## 2 Materials and Methods
### Surgical Technique  
Depending on TT-TG and CDI, different surgical techniques were chosen. In case of patellar instability with normal TT-TG (<16 mm) and normal CDI (<1.2), only MPFL reconstruction was performed. When TT-TG was pathologic and CDI was normal, ATA medialization osteotomy was performed. When CDI was pathologic and TT-GT was normal, ATA lowering osteotomy was preferred. When both TT-TG and CDI were altered, a medialization and lowering osteotomy was performed. The patellar lowering necessary to achieve a normal CDI was calculated in millimeters, starting from the preoperative CDI measured on lateral radiographs with a 20 mm calibration marker (Figure 1).  
MPFL reconstruction was added to ATA osteotomy when there was evidence of excessive patellar lateralization or evidence of MPFL rupture on MRI. Firstly, a diagnostic arthroscopy was routinely performed through standard medial and lateral portals to check possible chondal damages and the integrity of the trochlear groove, the anterior and posterior cruciate ligaments, as well as the medial and lateral compartments. MPFL reconstruction was performed using a gracilis muscle autograft. The gracilis muscle autograft was harvested and then prepared on a designated graft workstation (Arthrex, Inc. Naples, FL, USA). Two 2.4 mm guidelines were placed parallel on the patellar medial side at 1.5 cm from each other. A 4.5 mm cannulated drill was used to perform two bone tunnels on the patella and finally the graft ends were fixed in the tunnels using Arthrex Swivelock 4.5 mm velet system (Arthrex, Inc. Naples, FL, USA). Under fluoroscopic guidance, the femoral isometric \"Schottle Point\" was searched and identified using an eyelet guidewire. The Schottle point lays 2 mm anterior to the posterior cortical line, between the posterior aspect of the Blumensaat line and a parallel line which crosses the posterior transition point of the femoral cordyles [14,15]. The graft loop was passed bluntly between joint capsule and vastus medialis. Subsequently, a 7 mm femoral bone tunnel was performed with a cannulated drill, through which the graft loop was passed with eyelet guidewire and fixed using an Arthrex resorbable 6 \(\times\) 23 mm interference screw. When ATA transposition and MPFL reconstruction needed to be combined in the same surgical procedure, ATA transposition was performed firstly. When only ATA medialization  
Figure 1: Caton–Deschamps index (CDI) assessed in a preoperative radiography of the knee, lateral view. In this example, CDI corresponds to 1.38, being \(\frac{4}{8}=\frac{40}{29}=1.38\), which means patella alta. Knowing that CDI should not be more than 1.2, using the proportion B x 1.2, we obtained the value B\({}^{\prime}\) (29 \(\times\) 1.2 = 34.8 mm) which corresponds to the maximum hypothetical length of A so that the patella does not result high. Then, with a simple subtraction, \(\mathrm{B-B^{\prime}=40-34.8}=5.2\) mm, we calculated the distance of which we want to lower the bony brat during surgery. A 20 mm calibration marker is essential for making the correct lowering calculations.  
osteotomy was necessary, an incomplete ATA osteotomy was executed leaving the distal apex of the ATA intact. When also CDI was \(\geq\)1.2, a complete osteotomy, with both medial and lowering transposition of the ATA, was performed to obtain a better correction, or just a lowering transposition if only CDI was higher than 1.2. The ATA transposition was intraoperatively measured with a ruler. The osteosynthesis was obtained with two compression screws.