Alignment in knee flexion position during navigation-assisted total knee arthroplasty.

PURPOSE: The aim of this study was to demonstrate the lower limb alignment in knee flexion position after navigation-assisted total knee arthroplasty using the gap technique and to identify the correlative factors.

METHODS: One hundred and twenty consecutive osteoarthritic knees (120 patients) were prospectively enrolled for intraoperative data collection. All TKA surgeries were performed using the navigation system (OrthoPilot™, version 4.0; B. Braun Aesculap, Tuttlingen, Germany). Before and after final prosthesis implantation, the lower limb navigation alignment in both knee extension (0°) and knee flexion (90°) position was recorded. The knee flexion alignment was divided into three groups: varus, neutral and valgus alignment. To determine the factors of the alignment in knee flexion position, preoperative demographics, radiologic and intraoperative data were obtained. Pearson's correlation (r) analysis was performed to find the correlation. The Knee Society Score and Western Ontario and McMaster Universities Osteoarthritis Index were compared between groups.

RESULTS: Although all postoperative extension alignment was within neutral position (between -2° and +2°), postoperative knee flexion alignment was divided into three groups: varus (≤-3°), 24 cases (20 %); neutral (between -2° and +2°), 85 cases (70.8 %) and valgus (≥+3°) alignment, 11 cases (9.2 %). There were a good correlation of alignment in knee flexion position with the rotation of femoral component relative to posterior condylar axis (r = -0.502, p = 0.000) and weak correlations with posterior femoral cut thickness (lateral condyle) (r = 0.207, p = 0.026), medial flexion (90°) gap after femoral component rotation adjustment (r = 0.276, p = 0.003). Other variables did not show correlations. There were no statistical clinical differences between varus, neutral and valgus knee flexion alignment groups.

CONCLUSION: About 30 % of the cases showed malalignment of more than 3° in knee flexion position although with neutral alignment in extension position. The knee flexion alignment had a good correlation with the rotation of femoral component relative to posterior condylar axis. Neutral alignment in knee flexion position may be adjusted by femoral component rotation especially by the use of navigation system.

LEVEL OF EVIDENCE: IV.