Nicole M. Grosland

Load-sharing between anterior and posterior elements in a lumbar motion segment implanted with an artificial disc.

Study Design. A nonlinear three-dimensional finite element model of the osteoligamentous L3–L4 motion segment was used to predict changes in posterior element loads as a function of disc implantation and associated surgical procedures. Objectives. To evaluate the effects of disc implantation on the biomechanics of the posterior spinal elements (including the facet joints, pedicles, and lamina) and on the vertebral bodies. Summary of Background Data. Although several artificial disc designs have been used clinically, biomechanical data—particularly the change in loads in the posterior elements after disc implantation—are sparse. Methods. A previously validated intact finite element model was implanted with a ball-and-cup–type artificial disc model via an anterior approach. The implanted model predictions were compared with in vitro data. To study surgical variables, small and large windows were cut into the anulus, and the implant was placed anteriorly and posteriorly within the disc space. The anterior longitudinal ligament was also restored. Models were subjected to either 800 N axial compression force alone or to a combination of 10 N-m flexion–extension moment and 400 N axial preload. Implanted model predictions were compared with those of the intact model. Results. Facet loads were more sensitive to the anteroposterior location of the artificial disc than to the amount of anulus removed. Under 800 N axial compression, implanted models with an anteriorly placed artificial disc exhibited facet loads 2.5 times greater than loads observed with the intact model, whereas posteriorly implanted models predicted no facet loads in compression. Implanted models with a posteriorly placed disc exhibited greater flexibility than the intact and implanted models with anteriorly placed discs. Restoration of the anterior longitudinal ligament reduced pedicle stresses, facet loads, and extension rotation to nearly intact levels. Conclusions. The models suggest that, by altering placement of the artificial disc in the anteroposterior direction, a surgeon can modulate motion-segment flexuralstiffness and posterior load-sharing, even though the specific disc replacement design has no inherent rotational stiffness.

Impact of Impaired Wrist Motion on Hand and Upper-Extremity Performance

PURPOSE To quantify and compare the disabilities caused by reduced and absent wrist motion using objective measurements of task performance and perceived disability, and to assess the compensatory motions of the shoulder, elbow, forearm, and trunk caused by impaired wrist motion. METHODS A clinical study of 21 normal subjects was done to measure physical performance and to assess wrist function under conditions of reduced (30 degrees flexion and 30 degrees extension) and nearly absent wrist motion using established physical tests and questionnaires (Disabilities of the Arm, Shoulder, and Hand [DASH], Patient Rated Wrist Evaluation [PRWE], and a study-specific survey). The clinical study also measured compensatory motions of the shoulder, elbow, forearm, and trunk. RESULTS Average times to perform the Jebsen test and activities of daily living (ADLs) test increased for both motion-restricted conditions of the wrist but did not differ significantly between the conditions. Questionnaire scores regarding function were significantly worse for both motion-restricted conditions and poorest for nearly absent motion. Average compensatory motions in the extremity and trunk statistically increased for both motion-restricted conditions but were not marked and did not differ between the conditions. High variability among subjects occurred in all physical tests and questionnaires for both motion-restricted conditions. CONCLUSIONS Perceived disability from reduced wrist motion appeared greater than measured functional loss using common physical tests and outcome surveys.

Cartilage contact pressure elevations in dysplastic hips: a chronic overload model

BackgroundDevelopmental dysplasia of the hip (DDH) is a condition in which bone growth irregularities subject articular cartilage to higher mechanical stresses, increase susceptibility to subluxation, and elevate the risk of early osteoarthritis. Study objectives were to calculate three-dimensional cartilage contact stresses and to examine increases of accumulated pressure exposure over a gait cycle that may initiate the osteoarthritic process in the human hip, in the absence of trauma or surgical intervention.MethodsPatient-specific, non-linear, contact finite element models, constructed from computed tomography arthrograms using a custom-built meshing program, were subjected to normal gait cycle loads.ResultsPeak contact pressures for dysplastic and asymptomatic hips ranged from 3.56 – 9.88 MPa. Spatially discriminatory cumulative contact pressures ranged from 2.45 – 6.62 MPa per gait cycle. Chronic over-pressure doses, for 2 million cycles per year over 20 years, ranged from 0.463 – 5.85 MPa-years using a 2-MPa damage threshold.ConclusionThere were significant differences between the normal control and the asymptomatic hips, and a trend towards significance between the asymptomatic and symptomatic hips of patients afflicted with developmental dysplasia of the hip. The magnitudes of peak cumulative contact pressure differed between apposed articular surfaces. Bone irregularities caused localized pressure elevations and an upward trend between chronic over-pressure exposure and increasing Severin classification.

