The fibular shaft axis and medial cortex of the proximal fibula are reliable landmarks for the mechanical axis of the tibia in patients with knee osteoarthritis

Abstract Background This study aimed to evaluate the validity of proximal fibular anatomic landmarks for measuring the coronal tibial mechanical axis in patients with knee osteoarthritis and to investigate individual factors associated with their reliability. Methods A total of 106 knees in 96 patients were retrospectively reviewed. The angles between the tibial mechanical axis and fibular shaft axis (TFA), medial cortex of the proximal fibular shaft (MTA), and lateral cortex of the proximal fibular shaft (LTA) were measured from full-leg standing digital anteroposterior radiographs. An angle within three degrees was considered reliable. The association between the above three angles and individual factors, such as age, sex, body mass index (BMI), and varus–valgus knee malalignment, was determined to investigate individual factors associated with their reliability. Results The median TFA, MTA, and LTA were 1.52°, 1.56°, and 2.62°, respectively. The reliability rates of TFA, MTA, and LTA were 73.6% (95% CI: 65.19–81.98%), 82.1% (74.77–89.38%), and 58.5% (49.11–67.87%), respectively. The reliability of TFA and MTA was not associated with individual variables. The reliability of LTA was associated with BMI. Among patients with BMI greater than 25.3 kg/m2, LTA was considered reliable in 65.7%; this rate was significantly higher than that among patients with BMI less than 25.3 kg/m2. Conclusions The fibular shaft axis and medial cortex of the proximal fibular shaft are reliable landmarks of the mechanical axis of the tibia. However, the reliability of the lateral cortex of the proximal fibular shaft is less satisfactory, especially in patients with BMI less than 25.3 kg/m2.