Abstract
Introduction
The THR is the second most successful and cost-effective surgical procedure of all time. Data shows that hip cup failure is a significant problem. The aim of this study is to improve methods of cemented cup fixation through validation experiments and FEA.
Methods
Five Sawbones composite pelves with cemented UHMWPE cups were tested. Each pelvis was instrumented with triaxial strain gauges at four locations of predicted high strain. Each sample (n = 5) was bolted at the sacroiliac joint in a uniaxial testing machine. A load of 500 N was applied in the direction of the peak force during normal walking, for five repetitions. The directional surface strains were used to evaluate the equivalent strain. Specimen specific finite element models were developed based on CT scan data using ScanIP. Each mesh consisted of an average of 2.5 million linear tetrahedral elements and was solved in ANSYS.
Results
The experimentally measured strains were compared against the finite element predictions. The mean linear gradients and SD of the mean at each gauge location were: 1.00 (16%), 0.78 (17%), 0.90 (13%) and 1.05 (4%).
Discussion
The agreement between the predicted and experimental equivalent strains was good, but varied across the population. This was caused by the variation in mechanical properties between specimens, and the sensitivity of the gauges to location (steep surface strain gradients). This is most evident with the second strain gauge (0.78, 17%), which is at a suboptimal location.
This specific methodology of conducting finite element analyses of the pelvis based on CT image data has been validated. The same methodology has been used to develop a patient specific FEA model, including a bone remodelling algorithm and muscle forces, based on the CT images from the Virtual Human Project. This model is currently being used to optimize the cemented fixation and will be verified experimentally using composite pelves. This research is aimed at informing clinical practice and enhancing long-term cemented fixation. Reducing the need for revision surgery will greatly improve patient quality of life, whilst also reducing the burden on the healthcare delivery system.