IA-FEMesh: An open-source, interactive, multiblock approach to anatomic finite element model development

Finite element (FE) analysis is a valuable tool in musculoskeletal research. The demands associated with mesh development, however, often prove daunting. In an effort to facilitate anatomic FE model development we have developed an open-source software toolkit (IA-FEMesh). IA-FEMesh employs a multiblock meshing scheme aimed at hexahedral mesh generation. An emphasis has been placed on making the tools interactive, in an effort to create a user friendly environment. The goal is to provide an efficient and reliable method for model development, visualization, and mesh quality evaluation. While these tools have been developed, initially, in the context of skeletal structures they can be applied to countless applications.

The pathomechanism of spondylolytic spondylolisthesis in immature primate lumbar spines in vitro and finite element assessments.

Study Design Immature Chacma baboon (Papio ursinus) spine specimens were used to determine load-displacement behavior as related to disc injury. This was accomplished through the application of A-P shear force until failure of FSUs with pars defects. Several finite element models (FEMs) of the FSU were developed to study the mechanism of slippage in immature baboon lumbar spines. Objectives The purpose was to show that spondylolisthesis (olisthesis) always occurs through the growth plate using a model similar to immature human lumbar spines. Using FEMs, the roles of facet orientation, pars interarticularis thickness, and a weak growth-plate in producing slippage were examined. Summary of Background Data Progression from spondylolysis (lysis) to olisthesis occurs, most often, during the adolescent growth spurt. The biomechanical literature dealing with the slippage mechanism in the immature lumbar spine does not provide a clear understanding and is sparse. Methods Several groups of FSUs were subjected to A-P shear force until failure. The results provided displacement at failure as a function of disc injury and flexion-extension fatigue. A bilateral pars defect was created in each specimen prior to application of A-P shear force using an MTS machine. Failure sites were assessed radiographically and histologically. A nonlinear 3-D FEM of the intact L4–L5 was created from CT scans. The model was modified to predict the effects of a pars fracture, a thin pars, a weak growth plate, and facet orientation on the shear load through the growth plate and stresses in the pars. Results Experimentally, failures always occurred through the growth-plate in the disc intact and disc-incised groups. In the intact FEM, the growth plate carried21% of the applied A-P shear force. The load increased when the facets were more sagittally oriented. The effect of thin pars and/or weaker growth plate was an increase in stresses in the pars. Changes in the load through the growth plate were minimal. Conclusions The weakest link in immature baboon lumbar functional spinal units (FSUs) with lysis during an A-P shear load was the growth plate, between the cartilaginous and osseous end plates. Surgeons may assess this lesion on MRI views, thereby predicting the possible development and preventing progression of olisthesis. Finite element model results predict that more sagittally orientated facets and/or a pars fracture are prerequisites for olisthesis to occur.

Bicortical vs monocortical orthodontic skeletal anchorage

INTRODUCTION Case reports have documented the use of titanium miniscrews in providing skeletal anchorage for orthodontic tooth movement. Success rates as low as 50% have been reported for screw retention in either the facial or the lingual cortical plates (monocortical placement). The purpose of this in-vitro study was to test the hypothesis that bicortical miniscrew placement (across the entire width of the alveolus) gives the orthodontist superior force resistance and stability (anchorage) compared with monocortical placement. METHODS Forty-four titanium alloy screws, 1.5 x 15.0 mm, were placed in 22 hemi-sected maxillae and mandibular specimens between the first and second premolars. Half were placed monocortically, half were placed bicortically, and all were subjected to tangential force loading perpendicular to the miniscrew through a lateral displacement of 1.5 mm. Bone samples were sectioned and bone thickness at the screw sites measured. Statistical analyses, consisting of paired samples t tests, 2-samples t tests, Spearman rank correlation tests, and Fisher exact tests, were used to compare monocortical with bicortical screw force-deflection characteristics and stability. Additionally, 2-dimensional plane-stress finite-element models of bicortical and monocortical screw placement subjected to similar loading were analyzed. RESULTS As hypothesized, deflection force values were significantly greater for bicortical screws than for monocortical screws placed in both the maxilla and the mandible (P <0.01 in each instance). Furthermore, force values at mandibular sites were significantly greater than those at maxillary sites for both types of screws. No significant differences in deflection force values were found between the right and left sides of the jaws, or between coronal and apical alveolar-process screw positions. A significant increasing relationship was found between mandibular buccal bone thickness and deflection force for monocortical screws only, and no relationship was found between maxillary bone thickness and deflection force for monocortical or bicortical screws. Monocortical screws were significantly more mobile after force application than bicortical screws. Finite-element analysis indicated lower cortical bone stresses with bicortical placement than with monocortical placement, and these results were consistent with in-vitro experimental findings. CONCLUSIONS Bicortical miniscrews provide the orthodontist superior anchorage resistance, reduced cortical bone stress, and superior stability compared with monocortical screws.

