Mei Wang

Validation of a multisegment foot and ankle kinematic model for pediatric gait

This paper reports the development, accuracy, reliability, and validation protocol of a four-segment pediatric foot and ankle model. The four rigid body segments include: 1) tibia and fibula; 2) hindfoot-talus, navicular, and calcaneus; 3) forefoot-cuboid, cuneiforms, and metatarsals; and 4) hallux. A series of Euler rotations compute relative angles between segments. Validation protocol incorporates linear and angular testing for accuracy and reliability. Linear static system resolution is greatest in the Y orientation at 0.10/spl plusmn/0.14 mm and 0.05 level of significance and 99.96% accuracy. Dynamic linear resolution and accuracy are 0.43/spl plusmn/0.39 mm and 99.8%, respectively. Angular dynamic resolution computes to 0.52/spl plusmn/3.36/spl deg/ at 99.6% accuracy. These calculations are comparable to the Milwaukee adult foot and ankle model.

Second generation intramedullary nailing of subtrochanteric femur fractures: a biomechanical study of fracture site motion.

Objectives: To compare fracture site motion between different second-generation intramedullary nails used to fix subtrochanteric fractures of the proximal femur with and without femoral neck fractures. Design: Nondestructive mechanical testing of four types of femoral intramedullary nails was undertaken to evaluate fracture site motion using a model that simulated single-leg and double-leg stance. Methods: Three types of reconstruction nails (the Russell-Taylor Delta [Smith & Nephew, Memphis, TN], the Uniflex [Biomet, Warsaw, IN], Alta CFX [Howmedica-Osteonics, Rutherford, NJ]) and the Long Gamma nail (Howmedica-Osteonics, Rutherford, NJ), each measuring 11 × 380 mm, were inserted in fiberglass composite femurs. Four fracture patterns were studied (transverse subtrochanteric, subtrochanteric with posteromedial wedge comminution, subtrochanteric with one-centimeter gap, and a one-centimeter gap with a subcapital neck fracture). Single- and double-leg stance loading was simulated using a servohydraulic load frame (MTS, Eden Prairie, MN). Two-way analysis of variance and post hoc t tests were used to determine any statistically significant differences between groups. Results: In single-leg stance there were significant differences in coronal plane rotation, shear, and axial translation across the subtrochanteric fracture site between the different nail types and the different fracture patterns (p < 0.001). In double-leg stance there were significant differences in coronal plane rotation and femoral head vertical motion between the different nail types and the different fracture patterns (p < 0.001), and there were significant differences in shear and axial translation between the different fracture patterns (p < 0.001) but not the different nail types (p > 0.05). Conclusions: For simple, well-reduced fractures the choice of implant is not critical. As fracture severity increased (comminution, gap, and combined neck fracture), the choice of implant, particularly with reference to proximal nail dimensions and implant materials, was a significant factor in reducing fracture site motion. Therefore, our laboratory data suggest that when subtrochanteric fractures are unstable (e.g., comminution, segmental bone loss) and early weight bearing is desirable, the choice of implant is critical and should be restricted to implants that allow minimal fracture site motion (Long Gamma and Russell-Taylor).

Biomechanical changes at adjacent segments following anterior lumbar interbody fusion using tapered cages.

Study Design. A biomechanical evaluation of anterior cages in a calf lumbar spine model. Objectives. To determine changes in spinal motion and intradiscal pressures at immediately adjacent lumbar motion segments following anterior insertion of tapered cages. Summary of Background Data. Stand-alone anterior lumbar interbody fusion (ALIF) is an effective approach in the treatment of discogenic low back pain. A tapered lumbar (LT) cage design attempts to restore physiologic lordosis and sagittal balance. We are not aware of any previous biomechanical evaluation of the effects of LT cages on adjacent motion segments. Methods. Nine fresh calf spines (L2–L5) were procured for the study. Pure moments (up to 8.5 Nm) in flexion, extension, and lateral bending were applied to the L2 vertebra in five steps through a nonconstrained loading system. With each step of loading, three-dimensional rotation at three intervertebral disc levels was obtained through a three-camera motion analysis system, and intradiscal pressures within the nucleus pulposus of the two nonoperated discs were measured with miniature transducers. The spines were tested initially intact and following paired anterior LT cage insertion. Results. Following ALIF, small to moderate increase in motion was found at both adjacent segments in flexion (superior: 12.5%, P < 0.05; inferior: 11.3%, P < 0.02) and lateral bending (superior: 7.8%, P < 0.02; inferior: 6.6%, P < 0.02). An increase in intradiscal pressure was noted at the superior adjacent segment under flexion (21%, P < 0.01) and lateral bending (16%, P < 0.03). Intradiscal pressure changes at the inferior adjacent level were not significant. Conclusions. Statistically significant changes in intradiscal pressures and motion were found at the adjacent levels following a single-level stand-alone ALIF procedure using paired LT cages.

