Abstract
[Background]
Factors determining improvement of the long-term outcome of total knee arthroplasty include accurate reproduction of lower limb alignment. To acquire appropriate lower limb alignment, tibial component rotation is an important element for outcomes. We usually determine the tibial component rotation using the anatomical rotaional landmark of the proximal tibia and range of motion technique. In addition we followed by confirmation of overall lower limb alignment referring to the distal tibial index. When the tibia have a rotational mismatch between its proximal and distal AP axis, a larger error of the distal tibial index than those of other rotational landmark is of concern. The purpose of this study is to evaluate the reliability of the distal tibial AP axis as a reference axis of tibial compornent rotation in the intraoperative setting.
[Subjects and Methods]
The 86 patients (104 knees) with osteoarthritis of the knee who underwent primary TKA were evaluated with use of computerized tomography scans. A 3D images of the proximal tibial and ankle joint surfaces and foot were prepared, and the reference axis was set. In measurement, the images and reference axes were projected on the same plane. We measured the angle caluculated by the proximal and distal tibial AP axes (torsion angle) in preoperative 3D CT images. As a proximal tibial AP reference axis, AP-1 is a line connecting the medial margin of the tibial tubercle and Middle of the PCL attachment site and AP-2 is a line connecting the 1/3 medial site of the tibial tubercle and center of the PCL attachment site. As a distal tibial AP reference axis, D3 is a line connecting the anteroposterior middle point of the talus, D4 is a perpendicular line of transmalleoler axes, and D5 is the second metatarsal bone axis.
[Results]
AP-2 was 9.9±1.1°externally rotated relative to AP-1. And D4 was externally rotated relative to D3 in all cases, and the mean external rotation was 11.6°. The mean torsion angle of the distal tibial AP axis relative to AP-1 were all positive and D3:3.7°, D4:15.3°, D5:0.1° respectively. D5 was internally rotated relative to D3 in 67 cases, externally rotated in 31, consistent in only 6, and the mean torsion angle was 3.5°internal rotation.
The mean torsion angle of AP-2 axis were D3, 6.3°internal rotation; D4, 5.3°external rotation; and D5, 9.8°internal rotation respectively.
[Discussion]
In our department, after determining the rotation using the proximal tibial AP axis and ROM method, the alignment is confirmed referring to the distal tibial AP axis. However, a torsion angle of 25° or greater was noted in some cases, while it was within 3° in about 35–40% of cases, showing that the distal tibial AP axis was inappropriate as a reference axis for some cases. To perform TKA, it is important to identify the difference (torsion angle) between the proximal and distal tibial AP axes to prevent errors in the intraoperative setting of rotation.
