Before KA-TKA, full-length weight-bearing radiographs of the lower limbs were obtained from all patients. The hip-knee-ankle angle (HKAA) was measured from the full-length radiograph. HKAA is defined as the angle between the mechanical axes of the femur and the tibia. The value of varus HKAA is defined as positive, and the value of valgus HKAA is defined as negative. Other alignment parameter measurements followed the methods described by Paley [23]. The mechanical lateral distal femoral angle (mLDFA), medial proximal tibial angle (MPTA), and joint line convergence angle (JLCA) were measured and recorded (Fig. 3). The full-length CT data of all PSI-KA patients were retrieved. The angle between the surgical transepicondylar axis (TEA) and the posterior condylar axis (PCA) was determined using in-house maximum intensity projection (MIP) technology (SOMATOM Sensation, Siemens, Germany), and this angle was defined as the transepicondylar axis angle (TEAA) (Fig. 4a). In addition, TEAA was also measured and recorded using a protractor in all patients included in this study during the operation (Fig. 4b). The above parameters were used as independent variables in this study.

Schematic diagrams for the measurement of tibiofemoral joint anatomical parameters: Hip-knee-ankle angle (HKAA), b Mechanical lateral distal femoral angle (mLDFA), c Joint line convergence angle (JCLA), and d medial proximal tibial angle (MPTA)

Transepicondylar axis angles (TEAAs) were measured by preoperative superimposed CT (a) and a protractor in the operating room (b). a Determination of the recess of the medial epicondyle, the prominence of the lateral epicondyle, and the highest point of the medial and lateral posterior condyles by the MIP approach of CT. The TEAA value was automatically obtained by the in-house program. b Intraoperative measurement required electrocautery to mark the position of the medial and lateral epicondyles on the distal femoral resection surface. The arms of the protractor were parallel to the posterior pedals of instruments and the electrocautery marks

The distances from the lateral edge of the trochlear resection surface to the femoral component trial were measured at two levels: the corner of the anterior resection and the anterior chamfer resection (distal exposure, DISexposure) and the middle level between this corner and the apex of the anterior flange of the femoral trial (middle exposure, MIDexposure) (Fig. 5). The widths of the exposed trochlear resection at these two levels were used as the dependent variables for subsequent analysis.

Intraoperative caliper measurement of the exposed bone resection of the lateral trochlea. The white and blue arrows represent the width of the exposed resection at the distal (DISexposure) and middle (MIDexposure) levels of the trochlea, respectively

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