Computed tomographic correlation between Akagi's line and the extensor hallucis longus tendon: A descriptive study of 100 normal tibiae.

Abstract

BackgroundRotational alignment of the tibial component is critical for successful total knee arthroplasty (TKA). Akagi's line is a reliable anteroposterior reference, but its intraoperative applicability may be limited when the posterior cruciate ligament (PCL) is not visible after tibial resection. The extensor hallucis longus (EHL) tendon, as an extra-articular and consistently identifiable structure, may represent a practical distal landmark. This study evaluates the anatomical relationship between Akagi's line and the EHL tendon on computed tomography (CT) and analyzes its association with tibial morphometric parameters.MethodsA descriptive correlational study was conducted on CT scans of 100 anatomically normal tibiae. Akagi's line was defined as the line connecting the center of the PCL to the medial border of the patellar tendon. Its distal projection at the tibial plafond was compared with the center of the EHL tendon. Tibial length, morphology, posterior slope, sagittal distance, and the angle of Akagi's line relative to the femoral transepicondylar axis (TEA) were measured. Two independent observers performed all measurements. Associations were analyzed using Spearman's correlation coefficients with 95% confidence intervals.ResultsAkagi's line projected through the center of the EHL tendon in 60% of cases and medially in 40%, with no lateral deviations. The Akagi-EHL distance correlated strongly with tibial length (ρ = 0.71; p < 0.001) and sagittal distance (ρ = 0.70; p < 0.001), but showed no association with tibial slope. No significant correlation was observed between the Akagi-femoral TEA angle and morphometric parameters.ConclusionThe distal projection of Akagi's line consistently aligns with, or lies slightly medial to, the center of the EHL tendon. This predictable relationship supports the EHL as a reliable, extra-articular distal landmark for tibial rotational alignment in TKA when intra-articular references are obscured.Level of evidenceIV (Descriptive study).