Effect of age on upper limb, neck, and trunk kinematics during activities of daily living.

Motion analysis during activities of daily living has been conducted in numerous studies. However, information is lacking regarding age-related differences that affect clinical assessment and treatment goals. This study aimed to examine the effect of age on kinematics during activities of daily living. Three-dimensional motions of the shoulder, elbow, neck, and trunk of 12 younger adults (age, 29.8 ± 5.4 years; 7 men and 5 women) and 10 older adults (age, 69.5 ± 4.9 years; 6 men and 4 women) were measured during the acts of reaching for a table, bringing a glass to the mouth for drinking, wiping the buttocks, tying shoelaces, washing hair, washing the axilla, reaching for a high shelf, and reaching for the floor. The ranges of motion and sequential joint angles were compared between age groups by using discrete analysis and statistical parametric mapping, respectively. The ranges of motion of all joint angles in older and younger adults were comparable in the drinking, washing hair, washing the axilla, and reaching for the floor tasks. Statistical parametric mapping indicated that older adults had significantly poorer neck extension than did younger adults during the drinking (67-92% cycle time) and tying shoelaces (64-95% cycle time) tasks. Kinematics were mostly maintained in healthy older adults during activities of daily living. However, reduced motions were confirmed later during some tasks. The results indicated that existing knowledge combined with the current findings, which take age into account, could be used in clinical settings to assess the kinematics of activities of daily living and set treatment goals.

Exploring whole-body kinematics when eating real foods with the dominant hand in healthy adults.

Despite the importance of eating movements to the rehabilitation of neurological patients, information regarding the normal kinematics of eating in a realistic setting is limited. We aimed to quantify whole-body three-dimensional kinematics among healthy individuals by assessing movement patterns in defined phases while eating real food with the dominant hand in a seated position. Our cross-sectional study included 45 healthy, right-hand dominant individuals with a mean age of 27.3 ± 5.1 years. Whole-body kinematics (joint angles of the upper limb, hip, neck, and trunk) were captured using an inertial sensor motion system. The eating motion was divided into four phases for analysis: reaching, spooning, transport, and mouth. The mean joint angles were compared among the phases with Friedman's analysis of variance. The maximum angles through all eating phases were 129.0° of elbow flexion, 32.4° of wrist extension, 50.4° of hip flexion, 6.8° of hip abduction, and 0.2° of hip rotation. The mean shoulder, elbow, and hip joint flexion angles were largest in the mouth phase, with the smallest being the neck flexion angle. By contrast, in the spooning phase, the shoulder, elbow, and hip flexion were the smallest, with the largest being the neck flexion angle. These angles were significantly different between the mouth and spooning phases (p < 0.008, Bonferroni post hoc correction). Our results revealed that characteristic whole-body movements correspond to each phase of realistic eating in healthy individuals. This study provides useful kinematic data regarding normal eating movements, which may inform whole-body positioning and movement, improve the assessment of eating abilities in clinical settings, and provide a basis for future studies.

Shadow pitching deviates ball release position: kinematic analysis in high school baseball pitchers.

BACKGROUND: Although shadow pitching, commonly called "towel drill," is recommended to improve the throwing motion for the rehabilitation of pitching disorders before the initiation of a throwing program aimed at returning to throwing using a ball, the motion differs from that of normal throwing. Learning improper motion during ball release (BR) may increase shoulder joint forces. Abnormal throwing biomechanics leads to injures. However, there has been no study of shadow pitching focusing on the BR position. The purpose of the present study was to evaluate the BR position and kinematic differences between shadow pitching and normal throwing. In addition, the effect of setting a target guide for BR position on throwing motion was examined in shadow pitching. METHODS: The participants included in this study were 20 healthy male students who were overhand right-handed pitchers with no pain induced by a throwing motion. Participants performed normal throwing (task 1), shadow pitching using a hand towel (task 2), and shadow pitching by setting a target of the BR position (task 3). A motion capture system was used to evaluate kinematic differences in throwing motions, respectively. Examination items comprised joint angles and the differences in BR position. RESULTS: BR position of task 2 shifted significantly toward the anterior, leftward, and downward directions compared with task 1. The distance of BR position between tasks 1 and 2 was 24 ± 10%. However, task 3 had decreased BR deviation compared with task 2 (the distance between 3 and 1 was 14 ± 7%). Kinematic differences were observed among groups at BR. For shoulder joint, task 2 showed the highest value in abduction and horizontal adduction among groups. In spine flexion, left rotation and thorax flexion, task 2 was significantly higher than task 1. Task 3 showed small differences compared with task 1. CONCLUSIONS: The BR position of shadow pitching deviated significantly in the anterior, leftward, and downward directions compared with normal throwing. Furthermore, we demonstrated that the setting of BR target reduces this deviation. Thus, the target of BR position should be set accurately during shadow pitching exercises in the process of rehabilitation.

