered the resistance to cumulative compression with decreased peak magnitudes corresponding to both increased cumulative load tolerance and increased height loss. Static load periods did not alter the resistance of the spinal unit to cumulative compression or impact the number of cycles tolerated to failure. The insertion of static load periods impacted the total survival time to failure, but only for the 1000% static load group, an exposure unlikely to occur for most in vivo exposures. The insertion of static load periods decreased the amount of height loss during testing which may play a protective role by allowing load redistribution within the vertebral bone and intervertebral disc.

Correlation of end plate shape on MRI and disc degeneration in surgically treated patients with degenerative disc disease and herniated nucleus pulposus

BACKGROUND CONTEXT

The sagittal profile of the lumbar end plates on magnetic resonance imaging (MRI) has not been investigated in patients with degenerative disc disease (DDD) or herniated nucleus pulposus (HNP).

PURPOSE

To examine the shape of the end plates in patients treated surgically for a) low back pain or b) radiculopathy with HNP. Furthermore, to investigate the correlation between end plate shape and disc degeneration on the one, and end plate shape and symptoms on the other.

STUDY DESIGN/SETTING

Retrospective review of charts and radiographs.

METHODS

The charts, operative reports, preoperative lateral plain radiographs, and MRI scans of 178 patients (85 with low back pain and 93 with HNP) were reviewed. End plate shape was determined on midsagittal MRI cuts, disc degeneration was graded on T2 sequences, and disc height was measured on lateral plain radiographs from L1 to S1 in all patients. Student t-test and chi(2) test were used to detect significant differences and associations.

RESULTS

Flat and irregular levels were most common in the lower lumbar spine. The L5/S1 segment was flat in most cases, due to a flat sacral end plate. In DDD patients, disc degeneration on MRI and plain radiographs worsened from concave to flat, to irregular levels. In HNP patients, MRI demonstrated concave levels to be less degenerated, whereas no difference was detected between flat and irregular levels. Disc height of irregular levels was well preserved in HNP patients. Comparing the two groups, flat levels were more degenerated on MRI in HNP patients. Despite similar degrees of degeneration on MRI, concave and irregular levels in DDD patients had lower disc heights. A higher frequency of symptoms was found in flat and irregular levels for both patient groups.

CONCLUSIONS

The sagittal profile of end plates in the lumbar spine was described for patients with DDD on the one and HNP on the other. A higher association with symptoms was observed for flat and irregular levels in both patient groups. In DDD patients, disck degeneration on both MRI and plain radiographs increased from concave to flat, to irregular levels. In HNP patients, MRI demonstrated concave levels to be less degenerated, whereas no difference was detected between flat and irregular levels. Disc height of irregular levels was well preserved in HNP patients. Comparing the two groups of patients, flat levels were more degenerated on MRI in HNP patients. Despite similar degrees of degeneration on MRI, concave and irregular levels in DDD patients had lower disc heights. The correlation of symptoms and disc degeneration with the end plate shapes is not definitive evidence of end plate remodeling around degenerated discs. It may simply represent the higher rate of disc degeneration in the lower lumbar levels. This analysis did not provide any hints as to which degenerated discs are more likely to herniated and cause leg symptoms or cause predominantly low back pain.

Discogenic origins of spinal instability

STUDY DESIGN

Cadaveric motion segment experiment.

OBJECTIVE

To show how two physical aspects of disc degeneration (dehydration and endplate disruption) contribute to spinal instability.

SUMMARY OF BACKGROUND DATA

The origins of spinal instability and its associations with back pain are uncertain. METHODS.: Twenty-one cadaveric thoracolumbar motion segments aged 48 to 90 years were secured in cups of dental plaster and loaded simultaneously in bending and compression to simulate full flexion, extension, and lateral bending movements. Vertebral movements, recorded using a two-dimensional "MacReflex" motion analysis system, were analyzed to calculate neutral zone (NZ), range of motion (ROM), bending stiffness (BS), horizontal translational movements, and the location of the center of rotation (COR). Intradiscal "stresses" were measured by pulling a miniature pressure transducer through the disc along its midsagittal diameter. All experiments were repeated after each of two treatments, which simulated physical aspects of disc degeneration: creep loading to dehydrate the disc and compressive overload to disrupt the endplate. Results were analyzed using ANOVA and linear regression.

RESULTS

Motion segment height was reduced by 1.0 (SD 0.3) mm during creep and by a further 1.7 (0.6) mm after endplate disruption. In flexion and lateral bending, the combined treatments increased NZ and ROM by 89% to 298%, and increased the "instability index" (NZ/ROM) by 43% to 61%. Translational movements increased by 58% to 86%, whereas BS decreased by 42% to 48%. In extension, ROM and NZ were little affected, although the COR moved closer to the apophyseal joints. Measures of instability increased most in lateral bending, and following endplate disruption. Stress concentrations in the posterior anulus fibrosus increased markedly after endplate disruption.

CONCLUSIONS

Two physical aspects of disc degeneration (dehydration and endplate disruption) cause marked segmental instability. Back pain associated with instability may be attributable to stress concentrations in degenerated discs.

