Koichi Sairyo

Effects of charité artificial disc on the implanted and adjacent spinal segments mechanics using a hybrid testing protocol.

Study Design. Finite element model of L3–S1 segment and confirmatory cadaveric testing were used to investigate the biomechanical effects of a mobile core type artificial disc (Charité artificial disc; DePuy Spine, Raynham, MA) on the lumbar spine. Objective. To determine the effects of the Charité artificial disc across the implanted and adjacent segments. Summary of Background Data. Biomechanical studies of artificial discs that quantify parameters, like the load sharing and stresses, are sparse in the literature, especially for mobile-type core artificial disc designs. In addition, there is no standard protocol for studying the adjacent segmental effects of such implants. Methods. Human osteo-ligamentous spines (L1–S1) were tested before and after L5–S1 Charité artificial disc placement. The data were used to validate further an intact 3-dimensional (3-D) nonlinear L3–S1 finite element model. The model was subjected to 400-N axial compression and 10.6 Nm of flexion/extension pure moments (load control) or pure moments that produced the overall rotation of the L3–S1 Charité model equal to the intact case (hybrid approach). Resultant motion, load, and stress parameters were analyzed at the experimental and adjacent levels. Results. Finite element model validation was achieved only with the load-controlled experiments. The hybrid approach, believed to be more clinically relevant, revealed that Charité artificial disc leads to motion increases in flexion (19%) and extension (44%) at the L5–S1 level. At the instrumented level, the decrease in the facet loads was less than at the adjacent levels; the corresponding decrease being 26% at L3–L4, 25% at L4–L5, and 13.4% at L5–S1 when compared to the intact. Intradiscal pressure changes in the L4–L5 and L3–L4 segments were minimal. Shear stresses at the Charité artificial disc-L5 endplate interface were higher than those at S1 interface. However, in the load control mode, the increase in facet loads in extension was approximately 14%, as compared to the intact case. Conclusions. The hybrid testing protocol is advocated because it better reproduces clinical observations in terms of motion following surgery, using pure moments. Using this approach, we found that the Charité artificial disc placement slightly increases motion at the implanted level, with a resultant increase in facet loading when compared to the adjacent segments, while the motions and loads decrease at the adjacent levels. However, in the load control mode that we believe is not that clinically relevant, there was a large increase in motion and a corresponding increase in facet loads, as compared to the intact.

Pathomechanism of ligamentum flavum hypertrophy: a multidisciplinary investigation based on clinical, biomechanical, histologic, and biologic assessments.

Study Design. A multidisciplinary study involving clinical, histologic, biomechanical, biologic, and immunohistologic approaches. Objective. To clarify the pathomechanism of hypertrophy of the ligamentum flavum. Summary of Background Data. The most common spinal disorder in elderly patients is lumbar spinal canal stenosis, causing low back and leg pain, and paresis. Canal narrowing, in part, results from hypertrophy of the ligamentum flavum. Although histologic and biologic literature on this topic is available, the pathomechanism of ligamentum flavum hypertrophy is still unknown. Methods. The thickness of 308 ligamenta flava at L2/3, L3/4, L4/5, and L5/S1 levels of 77 patients was measured using magnetic resonance imaging. The relationships between thickness, age, and level were evaluated. Histologic evaluation was performed on 20 ligamentum flavum samples, which were collected during surgery. Trichrome and Verhoeff-van Gieson elastic stains were performed for each ligamentum flavum to understand the degree of fibrosis and elastic fiber status, respectively. To understand the mechanical stresses in various layers of ligamentum flavum, a 3-dimensional finite element model was used. Von Mises stresses were computed, and values between dural and dorsal layers were compared. There were 10 ligamenta flava collected for biologic assessment. Using real-time reverse transcriptase polymerase chain reaction, transforming growth factor (TGF)-β messenger ribonucleic acid expression was quantitatively measured. The cellular location of TGF-β was also confirmed from 18 ligamenta flava using immunohistologic techniques. Results. The ligamentum flavum thickness increased with age, however, the increment at L4/5 and L3/4 levels was larger than at L2/3 and L5/S1 levels. Histology showed that as the ligamentum flavum thickness increased, fibrosis increased and elastic fibers decreased.This tendency was more predominant along the dorsal side. Von Misses stresses revealed that the dorsal fibers of ligamentum flavum were subjected to higher stress than the dural fibers. This was most remarkably observed at L4/5. The largest increase in ratio observed between the dorsal and dural layer was approximately 5-fold in flexion at L4/5 in flexion. Expression of TGF-β was observed in all ligamenta flava, however, the expression decreased as the ligamentum flavum thickness increased. Immunohistochemistry showed that TGF-β was released by the endothelial cells, not by fibroblasts. Conclusions. Fibrosis is the main cause of ligamentum flavum hypertrophy, and fibrosis is caused by the accumulation of mechanical stress with the aging process, especially along the dorsal aspect of the ligamentum flavum. TGF-β released by the endothelial cells may stimulate fibrosis, especially during the early phase of hypertrophy.

