Table 1 Description of measurements

Tibial tuberosity–trochlear groove distance (TT–TG)

Line 1 was drawn through the bottom of the trochlear groove, and line 2 was drawn through the middle point of the tibial tubercle. Line 1 and line 2 were perpendicular to the tangent line of the dorsal femoral condylar line (line 3). The TT–TG distance was the distance between the two parallel lines (Fig. 1)

Congruence angle (CA)

It was defined as the angle with the line drawn through the lower pole of the patella and the deepest point of the chute (line 4) to the line on the side of the bisector defined the tackle angle (line 5) (Fig. 2)

Patellar tilt angle (PTA)

It was defined as the angle between the extension line of the maximum transverse diameter of the patella (line 6) and the tangent to the posterior condyles (line 7) (Fig. 3)

Medial–lateral width (MLW)

It was defined as the length between the medial (a) and lateral edge (b) of the epicondyle (Fig. 4)

Patellar width (PW)

It was defined as the length between the medial (A) and lateral edge (B) of the patella in the slide with the widest patellar diameter (Fig. 5)

Patellar thickness (PT)

It was defined as the length between the patellar front polar (C) and back polar (D) (Fig. 5)

Modified Wiberg index (AE/BE)

It was measured as the ratio of the transverse length of the lateral patellar facet (AE) to the medial patellar facet(BE) (Fig. 5)

Wiberg angle

The angle between the slopes of the medial and lateral patella (Fig. 6)

Relative patellar width (PW/MLW)

The ratio of length of patellar width to medial–lateral epicondyle width

Relative patellar thickness (PT/MLW)

The ratio of length of patellar thickness to medial–lateral epicondyle width

Lateral patellar facet angle

The angle between the patellar transverse axis (AB) and the lateral patellar facet tangent (Fig. 7)