Kenneth J. Fischer

A new methodology to measure load transfer through the forearm using multiple universal force sensors.

Previous approaches to measuring forces in the forearm have made the assumption that forces acting in the radius and ulna are uniaxial near the wrist and elbow. To accurately describe forces in the forearm and the forces in the interosseous ligament, we have developed a new methodology to quantitatively determine the 3-D force vectors acting in forearm structures when a compressive load is applied to the hand. A materials testing machine equipped with a six degree-of-freedom universal force-moment sensor (UFS) was employed to apply a uniaxial compressive force to cadaveric forearms gripped at the hand and humerus. Miniature UFSs were implanted into the distal radius and proximal ulna to measure force vectors there. A 3-D digitizing device was used to measure transformations between UFS coordinate systems, utilized for calculating the force vectors in the distal ulna, proximal radius, and the interosseous ligament (IOL). This method was found to be repeatable to within 3 N, and accurate to within 2 N for force magnitudes. Computer models of the forearm, generated from CT scans, were used to visualize the force vectors in 3-D. Application of this methodology to eight forearm specimens showed that the radius carries most of the load at the wrist while force in the IOL relieves load acting in the radius at the mid-forearm. For a 136 N applied hand force, the force in the IOL was 36 + 21 N. Advantages of this methodology include the determination of 3-D force vectors, especially those in the IOL, as well as computer generated 3-D visualization of results.

FEASIBILITY OF PARTIAL A2 AND A4 PULLEY EXCISION: RESIDUAL PULLEY STRENGTH

We investigated residual digital flexor pulley strengths after 75% excision of the A2 and A4 pulleys. For direct pull-off tests, A2 and A4 pulleys from cadaveric fingers were tested by pulling on a loop of flexor digitorum profundus tendon through the pulley. For functional loading tests, fingers were positioned with the metacarpophalangeal joint flexed to 90° for A2 testing, and with the proximal interphalangeal joint in 90° flexion for A4 testing (with all other joints in full extension). Excision of 75% of A2 and A4 pulleys reduced pulley strengths determined by both testing methods. For the functional loading tests, which are more clinically relevant, mean tendon forces at failure after partial excision of A2 and A4 pulleys were 224 and 131 N respectively, which is sufficient to withstand flexor tendon forces expected during activities of daily living.

Observations of convergence and uniqueness of node-based bone remodeling simulations

Some investigators have indicated that mathematical theories and computational models of bone adaptation may not converge and that the density solutions from such simulations are dependent on the initial density distribution In this study, two-dimensional finite element models were used to investigate the effect of initial density distribution on the final density distribution produced using a node-based bone remodeling simulation. The first model was a generic long bone, and the second was a proximal femur. For each model, we conducted time-dependent, node-based, linear rate-law bone remodeling simulations. Five initial density conditions were used with the generic long bone and three with the proximal femur. Remodeling simulations were performed, and the largest average nodal density differences at the end of the simulations were 0.000010 g/cm3 and 0.000006 g/cm3 for the generic long bone and proximal femur models, respectively. Results illustrate that, for a given set of loads and a given finite element model, the node-based bone adaptation algorithm can yield a unique density distribution. In conjunction with previous studies, this finding suggests that uniqueness of the density solution is dependent on both the mathematical theory and the computational implementation.

MRI-Based Modeling for Radiocarpal Joint Mechanics: Validation Criteria and Results for Four Specimen-Specific Models

The objective of this study was to validate the MRI-based joint contact modeling methodology in the radiocarpal joints by comparison of model results with invasive specimen-specific radiocarpal contact measurements from four cadaver experiments. We used a single validation criterion for multiple outcome measures to characterize the utility and overall validity of the modeling approach. For each experiment, a Pressurex film and a Tekscan sensor were sequentially placed into the radiocarpal joints during simulated grasp. Computer models were constructed based on MRI visualization of the cadaver specimens without load. Images were also acquired during the loaded configuration used with the direct experimental measurements. Geometric surface models of the radius, scaphoid and lunate (including cartilage) were constructed from the images acquired without the load. The carpal bone motions from the unloaded state to the loaded state were determined using a series of 3D image registrations. Cartilage thickness was assumed uniform at 1.0 mm with an effective compressive modulus of 4 MPa. Validation was based on experimental versus model contact area, contact force, average contact pressure and peak contact pressure for the radioscaphoid and radiolunate articulations. Contact area was also measured directly from images acquired under load and compared to the experimental and model data. Qualitatively, there was good correspondence between the MRI-based model data and experimental data, with consistent relative size, shape and location of radioscaphoid and radiolunate contact regions. Quantitative data from the model generally compared well with the experimental data for all specimens. Contact area from the MRI-based model was very similar to the contact area measured directly from the images. For all outcome measures except average and peak pressures, at least two specimen models met the validation criteria with respect to experimental measurements for both articulations. Only the model for one specimen met the validation criteria for average and peak pressure of both articulations; however the experimental measures for peak pressure also exhibited high variability. MRI-based modeling can reliably be used for evaluating the contact area and contact force with similar confidence as in currently available experimental techniques. Average contact pressure, and peak contact pressure were more variable from all measurement techniques, and these measures from MRI-based modeling should be used with some caution.

