A computed tomographic evaluation of femoral and tibial rotational reference axes in total knee arthroplasty.

Introduction:  The surgical trans epicondylar axis (sTEA) is considered the gold standard for optimum rotation of the femoral component; however, no consensus exists on tibial component positioning. The objectives of this study were to determine the relationship of sTEA to various femoral and tibial reference axes in varus osteoarthritis (OA) knees and (ii) to study the intra-observer and inter-observer variability of the axis relationships.

Materials and methods:  The study was done on preoperative computerised tomogram (CT) scans of 110 varus knees to assess the rotational relationships respectively of femoral side sTEA with whitesides line (WSL), posterior condylar axis (PCA), clinical trans epicondylar axis (cTEA) and on the tibial side sTEA with posterior tibial margin (PTM), anterior condylar axis (ACA), Akagi's line and line from the geometric centre of the tibial plateau to 1/3rd tibial tubercle (line GC 1/3rd TT).

Results:  On the femoral side the mean angles of sTEA with WSL, PCA, cTEA were 95.64° ± 2.85°, 1.77° ± 1.88°, 4.19° ± 0.99° respectively. On the tibial side, the mean angles of sTEA with, PTM, ACA, Akagi's line, and line GC 1/3rd TT were 1.10° ± 4.69°, 11.98° ± 4.51°, 2.43° ± 4.35°, 16.04° ± 5.93° respectively.

Conclusion:  Contrary to the generalization, TEA has variable relationships. The surgical trans epicondylar axis was not at the assumed 3° of external rotation to PCA in 85% of knees, nor perpendicular to WSL in >95% of knees. Of the four tibial axes, Akagi's line was the least variable with sTEA. Furthermore, surgeons should also be aware of the multiple reference axes and the range of deviation from sTEA to optimize the rotational alignment of components.