The effect of dorsal rim loss on the initial stability of the BioMedtrix cementless acetabular cup

Background: Loss of dorsal acetabular rim (DAR) is a common sequela to canine hip dysplasia. The purpose of this study is to evaluate the effect of DAR loss on the initial stability of the cementless (BFX) acetabular cup. BFX cups were implanted into foam blocks reamed to resemble acetabulae with simulated 0, 25, 50, and 75% DAR loss. Models were tested in edge loading of the lateral surface of the cup with an indenter, and in centered loading with an articulated femoral prosthesis. Additionally, cups were implanted into paired cadaveric canine hemipelves with either no DAR depletion, or removal of 50% of the DAR, and acutely loaded to failure with an articulated femoral prosthesis. Results: Mean load measured at 1 mm cup displacement during edge loading was not significantly different in foam blocks with loss of 0, 25, 50, and 75% DAR (360 +/- 124 N, 352 +/- 42 N, 330 +/- 81 N, 288 +/- 43 N, respectively; P = 0.425). Mean load to failure with centered loads was greatest in blocks with 0% DAR loss (2828 +/- 208 N; P < 0.001), but was not significantly different between 25, 50, and 75% DAR loss (2270 +/- 301 N, 1924 +/- 157 N, 1745 +/- 118 N). In cadaveric testing, neither mean load to failure (P = 0.067), stiffness (P = 0.707), nor energy (P = 0.228) were significantly different in control hemipelves and those with 50% depletion of the DAR. Failure in all acetabulae occurred due to acetabular bone fracture at forces in supraphysiologic ranges. Conclusions: BFX cup stability under normal physiologic loads does not appear to be compromised in acetabulae with up to 50% DAR loss.