We investigated the radiographic and clinical course of 31 patients in whom a bulk acetabular allograft had been used during the cementless revision of a total hip replacement. Two patients died and two were lost to follow-up within 24 months, but of the remaining 27 acetabular components, 12 (44%) showed radiographic evidence of instability at a mean of 46 months. Five of these have been revised. In the 12 failures, signs of instability had been noted at an average of 29 months (1 to 60). Failures during the first 24 months were usually due to technical errors, later failures to gradual migration of the cup into the graft. The cups with the greatest amount of their surface supported by grafts were most likely to migrate, but this migration was usually asymptomatic. Screw fixation of the cup, used in 24 cases, appeared to control the mechanism of failure. Femoral head allografts and distal femoral allografts had been used, with failure in 6 of 16, and 6 of 11 respectively; distal femoral allografts were used only for large defects. The insidious course of late cup migration and graft failure necessitates close radiographic follow-up of patients treated with bulk allografts.

Four hundred and fifteen patients with cementless acetabular components of either a smooth threaded (130) or porous surfaced (285) variety were compared for clinical symptoms and radiographic signs of component loosening. At a mean 4.8 year follow-up none of the patients with porous acetabular components had signs of component instability. At a mean 3.9 year follow-up 27 (21%) of the patients with a smooth threaded acetabular component showed radiographic signs of instability and 33 (25%) had clinical symptoms. The disappointing short-term results with these threaded cups in our hands have prompted us to abandon their use in favour of the porous surfaced hemispherical cups.

Threaded acetabular components are widely used in cementless total hip replacement, despite a poor understanding of the nature of the bone-implant interface. We have examined one case in which the threaded titanium ring appeared to be well incorporated with no discernible radiolucency. Microradiography and histology surprisingly showed that the threads were entirely encapsulated in fibrous tissue. This raises doubt about the relevance of plain radiography to the analysis of the acetabular interface.

Total hip replacement using porous-coated cobalt-chrome femoral implants designed for biological fixation has been evaluated in 307 patients after two years and in 89 patients after five years. Histological study of 11 retrieved specimens showed bone ingrowth in nine and fibrous tissue fixation in two. Fixation by bone ingrowth occurred in 93% of the cases in which a press fit of the stem at the isthmus was achieved, but in only 69% of those without a press fit. The clinical results at two years were excellent. The incidence of pain and limp was much lower when there was either a press fit of the stem or radiographic evidence of bone ingrowth. Factors such as age, sex, and the disease process did not influence the clinical results. Most cases showed only slight resorptive remodelling of the upper femur, but in a few cases with a larger, more rigid stem, more extensive bone loss occurred. The results after five years showed no deterioration with time. Fixation by the ingrowth of bone or of fibrous tissue both appeared to be stable, but bone ingrowth gave better clinical results.