History may differ depending on the mechanism of injury, but most describe pain and potentially a noticeable deformity of the knee. This is usually preceded by direct trauma to the knee or a sudden change in direction. Many also describe a sense of giving way or instability when the dislocation occurs. Patients often feel a pop or multiple pops and there may be generalized pain in the anteromedial knee.

On physical examination, the first step is to inspect the knee. An acutely dislocated patella is typically easy to see but there may be a large effusion or hemarthrosis that make it more difficult. The provider should also inspect for patella alta, tibial torsion, genu valgum or varum or general ligamentous laxity. General ligamentous laxity is assessed with the Beighton hypermobility score [2]. There has been a term dubbed “miserable malalignment syndrome” for patients with excessive femoral anteversion, excessive tibial outward rotation, and genu valgum [3]. The knee should be palpated including each pole of the patella, medial and lateral joint lines and retinaculum. The bulk of the VMO should be evaluated and quantified if possible. A palpable defect along the medial retinaculum or MPFL may be appreciable. Tenderness over the MPFL origin has been called the Bassett sign, which is consistent with ligamentous disruption [4,5]. The collateral ligaments should also be palpated and assessed along with the cruciate ligaments. Range of motion and patellar glide should be tested if pain permits. The patella can normally be moved medially and laterally between 25-50 percent the width of the patella. A “J sign” may also be seen with knee extension in which the patella tends to deviate laterally and may “pop” into the trochlear groove. Lastly, the moving patellar apprehension test is most specific and sensitive for instability. The examination begins with the knee held in full extension and the patella is manually translated laterally with the thumb. The knee is then flexed to 90 degrees and then brought back to full extension while the lateral force on the patella is maintained. For the second half of the test, the knee is started in full extension, brought to 90 degrees of flexion, and then back to full extension while the index finger is used to translate the patella medially. Part one should have apprehension and possibly tightening of quadriceps muscle and the second half should not elicit apprehension. Sensitivity was 100 percent with specificity 88 percent in one study with 51 patients [6].

The first step in imaging patellar dislocations is plan radiography. Standard anteroposterior weight bearing radiographs of both knees and posteroanterior radiographs at 45 degrees flexion may aid in the assessment of the coronal alignment of the tibiofemoral joint or possibly the presence of arthrosis. Lateral views and sunrise, also called Merchant, views should also be performed to help with assessment. The lateral and sunrise radiographs provide information in regards to trochlear morphology, patellar height, and patellar tilt. These radiographs will also help identify fractures of the patella or avulsion fractures, which sometimes occur with a rupture of the MPFL from the middle third of the patella. Loose bodies and sometimes large cartilage defects can also be seen with some dislocations or subluxations.

Other risk factors can also be quantified or identified with radiographs and are important in both acute and recurrent dislocations. Lateral patellar tilt is assessed by the lateral patellofemoral angle on sunrise view. This is an angle is measured between a line along the subchondral bone of the lateral trochlear facet and posterior femoral condyles with normal being an angle greater than 11 that opens laterally [7]. Abnormal angles are parallel or open medially. There are numerous methods to identify or measure patellar height. Patellar height can be assessed by direct or indirect methods. Indices such as Insall-Salvati (IS), Blackburne-Peel(BP) and Caton-Deschamps (CD) are indirect methods which measure patellar height by ratios based on the length of the patellar tendon or some reference points on the proximal part of the tibia (Figure 2). The Insall-Salvati ratio is the ratio measuring the length of the patella ligament and the patellar length. A normal ratio is 1.0; a ratio of 1.2 suggests patella alta and 0.8 patella baja. According to the description by Caton [8], the Caton-Deschamps index is the distance between the distal point of the patellar articular surface and the anterior superior margin of the tibia, divided by the patellar articular surface length. A normal ratio is 1.0; a ratio of less than 0.6 suggests patella baja and a ratio of 1.3 suggests patella alta. The Blackburne-Peel method (BP) measures the ratio of the height of the lower pole of the articular surface above a tibial plateau line to the articular surface length of the patella [9]. Usually range between 0.54- 1.06. A ratio of less than 0.54 is considered to be patella alta. The technique described by Blumensaat in 1938, which uses the roof of the intercondylar notch as a reference line, is one of the most commonly used direct methods for the assessment of patellar height [10, Figure 3]