The Effect of Dynamic Femoroacetabular Impingement on Pubic Symphysis Motion A Cadaveric Study

Background: A link between femoroacetabular impingement and athletic pubalgia has been reported clinically. One proposed origin of athletic pubalgia is secondary to repetitive loading of the pubic symphysis, leading to instability and parasymphyseal tendon and ligament injury. Hypothesis/Purpose: The purpose of this study was to investigate the effect of simulated femoral-based femoroacetabular impingement on rotational motion at the pubic symphysis. The authors hypothesize that the presence of a cam lesion leads to increased relative symphyseal motion. Study Design: Controlled laboratory study. Methods: Twelve hips from 6 fresh-frozen human cadaveric pelvises were used to simulate cam-type femoroacetabular impingement. The hips were held in a custom jig and maximally internally rotated at 90° of flexion and neutral adduction. Three-dimensional motion of the pubic symphysis was measured by a motion-tracking system for 2 states: native and simulated cam. Load-displacement plots were generated between the internal rotational torque applied to the hip and the responding motion in 3 anatomic planes of the pubic symphysis. Results: As the hip was internally rotated, the motion at the pubic symphysis increased proportionally with the degrees of the rotation as well as the applied torque measured at the distal femur for both states. The primary rotation of the symphysis was in the transverse plane and on average accounted for more than 60% of the total rotation. This primary motion caused the anterior aspect of the symphyseal joint to open or widen, whereas the posterior aspect narrowed. At the torque level of 18.0 N·m, the mean transverse rotation in degrees was 0.89° ± 0.35° for the native state and 1.20° ± 0.41° for cam state. The difference between cam and the native groups was statistically significant (P < .03). Conclusion: Dynamic femoroacetabular impingement as caused by the presence of a cam lesion causes increased rotational motion at the pubic symphysis. Clinical Relevance: Repetitive loading of the symphysis by cam impingement is thought to lead to increased symphyseal motion, which is one possible precursor to athletic pubalgia.

Does anterior plating of the cervical spine predispose to adjacent segment changes

Study Design. In a human cadaveric model, the effects of plate supplementation on the mechanical behaviors of adjacent segments were investigated. Objectives. The objective was to determine the effects of anterior cervical fusion and plating on the adjacent segments. Summary of Background Data. Increases in intradiscal pressure and intervertebral motion at adjacent segments have been reported in the lumbar spine following an instrumented fusion. It is unclear if the same phenomenon presents in the cervical spine. Methods. Seven human cadaveric cervical spines (C2–T1) were used, and fusion of the C5–C6 segment was chosen for the purpose of this study. Two miniature pressure transducers were implanted within each adjacent disc. Flexion, extension, lateral bending, and torsion loads up to 2.5 Nm were applied to the intact spine, and following each of the two procedures, anterior discectomy and grafted fusion, and anterior plating of the C5–C6 motion segment. Results. At the surgical level, a significant increase in segmental stiffness was observed after plating in all directions. Following the grafted fusion, there were no statistically significant changes at the superior adjacent segment, and there was a 13.7% increase in axial rotation in the inferior adjacent segment. Once anterior plating was applied, slight increase (<12%) over the intact spines was noted in lateral bending in both adjacent segments. However, there was no significant difference between the grafted fusion and anterior plated fusion at either adjacent segment. At both adjacent disc levels, the differences in intradiscal pressures between grafted fusion and plated fusion were less than 30% in all directions, and none of these differences was statistically significant. Conclusions. Intradiscal pressures and intervertebral motion at the adjacent levels are not significantly affected by the instrumented anterior fusion. The clinically observed degenerative change at adjacent segments in the cervical spine is more likely to be attributed to natural progression of the spondylotic process as opposed to biomechanical effect of the instrumentation or fusion.

Intradiscal pressure and kinematic behavior of lumbar spine after bilateral laminotomy and laminectomy

BACKGROUND CONTEXT Bilateral laminotomy has been proposed as an alternative to laminectomy for decompression of lumbar spinal stenosis. Preservation of the posterior midline ligaments with laminotomy is presumed to maintain spinal segment stability. There have been no previous studies that directly compare the amount of destabilization and increase in disc pressures between the two procedures. PURPOSE To quantify spinal segmental instability caused by bilateral laminotomy and laminectomy, and to compare the central and peripheral intradiscal pressures after the two procedures. STUDY DESIGN/SETTING Mechanical testing of the lumbar motion segments of calf spines. METHODS Nine fresh calf spines were tested under flexion, extension, lateral bending and axial rotation, intact first, then after laminotomy and laminectomy at the level of L4-L5. Four miniature pressure transducers were implanted in the central and peripheral disc at L4-L5 to measure intradiscal pressures. Three-dimensional motion was measured with motion analysis system. RESULTS Comparing with bilateral laminotomy, laminectomy showed significant increase in segmental motion at the surgical level in flexion (16%, p<.05), extension (14%, p<.04) and right axial rotation (23%, p<.03). In flexion, the stress at the anterior annulus increased a nonsignificant 20% after laminotomy, but significant 130% after laminectomy (p<.02). In the intact spine, the posterolateral annulus experienced the highest stress with lateral bending to the same side when compared with other loading directions. This stress remained unchanged after laminotomy but increased 9% after laminectomy (p<.06). In rotation, axial intradiscal stresses were evenly distributed and unchanged after each procedure. CONCLUSIONS Laminectomy causes more destabilization of a spinal motion segment than laminotomy and significantly increases disc stress in the anterior annulus.

