Abstract
Metal-backed tibial components in total knee arthroplasty (TKA) currently dominate the orthopaedic market due to intra-operative flexibility afforded by modularity. Metal-backing was first used in TKA as a method to potentially improve loading distributions over the tibial plateau at the interface between the prosthesis and the supporting cancellous bone. Many studies have compared metal-backed and all-polyethylene tibial components with variable survivorship. We have found decreased clinical survivorship with all-polyethylene Anatomic Graduated Component (AGC) TKA's (Biomet, Inc, Warsaw, Indiana) compared to the non-modular metal-backed design at 10-year follow up, 68% vs 98%, respectively. Loosening or bony collapse beneath the medial plateau accounted for 74% of failures in our AGC all-polyethylene cohort. We hypothesised that all-polyethylene tibial components may lead to increased strains in the proximal tibia with the AGC-TKA design, possibly correlating to osseous overload in the medial compartment and accounting for the increased observed rates of clinical failures in the all-polyethylene group. Finite element studies and our lab studies have shown that metal-backing reduces system stresses in the PMMA bone cement, as well as in the underlying cancellous bone. Overall, in every measurement region with a statistically significant difference in shear strain, higher strain was measured in the all-polyethylene implanted tibiae compared to metal-backed components. Statistically significant increases in strain from 126 μɛ (p=0.0131) to 745 μɛ (p=0.0011) and from 40% (p=0.0010) to 587% (p=0.0054) were seen in the all-polyethylene experimental group. We believe this may correlate with the higher failure rates we have observed in the AGC all-polyethylene cohort compared to the metal-backed cohort from our institution. Other all-poly TKA designs with varied articular congruities may afford improved or equal survivorship to metal backed implants at a reduced cost.