Acetabular reinforcement ring with additional hook improves stability in three-dimensional finite element analyses of dysplastic hip arthroplasty.

BACKGROUND: The stability of acetabulum reconstructions using reinforcement rings and hooks is important for successful replacement surgery. The objective of this study was to biomechanically determine the effects of the hook on stress and the related micromotions of the acetabular reinforcement ring during the immediate postoperative period. METHODS: Acetabular reinforcement ring models were developed using a nonlinear, three-dimensional, finite element method. Using a pre-prepared template, we constructed without-hook and bone graft models of varying volumes and material properties. RESULTS: The stress on the inferior margin of the acetabulum was higher in the with-hook model than in the without-hook model, especially with increased bone graft volumes, and the stiffness of the bone graft material was decreased. Relative micromotions in the without-hook model were higher than in the with-hook models. The highest relative micromotion was observed in the model with increased bone graft volume and lower stiffness of bone graft material. CONCLUSIONS: In biomechanical analyses, the hook effectively dispersed stress and improved the initial fixation strength of the acetabular reinforcement ring.

Finite element analysis of the tibial bone graft in cementless total knee arthroplasty.

BACKGROUND: Achieving stability of the tibial implant is essential following cementless total knee arthroplasty with bone grafting. We investigated the effects of bone grafting on the relative micromotion of the tibial implant and stress between the tibial implant and adjacent bone in the immediate postoperative period. METHODS: Tibial implant models were developed using a nonlinear, three-dimensional, finite element method. On the basis of a preprepared template, several bone graft models of varying sizes and material properties were prepared. RESULTS: Micromotion was larger in the bone graft models than in the intact model. Maximum micromotion and excessive stress in the area adjacent to the bone graft were observed for the soft and large graft models. With hard bone grafting, increased load transfer and decreased micromotion were observed. CONCLUSIONS: Avoidance of large soft bone grafts and use of hard bone grafting effectively reduced micromotion and undue stress in the adjacent area.

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 ß 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 Young's 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 Young's modulus from the proximal to the distal end, from 82.1 to 51.0GPa as calculated by a heat transfer simulation. The Young's 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 Young's modulus gradient may have bone-preserving effects and sufficient stem strength, without the excessive interface stress.

Influence of Gender Differences on Range of Motion and Joint Angles During Eating in Young, Healthy Japanese Adults.

OBJECTIVE: Eating is important in rehabilitation and is evaluated using joint angles that serve as kinematic information. Joint angles of the upper extremities during eating have been reported for men and for women; however, no study has investigated these joint angles in terms of gender differences. At present, no information is available on gender differences as individual factors of upper extremity joint angles during eating. Therefore, the present study investigated gender differences in upper limb joint angles during eating tasks involving a spoon or chopsticks. METHODS: We examined eating motions in 12 healthy Japanese men and 13 healthy Japanese women (aged 20-39 years) and compared the two groups. Motions were assessed using inertial sensors and three-dimensional motion analysis. RESULTS: We established, separately for men and women, the maximum angle and the range of motion (RoM) for each upper limb joint. Women generally had greater RoMs for all upper limb joints than men did. When subjects used a spoon, statistically significant differences and large effect sizes were observed for the maximum elbow joint flexion angle and the RoMs of the shoulder joint in abduction, flexion, and internal rotation. When subjects used chopsticks, statistically significant differences and large effect sizes were observed for maximum angles of shoulder joint internal rotation, elbow joint flexion, and wrist joint radial flexion and for the RoM of shoulder joint abduction. CONCLUSIONS: We concluded that there are significant gender differences in upper limb joint angles during eating, and that these differences are influenced by numerous factors.

