Polyethylene insert damage in unicondylar knee replacement: a comparison of in vivo function and in vitro simulation

Modification of knee joint wear simulation methods has included 'anatomic attachment' of unicondylar knee replacements (UKR) onto synthetic femurs with material properties and morphology similar to human femurs. The present study assesses the effect of such modification by comparing the damage patterns on UKR polyethylene inserts after in vitro simulation using standard and modified simulation methods with those on inserts retrieved after in vivo function. Three groups of UKR inserts were evaluated after retrieval (Explant Group, n = 17) or after knee joint wear simulation with the components attached to standard metal blocks (Standard Group, n = 6) or synthetic femurs (Anatomic Group, n = 6). All UKR had similar non-conforming articular surfaces. Articular damage patterns (mode, frequency, and area) were quantified using digital image photogrammetry. Although some common damage modes were noted, knee joint wear simulation with standard or 'anatomic' attachment did not generate damage pattern sizes similar to the explanted UKR. A focal damage pattern consistent with contact between the metal femoral articular surface and the polyethylene inserts was evident on all inserts, but only the Explant Group had evidence of dispersed damage dominated by abrasive modes. Synthetic femurs added complexity to the wear simulation without generating wear patterns substantially more similar to those observed on retrieved inserts.

Predictive role of the Λ ratio in the evaluation of metal-on-metal total hip replacement

The wear of metal-on-metal bearings is affected by various design parameters, such as the clearance or surface roughness. It would be very useful to have a significant indicator of wear according to these design parameters, such as the Λ ratio. Three different batches of cast high- and low-carbon cobalt—chromium hip implants (28 mm, 32 mm, and 36 mm diameters) were tested in a hip joint simulator for 2 × 106 cycles. Bovine calf serum was used as lubricant, and the samples were weighed at regular intervals during the test. The predictive role of the Λ ratio on the wear behaviour was investigated. Three different configurations were tested to explore the wear rate for a broad range of Λ ratios. The results of these studies clearly showed that the femoral heads of 36 mm diameter had the best wear behaviour with respect to the other two smaller configurations tested. From a predictive point of view, the Λ ratios associated with the configurations tested could clearly indicate that the femoral heads of 36 mm diameter worked in the mixed-lubrication regime (Λ>1); all the smallest configurations (28 mm size) had λ< 1, thus showing their aptitude to work in the boundary lubrication regime, with substantially higher volume depletion due to wear. The Λ values associated with the 32 mm size varied in a range around 1 (0.95<Λ<1.16), suggesting the possibility of operating in the mixed-lubrication regime.