Quantification of Caudal Osteointegration Surface Exposure after Uncemented Total Hip Replacement Stem Implantation: Effect of Stem Size and Implantation Depth.

OBJECTIVE: The objectives of this study are to quantify the area of exposed caudal osseointegration surface after implantation of Biomedtrix Biological Fixation (BFX) stems and study the effect of stem size and implantation depth on this measurement as well as to quantify caudal femoral bone defect changes during femoral canal preparation and stem exposure. STUDY DESIGN: This is an in vitro, original research. Using 10 canine femoral model specimens, an undersized, an appropriately sized, and an oversized uncemented BFX stems were implanted in each bone, using three-dimensional-printed guides, to three levels of progressive distalization. Measurements of porous surface exposure and caudal bone defect were obtained from photographs, for all stem sizes and implantation depth. RESULTS: In correctly sized, aligned, and distalized stems, 34.8% of the caudal porous surface remains exposed. Stem distalization significantly decreases caudal surface exposure for all stem sizes and increases the extent of the caudal femoral bone defect for all stem sizes. CONCLUSION: Complete, circumferential coverage of the proximal BFX stem is not possible. Stems should be properly distalized to reduce the exposed caudal integration surface while maintaining proper hip biomechanics. The caudal bone defect produced by femoral canal preparation progressively widens and migrates distally as stem size and depth increase. Familiarity with these features, not visible on a craniolateral approach, may be useful for surgeons performing canine uncemented hip replacement with the caudal approach.