Biomechanical rationale for using polyetheretherketone (PEEK) spacers for lumbar interbody fusion-A finite element study.

Study Design. To determine the effect of cage/spacer stiffness on the stresses in the bone graft and cage subsidence. Objective. To investigate the effect of cage stiffness on the biomechanics of the fused segment in the lumbar region using finite element analysis. Summary of Background Data. There are a wide variety of cage/spacer designs available for lumbar interbody fusion surgery. These range from circular, tapered, rectangular with and without curvature, and were initially manufactured using titanium alloy. Recent advances in the medical implant industry have resulted in using medical grade polyetheretherketone (PEEK). The biomechanical advantages of using different cage material in terms of stability, subsidence, and stresses in bone graft are not fully understood. Methods. A previously validated 3-dimensional, nonlinear finite element model of an intact L3–L5 segment was modified to simulate posterior interbody fusion spacers made of PEEK (“E” = 3.6 GPa) and titanium (“E” = 110 GPa) at the L4/5 disc with posterior instrumentation. Bone graft (“E” = 12 GPa) packed between the spacers in the intervertebral space was also simulated. The posterior lumbar interbody fusion spacer with instrumentation and graft represent a simulation of the condition present immediately after surgery. Results. The peak centroidal Von Mises stresses in the graft bone increased by at least 9-fold with PEEK spacers as compared to titanium spacer. The peak centroidal Von Mises stresses in the endplates increased by at least 2.4-fold with titanium spacers over the PEEK spacers. These stresses were concentrated at places where the spacer interfaced with the endplate. The stiffness of the spacer did not affect the relative motion (stability) across the instrumented (L4/5) segment. Conclusions. Spacers less stiff than the graft will: (1) provide stability similar to titanium cages in the presence of posterior instrumentation, (2) reduce the stresses in endplates adjacent to the spacers, and (3) increase the load transfer through the graft, as evident from the increase in stresses in graft.

MRI signal changes of the pedicle as an indicator for early diagnosis of spondylolysis in children and adolescents : A clinical and biomechanical study

Study Design. Clinical review of pediatric patients with lumbar spondylolysis and biomechanical analysis using finite-element lumbar spine model. Objectives. To evaluate the usefulness of the signal changes observed on MR images of the pedicle for the early diagnosis of spondylolysis, and to investigate the pathomechanism of the signal changes based on the stresses in pedicles, as predicted using finite-element analyses. Furthermore, to evaluate the usefulness of the signal change to predict the bony healing following conservative treatment. Summary of Background Data. Since early-stage spondylolysis can achieve osseous healing conservatively, it is important to diagnose this disorder as early as possible. Presently, there is no well-established, noninvasive, and reliable diagnostic tool for the early diagnosis. Methods. Thirty-seven pediatric patients with spondylolysis were included. Sixty-eight defects were examined and their stages as revealed on CT scans were recorded. High signal changes (HSC) of the pedicles on axial T2-weighted MRI were compared with the CT-based stages of the defect. Among them, 16 patients, including 15 boys and 1 girl, were treated conservatively for at least a 3-month period. Bony healing of the fracture site was evaluated on CT, and the results were compared between two groups with or without HSC at the initial consultation. Using a three-dimensional nonlinear finite-element model of the L3–L5 segment, stress distributions in the pars and pedicle regions were analyzed in response to 400 N compression and 10.6 Nm moment. Results. Based on CTs, 68 pars defects were classified as follows: 8 very early, 24 late-early, 16 progressive, and 20 terminal stages. All defects in very early and late-early stages (100%) showed HSC on T2-weighted MRI at the ipsilateral pedicle. Among 16 progressive stages, eight (50%) showed HSC, while no defects of the terminal stage (0%) were found to have HSC. In total, 29 pars defects were treated conservatively out of 16 patients. In 19 of the HSC positive defects, 15 (79%) showed bony healing after the conservative treatment, whereas none of the 10 HSC negative defects (0%) showed any healing. The results were statistically significant at P < 0.05 (&khgr;2). Stress results from the finite-element model indicated that pars interarticularis showed the highest value in all loading modes, and the pedicle showed the second highest. Conclusions. The correlation between the high stresses in the pedicle and the corresponding HSC suggest that signal changes in MRI could be used as an indicator for early diagnosis of spondylolysis. The HSC of the pedicle provided useful information to diagnose early stage spondylolysis. Furthermore, the HSC may be a good indicator as to whether a bony union will result from conservative treatment.

