The effect of femoral component rotation on the five-year outcome of cemented mobile bearing total knee arthroplasty

Abstract

Purpose: Performing total knee replacement, accurate alignment and neutral rotation of the femoral component are widely believed to be crucial for the ultimate success. Contrary to absolute bone referenced alignment, using a ligament balancing technique does not automatically rotate the femoral component parallel to the transepicondylar axis. In this context we established the hypothesis that rotational alignment of the femoral component parallel to the transepicondylar axis (0° ± 3°) results in better outcome than alignment outside of this range.

Methods: We analysed 204 primary cemented mobile bearing total knee replacements five years postoperatively. Femoral component rotation was measured on axial radiographs using the condylar twist angle (CTA). Knee society score, range of motion as well as subjective rating documented outcome.

Results: In 96 knees the femoral component rotation was within the range 0 ± 3° (neutral rotation group), and in 108 knees the five-year postoperative rotational alignment of the femoral component was outside of this range (outlier group). Postoperative CTA showed a mean of 2.8° (±3.4°) internal rotation (IR) with a range between 6° external rotation (ER) and 15° IR (CI 95). No difference with regard to subjective and objective outcome could be detected.

Conclusion: The present work shows that there is a large given natural variability in optimal rotational orientation, in this study between 6° ER and 15° IR, with numerous co-factors determining correct positioning of the femoral component. Further studies substantiating pre- and postoperative determinants are required to complete the understanding of resulting biomechanics in primary TKA.

Figures

Fig. 1

Relationship between Knee Society score and the rotation of the femoral component at the five-year follow up (CTA post). Negative values of CTA represent internal rotation (IR), positive values external rotation (ER). The single point marks the value of CTA on the X-axis, the neutral rotation group is marked in green. Presented values for femoral component rotation are consensus decisions

Fig. 2

Subjective outcome versus rotational alignment of the femoral component, internal rotation (IR) represented by negative values, external rotation (ER) by positive values, the neutral rotation group is marked in green. Subjective outcome: 3—excellent, 2—good, 1—satisfactory, 0—poor

Fig. 3

Axial radiographs at the five-year follow-up: on the left 7° external rotation (ER) of the femoral component, in the middle a femoral component with rotation parallel to the transepicondylar axis and on the right a patient with 8° internal rotation (IR) of the femoral component. All patients had excellent subjective and objective outcome