Adaptive bone remodelling of all polyethylene unicompartmental tibial bearings

Background: Failure of unicompartmental knees is either by progressive osteoarthritis or by failure of the prosthesis. Prosthesis failure can be because of either the actual component itself or its fixation method. Specifically, the tibial component may wear, loosen or subside into the tibial metaphyseal bone. We modelled all polyethylene tibial components and investigated the periprosthetic adaptive remodelling of the bone. Methods: Computed tomography scans were used to reconstruct the tibial geometry and a 3-D finite element mesh was created. The tibia loading was set at 45% of the gait cycle. The distal end of the tibia set as fixed. Implant orientations were in accordance with the manufacturer specifications. The bone mineral density changes at three regions of interest under the component were measured and plotted. Results: All regions of interest became stable at the 12-month time-point. Predictive bone mineral changes were minimal in both resorption and deposition. Zones 1-3 for the St Georg Sled (Waldermar Link, Hamburg, Germany) presented evidence of stress shielding and a maximum loss of approximately 6% bone mineral density. The Eius (Stryker Orthopaedics, Mahwah, NJ, USA) showed increased loading and caused an increase in bone mineral density for all regions of interests; the maximum being approximately 10%. Conclusion: These findings are consistent with the clinical experience of unicompartmental knee replacements, where the changes are small and have little effect on the outcome of the prosthesis. From a mechanical perspective, the results are also consistent when a compliant material is used to distribute loads. They do, however, support the biomechanical theory that a change in geometry influences the loading environment and as a consequence the adaptive response of the bone is also influenced.