Objective: In total hip arthroplasty the medial peak of the medullary cavity in the femoral neck is referred to as the bony index of stem anteversion. We measured the difference between stem anteversion upon stem insertion into the medial peak of a medullary cavity, and actual femoral neck anteversion (FNA) using three-dimensional preoperative software known as Athena® (Soft Cube, Osaka, Japan).
Methods: Computed tomography (CT) scans were obtained from 79 patients with osteoarthritis (64 women; 120 hips without degenerative changes) who were candidates for total knee arthroplasty. CT data of the femur were used to measure FNA using Athena.The proximal femoral shaft axis was defined as the line connecting the centroids of the transverse slices taken just below the lesser trochanter to the proximal third of the femur. The position of the femoral head center was defined as the center of the sphere of best fit to the head surface. We defined three points: A, the point at which the axis of the proximal femoral shaft passes over a slice perpendicular to the proximal femoral axis just 1 cm above the lesser trochanter; B and C, the medial peak of the medullary cavity and the point projected onto the femoral head center on the same slice, respectively. The neck axis was defined as the line connecting A and C. The stem anteversion axis was defined as the line connecting B and C. The α angle represented the difference between stem anteversion and FNA was the angle between the stem axis and the neck axis.
Result: The α angle was −0.1±2.3° (−5.7° to 4.9°), and an outlier was observed in two cases.
Conclusion: The α angle was approximately 0° and therefore, stem anteversion appeared to be highly correlated with actual FNA when the stem was inserted into the medial peak of the medullary cavity. In the two cases with outliers, the medial peak of the medullary cavity was difficult to identify because the medial shape of the neck was round.
