Abstract
Summary
The effect of polyethylene wear and lift-off between the tibial and femoral components on the mechanical axis was assessed in primary TKA (Total Knee Arthroplasty) based on retrieval data and full leg radiographs.
Introduction
Controversy exist regarding performing a TKA with component placement in physiologic versus neutral alignment. Some literature indicates good survivorship and superior clinical outcome in undercorrected TKA's for varus osteoarthritic knees. However, other literature indicates decreased survivorship and coronal plane alignment is still one of the contributing factors to wear in total knee arthroplasty (TKA). The two determinants of the intra-articular deformity in TKA arepolyethylene wear and lift-off between the tibial and femoral compartment. The goal of this study was to evaluate the impact of wear and lift-off on the mechanical axis in neutral and varus aligned TKA's.
Materials and methods
Seventy-six tibia inserts retrieved from neutral and varus aligned primary TKA's with a minimum 5 year in vivo time were assessed for the ratio of wear (RW) using a micrometer. Full-leg radiographs were assessed to determine the Hip-Knee-Ankle (HKA) and Condylar-Plateau (CP) angle, which is the intra-articular deformity. The HKA and CP angle was corrected for wear to a New-HKA angle (N-HKA) and the New-CP angle (New-CP), which was defined as lift-off. The RW and N-CP was subsequently assessed for neutral (0 ± 3°), mild varus (3 – 6°) and moderate varus (>6°) TKA's based on the mechanical axis.
Results
Demographics of the study are shown in table 1.
The RW correlated with frontal plane alignment, with increased wear being related to progressive varus alignment (p < 0.01).
The difference from CP to N-CP and HKA to N-HKA was significant for the mild varus (p<0.01) and moderate varus (p<0.01) group. (table 2 and 3)
The difference in N-CP (lift-off) among alignment groups was significant with increased lateral lift off with progressive varus alignment (p=0.02).
Discussion
After correction of the intra-articular angle and the mechanical axis by the ratio of wear, coronal plane alignment significantly changed in the mild and moderate varus aligned TKA population. Both polyethylene wear and lift-off contribute to this, with progressive values in higher varus aligned TKA's. These results are important when targeting an undercorrection in TKA but should also be taken into account when describing the mechanical axis of TKA's.
Conclusion
Lift-off between the tibia and femur and wear contribute significantly to the change in mechanical alignment in primary TKA's with higher values in progressive varus.