A novel protocol to test age-related decreases in sit-to-stand movement abilities in healthy subjects.

BACKGROUND: The development of simple approaches to quantitatively estimate functional motor performance in a wide range of ages is crucial for early detection of locomotive syndrome (LS). Sit-to-stand (STS) movements from a chair are important in activities of daily living (ADL), although such tasks are the most mechanically demanding among all ADL. Here, we propose a novel test to score STS abilities to estimate individual functional motor status and investigated age-related changes in STS scores as a method to evaluate reduction in performance. METHODS: A total of 606 healthy subjects (average age, 48.4 years; age range, 20-87 years) without neuromusculoskeletal disorders participated in the study. We designed a test to score STS abilities, based on STS determinant variables, including seat height, foot positioning, and both-leg- or one-leg-standing tasks. STS scores ranged from 0 to 14 points, depending on the difficulty or mechanical demands of the STS task. We evaluated the STS score of the subjects divided into seven 10-year age groups. RESULTS: A significant and negative correlation was observed between STS scores and subject age (men: r = -0.65, P < 0.001; women: r = -0.62, P < 0.001). The proportion of subjects able to stand on one leg from a chair seat height of 100% of knee height decreased with age from 100% for both men and women aged 20-29 years to 0.0% and 7.1% for men and women aged 80-87 years, respectively. A large variation in STS scores was observed for subjects aged >60 years. There was a negative linear correlation between mean STS score per age group and mean age (men: R(2) = 0.93, slope = -0.10, intercept = 16.63 points; women: R(2) = 0.92, slope = -0.08, intercept = 14.76 points). CONCLUSIONS: The ability to perform STS was negatively correlated with age among healthy subjects aged 20-87 years. For subjects aged >60 years, STS abilities decreased, but within-group variability increased. This test was useful to identify the deterioration of functional motor performance and prevent early LS.

Trade-off between stress shielding and initial stability on an anatomical cementless stem shortening: in-vitro biomechanical study.

Shortened cementless femoral stems have become popular with the advent of minimally invasive total hip arthroplasty (THA). Successful THA requires initial stem stability and prevention of stress shielding-mediated bone loss, although the effect of stem shortening is controversial. Here we experimentally examined whether stem shortening affects stress shielding and initial stability. Anatomical stems (length, 120 mm) were cut to an 80 mm or 50 mm length. Ten tri-axial strain gauges measured the cortical strain on each stem-implanted femur to evaluate stress shielding. Two transducers measured axial relative displacement and rotation under single-leg stance loading. The 50 mm stem increased the equivalent strains with respect to the original stem in the proximal calcar region (31.0% relative to intact strain), proximal medial region (63.1%), and proximal lateral region (53.9%). In contrast, axial displacement and rotation increased with a decreasing stem length. However, the axial displacement of the 50 mm stem was below a critical value of 150 µm for bone ingrowth. Our findings indicate that, with regard to a reduction in stem length, there is a tradeoff between stress shielding and initial stability. Shortening the stem up to 50 mm can promote proximal load transfer, but bone loss would be inevitable, even with sufficient initial stability for long-term fixation.

In-vitro biomechanical evaluation of stress shielding and initial stability of a low-modulus hip stem made of ß 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 ß-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 Young's 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 Young's 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.

Effect of acetabular reinforcement ring with hook for acetabular dysplasia clarified by three-dimensional finite element analysis.

The objective of this study was to biomechanically determine the effect of the severity of acetabular dysplasia, number and positions of screws and type of bone graft material used on the initial fixation strength of the acetabular reinforcement ring with hook (Ganz ring) using the finite element method. Relative micromotion increased as the severity of acetabular dysplasia increased and tended to decrease as the number of screws increased, but varied according to screw placement position. Increased strength of the bone graft material led to decreased relative micromotion. Biomechanically, the Ganz ring can be placed securely using 3 screws in patients with Crowe 1 dysplasia. However, in patients with Crowe 2 or higher dysplasia, it is necessary to spread at least 4 screws across an area of good host bone.

