Stability of uncemented acetabular components: Design, patient-dependent, and surgical effects and complications due to trapped interfacial fluid

Besides affecting initial implant stability and inhibiting bone ingrowth, immediate post-operative gaps at the bone-implant interface may allow debris and fluid accumulation in the periprosthetic joint space, which could lead to complications such as osteolysis. A previously developed specimen-specific finite element model of the human pelvis was augmented to assess the influence of design, patient-dependent, and surgical factors on the immediate post-operative stability of acetabular cups. Trapped interfacial fluid with and without a lesion was also incorporated to evaluate its effects on implant stability. Parametric analysis showed that contemporary designs such as hemispherical and elliptical cups performed as well, if not better, than hybrid designs such as the serrated cup, which had large bands of gaps around its periphery. Taking into account the variability in joint loading and surgical implantation, a cup design with 2 mm diametral equatorial and polar bone-implant interferences was found to minimize the change in gap volume and nearly maximize the total and rim contact areas, thereby improving implant stability and bone ingrowth conditions. The introduction of randomly reamed acetabular surfaces with different surface roughnesses demonstrated that roughness effects were significant. Also, large gaps tend to appear in regions with extensive over-reaming, highlighting the need to minimize reaming inaccuracies. In a follow-up stochastic analysis of cup stability that incorporated a comprehensive number of surgical and patient-dependent environmental factors, cup designs with 2 mm and 3 mm bone-implant interferences were found to maximize implant stability because they were sufficiently oversized to overcome acetabular reaming variations so as to provide the minimal amount of bone-implant interference for implant stability. Trapped interfacial fluid with and without a lesion caused contact areas to decrease by up to 23% and maximum gap sizes to increase by fourteen times. In the presence of a lesion, the elevated fluid pressure fluctuations increased pressure-induced strains by up to 57%, which may stimulate bone formation but may also cause bone trauma. These analyses demonstrate that implant stability is sensitive to a wide range of design and environmental factors and that complications may develop when fluid is trapped in the periprosthetic joint space.