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What is the diagnosis?

Lateral patellar dislocation with medial avulsion fracture noted on sunrise view. There is evidence of a knee effusion on imaging and findings to suggest trochlear dysplasia, coinciding with a history of possible patellar subluxation vs dislocation.

Image 2 . Sunrise view of the left knee. There is evidence of lateral patellar tilt and a surrounding effusion. There is evidence of a medial patellar avulsion fracture (yellow arrow). A borderline shallow sulcus angle is identified (144°). Author's own images and annotations.

Image 3. Lateral projection of the left knee suggesting a shallow trochlear floor as it intersects with the lateral condyle. Red: medial condyle. Blue: lateral condyle. Yellow: trochlear floor. Author's own images and annotations.

What is the mechanism of injury?

Lateral patellar dislocations (most common) are often a result of direct traumatic injury to the knee (often medially) or in the case of non-contact injuries, are seen frequently from sudden dynamic movements of the knee in flexion and valgus [1]. Nearly all traumatic dislocations are associated with disruption of the medial patellofemoral ligament (MPFL) [2].

What physical exam findings are expected?

Most patellar dislocations spontaneously reduce by time of visit. Examination may reveal a knee effusion, tenderness over the MPFL, increase in patellar translation on passive and active testing, or positive patellar apprehension. Care should be placed in evaluating for stability of the knee joint to rule out other causes of acute knee pain such as patellar/quadricep tendon rupture.

In the event that the patient is still dislocated, typically the patella is obviously identified lateral to the knee joint. The knee is often held in passive flexion.

Which imaging modalities can be used?

Should there be concern for patellar fracture or surrounding fracture patterns from high mechanism injury, x-ray imaging can be obtained though frequently is not required in simple dislocations.

In this case, post-reduction x-rays were obtained to reveal a medial avulsion fracture fragment and signs of trochlear dysplasia predisposing this patient to recurrent dislocations. They would eventually require outpatient surgical management.

• Pearl: X-ray imaging may reveal signs of trochlear dysplasia as demonstrated by a shallow sulcus angle on sunrise views (normal 128°, greater than 145° suggest dysplasia) [3]. Lateral knee projections may suggest a shallow trochlear floor as identified by the “Crossing Sign”, a radiographic finding where the trochlear floor intersects with the femoral condyles [1].

What is the management in the ED/when do you consult Orthopedics?

Obvious patellar dislocations should be reduced. After pain control is administered, gentle extension of the knee with lateral to medial force can be applied to the patella.

Post-reduction plain films can be used to evaluate for loose bodies or avulsion fragments (as is in this case) however are not required in the Emergency Department should the knee exam otherwise be unremarkable (i.e. extensor mechanism intact).

First time non-recurrent patellar dislocations without fractures are typically nonoperative. Fracture fragments typically require outpatient surgical intervention.

Traditional teaching emphasizes the placement of a knee immobilizer prior to outpatient follow up with orthopedic surgery in 1-2 weeks though emerging evidence has suggested that patellar stabilizing braces may be a better option. Some studies show comparable re-dislocation rates between both methods, while immobilizers are linked to greater quadriceps atrophy, restricted ROM, and worse functional outcomes [4][5]. Patellar stabilizing braces may not be available in all EDs thus knee immobilizers would be appropriate in this scenario.

Orthopedic consultation in the ED is not required.