Summary

The tibial anteroposterior axis "original Akagi’s line" defined on CT was not replicated intraoperatively and the intraoperative poor detection of Akagi’s line could be the reason for the tibial component rotational error and worse postoperative clinical outcomes in total knee arthroplasty

Abstract

Background

Malrotation of the tibial component can cause revision, lead to altered joint kinematics, and likely produce clinical issues in total knee arthroplasty (TKA). Many surgeons decide on the tibial component’s rotational angle with reference to the tibial anatomical anteroposterior (AP) axis, and they have adopted Akagi’s line, which is originally defined on computed tomography (CT) as the line connecting the middle of the posterior cruciate ligament to the medial border of the patellar tendon attachment, as the most important tibial AP axis intraoperatively. However, some surgeons experienced intraoperative difficulty in detecting Akagi’s line. The accuracy of the intraoperative identification of Akagi’s line and the effect of accuracy on postoperative clinical outcomes are unknown. Therefore, we evaluated the intraoperative reproducibility of the tibial AP axis “Akagi’s line,” which is originally defined on CT, and the effect of the reproducibility on postoperative clinical outcomes.

Methods

This prospective study included 171 knees of 160 patients who underwent TKA. We measured the difference between the intraoperative Akagi’s line and the original Akagi’s line defined on CT. The difference was measured by the tibial component rotation angle relative to the two kinds of Akagi’s line. In CT analyses, the original Akagi’s line was defined on preoperative CT and the tibial component rotation was measured on postoperative CT, which projected the preoperative Akagi’s line using a 3D software program. In intraoperative analyses, the intraoperative Akagi’s line was registered in the navigation system and the tibial component rotation was measured using the navigation system. The value and absolute value of the angular divergence of the intraoperative Akagi’s line relative to the original Akagi’s line were measured. Additionally, the effect of the angular difference on postoperative clinical outcomes (Knee injury and Osteoarthritis Outcome Score: KOOS, 2011 New Knee Society Score: NKSS) was evaluated.

Results

The absolute value of the angular difference of the intraoperative Akagi’s line to the original Akagi’s line was 5.5°. The range of the intraoperative Akagi’s line relative to the original Akagi’s line was internally rotated 22° to externally rotated 16°. Intraoperative Akagi’s line outliers (difference to original Akagi’s line >5° and >10°) occurred in 46% (78 knees) and 14% (24 knees). In the outlier analysis (>5°), the tibial component rotation angle was externally rotated 5.3° in the outlier group (>5°) and externally rotated 3.2° in the non-outlier group (=5°) and, there was a significant difference; however, there was no difference in the clinical outcomes between two groups. In the outlier analysis (>10°), the tibial component rotation angle was externally rotated 6.5° in the outlier group (=10°) and externally rotated 3.7° in the non-outlier group (=10°) and, there was significant difference. Additionally, the outlier group (=10°) showed lower pain scores in KOOS and lower symptom scores in NKSS.

Conclusion

The original Akagi’s line defined on CT was not replicated intraoperatively. The intraoperative poor detection of Akagi’s line could be the reason for the tibial component rotational error and worse postoperative clinical outcomes.