Athletes With Unilateral Spondylolysis Are at Risk of Stress Fracture at the Contralateral Pedicle and Pars Interarticularis A Clinical and Biomechanical Study

Background Unilateral spondylolysis is common in youths; its clinical and biomechanical features, especially effects on the contralateral side, are not fully understood. Hypothesis Unilateral spondylolysis predisposes the contralateral side to stress fracture, especially in athletes actively engaged in sporting activities involving torsion of the trunk. Study Design Case series and descriptive laboratory study. Methods Thirteen athletes younger than age 20 with unilateral spondylolysis were included. The contralateral pedicle and pars of spondylolytic vertebrae were examined using computed tomography and magnetic resonance imaging. Using a finite element model of the intact ligamentous L3-S1 segment, stress distributions were analyzed in response to 400-N axial compression and 10.6-N.m moment in flexion, extension, lateral bending, and axial rotation. Unilateral spondylolysis was created in the model at L5. The stress results from the unilateral defect model were compared to the intact model predictions and correlated to the contralateral defects seen in patients. Results Among 13 patients, there were 6 early-, 2 progressive-, and 5 terminal-stage defects. Three (23.1%) showed contralateral stress fracture. Among them, 2 belonged to the progressive-stage and 1 to the terminal-stage spondylolysis group. The remaining 4 patients in the terminal defect group showed stress reactions, such as sclerosis at the contralateral pedicle. In the finite element analysis model with an L5 left spondylolysis, the stresses at the contralateral and pars interarticularis were found to increase in all loading modes, with increases as high as 12.6-fold compared to the intact spine. Conclusions Unilateral spondylolysis could lead to stress fracture or sclerosis at the contralateral side due to an increase in stresses in the region. Clinical Relevance Surgeons should be aware of possibility of contralateral stress fractures in cases in which patients, especially athletes engaged in active sports, show unilateral spondylolysis and persistent low back pain complaints.

Union of defects in the pars interarticularis of the lumbar spine in children and adolescents THE RADIOLOGICAL OUTCOME AFTER CONSERVATIVE TREATMENT

Lumbar spondylolysis can heal with conservative treatment, but few attempts have been made to identify factors which may affect union of the defects in the pars. We have evaluated, retrospectively, the effects of prognostic variables on bony union of pars defects in 134 young patients less than 18 years of age with 239 defects of the pars who had been treated conservatively. All patients were evaluated by CT scans when first seen and more than six months later at follow-up. The results showed that the spinal level and the stage of the defects were the predominant factors. The site of the defects in the pars, the presence or development of spondylolisthesis, the condition of the contralateral pars, the degree of lumbar lordosis and the degree of lumbar inclination all significantly affected union.

Incidence of lumbar spondylolysis in the general population in Japan based on multidetector computed tomography scans from two thousand subjects.

