The biomechanics of plate fixation of periprosthetic femoral fractures near the tip of a total hip implant: cables, screws, or both?

• Published in: Proceedings of the Institution of Mechanical Engineers. Part H, Journal of engineering in medicine Vol. 225; no. 9; p. 845
• Main Authors: Shah, S, Kim, S Y R, Dubov, A, Schemitsch, E H, Bougherara, H, Zdero, R
• Format: Journal Article
• Language: English
• Published: England 01.09.2011
• Subjects: Biomechanical Phenomena; Bone Plates; Bone Screws; Bone Substitutes - chemistry; Computer-Aided Design; Femoral Fractures - physiopathology; Femoral Fractures - surgery; Femur - surgery; Fracture Fixation, Internal - instrumentation; Fracture Fixation, Internal - methods; Hip Prosthesis; Humans; Periprosthetic Fractures - physiopathology; Periprosthetic Fractures - surgery; Stress, Mechanical
• ISSN: 0954-4119
• DOI: 10.1177/0954411911413060

Femoral shaft fractures after total hip arthroplasty (THA) remain a serious problem, since there is no optimal surgical repair method. Virtually all studies that examined surgical repair methods have done so clinically or experimentally. The present study assessed injury patterns computationally by developing three-dimensional (3D) finite element (FE) models that were validated experimentally. The investigation evaluated three different constructs for the fixation of Vancouver B1 periprosthetic femoral shaft fractures following THA. Experimentally, three bone plate repair methods were applied to a synthetic femur with a 5 mm fracture gap near the tip of a total hip implant. Repair methods were identical distal to the fracture gap, but used cables only (construct A), screws only (construct B), or cables plus screws (construct C) proximal to the fracture gap. Specimens were oriented in 15 degrees adduction to simulate the single-legged stance phase of walking, subjected to 1000 N of axial force, and instrumented with strain gauges. Computationally, a linearly elastic and isotropic 3D FE model was developed to mimic experiments. Results showed excellent agreement between experimental and FE strains, yielding a Pearson linearity coefficient, R2, of 0.92 and a slope for the line of best data fit of 1.06. FE-computed axial stiffnesses were 768 N/mm (construct A), 1023 N/mm (construct B), and 1102 N/mm (construct C). FE surfaces stress maps for cortical bone showed Von Mises stresses, excluding peaks, of 0-8 MPa (construct A), 0-15 MPa (construct B), and 0-20 MPa (construct C). Cables absorbed the majority of load, followed by the plates and then the screws. Construct A yielded peak stress at one of the empty holes in the plate. Constructs B and C had similar bone stress patterns, and can achieve optimal fixation.