Koji Totoribe

Analysis of the effect of lumbar spine fusion on the superior adjacent intervertebral disk in the presence of disk degeneration, using the three-dimensional finite element method

The purpose of this study was to analyze the effect of lumbar spine fusion on the superior adjacent intervertebral disk in the context of disk degeneration, using a nonlinear three-dimensional finite element method. Detailed L3–L5 motion segment models of normal and degenerated intervertebral disks were developed. In fusion models, L4–L5 was fixed by either posterolateral fusion or posterior lumbar interbody fusion (PLIF). Various loading conditions such as compression loading, compression loading plus flexion moment loading, or compression loading plus extension moment loading were applied to study the corresponding stress. Tresca stress on the posterolateral part of intervertebral annulus fiber and von Mises stress on the vertebral endplate (the superior and inferior sides of L3 and L4) were reduced in all degenerated disk models compared with the normal disk models. The PLIF model showed an increase in the percentage change of stress on the vertebral endplate and on the intervertebral annulus fibrosus when flexion and extension moment loadings were applied. This finding suggests that surgeons should consider the risk of exacerbating degeneration of intervertebral disks by undertaking lumbar spine fusion, when degeneration is found in intervertebral disks adjacent to vertebrae requiring fusion.

A biomechanical study of lumbar spondylolysis based on a three-dimensional finite element method

Biomechanical analyses under compression only, and for a combination of flexion, extension, rotation, and lateral bending were performed to evaluate the stress of the interarticular portion of the lumbar vertebra using a nonlinear three‐dimensional finite element method. A detailed three‐dimensional L4–L5 motion segment model was developed that took into consideration the material nonlinearities of ligaments and annular fibers and the contact nonlinearities of facet joints. For a more accurate examination, the separation of cortical bone and cancellous bone for both posterior and anterior elements were also considered. The stress in the pars interarticularis was weakest under compression alone, but stronger under compression with lateral bending loading, with flexion, with rotation, and with extension. Under each loading condition, the region of the stress concentration was consistent with the separated region of the spondylolysis observed in clinical situations. Since the stress in the pars interarticularis was high under extension and rotation in particular, those loadings were suggested to be relatively high risk factors leading to spondylolysis.

Effect of periacetabular osteotomy for acetabular dysplasia clarified by three-dimensional finite element analysis.

BackgroundFinite element analysis (FEA) has been applied for the biomechanical analysis of acetabular dysplasia, but not for biomechanical studies of periacetabular osteotomy (PAO) or those performing analysis taking into consideration the severity of acetabular dysplasia. This study aimed to perform biomechanical evaluation of changes in stress distribution following PAO and to determine the effect of the severity of developmental dysplasia of the hip (DDH) using three-dimensional FEA.MethodsA normal model was designed with a 25° center-edge (CE) angle and a 25° vertical-center-anterior margin (VCA) angle. DDH models were designed with CE and VCA angles each of 10, 0, or −10°. Post-PAO models were created by separating each DDH model and rotating the acetabular bone fragment in the anterolateral direction so that the femoral head was covered by the acetabular bone fragment, with CE and VCA angles each at 25°.ResultsCompared to the normal hip joint model, the DDH models showed stress concentration in the acetabular edge and contacting femoral head, and higher stress values; stress increased with decreasing CE and VCA angles. Compared to the DDH models, the post-PAO models showed near-normal patterns of stress distribution in the acetabulum and femoral head, with stress concentration areas shifted from the lateral to medial sides. Stress dispersion was especially apparent in the severe acetabular dysplasia models. PAO provided greater decreases in the maximum values of von Mises stress in the load-bearing area of the acetabulum and femoral head when applied to the DDH models of higher degrees of severity, although the values increased with increasing severity of DDH.ConclusionsPAO is expected to provide biomechanical improvement of the hip joint and to be particularly effective in patients with severe preoperative DDH, although the results also suggested a limitation in the applicability of PAO for these patients.

