CT-based accuracy of implanting custom-made endoprostheses

Background. To optimize physiological load transfer and mechanical stability a close geometric fit between cementless stems and the bone stock is essential. To solve the problem of a geometric mismatch between the anatomic shape of the femoral canal and conventional stems custom-made implants were developed. This study determined the accuracy of implanting custom-made press-fit straight femoral stems compared to conventional cementless stems. Methods. Nine pairs of human cadaveric femurs were used to determine the extent of the endocortical area in contact with the stem. The bone–femoral component interface was measured in vitro using CT-data analysis. A software program was developed to describe the periprosthetic inner cortical bone structure, the stem surface and the proportionate implant–endosteal bone contact. Findings. The mean endosteal bone contact of both prostheses was 21.6%. The proportionate implant–endosteal bone contact for straight custom-made femoral prostheses averaged 20.98%. For the conventional cementless stems the mean proportionate contact was 22.15%. Especially in the proximal femur the desirable contact of customized implants was not achieved by closed distal press-fit. Interpretation. Compared to conventional straight cementless stems custom-made stems did not extend the endocortical contact area. The desirable “fit and fill” of the proximal femur was not achieved with straight custom-made femoral components in femurs of regular geometry.