In TKA, we have used the “projected SEA”, which is obtained by projecting the “true SEA” on the distal femoral cutting plane in clinical practice to determine the femoral component rotation. There are no reports examining the accuracy of the “projected SEA”. In this study, we investigated the difference between the “true SEA” and “projected SEA”.
The present study was a CT-based computer-simulated case series. We evaluated 34 knees without osteoarthritis changes. These patients were selected from the operative schedule prior to THA. We defined the “true SEA” on the 3D model and the “projected SEA” on the cutting plane parallel to the distal femoral axis obtained based on the “true SEA”. We changed the cutting angles from 20° flexion to 20° extension, and from five degrees varus to five degrees valgus. We measured the “true SEA angle” and “projected SEA angle” regarding the posterior condylar axis (PCA).
The mean “true SEA angle” was 3.04° ± 1.34° (0.6–5.0°). The mean “projected SEA angle” was 3.43° ± 1.58° at 20° flexion, 3.42° ± 1.56° at 0° flexion, 3.43° ± 1.52° at 20° extension, 3.39° ± 1.59° at five degrees valgus, and 3.39° ± 1.50° at five degrees varus. At each cutting angle, the “projected SEA angle” was significantly larger than the “true SEA angle” (p < 0.001). There was no significant difference between any of the “projected SEA angles” (p > 0.001).
We found that the true SEA and projected SEA do not differ significantly (0.39° ± 0.29° [range 0–1.0°]).