Femoral Component Varus Malposition is Associated with Tibial Aseptic Loosening After TKA

Abstract

Background: The notion that neutral alignment is mandatory to assure long-term durability after TKA has been based mostly on short-film studies. However, this is challenged by recent long-film studies.

Questions/purposes: We conducted this long-film study to know (1) whether the risk of aseptic revision for nontraumatic reasons was greater among knees with greater than 3° varus or valgus (defined as "outliers") than those that were aligned within 3° of neutral on long-standing mechanical axis (hip to knee) radiographs; and (2) what the failure mechanisms were and whether the malalignment was femoral or tibial in origin, or both, among those in the outlier group.

Methods: Between November 1998 and January 2009 we performed 1299 cemented, posterior cruciate ligament-substituting TKAs in 867 patients for primary osteoarthritis. We had inadequate long-standing radiographs to analyze postoperative alignment for 124 of those knees, and an additional 24 were excluded for prespecified reasons. Consequently, 1151 knees were enrolled in our study. Of these, 982 (85%) in 661 patients (620 women and 41 men) who had followup greater than 24 months were analyzed. The knees were divided according to whether the postoperative mechanical axis was neutral (0° ± 3°), varus (> 3°), or valgus (< -3°) alignment on long-standing radiographs. The survivorships free from aseptic revision for nontraumatic reasons were compared among groups. The mechanical femoral and the tibial component alignment (MFCA and MTCA, respectively) were investigated to know the origin of overall mechanical malalignment of the outlier knees. The mean duration of followup was 8 ± 4 years (range, 2-17 years). Thirty-five knees (4%) showed aseptic loosening at 7 ± 4 years (range, 0.1-14 years) and five (1%) showed polyethylene wear at 12 ± 1 years (range, 10-13 years). Tibial loosening (73%) was the most common reason for aseptic revision followed by femoral loosening (30%). Of this cohort, 687 (70%), 250 (25%), and 45 (5%) knees had overall mechanical neutral, varus, and valgus alignment, respectively. Factors associated with the risk of aseptic revision were identified by Cox regression.

Results: The varus outliers (but not the valgus outliers) failed more often than the neutral knees (10% [25 of 250] versus 2% [13 of 687]; odds ratio [OR], 5.8, 95% CI, 2.9-11.5; p < 0.001). Ten-year survivorship free from aseptic revision was lower among varus outliers than among knees with neutral alignment (87% [95% CI, 80%-93%] versus 98% [95% CI, 97%-99%]; p = 0.001). Femoral component varus malpositioning was the main origin of the varus outliers (MFCA = 4.2° ± 2.0°; MTCA = 0.9° ± 1.7°) and was a risk factor for aseptic revision compared with neutral femoral positioning (OR, 14.0; 95% CI, 1.9-105.6; p < 0.001).

Conclusions: This long-film study corresponds to previous short-film studies for the notion that varus malalignment is associated with inferior long-term implant survivorship. Although aseptic loosening occurred most commonly on the tibial side, the primary origin of the overall varus malalignment was femoral component varus malpositioning. Aiming for neutral alignment in TKA still seems to be a reasonable strategy in clinical practice.

Level of evidence: Level III, therapeutic study.

Conflict of interest statement

Each author certifies that he has no commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.

Figures

Fig. 1

The distribution of postoperative overall mechanical alignment on long-standing hip-to-ankle radiographs is shown. The aseptic loosening and wear cases are indicated by the red and yellow bars, respectively. PE = polyethylene.

Fig. 2

Survival of the implants in the neutral, varus, and valgus groups during followup is shown. The varus outliers had lower survivorship than the neutral group (Gehan-Wilcoxon test, p = 0.002), but there was no difference between the neutral and valgus groups (Gehan-Wilcoxon test, p = 0.241). However, the neutral and varus groups did not differ, with the numbers available, in implant survival until 10 years after the index operation.

Fig. 3 A-B

The distributions of (A) mechanical femoral component alignment (MFCA) and (B) mechanical tibial component alignment (MTCA) in the varus outlier group are shown. The histograms indicate that these knees had a propensity toward varus-deviated MFCA and well-aligned MTCA. This suggests that overall malalignment of the varus outlier knees was the result of varus MFCA. Moreover, the knees with MFCA greater than 3° showed aseptic loosening (red bars) more frequently (11%) than the knees with neutral MFCA (1%). PE = polyethylene.