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

BACKGROUND: Finite 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. METHODS: A 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°. RESULTS: Compared 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. CONCLUSIONS: PAO 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.

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 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.

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.

Biomechanical analysis for stress fractures of the anterior middle third of the tibia in athletes: nonlinear analysis using a three-dimensional finite element method.

We evaluated stresses in the anterior middle third of the tibia that have been reported to predict a poor prognosis for tibial stress fractures compared to other predominant sites (posteromedial regions of the distal third and proximal third). The effect of two different loads (bending-compression load and torsional load) on three sites was investigated using a three-dimensional finite element method. The model was constructed using the tibia, fibula, proximal tibiofibular joint, interosseous membrane, and tibiofibular ligament based on computed tomography scans obtained at 4-mm intervals of the lower leg of a 20-year-old woman who exhibited no abnormal findings on roentgenograms. First, a normal model was constructed using normal material properties, and then the model was modified to produce fracture models by varying the mechanical properties of each predominant site and expanding the area in three gradual phases on the assumption that the fracture advanced in three phases. Each model was tested against the same two loads, and stresses at the nodal points on the border of the fracture area and normal area were compared in each cross section to determine the effect of the load on fracture advancement. In response to torsional load, both the normal model and fracture models tended to show higher values for the posteromedial distal third than the anterior middle third. By examining the bending-compression load it could be seen that the mean peak value significantly decreased between the first and second phases in fracture models of the anterior middle third. This finding was inconsistent with our previous belief that the bending-compression load would have more serious consequences than the torsional load. In contrast, when the area of fracture was expanded into the third phase, maximum values were significantly higher than during the second phase. No similar finding was observed for the posteromedial distal third, suggesting that the anterior middle third may have the same stable biomechanical conditions as the posteromedial distal third at an earlier stage and thus have little influence on fractures. When the fracture is more advanced, however, the conditions change suddenly, and a bending-compression load may adversely affect the mechanical conditions in this area and thereby cause complete fracture.

Effects of lumbar spinal fusion on the other lumbar intervertebral levels (three-dimensional finite element analysis).

The risk of accelerating the degeneration of adjacent disc levels after lumbar spinal fusion is a controversial issue. A finite element model consisting of L1 to L5 lumbar spines was used to assess the effect on adjacent disc level after lumbar spinal fusion. We compared intact, L4/5 posterior interbody fusion (PLF), and L4/5 posterior lumbar interbody fusion (PLIF) models. The loading conditions applied were compressive force, compressive force plus flexion moment, and compressive force plus extension moment. Evaluations were made for von Mises stress on each vertebral end-plate, Tresca stress of all the annulus fibrosus, and Tresca stress of the annulus fibrosus from the posterior surface of the disc to the neural foramen. As the result, the von Mises stress adjacent to the fusion level was higher than the other nonfusion levels; it was higher under conditions of flexion moment loading plus compression loading [112% (2.59 PMa) in the PLF model and 117% (2.72 Mpa) in the PLIF model] than in the intact model. The Tresca stress of all the annulus fibrosus adjacent to the fusion level was higher than that on other nonfusion intervertebral levels; it was higher under conditions of flexion moment loading plus compression loading [127% (0.57 PMa) in the PLF model and 209% (0.89 Mpa) in the PLIF model] than in the intact model. The Tresca stress of the annulus fibrosus from the posterior surface of the disc to the neural foramen adjacent to the fusion level was higher than that on other nonfusion intervertebral levels; and it was higher under conditions of flexion moment loading plus compression loading [107% (1.48 PMa) in the PLF model and 112% (1.54 Mpa) in the PLIF model] than in the intact model. These findings demonstrate that with lumbar fusion, stresses on the vertebral end-plate and the annulus fibrosus were high adjacent to the fusion level; furthermore, stresses were higher in the PLIF model than in the PLF model. These results suggested that lumbar spinal fusion might bring with it a risk of damage to the annulus fibrosus and the vertebral end-plate adjacent to the fusion level.

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.