A mechanical review of postoperative stem fractures of cemented total hip arthroplasty implants without femoral fracture

Abstract

Background: We encountered five patients of post-THA femoral stem fracture without femoral fracture. The purpose of this study was to elucidate the mechanism underlying the observed femoral stem fractures. Methods: We performed an assessment of the cement mantle using plain radiographic images. We analyzed the state of the fractured stems and fractured surfaces of these stems during the revision surgery. Additionally, we replicated the same prosthesis support conditions that were present around the fractured stems and verified the analytic results by using mechanical tests (bending test, fatigue test), finite element method (FEM) analysis and material tests. Bending tests were performed at a loading rate of 5 mm/min. In contrast, for fatigue testing, the bending tests were repeated under a loading of 2300 N at a frequency of 5 Hz. The cemented distal portions of the fractured stems were all firmly attached, but the proximal portions were unsupported. During these simulation tests, the distal portion was fixed at a distance of 80 mm from the center of the femoral head using a 36-mm femoral head. Results: The plain radiographic images at the time of the stem fracture showed that radiolucencies were observed in all patients around the proximal regions of the stem, although good fixation was obtained at the distal portions. We could easily remove the parts of the stem proximal to the fracture site in all patients during surgery, while the distal parts were solidly cemented. Examination of the fracture surfaces showed that the fracture origin was on the lateral surface in the middle of the stem. The middle portion of the stem bent during the compression-bending test, with this position corresponding to the fracture site. FEM analysis showed that the area corresponding to the stem fracture origin was the same as predicted from examining the fractured surface. Conclusion: Bending test and FEM showed that only the distal part of Co–Cr alloy stem was firmly fixed and the internal stress was concentrated at a point just proximal to the fixed part. Therefore, this point coincided with the fractured point of the stem.

Keywords

• hip replacement arthroplasty
• femoral fractures
• finite element analysis
• osteoarthritis
• cementation