Fig. 2

a Scheme of the patellar bone position (PBP) grading system: the patella bone position is evaluated relative to the supraconylar spur (arrows). Grade 1 (b) is defined as a fully distally located PB to the spur, grade 2 (c) with most of the PB length being distal to the spur, grade 3 (d) with half of the PB being on top of the spur, grade 4 (e) with most of the PB length being proximal to the spur and grade 5 (f) with the whole PB located proximal to the spur. The representative X-ray images also show an increase of the measured patello-tibial distance (PTD). These were measured from the distal pole of the PB to the tibial tuberosity (white lines). The dashed lines illustrate the probable graft plane. At grades 4 and 5 that line crosses the condyles (marked area), implying that the graft is bent over the patellofemoral surface of the femur