Study Design. Epidemiological analysis using CTs. Objective. To investigate the true incidence of lumbar spondylolysis in the general population in Japan. Summary of Background Data. Although there have been several reports on the incidence of lumbar spondylolysis, they had some weakness. One of them concerns the subjects investigated, because the incidence of lumbar spondylolysis varies considerably, and some patients are asymptomatic. In addition, most of the past studies used plain radiograph films or skeletal investigation. Therefore, the past reported incidence may not correspond to that of the general population. Methods. We reviewed the computed tomography (CT) scans of 2000 subjects (age: 20–92 years) who had undergone abdominal and pelvic CT on a single multidetector CT scanner for reasons unrelated to low back pain. We reviewed them for spondylolysis, spondylolytic spondylolisthesis, and spina bifida occulta (SBO) in the lumbosacral region. The grade (I–IV) of spondylolisthesis was measured using midsagittal reconstructions. Results. Lumbar spondylolysis was found in 117 subjects (5.9%). Their male-female ratio was 2:1. Multiple-level spondylolysis was found in 5 subjects (0.3%). Among these 117 subjects, there were 124 vertebrae with spondylolysis. Of them, 112 (90.3%) corresponded to L5, and 26 (21.0%) had unilateral spondylolysis. SBO was found in 154 subjects. Of them, 25 had spondylolysis (16.2%), whereas, in 1846 subjects without SBO, 92 had spondylolysis (5.0%). The incidence of spondylolysis among the patients with SBO was significantly higher than that in subjects without SBO (Odd ratio was 3.7-fold). Of 124 vertebrae with spondylolysis, 75 (60.5%) showed low-grade (Meyerding grade I or II) spondylolisthesis, and no subject presented high-grade spondylolisthesis. Spondylolisthesis was found in 74.5% of the subjects with bilateral spondylolysis, and in 7.7% of those with unilateral spondylolysis. Conclusion. The incidence of lumbar spondylolysis in the Japanese general population was 5.9% (males: 7.9%, females: 3.9%).

Pathomechanism of loss of elasticity and hypertrophy of lumbar ligamentum flavum in elderly patients with lumbar spinal canal stenosis.

Study Design. A histologic, biologic, and immunohistochemical assessment using human samples of lumbar ligamentum flavum. Objective. To clarify the pathomechanism of loss of elasticity and hypertrophy of the lumbar ligamentum flavum (LF) in the elderly population. Summary of Background Data. The most common spinal disorder in elderly patients is lumbar spinal canal stenosis, causing low back and leg pain, and paresis. Canal narrowing, in part, results from hypertrophy of the LF. Although histologic and biologic literature on this topic is available, the pathomechanism of loss of elasticity and hypertrophy of the LF is still unknown. Methods. One fetus, 5 young, and 5 elderly LF were obtained for histologic study. Hematoxylin and eosin, Alcian blue, Masson Trichrome, and Elastica Van Gieson stains were performed for each LF. Nine LF were collected and were used for biologic study of real time RT-PCR to quantitatively measure mRNA expression of Type I collagen and elastin in each LF. Results. In the LF of the fetus, elastic fibers accounted for about 75% of the entire area. In the dural aspect of the LF in the young and elderly group, the ratio was also around 75%; however, the ratio of the dorsal aspect decreased with age. Almost half of the area showing loss of elastic fibers was shown to be converted to cartilaginous tissue producing Type II collagen and proteoglycan by Alcian blue and Type II collagen immunohistochemistry. The area, which did not stain black with EV nor blue with AB stain, was positively stained blue with T stain, indicating scarring. The area of the normal dural layer was 18.0 ± 2.3 and 33.8 ± 4.3 (mm2), for young and elderly group, respectively. Accordingly, it was 3.2 ± 0.8 and 18.0 ± 10.2 (mm2), for the dorsal abnormal layer. Elastin mRNA showed a relatively strong correlation (r = 0.44) with age; however, the slope was very gentle. Type I collagen mRNA showed a very strong correlation (r = 0.80) with age. The slope was steeper, and the value reached at 1000% (10-fold) around 65 years old when compared with the LF from younger patient. Elastin mRNA showed a weak correlation (r = 0.36) with thickness, and the slope was gentle. Type I collagen mRNA showed relatively strong correlation (r = 0.52) with thickness. The slope was steeper, and the line reached at 1000% (10-fold) around 6.5 (mm) when compared with a thin LF. Conclusion. Decreased elasticity of LF in the elderly is due to the loss of elastic fibers and a concomitant increase of collagenous fibers in the dorsal aspect. LF hypertrophy could be due to the thickening of the normal elastic layer as well as of the abnormal collagenous layer.

