Can Achilles tendon be used as a new distal landmark for coronal tibial component alignment in total knee replacement surgery? An observational MRI study

Background: In total knee arthroplasty, it is better to use more than one reference point for correct alignment of the components. By measuring the distances of Achilles tendon (AT) and other conventional landmarks from the mechanical axis in magnetic resonance imaging (MRI) of the ankle, we aimed to demonstrate that, as a novel landmark which can help for correct alignment in the coronal plane, AT is a better option than other landmarks. Materials and methods: This retrospective study was done on 53 ankle MRIs that met the criteria for inclusion to the study among 158 ankle MRIs. After identification of the mechanical axis, the distances of distal landmarks, which were extensor hallucis longus tendon (EHLT), tibialis anterior tendon (TAT), dorsalis pedis artery (DPA), AT, extensor digitorum longus tendon (EDLT), and malleoli, were measured from the mechanical axis and were statistically evaluated. Results: In proximal measurements, the distances of the landmarks to the mechanical axis (on average) were AT, 2.64 +/- 1.62 mm lateral; EHLT, 3.89 +/- 2.45 mm medial; DPA, 4.69 +/- 2.39 mm medial; TAT, 8.24 +/- 3.60 mm medial; and EDLT, 14.2 +/- 4.14 mm lateral (P<0.001). In distal measurements, the distances of the landmarks to the mechanical axis (on average) were AT, 1.99 +/- 1.24 mm medial; EHLT, 4.27 +/- 2.49 mm medial; DPA, 4.79 +/- 2.10 mm medial; TAT, 12.9 +/- 4.07 mm medial; and EDLT, 12.18 +/- 4.17 mm lateral (P<0.001). Conclusion: In this study, the mechanical axis line, which is the center of talus, passes through the AT. Our MRI investigations showed that the AT, EHLT, DPA, and malleolar center (3-5 mm medial) may help in correct alignment.