Effects of pre-operative measurement of proximal tibia varus angle on the prediction for condylar twist angle in total knee arthroplasty

Abstract

Objective To assess the effects of pre-operative measurement of proximal tibia varus angle (PTA) on predicting condylar twist angle (CTA) in total knee arthroplasty (TKA). Methods The CT-scan of lower limb was performed in sixty-one health female volunteers aged 31.5 (18-38) years with 160 (153-170) cm heights from March 2016 to August 2016. The PTA, medial proximal tibial angle (MPTA) and CTA were determined. The PTA was defined between a line perpendicular to the mechanical axis of the tibia and the tangent on the internal tibial plateau. The MPTA was the medial angle between the mechanical axis of the tibia and the tangent on the internal tibial plateau. The CTA was the angle between the sTEA and the posterior condylar line. The correlations between these parameters were analyzed statistically by linear regression. Results The PTA, MPTA and CTA parameters were conformed to the normal distribution. The PTA was 2.30°±1.86°, ranged from 1.40° to 6.50°. The MPTA was 87.7°±1.86°, ranged from 83.50° to 88.60°. The CTA was 3.4°±1.27°, ranged from 0.50° to 5.80°. The linear regression analysis showed a strong positive correlation between the CTA and PTA in health female knees (r=0.58, P=0.048), while negative correlation was found between the CTA and MPTA in health female knees (r=-0.58, P=0.048). There were fifty-two (85.2%) volunteers with PTA that was more than 2° and less than 4°. Six (9.8%) volunteers had PTA less than 2°, while three (4.9%) had PTA more than 4°. There were eight (13.1%) volunteers with CTA less than 3°. Forty-three (70.5%) volunteers had CTA more than 3° and less than 4°. CTA more than 4° was found in ten volunteers (16.4%). Based on these above results, two formulas were demonstrated for predicting the CTA: CTA=2.52+0.395PTA, CTA=38.06-0.390MPTA. Conclusion The CTA can be predicted by calculating the PTA by CT-scan and standard long-leg standing radiographs before TKA. Using the present method could reduce the risk of femur component malrotation and the flexion gap balancing during TKA. Key words: Arthroplasty, replacement, knee; Femur; Rotation; Tomography