Lumbar ligamentum flavum hypertrophy is due to accumulation of inflammation-related scar tissue

Study Design. A histologic, biologic, and immunohistochemical assessment using human samples of the lumbar ligamentum flavum. Objective. To prove our hypothesis that hypertrophy of the ligamentum flavum is caused by accumulation of inflammation-related scar tissue. Summary of Background Data. Lumbar spinal canal stenosis is 1 of the most common spinal disorders in elderly patients. Canal narrowing, in part, results from hypertrophy of the ligamentum flavum. The hypertrophy mechanism remains unclear. Based on our preliminary analyses, we have previously proposed that the hypertrophy may be due to accumulation of scar tissue in the ligament. Scar tissue is reported to develop after inflammation; however, there is no report, including our previous study, on inflammation in the ligamentum flavum. There is a need for an in-depth investigation of any relationship between inflammation and scar formation in the ligamentum flavum. If inflammation is related to hypertrophy, we may control/delay the hypertrophy by inhibiting the inflammation. Methods. Twenty-one ligamentum flavum samples were obtained for the histologic study. Trichrome and Verhoeff-van Gieson stains were used to assess the degree of fibrosis (scarring) and content of elastic fibers, respectively. Two ligamentum flavum samples, hypertrophied and thin control ligaments, were used for a global genetic assessment by oligonucleotide gene array technology with gene chips. Messenger ribonucleic acid expression of cyclooxygenase (COX)-2 was quantitatively measured from 16 ligamentum flavum samples using real-time reverse transcriptase polymerase chain reaction. Immunohistochemistry evaluated the cellular location of COX-2 in ligamentum flavum. Results. In the hypertrophied ligament, severe fibrosis (scarring) was observed in the entire area of the ligamentum flavum, and the severity of scarring showed a significant (r = 0.79; P < 0.0001) and positive linear correlation with ligamentum flavum thickness. Gene array results showed in both thin/control and hypertrophied ligaments expression of inflammation-related genes such as COX-2, tumor necrosis factor-α, and interleukin-1, 6, 8, and 15. Real-time polymerase chain reaction showed COX-2 messenger ribonucleic acid expression in all ligamentum flavum samples. Its expression showed weak positive linear correlation with the thickness of ligament. COX-2 was released from vascular endothelial cells in ligamentum flavum as per the immunohistochemical analysis. Conclusions. Accumulation of fibrosis (scarring) causes hypertrophy of the ligamentum flavum. Inflammation-related gene expression is found in the ligamentum flavum. It might be possible to prevent the hypertrophy of ligamentum flavum with antiinflammatory drugs.

Development of spondylolytic olisthesis in adolescents.

BACKGROUND CONTEXT Although it has been well documented that slippage in patients with spondylolysis is most prevalent during the growth period, the exact time when slippage initiates and halts during the growth period is still unknown. Moreover, the contribution of spinal deformities, such as wedging of the vertebral body to the slippage, remains controversial. PURPOSE To clarify when slippage in pediatric spondylolysis initiates and halts. STUDY DESIGN Retrospective study. PATIENT SAMPLE We radiographically examined 46 athletes under 18 years of age with spondylolysis at the fifth lumbar vertebra (L5). The mean age at the first consultation was 13.3 years. The average follow-up period was 6.0 years. OUTCOME MEASURES Longitudinal observation of slippage at L5 on radiogram in correlation with the maturity of the lumbar spine. METHODS From a lateral radiogram of each patient, percent slippage, lumbar index (LI), and skeletal age of the affected vertebra were measured. Changes in the percent slippage over time were investigated, and the correlation between the percent slippage and LI was analyzed. RESULTS From the cartilaginous stage to the apophyseal stage, the slippage increased in 80.0% of the patients (16 of 20). From the cartilaginous stage to the epiphyseal stage, slippage increased in 11.1% of the patients (3 of 27). None of the patients (0 of 22) showed an increase after the epiphyseal stage. In 20 patients in whom slippage increased during the follow-up period, the percent slippage at the final consultation and the LI at the first consultation showed no significant correlation; however, the percent slippage and the LI at the final consultation were significantly (p<.01) correlated. CONCLUSION In conclusion, slippage was more prevalent in individuals of a younger skeletal age whose lumbar spine was immature, and it halted during the epiphyseal stage when the growth period was over and the vertebra matured. Furthermore, the results suggest that wedge deformity of an affected vertebra might be the result rather than the cause of slippage.