Does a kinematically aligned total knee arthroplasty restore function without failure regardless of alignment category?

Abstract

Background: Kinematically aligned TKA restores function by aligning the femoral and tibial components to the normal or prearthritic joint lines of the knee. However, aligning the components to the joint lines of the normal knee also aligns the tibial component in varus, creating concern that varus alignment might result in poor function and early catastrophic failure.

Questions/purposes: We therefore determined whether function and the incidence of catastrophic failure were different when the tibial component, knee, and limb alignment were in a specified normal range, varus outlier, or valgus outlier.

Methods: We prospectively followed all 198 patients (214 knees) who underwent TKAs between February and October 2008. We treated each knee in this cohort of patients with a kinematically aligned, cruciate-retaining prosthesis implanted using patient-specific guides. From a long-leg scanogram, we measured and categorized alignment of the tibial component as in range (≤ 0°) or a varus outlier (> 0°), alignment of the knee as in range (between -2.5° to -7.4° valgus) or a varus (> -2.5°) or valgus (< -7.4°) outlier, and alignment of the limb as in range (0° ± 3°) or a varus (> 3°) or valgus (< -3°) outlier. We assessed function using the Oxford Knee Score and WOMAC score, and reported catastrophic failure as the incidence of revision attributable to loosening, wear, and instability of the femoral or tibial components. The minimum followup was 31 months (mean, 38 months; range, 31-43 months).

Results: The mean Oxford Knee Score of 43 and WOMAC score of 92 were similar between the three alignment categories. The incidence of catastrophic failure in each alignment category was zero.

Conclusions: Kinematically aligned TKA restores function without catastrophic failure regardless of the alignment category. Because 75% of patients had their tibial component categorized as a varus outlier and also had high function and a zero incidence of catastrophic failure, the concern that kinematic alignment compromises function and places the components at a high risk for catastrophic failure is unfounded and should be of interest to surgeons committed to cutting the tibia perpendicular to the mechanical axis of the tibia.

Level of evidence: Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Fig. 1A–C

CT scanograms from three patients shows the lines used to measure the alignment of the (A) tibial component, (B) knee, and (C) limb of the kinematically aligned TKA. Because the measurements made on the two extremities in each patient are identical, the femoral and tibial components appear aligned to the normal joint lines. Varus alignment of the tibial components is anatomic and not excessive because it matches the angle of the tibial joint line in the contralateral normal knee. Imaging the extremity when the flange was between the posterior femoral condyles of the femoral component standardized rotation of the knee.

Fig. 2

Scatterplots show distribution of the tibial component, knee, and limb alignment in the in-range, varus outlier, and valgus outlier categories. Few limbs (6%) were in the varus outlier category, whereas most tibial components (75%) were.

Fig. 3

Box plots compare the Oxford Knee Scores and WOMAC™ scores for the knees in our study (N = 204) at a minimum of 31 months followup with a published randomized control trial (RCT) of kinematically (N = 41) and mechanically (N = 41) aligned TKAs at an average 6-month clinical followup [5]. The gray center line indicates the median; the two green lines indicate the limits of the 95% CI of the mean. The limits of the 95% CI of the mean indicate no difference in the Oxford Knee Scores and WOMAC™ scores between the kinematically aligned TKAs in each study, and differences in the Oxford Knee Scores and WOMAC™ scores between the kinematically aligned TKAs and mechanically aligned TKAs (boxplots with dissimilar letters indicate a difference of p < 0.05).