In a simplified manner, recurrent lateral patellar instability occurs because of an imbalance among the forces acting on the patella. This occurs when a large enough laterally directed force vector overcomes the medial “pull” of the soft tissue restraints and the “push” of the lateral femur's bony constraint. There are numerous factors that can contribute to this imbalance, and it is helpful to group them based on how they affect patellar tracking, patellar restraint, or patellar constraint. A laterally directed patellar tracking path from an increased tibial tubercle to trochlear groove (TT-TG) distance, an excessively lateral patella “entry point” into the trochlea (quantified by Yu et al.’s entry point-transition point, or EP-TP, measurement) [1], excessive femoral internal rotation, excessive tibial external rotation, or genu valgum can result in the loss of optimal co-linearity and congruence with the trochlear groove 2, 3, 4, 5. A decrease in medial patellar restraint can occur in situations of patellofemoral complex (MPFC) incompetence or quadriceps weakness, while those of decreased constraint can occur in settings of trochlear dysplasia or patella alta [2,5,6]. The factors that contribute to each notion of tracking, restraint, and constraint are not mutually exclusive, and it is more appropriate to consider that they can have additive effects with potential interplay amongst them [5].

Numerous risk factors contribute to the overall picture of instability, each occurring along its own spectrum of severity. Notably, a TT-TG >20mm, the presence of trochlear dysplasia, patella alta, or young age and skeletal immaturity have all been identified as major risk factors, and thus have been used to develop models that can help stratify a patient's risk for patellar instability 7, 8, 9. Understandably, it can be challenging to determine if, and when, a given risk factor contributes enough influence towards persistent instability to warrant surgical correction [6].

It remains prudent to consider the chronicity of the patient's symptoms and dislocation events at the time of presentation. This facilitates the differentiation of a first-time patellar dislocation from ongoing recurrent instability or dislocations. Most first-time dislocations occur in young, active patients, typically during sport-related activities [10]. In the absence of an operative osteochondral fracture fragment, the mainstay of treatment in this setting does not typically involve surgery. Treatment often consists of relocation (if necessary), bracing, and lower extremity strengthening. Despite proper non-operative management for first-time dislocations, approximately 17-43% of these patients will develop symptoms of recurrent instability or dislocations 10, 11, 12. When recurrence develops, operative treatment is often indicated [13].

To date, numerous surgical procedures have been described for treating recurrent lateral patellar instability. Given the multifactorial nature of patellar instability, this comes as no surprise. Because of this heterogeneity, it is important to identify the risk factors present, gauge their magnitude, and then develop a sustainable, individualized surgical plan.

With the tibial tubercle being the distal attachment site of the extensor mechanism, an osteotomy carries a long lever arm that can be used to manipulate the patellar tracking vector. Effectively, moving the distal point along the line of tracking can treat patellar instability from a variety of different anatomic etiologies that cause a functionally lateralized extensor mechanism, such as an increased TT-TG, tibial torsion, genu valgum, patella alta, or femoral anteversion. An added benefit of a TTO is the capacity to alter patellofemoral contact pressures (based on the angle of the oblique osteotomy) or patellar height (by concomitant distalization). The combined benefits of realignment and articular unloading are influencing the rising success rates seen for cartilage restoration procedures within the patellofemoral compartment [14].

TTOs are classified by the direction in which the tubercle is shifted. In most situations, the tibial tubercle is moved in one of three main directions: anteriorly, medially, or distally. Often a combination of these directions is utilized, such as anteromedialization (AMZ) with or without distalization [15,16]. The versatility of AMZ TTOs stems from the ability to manipulate the oblique osteotomy cut angle to produce differing amounts of anteriorization and medialization while also facilitating the surgeon's control over the amount of distalization through modifications to either the proximal or distal aspects of the oblique cut 16, 17, 18.