Anatomic Risk Factors for Lateral Patellar Instability

With an incidence of approximately 42 per 100,000 persons annually, patellar instability is a debilitating condition that can result in dysfunction of the normal patellar tracking and potential cartilage damage. The stability of the patellofemoral (PF) joint involves an intricate relationship between muscular forces, soft tissues, trochlear and patellar geometry, and limb alignment. Several anatomic patellar risk factors (APRFs) have been identified including patella alta (Caton Deschamps >1.2; Insall-Salvati >1.2), rotational malalignment (femoral anteversion >30°, knee rotation >10°, and tibial rotation >35°), genu valgum (Valgus: zone 2 or greater), a lateralized tibial tubercle (tibial tubercle-trochlear grove distance >17 mm; tibial tubercle-posterior cruciate ligament distance >21 mm), and trochlear dysplasia. The importance of APRFs is highlighted by their overwhelming association with patellar instability; >80% of patients with patellar instability have at least 1 risk factor. Biomechanically, these risk factors increase lateralizing forces on the patella, increase maltracking (patellar tilt and subluxation), decrease contact area, and increase pressure in the PF joint. In addition, there is greater anisometry of the medial PF ligament reconstruction. Clinically, the presence of APRFs increases the chances of recurrence after a first episode as well as failure rates of medial PF ligament reconstruction. Initially described by Dejour on lateral radiographs, current APRF evaluation includes standard radiographs along with axial slice imaging, with magnetic resonance imaging being more commonly used currently. In some instances, mechanical axis radiographic views and axial computed tomography rotational alignment studies may be indicated. Each risk factor can be assessed independently, as there are good-quality studies defining abnormal thresholds for individual APRF. However, there is a lack of robust clinical data defining use of these thresholds for guiding decisions regarding nonsurgical/surgical treatment, specifically, which factors need to be surgically managed and at what threshold for optimal outcomes. It is important to understand that there is an intricate and complex interaction between risk factors that need to be considered during PF evaluation. Overall, evaluation of APRF is one of the core elements of PF instability management.