Proximal Femoral Density Patterns are Consistent with Bicentric Joint Loads.

We developed an alternate method for density-based load estimation and applied it to estimate hip joint load distributions for two femora. Two-dimensional finite element models were constructed from single energy quantitative computed tomography (QCT) data. Load estimation was performed using five loading regions on the femoral head. Within each loading region, individual nodal loads, normal to the local surface, were supplied as input to the load estimation. An optimization procedure independently adjusted individual nodal load magnitudes in each region, and the magnitudes of muscle forces on the greater trochanter, such that the applied tissue stimulus approached the reference stimulus throughout the model. Dominant estimated load resultant directions were generally consistent with published experimental data for loads during gait. The estimated loads also suggested that loads near the extremes of the articulating surface may be important (even required) for development and maintenance of normal bone architecture. Estimated load distributions within nearly all regions predicted bicentric loading patterns, which are consistent with observations of hip joint incongruity. Remodeling simulations with the estimated loads predicted density distributions with features qualitatively similar to the QCT data sets. This study illustrates how applications of density-based bone load estimation can improve understanding of dominant loading patterns in other bones and joints. The prediction of bicentric loading suggests a very fine level of local adaptation to details of joint loading.

Scapholunate ligament injury adversely alters in vivo wrist joint mechanics: an MRI-based modeling study.

We investigated the effects of scapholunate ligament injury on in vivo radiocarpal joint mechanics using image‐based surface contact modeling. Magnetic resonance images of 10 injured and contralateral normal wrists were acquired at high resolution (hand relaxed) and during functional grasp. Three‐dimensional surface models of the radioscaphoid and radiolunate articulations were constructed from the relaxed images, and image registration between the relaxed and grasp images provided kinematics. The displacement driven models were implemented in contact modeling software. Contact parameters were determined from interpenetration of interacting bodies and a linear contact rule. Peak and mean contact pressures, contact forces and contact areas were compared between the normal and injured wrists. Also measured were effective (direct) contact areas and intercentroid distances from the grasp images. Means of the model contact areas were within 10 mm2 of the direct contact areas for both articulations. With injury, all contact parameters significantly increased in the radioscaphoid articulation, while only peak contact pressure and contact force significantly increased in the radiolunate articulation. Intercentroid distances also increased significantly with injury. This study provides novel in vivo contact mechanics data from scapholunate ligament injury and confirms detrimental alterations as a result of injury.

PRELIMINARY VALIDATION OF MRI-BASED MODELING FOR EVALUATION OF JOINT MECHANICS

The objective of this study was to perform preliminary validation of MRI-based joint contact modeling methodology in the radiocarpal joints by comparison with the results of invasive radiocarpal contact measurements in three cadaver experiments. For each experiment, either Pressurex film or a Tekscan sensor was placed into the radiocarpal joints during a simulated grasp. Computer models were based on magnetic resonance imaging (MRI) of the cadaver specimens without load as well as on images acquired with the same loading used for the direct measurements. Geometric surface models of the radius, scaphoid, and lunate (including cartilage) were constructed from the images acquired without load. The carpal bone motions from the unloaded to the loaded state were determined using three-dimensional (3D) voxel image registration. Cartilage thickness was assumed to be uniform at 1.0 mm with an effective compressive modulus of 4 MPa. Resulting data included peak contact pressure, contact area, and contact force in the radioscaphoid and radiolunate joints. Contact area was also measured directly from MR images acquired with load and compared to model data. Qualitatively, there was good correspondence between the MRI-based model data and experimental data, with consistent relative size, shape, and location of radioscaphoid and radiolunate contact areas. Quantitative comparison of model and experimental data was reasonable, but less consistent. Contact area from the MRI-based model was always similar to the contact area measured directly from the MR images. With additional experiments, we believe that MRI-based joint contact modeling will soon be fully validated in the radiocarpal joints.

