57 COPYRIGHT © 2011 BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED Fracture of Cementless Femoral Stems at the Mid-Stem Junction in Modular Revision Hip Arthroplasty Systems By Dror Lakstein, MD, Noam Eliaz, PhD, MBA, Ofer Levi, MSc, David Backstein, MD, MEd, FRCSC, Yona Kosashvili, MD, MHA, Oleg Safir, MD, MEd, FRCSC, and Allan E. Gross, MS, FRCSC Investigation performed at the Division of Arthroplasty, Orthopedic Department, Mount Sinai Hospital, Toronto, Ontario, Canada, and the Biomaterials and Corrosion Laboratory, Materials and Nanotechnologies Program, Tel-Aviv University, Tel-Aviv, Israel Background: Mechanical failure of femoral stems at the modular junction of revision hip arthroplasty systems has been reported only Infrequently. In the current study, the cause of six stem fractures, which occurred in vivo, was analyzed with use of clinical data and failure analysis. Methods: Six patients with a fracture at the mid-stem junction of a modular revision hip implant were identified in our database of patients who had undergone revision arthroplasty. The characteristics of the patients with a fractured stem were compared with those of 165 patients from the same prospective database who had a modular stem implanted, had at least two years of follow-up, and had not had a fracture of the stem. Failure analysis of three implants (six fracture surfaces) was carried out, with use of microscopic, chemical, and microhardness characterization techniques. Results: Patients with a fractured stem had significantly higher body mass indices than patients without a stem fracture. Radiographs demonstrated that these femoral implants lacked adequate osseous support of the junction area of the stem. All stems failed approximately 1 to 2 mm proximal to the body-stem junction, thus indicating the presence of a bending moment. The chemical composition and microhardness matched those of TÍ-6AI-4V, Evidence of wear and fatigue were found on the fracture surface. A wear strip was also observed along the circumference ofthe stem near the junction. Conclusions: We concluded that the stem failure was initiated by a fretting fatigue mechanism and was propagated by a pure bending fatigue mechanism. Risk factors for fractures of the modular junction include excessive body weight and inadequate proximal osseous support because of trochanteric osteotomy, reduced preoperative bone stock, osteolysis, loosening, and/or implant undersizing. Surgeons should consider the use of implants with strengthened junctions when using modular stems in such patients. M echanical failure of the femoral component after total hip arthroplasty is not a common occurrence, but fractures of femoral stems after hip arthroplasty have been previously described' \ Factors that predispose to this form of stem failure include excessive patient weight, high levels of physical activity, deficient osseous support, malposi- tion or loosening of the stem, the presence of a stress-riser, and reduced cross-sectional area within the stem"^ ', Cxmentless revision stems have become an appealing option for the arthroplasty surgeon in treating patients with a deficient femur during the last three decades" '". The fixation options available for revision femoral components include distal fixation as well as extensive (both distal and proximal) fixation stems. Both methods have been reported on and both exhibit survival rates of >90% at five to fifteen years of follow- up" '". These stems are sLiitable for use with various types of bone deficiency and are compatible with extended trochanteric osteotomies". Modularity offers the advantage of adjustment and restoration of joint kinematics including leg length, ver- sion, and offset, regardless ofthe exact position ofthe di.stal part ofthe stem. In addition to the higher cost associated with modularity, concerns have been raised with regard to the potential com- plications of fretting and fracture at the modular junction"'\ Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. One or more ofthe authors, or a member of his or her immediate family, received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Zimmer). JBoneJointSurgAm. 2011;93:57-65 • doi:10,2t06/JBJS.I.01589