The accuracy of posterior condylar angles measured by one MR image

The posterior condylar axis is used to establish femoral rotational alignment during total knee arthroplasty. One magnetic resonance image or computed tomography image often is used to determine the posterior condylar angle. However, the bony landmarks for determining the posterior condylar angle are not always present or obvious. We hypothesized measuring the posterior condylar angle on one slice would be no different when compared with using superimposed images with bony landmarks accurately identified in one or more slices. We analyzed magnetic resonance images of 95 knees without any bone disorders. Only 20 knees had all four bony landmarks apparent on any single slice. We compared the posterior condylar angles measured from one slice in which the medial sulcus, lateral epicondyle, or both posterior condyles were accurately identified with the angle measured from superimposed images. The average posterior condylar angle in the remaining 75 knees measured from superimposed images was 4.27 degrees +/- 1.95 degrees, which was similar to the angle measured from one in which the lateral epicondyle was the most accurately identified (3.95 degrees +/- 1.93 degrees). However, the largest difference (-3.53 degrees and 4.29 degrees) between three-dimensional and two-dimensional posterior condylar angle measurements in the worse-case scenario was unacceptable in clinical practice. We could not confirm our hypothesis: using one two-dimensional image to measure the posterior condyle angle is not accurate enough to determine its three-dimensional angle.