Recurrent patellofemoral instability is a multifactorial pathology based on limb alignment, soft tissue constraints as well as patella and trochlear groove abnormal anatomy. For a correct and effective surgical approach, an accurate radiological evaluation is essential, which consists of the evaluation of three main factors of instability: trochlear dysplasia, abnormal height of the patella and pathological distance of the anterior tibial apophysis and trochlear throat (TA-GT). Concerning patellar height, one of the most used methods is the calculation of the Caton-Deschamps radiographic index. However, to date there is no study that certifies its reproducibility and reliability from a pre-operative assessment and planning perspective.

The patellofemoral joint (PFJ) consists of two bony components, the patella and the femoral trochlea, sharing the same joint surface (1). It may be prone to instability, that is when the patella dislocates, usually on the lateral side of the knee. The incidence of primary patellar dislocation has been reported to be 5.8 cases per 100,000 in the general population, with the highest incidence occurring in the 2nd decade of life (29 per 100,000) (2). Patellar instability can be classified as traumatic, when the dislocation is caused by an external force, or nontraumatic, when native anatomy of the knee predisposes to instability. Non-operative treatment is usually taken into account for first episode of lateral patellar dislocation, unless the presence of loose bodies is verified, and consists of knee immobilization, physical therapy, non-steroidal anti-inflammatory drugs (NSAIDs) administration (3). Recurrence rate after conservative management can be up to 15-44% and patients with a clinical history of two or more dislocations have a 50% chance of recurrent dislocation episodes (4). Recurrent instability is a multifactorial problem that rely on limb alignment, bone anatomy of the patella and the trochlear groove, soft-tissue constraints, especially the medial patello-femoral ligament (MPFL). The management of recurrent patellar instability is difficult for many reasons, including heterogeneous patient population, skeletal maturity, challenging surgical approach and lack of long-term and robust clinical outcome studies (5,6). Radiologic evaluation should study the three principal factors of instability: trochlear dysplasia, abnormal patellar height, pathological tibial tubercle-trochlear groove (TT-TG) distance (7). Patellar height can be easily assessed, through standard lateral view radiographs. Several indexes for studying and calculating patellar height have been described in the literature (8-10). In patellar instability, the use of Caton-Deschamps index CDI to measure patellar height one of the most employed. Specifically, CDI relies upon the length of the articular surface of the patella and its distance from the tibia. The purpose of our study is to evaluate the reliability and reproducibility of CDI in measuring patellar height for planning patella stabilization surgery. The secondary target is to hypothesize a quantitative method of preoperative planning for anterior tibial apophysis transposition surgery, by calculating CDI.

Inclusion Criteria:

• Diagnosis of recurrent patellar instability (≥ 2 episodes) at the time of surgery
• Indications for patellar stabilization surgery
• Availability of X-rays in digital format
• Patients who do not meet the inclusion criteria

Exclusion Criteria:

• Clinical history of <2 patella dislocations
• Non-availability of X-rays in digital format
• Not indicated for patellar stabilization surgery
• Contraindications to surgery
• Patients with osteoarthritis

Patients suffering from recurrent patellar instability (≥ 2 episodes) with indication of patellar stabilization surgery by reconstruction of the medial patellofemoral ligament and / or transposition of the anterior tibial tuberosity.