Validation of a C2–C7 cervical spine finite element model using specimen-specific flexibility data

This study presents a specimen-specific C2-C7 cervical spine finite element model that was developed using multiblock meshing techniques. The model was validated using in-house experimental flexibility data obtained from the cadaveric specimen used for mesh development. The C2-C7 specimen was subjected to pure continuous moments up to +/-1.0 N m in flexion, extension, lateral bending, and axial rotation, and the motions at each level were obtained. Additionally, the specimen was divided into C2-C3, C4-C5, and C6-C7 functional spinal units (FSUs) which were tested in the intact state as well as after sequential removal of the interspinous, ligamentum flavum, and capsular ligaments. The finite element model was initially assigned baseline material properties based on the literature, but was calibrated using the experimental motion data which was obtained in-house, while utlizing the ranges of material property values as reported in the literature. The calibrated model provided good agreement with the nonlinear experimental loading curves, and can be used to further study the response of the cervical spine to various biomechanical investigations.

Influence of articular geometry on prosthetic wrist stability.

Ellipsoid and toroid-shaped articulations for total wrist prostheses were evaluated using computer modeling and laboratory experiments. An ellipsoidal design was found to accommodate greater width of the concave proximal component, resulting in better capture and prosthetic stability than a toroid shape. An ellipsoid articulation also provides greater contact area through the available arc of motion. An ellipsoidal articulation is a reasonable design for total wrist arthroplasty.

Stability Analysis of an Enhanced Load Sharing Posterior Fixation Device and Its Equivalent Conventional Device in a Calf Spine Model

STUDY DESIGN An in vitro test of calf spine lumbar segments to compare biomechanical stabilization of a rigid versus a dynamic posterior fixation device. OBJECTIVES To compare flexibility of a dynamic pedicle screw fixation device with an equivalent rigid device. SUMMARY OF BACKGROUND DATA Dynamic pedicle screw device studies are not as prevalent in the literature as studies of rigid devices. These devices contain the potential to enhance load sharing and optimize fusion potential while maintaining stability similar to that of rigid systems. METHODS Load-displacement tests were performed on intact and stabilized calf spines for the dynamic and rigid devices. Stability across a destabilized L3-L4 segment was restored by insertion of either a 6 mm x 40 mm dynamic or rigid pedicle screw fixation device across the L2-L4 segment. The screws then were removed, 7 mm x 45 mm pedicle screws of the opposite type were inserted, and the construct then was re-tested. Axial pull-out tests were performed to assess the likely effects of pedicle screw replacement on the load-displacement data. RESULTS Results indicated a 65% reduction in motion in flexion-extension and a 90% reduction in lateral bending across the destabilized level for both devices, compared with intact spine values. Reduction in axial rotation motion was much smaller than in other modes. Axial pull-out tests showed no weakening of the bone-screw interface. CONCLUSIONS Both devices provided significant stability of similar magnitudes in flexion, extension, and lateral bending. In axial rotation, the devices only could restore stability to levels similar to those in an intact spine. The dynamic device offers a design that may enhance load sharing without sacrificing construct stability.

Biomechanical studies on two Anterior Thoracolumbar implants in cadaveric spines

STUDY DESIGN A biomechanical comparison of two commonly used anterior spinal devices: the Smooth Rod Kaneda and the Synthes Anterior Thoracolumbar Spinal Plate. OBJECTIVES To compare the stability imparted to the human cadaveric spine by the Smooth Rod Kaneda and Synthes Anterior Spinal Plate, and to assess how well these devices withstand fatigue and uni- and bilateral facetectomy. SUMMARY OF BACKGROUND DATA Biomechanical studies on the aforementioned and similar devices have been performed using synthetic, porcine, calf, or dog spines. As of the time of this writing, studies comparing anterior spinal implants using human cadaveric spines are scarce. METHODS An L1 corpectomy was performed on 19 spines. Stabilization was accomplished by an interbody wooden graft and the application of the Smooth Rod Kaneda in 10 spines and the Synthes Anterior Spinal Plate in the remaining 9. Biomechanical testing of the spines was performed in six degrees of freedom before and after stabilization, and after fatiguing to 5000 cycles of +/- 3 Nm of flexion and extension. Testing was repeated after uni- and bilateral facetectomy. RESULTS After stabilization, the Smooth Rod Kaneda was significantly more rigid than the anterior thoracolumbar bar spinal plate in extension. After fatigue, the Smooth Rod Kaneda was significantly stiffer than the anterior thoracolumbar spinal plate in flexion, extension, right lateral bending, left lateral bending, and right axial rotation. A significant decrease in stiffness was noted with the Synthes device in flexion after bilateral facetectomy compared with the stabilized spine. CONCLUSIONS The smooth Rod Kaneda device tends to be stiffer than the anterior thoracolumbar spinal plate, particularly in extension, exceeding the anterior thoracolumbar spinal plate in fatigue tolerance. The spine stabilized with the anterior thoracolumbar spinal plate is more susceptible to the destabilizing effect of bilateral facetectomy than than that stabilized with the Smooth Rod Kaneda. The additional rigidity encountered with the Smooth Rod Kaneda must be weighed against the simplicity of anterior thoracolumbar spinal plate application.