Finite element (FE) models are frequently used in biomechanics to predict the behaviour of new implant designs. To increase the stability after severe bone loss tibial components with long stems are used in revision total knee replacements (TKR). A clinically reported complication after revision surgery is the occurrence of pain in the stem-end region. The aim of this analysis was the development of a validated FE-model of a fully cemented implant and to evaluate the effect of different tibial stem orientations. A scanned 4th generation synthetic left tibia (Sawbones) was used to develop the FE-model with a virtually implanted fully cemented tibial component. The 500 N load was applied with medial:lateral compartment distributions of 60:40 and 80:20. Different stem positons were simulated by modifying the resection surface angle posterior to the tibias shaft axis. The results were compared with an experimental study which used strain gauges on Sawbones tibias with an implanted tibial TKR component. The locations of the experimental strain gauges were modelled in the FE study.Background
Methods
There has been an unprecedented increase in total knee replacement in recent years. The UK national joint registry recorded over 80,000 total knee replacements per year with a generally successful outcome. Improvements in modern knee replacement designs and surgical techniques has resulted in more and more young and active patients having knee replacements. Their more active lifestyles and increased life expectancy is also leading to a rise in revision knee surgery. The most common reason for revision knee replacement is for loosening as a result of wear and/or bone resorption. Revision knee tibial components typically use long stems to increase the stability in the presence of the proximal bone loss associated with implant removal and loosening. The stem design has been cited as a possible cause of the clinically reported pain at the stem end region. The aim of this study was to experimentally validate a finite element (FE) model and the analysis different load conditions and stem orientations in a stemmed tibial component. CT-scans of a composite tibia (Sawbones) were utilized to form a multi-body solid consisting of cortical bone and cancellous bone with an intramedullary canal. A fully cemented tibial component (Stryker) was virtually implanted in the composite tibia with the stem-end centred in the cancellous bone. The tibial compartment loads were distributed with a 60:40 (Medial: Lateral) and 80:20 ratio to simulate a normal and varus type knee. Several stem-end positions were developed with the modification of the tibias proximal resection angle. An experimental study using strain gauges applied to the same composite tibia was used to compare the results with the FE-model. The model was validated with the strain gauged experimental test specimens demonstrating a similar pattern and magnitude of predicted strains. The simulation of different stem-end orientations revealed an increase in strain to the posterior cortex below the stem-end with the stem in direct contact to the posterior cortical bone. A tibial stem fully surrounded by cancellous bone demonstrated a small increase to the proximal strains. The simulation of a varus aligned knee with a 80:20 (Medial: Lateral) load distribution shifted strain overall to the medial side and revealed a large increase of strain to the posterior-medial in the proximity of the stem-end. The intensification of the load on one side of the tibial plateau, associated with a varus aligned knee, developed the largest increase in strain beneath the stem-end region and is possibly a factor in the reported pain after surgery. The stem in close proximity to the posterior cortical bone is also a possible contributing factor to pain due to the increase of strain in the vicinity of the stem-end.
