What is the diagnosis?

Acute inferior patellar sleeve avulsion (PSA) fracture with associated intra-articular effusion in setting of likely chronic Sinding-Larsen-Johansson Disease (SLJ). 

• Pearl: PSA fractures (named because of the relatively large sleeve of unossified patellar cartilage that accompanies these small bony fragments) occur in the pediatric population between 8 and 16 years of age, with a peak incidence at ~13 years [6,16,17]. Adolescents are more susceptible due to rapid growth, increased sports activity, and relative patella instability.

• Pearl: These avulsion injuries occur at the attachment sites of the patellar/quadriceps tendons, and are more common at the inferior pole. 

• Pearl: SLJ is osteochondrosis/traction apophysitis of the proximal attachment of the patellar tendon caused by repetitive microtrauma to the tendon at the insertion to the lower pole of the patella. Oschgood Schlatter, its more famous cousin, has similar pathology but at the tibial insertion of the patellar tendon. 

What is the mechanism of injury?

The main mechanism of injury in PSA is indirect, acute forceful muscle contraction of the quadriceps muscle typically seen during jumping activities [8,10,18].

• Pearl: In adolescents, the patella is hypermobile, and the relatively high cartilage-bone ratio at the transformation zone makes it more vulnerable to acute and chronic eccentric loads and shear forces resulting in PSA [18].

What physical exam findings are expected?

Pain at the avulsion site, swelling, and decreased range of motion secondary to pain and hemarthrosis. Additionally, a palpable defect of the patellar tendon, inability to perform a straight-leg raise, assessment of patellar alignment (alta/baja), location of soft-tissue swelling/ossific fragments and joint effusion can provide clues to help differentiate operative versus non-operative treatment [15].

• Pearl: An appropriate history of an acute event with or without previous pain, as well as physical exam, helps to differentiate an acute minimally displaced patellar sleeve avulsion from the chronic avulsion changes of Sinding–Larsen–Johansson disease [15].

Which imaging modalities can be used?

Plain radiographs, ultrasound, and cross-sectional imaging (CT vs. MRI) all can be used as needed, usually in that order depending on provider gestalt. 

• Pearl: Ultrasound can be helpful in the absence of visible fracture fragments on x-ray as a disruption of the cartilage may be seen and degree of separation estimated [4]. It can also verify an intact tendon. 

• Pearl: Patellar sleeve fractures can lack specific radiographic features, except for a high-riding patella, or patella alta, visualized on lateral plain radiographs. When a patellar sleeve fracture is suspected, other imaging modalities, such as MRI, can be used to confirm its diagnosis and evaluate the degree of chondral injury [19].

What is the management in the ED?

Pain control, supportive care, and image modalities as above to assist with proper diagnosis. Non operative management via a cylinder cast or knee immobilizer for six weeks is indicated for nondisplaced fractures with intact extensor mechanisms [1,7].

When do you consult Orthopedics?

PSA fractures can be managed as an outpatient. Most authors agree that surgical treatment is recommended when there is significant displacement (>2 mm) of the displaced osteo/chondral fragment [8,10,11]. Early surgical intervention is considered essential for displaced cases [19]. Minimally displaced fractures such as those seen with SLJ syndrome are almost always managed conservatively and nonoperatively [18].

• Pearl: If left untreated, a distal pull of the potent bone-forming tissue at the distal pole may go on forming bone and lead to an enlarged or even duplicated patella, active extensor lag and quadriceps muscle atrophy [3,9]. Additional complications of this trauma include nonunion, degenerative changes, hypertrophic changes, transient ischemic changes, the development of extensive ectopic bone within the knee, a limited range of movement in the knee joint, and patella alta [12,14].