Fatigue Fracture of Co-Cr-Mo Femoral Stems after Canine Cemented Total Hip Replacement

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Introduction: Complications of canine total hip replacement have been reported in 3 to 11% of patients. Fatigue fracture of the femoral stem following cemented THR is an infrequent complication reported in 1.5% of cases. Two fractured stems underwent metallographic analysis after explantation to determine if material factors contributed to implant failure. Materials and Methods: Cases of Cobalt-Chromium-Molybdenum (Co-Cr-Mo) femoral stem fracture were identified in dogs receiving cemented THR. Two Co-Cr-Mo stems were explanted after mid-stem fracture and subjected to metallographic analysis. Scanning electron microscopy utilizing secondary and backscattered electron imaging modes was used to investigate primary fracture surfaces, adjacent exterior stem surface, and transverse and longitudinal cross-sections. Results: Cemented THR was performed on two juvenile canines. Orientation, sizing, and mantle quality were deemed appropriate postoperatively. Transverse fractures of the femoral stems occurred 3 years postoperatively and subsequently explanted. Metallographic analysis identified clear signs of fatigue failure causing stem fracture, namely fatigue striations and secondary cracking. Cross-sectional analysis identified significant porosity and propagation of cracks through these pores. Discussion/Conclusion: Metallographic analysis confirmed that the mechanism of stem fracture was fatigue failure. Quality control during manufacturing should be emphasized to reduce material imperfections. Both patients demonstrated substantial postoperative weight gain that exacerbated alloy imperfections by increasing cyclic loading of the THR, pushing stems to fatigue threshold. Lean body weight should be emphasized postoperatively to prevent excessive stem loading and potential failure. Implants used in these cases have been re-designed and are now obsolete. Acknowledgement: There was no proprietary interest of funding provided for this project.

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