Biomechanical Evaluation of Acetabular Cup Implantation in Cementless Total Hip Arthroplasty

Objective To report biomechanical properties of the Biologic Fixation System (BFX) acetabular cup impacted into a normal canine pelvis and to compare the effect of implant positioned to and beyond the medial acetabular wall. Study Design In vitro cadaveric study. Animals Hemipelves of mature, large-breed dogs (n=6). Methods For each dog, 1 hemipelvis was reamed to the depth of the acetabular wall (group A) and 1 was reamed an additional 6 mm after penetration of the medial cortex of the acetabulum (group B). The hemipelves were implanted with acetabular cups and loaded in compression through a matching femoral prosthetic component until failure. Specimen stiffness, and failure displacement, load, and energy were determined from load and displacement data and results between groups compared with a paired t-test. Results Mean failure load was greater in group A (3812 +/- 391 N) than group B (2924 +/- 316 N; P <.014). No other differences (P >.05) were observed between groups. Bone fracture (n=5) and cup displacement (1) occurred in group A whereas in group B there were 3 fractures and 3 cup displacements. Conclusions Although medial placement of the BFX cup affected compressive failure loads, failure loads for both groups exceeded normal physiologic loads. Clinical Relevance Medial positioning of the acetabular cup does not appear to compromise acetabular implant-pelvic stability under normal physiologic loads. Because arthroplasty candidates often have abnormal acetabular architecture, mechanical properties of the cup placed in acetabula without a dorsal rim should be investigated.