A complex interaction between muscles, ligaments, bone morphology, and lower extremity alignment influences patellar motion. The most important stabilizer of the patella beyond 30° of knee flexion is bony stability of the femoral trochlea. Additional stability is provided by active and passive restraint. Actively, the rectus femoris and vastus intermedius muscles act along the femoral axis, and the vastus lateralis and medialis help stabilize the patella in the medial/lateral plane. Passive restraints include patellar tendon, the medial menisco-patellar ligament, and the medial patellofemoral ligament (MPFL), a thin but relatively consistent ligament [4, 5]. The MPFL extends from the medial margin of the patella and attaches to the femur between the adductor tubercle and the medial epicondyle [6, 7]. The MPFL is important in initiating smooth entry into the trochlea with knee flexion and seems to be the major ligamentous restraint against patellar dislocation [8]. Traumatic primary patellar dislocation is almost always associated with hemarthrosis, MPFL injury, and medial retinacular disruption [3]. Clinically, 94–100 % of patients suffer from MPFL rupture after acute patellar dislocation [8].

The initial evaluation of a first-time patellar dislocation should include an appropriate patient history, family history of patellar dislocation or hyperlaxity, and physical examination. Patients frequently report that their knee “gave way” and that they “felt a pop.” Clinical findings include large effusion with tenderness about the medial retinaculum and hemarthrosis. The patella is usually spontaneously reduced, making the diagnosis less clear if the athlete cannot clearly identify what happened during injury. The clinician can rarely assist the reduction by applying a medially directed force with knee extension to push the patella over the lateral trochlea and back into the trochlear groove. If the patella does not easily reduce, the procedure should be aborted. A fracture fragment of the patella or condyle may limit the reduction and can be verified with radiographic studies [9].

Aspiration of the knee joint should be performed in patients with moderate to severe effusions. The presence of a hemarthrosis raises the likelihood that a significant osteochondral fracture has occurred. Clinical findings are not specific, so careful examination should be undertaken for anterior cruciate ligament, posterior cruciate ligament, and collateral and rotational laxity. Patellar apprehension and mobility should be assessed by medial and lateral patellar translation. A common physical examination sign to asses patella laxity is measuring the movement of the patella when passively stressed, recording the movement in quadrants of mobility as well as its endpoint firmness [10]. Increased lateral patella translation is suggestive of a lateral patellar dislocation.

Plain radiographs are always needed to evaluate patellar position and to asses osteochondral fractures, in particular, an AP-extended knee weight-bearing view, a Mercer-Merchant view (45° flexion weight-bearing view), and a 30° flexion lateral view. Both patellae should be included in the axial view. Even when the radiographs are normal, an osteochondral fracture may have occurred. Osteochondral fracture has been reported to be missed in 30–40 % of initial radiographs in both surgical and MRI studies [11, 12]. MRI is recommended to be performed quite soon after the injury to verify the diagnosis, evaluate additional injuries, asses the cartilage more precisely, classify the MPFL injury, and describe the anatomic factors of the patellofemoral joint (Fig. 49.2). MPFL injuries have been classified into three categories based on location: at the level of the MPFL patellar insertion, at the midsubstance, and at the femoral origin of the MPFL [13, 14]. Between 40 and 90 % of MPFL disruptions are located in the femoral attachment, whereas some studies have reported figures up to 50–60 % at the patellar insertion. The MPFL midsubstance region seems to be less frequently affected [15]. Recently, Sillanpää proposed a new classification for patellar insertion MPFL injuries: type P0 with ligamentous disruption, type P1 with bony avulsion fragment, and type P2 with bony avulsion involving articular cartilage surface from the medial facet of the patella [15].

The initial management of a first-time traumatic patellar dislocation is controversial with no evidence-based consensus to guide decision-making. Conservative treatment has been historically suggested for patients with primary patellar dislocation. A short immobilization period is used for patient comfort and is followed by formal physiotherapy. Therefore, the optimal conservative management has yet to be established [16]. The surgical treatment should be considered in specific conditions. These include the finding of an osteochondral fracture, a substantial disruption of the MPFL, and a laterally subluxated patella with normal alignment of the contralateral knee [17].