Micromotion measurements with hip center and modular neck length alterations.

Hip center relocation often is necessary because of acetabular deformity or in revision surgery. Superolateral relocation of the acetabular component increases the hip joint reaction forces and has been associated with early femoral implant loosening. In addition, relocation can necessitate the use of extended femoral neck lengths. The purpose of this study was to compare the initial stability (micromotion) of an anatomically placed femoral component with that of a superolaterally relocated component and with a component having an extended neck length. A six-degree of freedom device was constructed to measure three-dimensional micromotion at the proximal and distal regions of the femoral component. The instrumented femur was loaded using a unique loading device that included musculature necessary to simulate stairclimbing. Results showed that superolateral relocation of the hip center (25 mm) only moderately increased femoral component micromotion (13%). However, it was found that extending the neck length 12.5 mm produced a dramatic increase in micromotion (38%). Clinically this suggests that hip center lateralization and the use of long modular neck lengths should be avoided.

Improvement of accuracy in a high-capacity, six degree-of-freedom load cell: application to robotic testing of musculoskeletal joints.

This study investigated a previously unaccounted for source of error in a high-capacity, six degree-of-freedom load cell used in multi-degree-of-freedom robotic testing of musculoskeletal joints, an application requiring a load cell with high accuracy in addition to high load capacity. A method of calibration is presented for reducing the error caused by changes in universal force-moment sensor (UFS) orientation within a gravitational field. Uncorrected, this error can exceed a magnitude of 1% of the full-scale load capacity-the manufacturer-stated accuracy of the UFS. Implementation of the calibration protocol reduced this error by approximately 75% for a variety of loading conditions. This improvement in load cell accuracy (while maintaining full load capacity) should improve both the measurement and control of specimen kinetics by robotic/UFS and other biomechanical testing systems.

Effect of superior and superolateral relocations of the hip center on hip joint forces. An experimental and analytical analysis.

With the extensive use of uncemented acetabular components in total hip arthroplasty, relocation of the hip center has become increasingly necessary to avoid bulk grafts and to promote contact between the porous prosthetic surface and bone. Compared with the anatomic hip center, superolateral relocation theoretically results in higher hip joint forces and has been shown in cemented acetabular components to result in an increased clinical failure rate. This study experimentally and analytically compared the hip joint forces in normal, superior, and superolateral hip center locations during both single-leg stance and stairclimbing, performing this comparison over a wide range of hip joint applied flexion moments. An advanced loading fixture was designed to allow any applied moment to be set independently of femoral position, incorporating all three major muscle groups active in stairclimbing position: extensors, abductors, and adductors. For all positions and moments tested, it was found that superolateral relocation caused significant increases in the total hip joint force, but did not affect the nonsagittal force component. Also, superior-only hip center relocation did not significantly affect the total joint force magnitudes or directions. The force increase on hip center lateralization can be attributed to a corresponding increase in the adduction moment. Results from the static analytical model developed supported these findings. The results of this study suggest that superolateral hip center relocation should be avoided and that superior-only relocation may be mechanically acceptable within the confines of the osseous anatomy of the acetabulum.