Kazuhiko Sawai

Analysis of the Endosteal Geometry of the Proximal Femur in Japanese Patients with Osteoarthritic Hips: Use in Femoral Stem Design

A study was undertaken to establish a basic design for a straight femoral stem to be used in total hip arthroplasty (THA). This stem would be fitted to the endosteal geometry of femurs in Japanese patients with advanced osteoarthritis of the hip due to congenital displacement of the hip (CDH). Using our three-dimensional computer-aided design (3D-CAD) system, we investigated the geometry of the proximal femur, based on computed tomography (CT) slices obtained from 30 femurs in patients with advanced osteoarthritis and 20 femurs in healthy controls. We found that almost 90% of the femurs in the patients with advanced osteoarthritis had a straight section in the marrow cavity, between 40 and 60 mm below the tip of the lesser trochanter. An adequate basic axis, as a standard for measuring and as a guide for reaming the marrow cavity of the femur, could be obtained by extending the connecting line of the inner center of each slice in this specific straight section. Reaming along our basic axis, simulated by the 3D-CAD system, cut off less cortical bone (79 mm3) than reaming along a conventional axis determined by connecting the center of two cross sections, one at the entry point of the marrow cavity and the other at the maximum depth of reaming (203 mm3). Excessive variations were observed in the cross-sectional geometry of the upper part of the canal of the proximal femur. It was found that, in THA, to obtain a standard straight marrow cavity with no excessive variation and under-cut, the femoral neck should be cut off less than 15 mm above the tip of the lesser trochanter, with a cutting angle of less than 20°. Two radiuses of curvature (R1, 100 mm; R2, 150 mm) were proposed to simplify the curve on the medial flare of the femoral canal between the levels of 15 mm above and 40 mm below the tip of the lesser trochanter. Two taper angles (T1, 2°; T2, 1°) were proposed, to simplify the inclination of the medial femoral canal between 40 and 80 mm below the tip of the lesser trochanter. Correlations were found between patient’s canal flare indices (CFIs) and canal widths 60 mm below the tip of the lesser trochanter in both the frontal (r = −0.6697) and sagittal (r = −0.7961) dimensions. No correlation, however, was revealed between frontal and sagittal CFIs.

—Overview— Clinical Gait Analysis in Hip Patients

The relationship between ascending and descending trunk movements and the biphasic pattern of the vertical reaction force was demonstrated by practical accelerometry and reaction force measurement. The vertical reaction force directly represents the kinematic acceleration of vertical body movement. The abnormal trunk movement of hip patients was investigated with a TV motion analysis system. Patients with lateral trunk bending and shifting showed considerable lateral trunk displacement, shortening the lever arm for the dynamic weight due to body mass, gravitation, and vertical acceleration. This bending and shifting may reduce the required abductor force and the hip resultant force. In clinical gait analysis, it is important to correctly interpret gait parameters and abnormal gait patterns in terms of biomechanical considerations, and to establish criteria for gait disorders.

Simple Solid Models for Surgical Planning in Hip Arthroplasties

Our simple solid model, made of foam polystyrol, is helpful in understanding the complex shape of bony geometry (such as that found in hip joints) and is useful for carrying out surgical planning and rehearsal operations.