Loss in mechanical contact of cementless acetabular prostheses due to post-operative weight bearing: a biomechanical model.

The primary stability of cementless acetabular components is a prerequisite for their clinical success. The target of the present study was to analyse possible effects of post-operative joint loading on the initial mechanical stability of a press-fitted acetabular prosthesis. For this purpose, a three-dimensional finite element model of the pelvic bone with acetabular reconstruction was set-up. The analysis included two steps: (1) simulation of the prosthesis press-fit implantation and (2) simulation of the instant of peak resultant hip loading during the one-legged stance. The difference between the contact pressures at the bone/implant interface, at the end of the second step and those at the end of the first step was calculated and assumed as an index of variation in mechanical contact due to post-operative weight bearing. The results show that, due to hip loading, contact pressures given by press-fit increase in the postero-superior acetabular region but decrease in the antero-inferior acetabular region. The calculated area in which the contact pressures decrease extend to about 30% of the total contact surface. These results imply that post-operative joint loading significantly reduces the mechanical stability given by press-fit. The decrease in contact pressures at the bone/implant interface may result in a lack of osteointegration, possibly hindering the implant secondary stability. It may also create a route for wear debris, possibly favouring periprosthetic osteolysis, which may lead to further loss in contact and clinical failure of the implant due to loosening.