Early shape change behaviour of an uncemented contemporary hip cup: A cadaveric experiment replicating host bone...

Modular uncemented acetabular components are in common use. Fixation is dependent upon press-fit but the forces necessary to achieve initial stability of the construct at implantation may deform the shell and prevent optimal seating of the polyethylene liner insert. Previous work using single-time point measurements in uncontrolled ambient temperature poorly replicates the native state. A controlled study was performed to observe the time-dependent behaviour of an uncemented acetabular shell in the early phase after implantation into the human acetabulum at near physiological temperature. Using a previously validated cadaveric hip model at controlled near physiological temperature with standardised surgical technique, immediate and delayed shell geometry was determined. Eight custom made 3-mm-thick titanium alloy (TiAl6V4) shells were implanted into four cadavers (eight hips). Time-dependent shell deformation was determined using the previously validated ATOS Triple Scan III (ATOS) optical measurement system. The pattern of change in the shape of the surgically implanted shell was measured at three time points after insertion. We found a consistent pattern for quantitative and directional deformation of the shells. In addition, there was consistency for relaxation of the deformation with time. Immediate mean change in shell radius was 104 µm (standard deviation 32, range 67-153) relaxing to mean 96 µm (standard deviation 32, range 63-150) after 10 min and mean 92 µm (standard deviation 28, range 66-138) after 20 min. The clinical significance of this work is the finding of a time-dependent early deformation of acetabular titanium shells on insertion adjusted for near physiological temperature-controlled host bone.