[Custom made hip prostheses : principles and design]

The design of a custom made hip prosthesis includes two parts: intramedullary and extramedullary. The design requires X-ray data including a full weight bearing view of the limbs, a frontal pelvis, frontal and sagittal views of the hip. Several CT-Scan cuts are also necessary at every 5 mm of the upper femur and then four cuts are important to design the extramedullary part: at the level of the true acetabulum, above the lesser trochanter to assess the torsion of the upper femur, the knee condylar axis, and the second metatarsal axis in the foot. The intramedullary part is designed using the numerical data of the CT and the inner contour of the femoral cavity is reconstructed by density gradient method. The extramedullary part reproduces all the elements of the preoperative planning: center of rotation, lever arm, lengthening, and anteversion axis. This is specially important in case of severe upper femoral deformity when excessive anteversion of the upper part of the femur often occurs.