Cone prosthesis for the hip joint.

The shape of the proximal segment of the femur must be taken into account when implanting femoral endoprostheses, especially those intended for cementless anchorage. Numerous femoral prostheses are available for the proximally broadly extending, "trumpet-shaped" morphology. However, the femur often has a narrow, more cylindrical configuration, as is frequently seen with dysplastic hip joints, but variants of the anatomical constitution or ethnic variants are also found. Conventional femoral prostheses with a proximal transverse oval or rectangular cross-section are often incorrectly positioned in those cases because they can fracture the narrow bones. In many instances, even a pathological anteversion attachment cannot be adequately corrected. The cone prosthesis is ideal for this morphology when pre-operative planning indicates good contact between the bone cortex and the middle third of the prosthetic stem. The tapered anchorage of the cone stem in the medullary cavity reamed to a cone shape promotes primary stability, which is a fundamental prerequisite for the osseointegration of a coarse blasted titanium implant. The sharp longitudinal ridges on the prosthetic stem, which tend to cut into the bone, ensure extensive rotational stability, which explains why thigh pain is not associated with the cone prosthesis. The cone prosthesis has proved its worth in 635 implants performed over 9 years, with highly satisfactory clinical and X-ray results. The surgical technique is relatively straightforward, and complications are rare. The patients' subjective satisfaction is particularly remarkable. The success of the operation lies in correct preoperative planning, which ensures that the morphology of the selected femur guarantees contact between the bone cortex and the middle third of the prosthetic stem.