Osseointegration in hip prostheses: experimental study in sheep

Sixteen 2- to 3-year-old sheep were submitted to a hemiarthroplasty of the hip joint with a specially designed femoral component. The proximal two thirds of the stem had a circumferential, plasma-sprayed, porous coating with hydroxyapatite. The animals where killed a 15, 30, 60, 90, 120, 180, 200, 270, 360, and 540 days after surgery. Femurs were submitted to plain radiographs, computerised tomography (CT) scan, and dual energy X-ray absorptiometry (DEXA). Cross-sections were obtained at four different levels and studied using scanning electron microscopy. In the coated portion of the stem, apposition of woven immature bone was evident at 15–30 days and mature lamellar bone by 30 days. With time, the gap between the endosteum and the coated surface was filled by bridges of lamellar bone with a marked trabecular orientation. In the distal uncoated portion of the stem, the implant was initially surrounded by fibrous tissue that, with time, transformed into lamellar bone.