The most important finding of the present study was that the medial tangent of the proximal tibia was a suitable extra-articular landmark in determining the tibial anteroposterior axis for TKA, and the MTAT was approximately 45° at level A.
In the past, tibial rotational alignment in TKA had been determined in reference to the medial one third of the tibial tubercle [12]. However, there was no strong theoretical justification for this method. Akagi et al. demonstrated that the line connecting the middle of the PCL to the medial edge of the patellar tendon attachment was perpendicular to the surgical epicondylar axis, and this line has since been used as the tibial AP axis for primary TKA [11, 18, 19]. Since then, variants of this line have been reported [20, 21]. Kim et al. reported that the line connecting the anterior border of the proximal third of the tibia to the middle of the PCL was also perpendicular to the surgical epicondylar axis, and could also be used as the tibial AP axis [22]. These lines are very useful for primary TKA in which the PCL is recognized directly. For primary medial unicompartmental knee arthroplasty, Tsukamoto et al. showed that the line connecting the medial border of the patellar tendon at the articular surface level and the medial intercondylar tubercle was suitable as the tibial AP axis [23]. However, the above lines cannot be used for knees in which the PCL disappears due to severe inflammation or for revision TKA in which joint structures have already been resected. Additional landmarks for tibial rotational alignment independent of intra-articular condition are needed for such cases.
In techniques referencing the extra-articular structure, the transmalleolar axis of the ankle and the second metatarsus bone axis of the foot have been used to determine the tibial AP axis conventionally [12]. However, it has been reported that these techniques exhibit individual variability [14, 24, 25]. Though reproducibility of the MTAT value at level C in the present study was high, the interquartile range of the MTAT at level C was the largest among the three groups (Table 1). Individual variability of the tibial torsion was suggested even in the present study. This shows that extra-articular reference in the determination of the tibial AP axis should be close to the knee. Therefore, the medial tangent of the proximal tibia was determined to be a suitable landmark. However, it was also considered that the MTAT at a level more proximal than level A would also vary between individuals because of the individual variability that exists in the protrusion of the tibial tubercle. In addition, medial bony defect or spur was often observed at levels more proximal than level A. Therefore, the MTAT was evaluated at three levels (A, B and C) in the present study. The median distance between the top of the tibia tubercle and the distal edge of the tibial tubercle was 17.8 mm (IR 15.7-19.8mm), or about one finger in the present study. Therefore, if it is difficult to identify the distal edge of the tibial tubercle, the MTAT at a level one finger distal to the top of the tibia tubercle may be used as a reference.
Even when the identification of the lines illustrated above is easy, it is difficult to confirm whether such lines do in fact match the preoperatively planned tibial axis of rotation. Mitsuhashi et al. showed that the number of outliers for rotational alignment of the tibial component was significantly higher in conventional TKA than in TKA using a navigation system [26]. However, because TKA using a navigation system involves higher costs, there are some hospitals at which such systems are not used. In addition, the tracker pins in such systems have been known to cause infection and fracture, although incidence rates are low [27, 28]. Therefore, intraoperative landmarks not involving a navigation system are more practical for the determination of tibial rotational alignment.
The distal edge of the tibial tubercle exists at, or very close to, the surgical area. The main medial anatomical structure on the horizontal plane at level A is the just inferior portion of the pes anserinus, which inserts into the tibia tubercle 6±5 mm distally [29, 30]. Therefore, by inserting a wire along the surface of the cortex through the thin portion of the pes anserinus, reproduction of the medial tangent of the tibia during surgery can be achieved (Fig. 4). In addition, because the MTAT at level A was found to be approximately 45° in the present study, the value can be easily used as an intraoperative reference for the tibial AP axis. Even in conventional TKA not involving a navigation system, the MTAT at level A can be a very useful landmark.
The medial surface of the tibia was smooth, but not completely flat (Table 2). In all lower limbs, the tibial medial surface at level A was found to be of the valley type due to the fact that the distal edge of the tibial tubercle is still slightly prominent at this level (Table 2). In addition, the two points forming the valley were bony ridges. Therefore, the medial tangent of the tibia at level A could be easily determined. As a result, the reproducibility of the MTAT value at level A was the highest among the three groups (Table 1).
There are some limitations to the present study. First, individual variability existed, even at level A. However, because the interquartile range of the MTAT at level A in our study was small, we believe that a value of 45° may be used by surgeons intraoperatively. Surgeons may also choose to reproduce the accurate MTAT intraoperatively, using the value of the MTAT at level A measured on preoperative CT scans. In revision cases in which the MTAT cannot be measured in preoperative CT, the MTAT on the contralateral side may be used as a reference since no significant difference between the right and left lower limbs was noted in our study (Table 1). Second, the patients in the present study were patients scheduled to undergo total hip arthroplasty. Although patients with moderate to severe OA in the knee were excluded due to the fact that degenerative changes were considered likely to affect the identification of the tibial AP axis on CT images, future studies should focus on patients scheduled to undergo knee arthroplasty. Furthermore, since the aim of our study was to investigate the MTAT using CT images, we deemed it unjustifiable to expose healthy subjects to unnecessary radiation exposure, and selected patients already scheduled to undergo lower limb CT scans. Third, the patients in the present study were all Japanese. Although the median MTAT at level A was approximately 45° in the present study, it is possible that different bony shapes in the lower limbs exist in different ethnicities. In the future more research involving subjects with osteoarthritic knee and in other ethnic groups should be conducted.