PURPOSE

To investigate the conversion formulas between the Miniaci angle, presurgery parameters, and changes in presurgery parameters in open-wedge high tibial osteotomy (OWHTO), including hip-knee-ankle (HKA) angle, weightbearing line (WBL) percentage, mechanical medial proximal tibial angle (mMPTA), ΔHKA angle, ΔWBL percentage, ΔmMPTA, and other parameters.

METHODS

From January 2012 to December 2019, 247 lower limbs of 144 patients with medial unicompartmental knee osteoarthritis combined with proximal tibia vara were enrolled. Inclusion criteria were adults, medial unicompartmental knee osteoarthritis, Kellgren-Lawrence classification grade ≤ III, mMPTA ≤ 85°, normal mechanical lateral distal femoral angle (85°-90°), and patella facing anterior in the bipedal standing position. Exclusion criteria were history of fracture, trauma, or orthopaedic surgery; developmental dysplasia of the hip or femoral head necrosis; femoral bowing deformity; deformity of the tibial shaft; and leg length discrepancy. Using standing whole-leg radiographs, an OWHTO simulation was performed to determine the Miniaci angle by delivering the WBL to the Fujisawa point. The relationship of the Miniaci angle, the presurgery parameters, and the changes in presurgery parameters were analyzed by Spearman's correlation and linear regression analyses. The relationship between the postsurgery HKA angle and presurgery parameters was analyzed by multiple linear regression model.

RESULTS

The Miniaci angle showed a near-perfect correlation with the presurgery HKA angle (y = -1.05x + 192.10, r = 0.99), presurgery WBL percentage (y = -0.25x + 15.14, r = 0.97), ΔHKA angle (y = 1.04x - 0.03, r = 1.00), ΔWBL percentage (y = 0.25x - 0.52, r = 0.97), and ΔmMPTA (y = 1.04x - 0.03, r = 1.00). The ΔHKA angle showed nearly perfect correlation with the ΔmMPTA (y = 1.00x, r = 1.00), and ΔWBL percentage (y = 0.24x - 0.47, r = 0.97).

CONCLUSIONS

The presurgery HKA angle, presurgery WBL percentage, ΔHKA angle, ΔWBL, and ΔmMPTA percentage are nearly perfectly correlated to the Miniaci angle, whereas the ΔmMPTA and ΔWBL percentage are nearly perfectly correlated to the ΔHKA angle. With the conversion formulas determined in the current study, surgeons can calculate the Miniaci angle based on the presurgery parameters without the assistance of digital software for complex surgical simulation. The Miniaci angle is closely related to the gap of the medial opening wedge. Based on the Miniaci angle and the depth of the osteotomy, surgeons can calculate the gap required before surgery using trigonometric functions and then simply measure the gap during surgery.