Micromotion, fit, and fill of custom made femoral stems designed with an automated process.

This study presents an automated process for the design of custom made femoral stems. Two software programs have been developed to obtain the inner bone contours from computed tomography scan images and to design the optimal stem that can be inserted into the femur. This process requires only 2 hours of computer use, therefore reducing the price of the stem. It is also possible to control the amount of bone sacrifice necessary to facilitate the insertion of the stem. Micromotion for 6 specimens was measured using a special machine simulating the load supported by the hip joint during single-limb stance. Three prostheses were tested successively: anatomic cementless stem, custom made stem, and modified custom made stem in which the distal part was thinned out. To be stabilized, the anatomic prosthesis required a greater number of cycles than custom made stems associated with a more important total vertical migration. The micromotion of custom made prostheses was significantly less than that of the other prostheses. The fill of custom made stems also was measured, having a range from 93% to 100% (mean, 97.5%-98.8%) of the medullary canal. The metaphyseal fill was significantly linked with the vertical and rotational components of instability. These results showed that it was possible, using a computerized automated and controlled process, to obtain low price femoral stems well fitted to the medullary canal that provide an excellent primary stability.