In-vitro biomechanical evaluation of stress shielding and initial stability of a low-modulus hip stem made of β type Ti-33.6Nb-4Sn alloy

Stress shielding-related proximal femoral bone loss after total hip arthroplasty occurs because of the different stiffness of metallic alloy stems and host bone. To overcome this, we fabricated a low-modulus cementless hip stem from β-type Ti-33.6Nb-4Sn alloy (TNS). Then we evaluated its stiffness, stress shielding, and initial stability compared with a similar Ti-6Al-4V alloy stem. Stiffness was determined by axial compression and cantilever-bending tests. Thirteen triaxial strain gages measured cortical strain. Stress shielding was defined as the percentage of intact strain after stem insertion. To evaluate initial stability, displacement transducers measured axial relative displacement and rotation. Intact and implanted femurs underwent single-leg-stance loading. Axial stiffness was 56% lower in the TNS stem than in the Ti-6Al-4V stem, and bending stiffness of the TNS stem decreased gradually from the proximal region to the distal region, being ≤ 53% that of the Ti-6Al-4V stem, indicating gradation of Youngs modulus. The TNS stem decreased stress shielding in the proximal calcar region (A1: 83%, B1: 85% relative to intact cortical strain) without affecting the proximal lateral region (B3: 53%). The initial stabilities of the stems were comparable. These findings indicate that the TNS stem with gradation of Youngs modulus minimizes proximal femoral bone loss and biological fixation, improving long-term stability.

Load-transfer analysis after insertion of cementless anatomical femoral stem using pre- and post-operative CT images based patient-specific finite element analysis

Periprosthetic bone remodeling is commonly seen after total hip arthroplasty, but the remodeling pattern differs among patients even in those implanted with the same stem. Remodeling occurs mainly because of the difference in load transmitted from the stem to the femur. In this study, we evaluated the load-transfer pattern in eight female patients implanted with an anatomical stem on an individual basis by patient-specific finite element analysis that is based on pre- and postoperative computed tomography images. Load transfer was evaluated using interface stress between the stem and bone. One of eight patients demonstrated proximal dominant load transfer, while the other patients demonstrated a distal dominant pattern. The results of our biomechanical simulations reveal the differences in load-transfer pattern after surgery among patients with the same anatomical stem.

Hydroxyapatite block for use in posterolateral lumbar fusion: a report of four cases.

Autologous bone grafts for posterolateral lumbar fusion are harvested from the iliac crests. Recently, several alternatives to autologous bone have been evaluated (such as graft substitutes, graft extenders, or both) with variable results. However, no clinical long-term studies have validated the efficacy of these techniques in posterolateral lumbar fusion. This study evaluated radiographic and histologic findings in four patients (mean age, 66 years) during the first 5 years after posterolateral fusion with an hydroxyapatite block. The mean followup was 7 years 1 month. Radiologic evaluation was by plain radiographs and computed tomography scans. Histologic evaluation was done in one patient. Capillaries extended into the porous structure of the hydroxyapatite substrate, and some of the pores were replaced by newly formed bone tissue. The long-term results of graft substitutes were stable and hydroxyapatite appeared to have some potential to achieve union in posterolateral lumbar fusion. However, hydroxyapatite block alone has not functioned effectively as a complete graft substitute in posterolateral lumbar fusion. Thus, a suitable osteogenic material is required to induce the formation of new bone and achieve a solid union.

A biomechanical study of lumbar fusion based on a three-dimensional nonlinear finite element method.

Biomechanical analyses under compression, flexion, and extension loading were performed to evaluate the stability of interbody, posterolateral, posterior, and facet fusions using a nonlinear three-dimensional finite element method. The effects of facet fusion on other lumbar fusions were also examined. A three-dimensional L4–L5 motion segment model was developed that took into consideration the material nonlinearities of ligaments and annular fibers and the contact nonlinearities of facet joints. Of all models of fusion, maximum rigidity was obtained in the interbody fusion model. In the posterolateral, posterior, and facet fusion models under compression, axial displacement and flexion rotation were induced. In combination with facet fusion, the interbody, posterolateral, and posterior fusion models demonstrated a decrease in axial displacement of about 6%, 1%, and 5%, respectively, under compression and a decrease in rotation angle of about 22%, 12%, and 48%, respectively, under flexion-extension loading. Stress concentration moved principally toward the fusion site, indicating increased load transfer across the fusion mass. Our findings suggest that a more solid fixation can be expected from lumbar fusion—especially in posterior fusion—if facet fusion is performed.

Comparative biomechanical analysis of a cervical cage made of an unsintered hydroxyapatite particle and poly-L-lactide composite in a cadaver model.