An upper extremity inverse dynamics model for pediatric Lofstrand crutch-assisted gait

The objective of this study was to develop an instrumented Lofstrand crutch system, which quantifies three-dimensional (3-D) upper extremity (UE) kinematics and kinetics using an inverse dynamics model. The model describes the dynamics of the shoulders, elbows, wrists, and crutches and is compliant with the International Society of Biomechanics (ISB) recommended standards. A custom designed Lofstrand crutch system with four, six-degree-of-freedom force transducers was implemented with the inverse dynamics model to obtain triaxial UE joint reaction forces and moments. The crutch system was validated statically and dynamically for accuracy of computing joint reaction forces and moments during gait. The root mean square (RMS) error of the system ranged from 0.84 to 5.20%. The system was demonstrated in children with diplegic cerebral palsy (CP), incomplete spinal cord injury (SCI), and type I osteogenesis imperfecta (OI). The greatest joint reaction forces were observed in the posterior direction of the wrist, while shoulder flexion moments were the greatest joint reaction moments. The subject with CP showed the highest forces and the subject with SCI demonstrated the highest moments. Dynamic quantification may help to elucidate UE joint demands in regard to pain and pathology in long-term assistive device users.

Comparison of upper extremity kinematics in children with spastic diplegic cerebral palsy using anterior and posterior walkers

This prospective study analyzes the upper extremity kinematics of 10 children with spastic diplegic cerebral palsy using anterior and posterior walkers. Although both types of walkers are commonly prescribed by clinicians, no quantitative data comparing the two in regards to upper extremity motion has been published. The study methodology included testing of each subject with both types of walkers in a motion analysis laboratory after an acclimation period of at least 1 month. Overall results showed that statistically, both walkers are relatively similar. With both anterior and posterior walkers, the shoulders were extended, elbows flexed, and wrists extended. Energy expenditure, walking speed and stride length was also similar with both walker types. Several differences were also noted although not statistically significant. Anterior torso tilt was reduced with the posterior walker and shoulder extension and elbow flexion were increased. Outcomes analysis indicated that differences in upper extremity torso and joint motion were not dependent on spasticity or hand dominance. These findings may help to build an understanding of upper extremity motion in walker-assisted gait and potentially to improve walker prescription.

Mechanical characterization of fourth generation composite humerus.

Mechanical data on upper extremity surrogate bones, supporting use as biomechanical tools, is limited. The objective of this study was to characterize the structural behaviour of the fourth-generation composite humerus under simulated physiologic bending, specifically, stiffness, rigidity, and mid-diaphysial surface strains. Three humeri were tested in four-point bending, in anatomically defined anteroposterior (AP) and mediolateral (ML) planes. Stiffness and rigidity were derived using load–displacement data. Principal strains were determined at the anterior, posterior, medial, and lateral surfaces in the humeral mid-diaphysial transverse plane of one specimen using stacked rosettes. Linear structural behaviour was observed within the test range. Average stiffness and rigidity were greater in the ML (918 ± 18 N/mm; 98.4 ± 1.9 Nm2) than the AP plane (833 ± 16 N/mm; 89.3 ± 1.6 Nm2), with little inter-specimen variability. The ML/AP rigidity ratio was 1.1. Surface principal strains were similar at the anterior (5.41 µε/N) and posterior (5.43 µε/N) gauges for AP bending, and comparatively less for ML bending, i.e. 5.1 and 4.5 µε/N, at the medial and lateral gauges, respectively. This study provides novel strain and stiffness data for the fourth-generation composite humerus and also adds to published construct rigidity data. The presented results support the use of this composite bone as a tool for modelling and experimentation.

Evaluation of 70/30 D,L-PLa for use as a resorbable interbody fusion cage.

Titanium lumbar interbody spinal fusion devices are reported to be 90% effective for single-level lumbar interbody fusion, although radiographic determination of fusion has been debated. Using blinded radiographic, biomechanic, histologic, and statistical measures, researchers in the present study evaluated a radiolucent 70/30 poly(L-lactide-co-D,L-lactide) (70/30 D,L-PLa) interbody fusion device packed with autograft or rhBMP-2 on a collagen sponge in 25 sheep at 3, 6, 12, 18, and 24 months. A trend of increased fusion stiffness, radiographic fusion, and histologic fusion was demonstrated from 3 months to 24 months. Device degradation was associated with a mild to moderate chronic inflammatory response at all postoperative sacrifice times.