The Effects of Interlocking a Universal Hip Cementless Stem on Implant Subsidence and Mechanical Properties of Cadaveric Canine Femora

Objective: To determine if an interlocking bolt would limit subsidence of the biological fixation universal hip (BFX^®) femoral stem under cyclic loading and enhance construct stiffness, yield, and failure properties. Study Design: Ex vivo biomechanical study. Animals: Cadaveric canine femora (10 pairs). Methods: Paired femora implanted with a traditional stem or an interlocking stem (constructs) were cyclically loaded at walk, trot, and gallop loads while implant and bone motions were captured using kinematic markers and high-speed video. Constructs were then loaded to failure to evaluate failure mechanical properties. Results: Implant subsidence was greater (P=037) for the traditional implant (4.19mm) than the interlocking implant (0.78mm) only after gallop cyclic loading, and cumulatively after walk, trot, and gallop cyclic loads (5.20mm vs. 1.28mm, P=038). Yield and failure loads were greater (P=029 and.002, respectively) for the interlocking stem construct (1155N and 2337N) than the traditional stem construct (816N and 1405N). Version angle change after cyclic loading was greater (P=020) for the traditional implant (3.89 degrees) than for the interlocking implant (0.16 degrees), whereas stem varus displacement at failure was greater (P=008) for the interlocking implant (1.5 degrees) than the traditional implant (0.17 degrees). Conclusion: Addition of a stabilizing bolt enhanced construct stability and limited subsidence of a BFX^® femoral stem. Use of the interlocking implant may decrease postoperative subsidence. However, in vivo effects of the interlocking bolt on osseointegration, bone remodeling, and stress shielding are unknown.