Study Design. A new cage made from a forged composite of unsintered hydroxyapatite particles and poly-L-lactide (F-u-HA/PLLA) is compared biomechanically with the Ray threaded fusion cage. Objectives. To compare the stability imparted to the human cadaveric spine by two different threaded cervical cages and the effect of cyclic loading on construct stability. Summary of Background Data. Threaded cages have been developed for use in anterior cervical interbody fusions to provide initial stability during the fusion process. However, metallic instrumentation has several limitations. Recently, totally bioresorbable bone fixation devices made of F-u-HA/PLLA have been developed, including a cage for spinal interbody fusion. However, no biomechanical study has compared the F-u-HA/poly-L-lactide (PLLA) cage with metallic cages. Methods. For this study, 12 fresh ligamentous human cervical spines (C4–C7) were used. After anterior discectomy across C5–C6, stabilization was achieved with the F-u-HA/PLLA cage in six spines and with the Ray threaded fusion cage in the remaining six spines. Biomechanical testing of the spines was performed with six degrees of freedom before and after stabilization, and after cyclic loading of the stabilized spines (5000 cycles of flexion–extension at 0.5 Nm). Results. The specimens stabilized with either the F-u-HA/PLLA cage or the Ray cage were significantly more stable than the discectomy case in all directions except in extension. In extension, both groups were stiffer, although not at a significant level (P > 0.05). After fatigue, the stiffness, as compared with that in the prefatigue case, decreased in both groups, although not at a significant level. The Ray cage group exhibited better stability than the F-u-HA/PLLA cage group in all directions, although a significant difference was found only in right axial rotation. Conclusions. The F-u-HA/PLLA cage has the possibility to supplant the use of metallic devices in interbody fusions of the cervical spine.

Quantification of the sit-to-stand movement for monitoring age-related motor deterioration using the Nintendo Wii Balance Board

Simple methods for quantitative evaluations of individual motor performance are crucial for the early detection of motor deterioration. Sit-to-stand movement from a chair is a mechanically demanding component of activities of daily living. Here, we developed a novel method using the ground reaction force and center of pressure measured from the Nintendo Wii Balance Board to quantify sit-to-stand movement (sit-to-stand score) and investigated the age-related change in the sit-to-stand score as a method to evaluate reduction in motor performance. The study enrolled 503 participants (mean age ± standard deviation, 51.0 ± 19.7 years; range, 20–88 years; male/female ratio, 226/277) without any known musculoskeletal conditions that limit sit-to-stand movement, which were divided into seven 10-year age groups. The participants were instructed to stand up as quickly as possible, and the sit-to-stand score was calculated as the combination of the speed and balance indices, which have a tradeoff relationship. We also performed the timed up and go test, a well-known clinical test used to evaluate an individual’s mobility. There were significant differences in the sit-to-stand score and timed up and go time among age groups. The mean sit-to-stand score for 60s, 70s, and 80s were 77%, 68%, and 53% of that for the 20s, respectively. The timed up and go test confirmed the age-related decrease in mobility of the participants. In addition, the sit-to-stand score measured using the Wii Balance Board was compared with that from a laboratory-graded force plate using the Bland–Altman plot (bias = −3.1 [ms]-1, 95% limit of agreement: −11.0 to 3.9 [ms]-1). The sit-to-stand score has good inter-device reliability (intraclass correlation coefficient = 0.87). Furthermore, the test–retest reliability is substantial (intraclass correlation coefficient = 0.64). Thus, the proposed STS score will be useful to detect the early deterioration of motor performance.

Improving stress shielding following total hip arthroplasty by using a femoral stem made of β type Ti-33.6Nb-4Sn with a Young's modulus gradation

Stress shielding-related bone loss occurs after total hip arthroplasty because the stiffness of metallic implants differs from that of the host femur. Although reducing stem stiffness can ameliorate the bone resorption, it increases stress at the bone-implant interface and can inhibit fixation. To overcome this complication, a novel cementless stem with a gradient in Youngs modulus was developed using Ti-33.6Nb-4Sn (TNS) alloy. Local heat treatment applied at the neck region for increasing its strength resulted in a gradual decrease in Youngs modulus from the proximal to the distal end, from 82.1 to 51.0GPa as calculated by a heat transfer simulation. The Youngs modulus gradient did not induce the excessive interface stress which may cause the surface debonding. The main purpose of this study was to evaluate bone remodeling with the TNS stem using a strain-adaptive bone remodeling simulation based on finite element analysis. Our predictions showed that, for the TNS stem, bone reduction in the calcar region (Gruen zone 7) would be 13.6% at 2years, 29.0% at 5years, and 45.8% at 10years postoperatively. At 10 years, the bone mineral density for the TNS stem would be 42.6% higher than that for the similar Ti-6Al-4V alloy stem. The stress-strength ratio would be lower for the TNS stem than for the Ti-6Al-4V stem. These results suggest that although proximal bone loss cannot be eliminated completely, the TNS stem with a Youngs modulus gradient may have bone-preserving effects and sufficient stem strength, without the excessive interface stress.