Stress Analysis of a failed Stainless Stell femoral plate

João Paulo Pascon¹; Carlos Antonio R.P. Baptista¹; José Benedito Marcomini¹

¹ EEL/USP - Depto Eng Materiais

doi:10.20906/CPS/COB-2015-0858

Resumo

Surgical implants are exposed to severe working conditions and therefore a wide range of failure mechanisms may occur, including fatigue, corrosion, wear, fretting and combinations of them. The mechanical failures of metallic implants may also be influenced by several other factors, including the design, material, manufacturing, installation, postoperative complications and misuse. An 83-year-old patient suffered an oblique femoral shaft fracture due to a fall at home. A stainless steel locking compression plate (LCP) was employed in the fracture reduction and failed after six months. A failure analysis of the femoral LCP was performed, including chemical analysis, metallography, hardness test and SEM fractography. There was no presence of rusts or corrosion on the failed plate surface, which presented a typical stainless steel brightness. The plate fractured along one of the combination holes, approximately aligned to the femur fracture and with no screw locked into it. A FEM model of the orthopaedic implant was developed in order to estimate the stress distribution along the femoral plate. The numerical simulation was done with a computer code developed by one of the authors. The stress analysis was aimed to study the influence of the screws on the stress distribution in the femoral plate, as well as the effect of an empty hole on the stress concentration. The adopted finite element is the isoparametric solid tetrahedron of quadratic order. Two models have been numerically analyzed: one with four screws, and one with three screws (that is, with an empty hole on the femoral plate). For the first case, the mesh employed has 18785 nodes and 12096 tetrahedral elements. The mesh of the second model has 18077 nodes and 11520 tetrahedra. The distribution of the von Mises stress was obtained at the end of the simulation. The numerical results show that the removal of one of the screws leads to a stress increase around the empty hole. The maximum value of the von Mises stress for the model with an empty hole is 125.3 MP

Palavras-chave: Failure Analysis; Stress Analysis; Orthopaedic Implants