Understanding failures of artificial joints through engineering analysis

Abstract

The aim of joint arthroplasty is to reduce pain and improve the range of motion and functionality in joints affected by diseases such as osteoarthritis and rheumatoid arthritis. Data recorded in National Joint Registries offers the clinical perspective in relation to prosthesis failures; however, this does not explain why a prosthesis has failed. Surgeons performing revision surgery for different implants often report similar findings, despite designs of prostheses and the natural joints having numerous differences, including anatomy, loading and range of movement. The underlying factor in the majority of cases of implant failure is complications arising as a result of wear debris. To understand the failures of artificial joints, a series of studies were performed examining hip prostheses in pre-clinical and post-clinical scenarios and finger prostheses in a post-clinical scenario. The pre-clinical studies focussed on areas including: the effect of acetabular shell deformation; and validating a method to measure volumetric wear from femoral stem trunnions. The deformation studies included an investigation of how bone strength influenced deformation. The post-clinical studies involved analysing retrieved finger and hip prostheses, to quantify the damage surfaces had sustained in vivo. Analysis of the finger prostheses involved the use of a non-contacting surface profilometer, to determine the surface roughness, whilst for the hip prostheses a coordinate measuring machine was used to quantify the volumetric wear. The deformation studies found that the maximum deformation was 340 μm, which could be sufficient to disrupt the assembly process of modular acetabular components. The strength of the bone was not found to correlate with the size of the deformation. The validation study found that the coordinate measuring machine was able to measure trunnions with a maximum error of 0.13 mm³ compared with gravimetric measurements. The ex vivo cohort of trunnions had a median wear volume of 0.14 mm³ (range 0.04 – 0.28 mm³). The first finger study analysed coated, metal-on-metal prostheses finding that prostheses had suffered extensive wear on the articulating surfaces. This was hypothesised to be due to the failure of the coating interface, resulting in a hard ii “grinding paste” that wore the articulating surfaces. The second finger study examined a cohort of explanted pyrolytic carbon prostheses. Even after use in vivo the roughness average (Ra) for the articulating surfaces was below the 50 nm specified by British Standards as the maximum Ra for orthopaedic implants manufactured from metal or ceramic.