Patellar Stabilization Surgery: Can the Caton-Deschamps Index be Useful in Planning?

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.