A new technique for morphologic measurement of the femur. Its application for Japanese patients with osteoarthrosis of the hip.

The cementless femoral prostheses developed in Western countries have commonly been used for Japanese patients, but it has been noted that it is difficult to achieve a good fit between the cementless femoral prostheses and some femora. In Japan, there are many patients with secondary osteoarthrosis (OA) of the hip, but Western designed femoral prostheses seem to be mainly for use with primary OA of the hip. To design femoral prostheses better suited to Japanese patients, a new method to study the morphometry of the femur is required--one which can determine the particular features of the femur in Japanese patients. In this study, a new morphologic measurement technique of the femur is proposed and applied to a number of Japanese OA patients in order to demonstrate its efficacy. In this technique, a three-dimensional solid model based on the outer and inner borders of the femoral cortex is constructed from 3-D CT images and computer-aided design (CAD) software. The bone axis is defined as the line of best fit to the centers of gravity of the femur cross sections in each CT plane by using the least square method. A new femur coordinate system is considered based on this bone axis. The corrected outer borders of the cortex are reconstructed in sections perpendicular to the bone axis. The angle between the principal axis of inertia of the sections and a plane defined by the femur coordinate system is selected as providing a good measurement of the torsion value to characterize the femur morphology. The angles of principal axis were measured in 110 OA and 36 normal hips, and then compared with each other. A significant difference between the OA and normal group was observed especially in the proximal region above the section 6 mm below the level of the lesser trochanter. It is suggested that the anteversion of the femoral neck influences the torsion of the parts below the lesser trochanter. From the results, it is demonstrated that the proposed measurement technique is a reliable and efficient means of determining femur morphology.