The initial recommendation for the postoperative regime after uncemented total hip arthroplasty (THA) was 6-12 weeks of partial weight bearing (PWB) to obtain a stable implant position during bone ingrowth. In recent years patients with uncemented THA have increasingly practiced full weight bearing (FWB) after surgery, which has largely been based on clinical experience rather than on scientific evidence. The aim of this thesis was to investigate the effects of FWB versus PWB for 3 months on the stability of the implants and on bone mineral density (BMD), as well as body composition (BC) of the lower extremities. We used radiostereometric analysis (RSA) to measure implant micromotion and dual X-ray absorptiometry (DXA) to measure BMD and BC. Forty-six patients with strictly unilateral osteoarthritis of the hip (OAH) received uncemented THA. These patients were then randomized to the FWB or PWB groups and followed for 5 years. In a preoperative cross-sectional study the BMD of the hip and heel were compared between the OAH-affected side and the healthy side. The study showed an increase of BMD at the femoral neck and a decrease at the total hip and trochanter. The results of a RSA study of cup stability showed that there might be minimal movement in medial and proximal directions during the first postoperative week. These results indicate that the RSA baseline investigation of uncemented cups should be performed as early as possible after the first postoperative day. FWB had no adverse effects on the stability of the uncemented press-fit cups or the uncemented cementless Spotorno (CLS) femoral stems after a 5-year follow-up. There was no difference in periprosthetic BMD around the CLS stem regardless of the postoperative weight bearing regime. All zones around the femoral stem indicated a recovery in BMD toward baselines, except the calcar region, which showed progressive loss in BMD to -22% at 5 years post-surgery. FWB had no effect on the changes in BC after surgery. In conclusion, FWB is safe in uncemented THA in terms of stability, BMD and BC. Furthermore, THA apparently counteracts age-related changes in BC but not in BMD.

This thesis, comprising 5 separate studies, is concerned with fixation of prosthetic components in total hip arthroplasty. The results and conclusions of the studies fol-low; The initial stability of femoral revision components, the long cementless PCA stem and the Exeter standard stem cemented in a bed of impacted bone graft, was com-pared in an experimental study. The PCA stem was more stable than the Exeter stem. However, for both stems initial stability may not be sufficient to allow bone ingrowth. Initial fixation is especially vulnerable to torsion. Identical femoral stems with or without HA-coating were compared in a prospec-tive randomized clinical trial. The long-term stable fixation of a cementless Link RS femoral component was improved by application of hydroxyapatite coating to the femoral stem. In a clinical study the method of extensive impaction of morsellized bone allograft and a hydroxyapatite-coated cementless Mallory-Head acetabular component was found to be advantageous for acetabular revision in the presence of contained or acetabular wall defects. The limited contact between the HA-coated implant and living host bone did not seem to compromise long-term stable fixation. Two different cup designs were compared in a prospective randomized RSA study. At 3 years after implantation the cemented low profile Lubinus FAL cup performed as well as the cemented Lubinus Standard cup in terms of migration and polyethyl-ene wear. In a study of the relationship between radiolucent lines and migration the Lubinus FAL cup displayed more radiolucent lines in the cement bone interface than the Lubinus Standard cup but no difference in migration was found. Early appearance of such radiolucent lines represents an unspecific finding without reliable correla-tion to 3-year migration of the acetabular component.