Mechanical and anatomical axis of the lower limb in total ankle arthroplasty.

Introduction: Inadequate correction of mechanical alignment may lead to failure of Total Ankle Replacements (TAR). The mechanical axis of the lower limb (MAL), the mechanical axis of the tibia (MAT) and the anatomical axis of the tibia (AAT) are three well described coronal plane measurements using plain radiography. The assumption is that the MAL, MAT and AAT are equivalent. The relationship between these axes can vary in the presence of proximal deformity. The purpose of this study was to assess the relationship between MAL, MAT and AAT in a cohort of patients considered for TAR.

Methods: 75 consecutive standardised preoperative long leg radiographs of patients with end stage ankle osteoarthritis, between 2016 and 2017 at a specialist tertiary center for elective orthopedic surgery were analysed. Patients were split into 2 groups. The first group had a clinically and radiologically detectable deformity proximal to the ankle (such as previous tibial or femoral fracture, severe arthritis, or previous reconstructive surgery), whereas the second (normal) group did not. The MAL, MAT and AAT were measured and the difference between these values were calculated.

Results: There were 54 patients in the normal group, and 21 patients in the deformity group. The mean difference between the MAL and AAT was 1.7 ± 1.3° (range, 0.1-5.4°). In the normal group, 15 patients (27%) had a difference of >2° between the MAL and AAT, compared with 52% in the deformity group. The mean difference between the MAL and MAT was 0.9 ± 1.7° (range, -4 to -3.5°). In the deformity group, 42% of patients had a difference between MAT and MAL of >2°, compared with 20% in the normal group.

Conclusion: MAT, MAL and AAT should not be assumed to be the same in all patients. The authors recommend considering the use of full-length weightbearing lower limb radiographs to plan TAR.