Fig. 1

Preoperative planning. a Midsagittal section of the patella, blue and purple axis intersect at distal tunnel direction. b Midaxial section of the patella, F is the point of the rim of the anterior cortex at the lateral patella margin and C is the point of the rim of articular surface at the lateral patella margin. R1 is defined as the angle between F-D1-C. c Midcoronal section of the patella, L1 is the maximal patellar length. W is defined as patellar width (perpendicular to and at midpoint of L1). D1 is the distal tunnel entry point (where W touches the medial patellar margin) and D2 is the proximal tunnel entry point. d Midsagittal section of the patella, blue and purple axis intersect at proximal tunnel direction. e R2 is the angle between F-D2-C. f Wf is patella width at the point which a line parallel to W reaches the anterior cortex of proximal patella pole. D2 is the proximal tunnel entry point (where Wf touches medial patella margin). L2 iss the distance of proximal patella pole from Wf