Purpose The sulcus line (SL) is a three-dimensional curve produced from multiple points along the trochlear groove. Whiteside’s Line, also known as the anteroposterior axis (APA), is derived from single anterior and posterior points. The purposes of the two studies presented in this paper are to (1) assess the results from the clinical use of the SL in a large clinical series, (2) measure the SL and the APA on three-dimensional CT reconstructions, (3) demonstrate the effect of parallax error on the use of the APA and (4) determine the accuracy of an axis derived by combining the SL and the posterior condylar axis (PCA).MethodsIn the first study, we assessed the SL using a large, single surgeon series of consecutive patients undergoing primary total knee arthroplasties. The post-operative CT scans of patients (n = 200) were examined to determine the final rotational alignment of the femoral component. In the second study, measurements were taken in a series of 3DCT reconstructions of osteoarthritic knees (n = 44).Results The mean position of the femoral component in the clinical series was 0.6° externally rotated to the surgical epicondylar axis, with a standard deviation of 2.9° (ranges from −7.2° to 6.7°). On the 3DCT reconstructions, the APA (88.2° ± 4.2°) had significantly higher variance than the SL (90.3° ± 2.7°) (F = 5.82 and p = 0.017). An axis derived by averaging the SL and the PCA+3° produced a significant decrease in both the number of outliers (p = 0.03 vs. PCA and p = 0.007 vs. SL) and the variance (F = 6.15 and p = 0.015 vs. SL). The coronal alignment of the SL varied widely relative to the mechanical axis (0.4° ± 3.8°) and the distal condylar surface (2.6° ± 4.3°).ConclusionsThe multiple points used to determine the SL confer anatomical and geometrical advantages, and therefore, it should be considered a separate rotational landmark to the APA. These findings may explain the high degree of variability in the measurement of the APA which is documented in the literature. Combining a geometrically correct SL and the PCA is likely to further improve accuracy.

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Femoral component rotation is very important in avoiding complications and maximizing knee function following total knee arthroplasty. There is continuing debate over the most accurate axis to select intraoperatively to obtain the best results. There is considerable doubt over the ability of commonly used bony landmarks to produce a line parallel to the flexion-extension axis of the femur, either due to anatomical variation or due to poor reproducibility of the axes. Computer navigation was used to accurately measure the relative rotation indicated intraoperatively between bony landmarks, including a previously uninvestigated axis on the anterior surface of the femur. Measurements were taken during 193 total knee arthroplasties. There was a significant association between the Whiteside's Line and the axis on the anterior surface of the femur. The Posterior Condlylar Axis was extremely variable when compared with the other measurements, with a wide range centered on a mean value of 4.1 degrees internally rotated to the perpendicular of Whiteside's Line. There were significant differences in both the Posterior Condylar Axis and Anterior Femoral Axis when comparing varus and valgus knees. The Posterior Condylar Axis was shown to be an inconsistent measurement in comparison to other bony landmarks, particularly in valgus knees. The strong correlation between the Whiteside's Line and the Anterior Femoral Axis raises the possibility that the Anterior Femoral Axis maybe a useful landmark in identifying correct femoral component rotation.

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