Abstract
Purpose
Femoral component malrotation is a common cause for persisting symptoms and revision following total knee arthroplasty (TKA). There is ongoing debate about the most appropriate use of femoral landmarks to determine rotation. The Sulcus Line (SL, See Figure 1) 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 three studies presented are to i) assess the SL in a large clinical series, ii) demonstrate the effect of parallax error on rotational landmarks, and iii) assess the accuracy of a device which transfers a geometrically corrected SL onto the distal cut surface of the femur.
Methods
The first study assessed the SL using a large, single surgeon series of consecutive patients (n=200) undergoing primary TKA. The postoperative CT scans of patients 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) comparing the rotational landmarks measured along either the mechanical axis or the coronal axis of the trochlear groove. The third study assessed the accuracy of a novel trochlear alignment guide (TAG) using cadavers (n=10)
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° (range −7.2° to 6.7°). On the 3DCT reconstructions the APA (88.2°±4.2°) had significantly higher variance when compared with the SL (90.3°±2.7°) (F=5.82, 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, p=0.007 vs SL) and the variance (F=6.15, 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°). The results of the cadaver study found that using the TAG and the SL produced less variability than the APA (SD 2.0° compared to 3.7°). In addition, this level of accuracy was maintained when using the TAG to transfer the SL onto both the distal femoral condyles and the distal cut surface of the femur.
Conclusions
The 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 suggest that much of the variability in the measurement of the APA, documented in the literature, is caused by parallax error. A new device, the TAG, is able to accurately transfer a geometrically correct SL on to the distal cut surface of the femur. This allows accurate comparison between the SL and other landmarks, including the PCA, which is likely to decrease the risk of femoral component malrotation.
