Failure analysis of a 316L stainless steel femoral orthopedic implant

• A 316L stainless steel locking compression plate failed prematurely in fatigue.
• Crack initiated at a combi-hole, which is a stress raiser.
• Bending stresses were responsible for crack initiation and propagation.
• Plate installation may cause pre-stresses reducing the implant durability.

This study presents a failure analysis of a femoral orthopedic implant. This implant is a locking compression plate that was fixed to a broken femur using two types of screws: locking and compression screws. The study elucidates the causes of an in situ premature failure of the plate and screws. Chemical analysis, hardness tests, and microstructural analysis confirmed that the implant was manufactured from cold-worked stainless steel 316L. The macro and micro fractographic analyses revealed that the failure mechanism was high-cycle fatigue and that the implant underwent approximately 106 loading cycles before failure. A finite element analysis of the assembly indicated that the crack initiation sites are located in the region where the highest stresses are observed. This numerical analysis confirmed that walking induces the loading condition needed for this specific failure. According to this loading condition and to the material properties, the stress amplitude that initiated and propagated the crack is estimated to be in the range of 400 MPa. Several considerations, both mechanical and medical, are discussed in order to explain the failure and to improve the system durability. From an engineering perspective, implant geometry and installation procedure could be optimized in order to reduce the stress concentrations that developed near the crack origin.