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.