DENSITY-BASED LOAD ESTIMATION PREDICTS ALTERED FEMORAL LOAD DIRECTIONS FOR COXA VARA AND COXA VALGA

Quantifying differences in joint loading for coxa vara and coxa valga is important for understanding what constitutes a pathological deformity. Prior free-body analyses for varus and valgus femora suggest that the loading direction in single-leg stance becomes more vertical for coxa valga and more horizontal for coxa vara. The objectives of this study were: 1) to apply a density-based load estimation technique to varus and valgus femora; 2) to infer potential differences in femoral loading for varus and valgus femora from the density; and 3) to compare the results with previous studies of femoral loading for single-leg stance. Representative valgus, normal, and varus femora from male cadavers were scanned in the plane of the femoral neck using computed tomography. A two-dimensional finite element model, including the density data from the CT scans, was constructed for each femur. A density-based bone load estimation method was used to determine the dominant loading pattern, and an average load direction was calculated. The average load direction varied consistently from more vertical for coxa valga to more horizontal for coxa vara. The results indicate that the differences in loading directions reduce the risk of epiphyseal slip or neck fracture in coxa vara and increase the tendency for subluxation or dislocation in coxa valga. Agreement between relative load angles from the density-based load estimation and free-body analyses confirms that internal femoral density is adapted to applied loads regardless of external femoral geometry.

Computationally Efficient Magnetic Resonance Imaging Based Surface Contact Modeling as a Tool to Evaluate Joint Injuries and Outcomes of Surgical Interventions Compared to Finite Element Modeling

Joint injuries and the resulting posttraumatic osteoarthritis (OA) are a significant problem. There is still a need for tools to evaluate joint injuries, their effect on joint mechanics, and the relationship between altered mechanics and OA. Better understanding of injuries and their relationship to OA may aid in the development or refinement of treatment methods. This may be partially achieved by monitoring changes in joint mechanics that are a direct consequence of injury. Techniques such as image-based finite element modeling can provide in vivo joint mechanics data but can also be laborious and computationally expensive. Alternate modeling techniques that can provide similar results in a computationally efficient manner are an attractive prospect. It is likely possible to estimate risk of OA due to injury from surface contact mechanics data alone. The objective of this study was to compare joint contact mechanics from image-based surface contact modeling (SCM) and finite element modeling (FEM) in normal, injured (scapholunate ligament tear), and surgically repaired radiocarpal joints. Since FEM is accepted as the gold standard to evaluate joint contact stresses, our assumption was that results obtained using this method would accurately represent the true value. Magnetic resonance images (MRI) of the normal, injured, and postoperative wrists of three subjects were acquired when relaxed and during functional grasp. Surface and volumetric models of the radiolunate and radioscaphoid articulations were constructed from the relaxed images for SCM and FEM analyses, respectively. Kinematic boundary conditions were acquired from image registration between the relaxed and grasp images. For the SCM technique, a linear contact relationship was used to estimate contact outcomes based on interactions of the rigid articular surfaces in contact. For FEM, a pressure-overclosure relationship was used to estimate outcomes based on deformable body contact interactions. The SCM technique was able to evaluate variations in contact outcomes arising from scapholunate ligament injury and also the effects of surgical repair, with similar accuracy to the FEM gold standard. At least 80% of contact forces, peak contact pressures, mean contact pressures and contact areas from SCM were within 10 N, 0.5 MPa, 0.2 MPa, and 15 mm2, respectively, of the results from FEM, regardless of the state of the wrist. Depending on the application, the MRI-based SCM technique has the potential to provide clinically relevant subject-specific results in a computationally efficient manner compared to FEM.

Effectiveness of surgical reconstruction to restore radiocarpal joint mechanics after scapholunate ligament injury: An in vivo modeling study

Disruption of the scapholunate ligament can cause a loss of normal scapholunate mechanics and eventually lead to osteoarthritis. Surgical reconstruction attempts to restore scapholunate relationship show improvement in functional outcomes, but postoperative effectiveness in restoring normal radiocarpal mechanics still remains a question. The objective of this study was to investigate the benefits of surgical repair by observing changes in contact mechanics on the cartilage surface before and after surgical treatment. Six patients with unilateral scapholunate dissociation were enrolled in the study, and displacement driven magnetic resonance image-based surface contact modeling was used to investigate normal, injured and postoperative radiocarpal mechanics. Model geometry was acquired from images of wrists taken in a relaxed position. Kinematics were acquired from image registration between the relaxed images, and images taken during functional loading. Results showed a trend for increase in radiocarpal contact parameters with injury. Peak and mean contact pressures significantly decreased after surgery in the radiolunate articulation and there were no significant differences between normal and postoperative wrists. Results indicated that surgical repair improves contact mechanics after injury and that contact mechanics can be surgically restored to be similar to normal. This study provides novel contact mechanics data on the effects of surgical repair after scapholunate ligament injury. With further work, it may be possible to more effectively differentiate between treatments and degenerative changes based on in vivo contact mechanics data.