Interbody device shape and size are important to strengthen the vertebra-implant interface

STUDY DESIGN

An in vitro cadaveric study to compare compressive failure load, strength, and stiffness of the implant-vertebra interface.

OBJECTIVES

To determine the effect of cage shape (kidney, cloverleaf, or oval) and cage surface area on endplate failure strength and secondly to determine the extent and pattern of trabecular failure adjacent to an interbody device.

SUMMARY OF BACKGROUND DATA

Recent studies indicate that the posterolateral and peripheral regions of the endplate are stronger than the central. Current implants are not designed to take advantage of these stronger regions of the endplate. The zone of trabecular failure that results from interbody device subsidence has not been reported extensively in the literature.

METHODS

Uniaxial compression testing with unrestricted rotation was carried out on the superior endplates of 48 thoracolumbar (T9-L2) vertebrae with 1 of 3 shaped indentors covering 20% or 40% of the endplate area. Failure load, failure strength, and stiffness were compared. Quantitative computed tomography scans were carried out before and following indentation tests to identify areas of trabecular densification that indicate localized failure.

RESULTS

The cloverleaf-shaped indentors resulted in significantly higher (P < 0.001) failure loads (by >45%), strength (>49%), and construct stiffness (>35%) for both the 20% and 40% cross-sectional area sizes. Trabecular bone failure occurred in a semielliptical zone underlying the interbody devices, leaving the endplate and underlying cancellous bone intact.

CONCLUSIONS

The cloverleaf-shaped indentor displayed an improved strength and stiffness profile when compared to oval or kidney-shaped indentors of similar surface areas.

Design and evaluation of the FlexiCore metal-on-metal intervertebral disc prosthesis

BACKGROUND CONTEXT

The technical difficulties associated with the development of an intervertebral disc prosthesis include endurance demands on the device, lack of consensus concerning the biomechanical principles governing the articulation of the spinal joint and the performance of materials available for implantation.

PURPOSE

Although biologically based disc prostheses and augmentations may be the endpoint of spinal disc replacements, these devices and associated technologies will still require decades of work in order to achieve fruition. The more immediate solution will require a durable, biocompatible device capable of restoring range of motion. The evaluation of such a device must include failure testing of critical components as well as a series of fatigue experiments under overloaded conditions.

STUDY DESIGN/SETTING

Recent literature citing adjacent level degeneration associated with segmental mobilization and a lack of correlation between successful fusion and clinical success has prompted the need for a dynamic intervertebral disc prosthesis.

METHODS

Combined with a better understanding of the biomechanics and the prospect of an increasing percentage of more elderly patients, the future of spinal fusion for pain and instability may need to be reexamined. The authors propose a novel metal-on-metal design for an intervertebral device that features a fixed center of rotation, a mechanical torsional limit, a unique feature to allow for the location of the device in a patient-specific manner, and means by which the device may be implanted directly anterior or anterolaterally.

RESULTS

Physiological ranges of motion are retained by the prosthesis. In addition, initial and long-term fixations are achieved through spines and bone ingrowth coating. The device comprises a retained ball and socket positioned between two baseplates. The ball and socket joint permits articulation through the appropriate physiological range of motion and center of rotation as the baseplates provide a stable platform for implantation.

CONCLUSIONS

An intervertebral disc prosthesis has been designed and has demonstrated mechanical performance beyond what is required physiologically under preliminary testing.

Biomechanics of the aging spine

The human spine is composed of highly specific tissues and structures, which together provide the extensive range of motion and considerable load carrying capacity required for the physical activities of daily life. Alterations to the form and composition of the individual structures of the spine with increasing age can increase the risk of injury and can have a profound influence on the quality of life. Cancellous bone forms the structural framework of the vertebral body. Individual trabeculae are oriented along the paths of principal forces and play a crucial role in the transfer of the predominantly compressive forces along the spine. Age-related changes to the cancellous core of the vertebra includes a loss of bone mineral density, as well as morphological changes including trabecular thinning, increased intratrabecular spacing, and loss of connectivity between trabeculae. Material and morphological changes may lead to an increased risk of vertebral fracture. The vertebral endplate serves the dual role of containing the adjacent disc and evenly distributing applied loads to the underlying cancellous bone and the cortex of the vertebra. With aging, thinning of the endplate, and loss of bone mineral density increases the risk of endplate fracture. Ossification of the endplate may have consequences for the nutritional supply and hydration of the intervertebral disc. The healthy intervertebral disc provides mobility to the spine and transfers load via hydrostatic pressurization of the hydrated nucleus pulposus. Changes to the tissue properties of the disc, including dehydration and reorganization of the nucleus and stiffening of the annulus fibrosus, markedly alter the mechanics of load transfer in the spine. There is no direct correlation between degenerative changes to the disc and to the adjacent vertebral bodies. Furthermore, advancing age is not the sole factor in the degeneration of the spine. Further study is crucial for understanding the unique biomechanical function of the aging spine.