The case
A teenage male was running in gym class when he felt/heard a pop in his left knee. He crumpled to the ground in excruciating pain and held his knee in a flexed position. He would not move it due to pain, and swelling developed shortly thereafter. Pain is greatest in the anterior and medial portion of his knee. He has intact distal pulses, perfusion and sensation. Despite IV morphine he will not extend his leg, nor will he allow you do do so. You get an X-Ray.
The Diagnosis
OK, so obviously this is not Osgood-Schlatter disease. The X-Ray reveals that there is an avulsion fracture of the tibial tuberosity extending through the physis. Essentially what happened is that his patellar tendon pulled the tibial tuberosity right off of the anterior surface of the bone.
Discussion
This is clearly a painful injury. Patents will experience immediate and immense pain and have significant swelling/effusion from hemarthroses and surrounding fluid. It should be differentiated from patellar dislocation, knee dislocation and Osgood-Schlatter disease. Recall that patellar dislocations will generally find the patella displaced laterally with the knee held in a flexed position. See this prior post from PEMBlog for more. A free floating patella with a straight leg position indicates that the patellar tendon has pulled away from the inferior pole of the patella, or less often the apophysis. This case shows a much more significant avulsion, whereby a large portion of the tibia itself was avulsed away. A knee dislocation is an orthopedic emergency, and will see the tibia/fibula displaced posteriorly. This has a high risk of arterial compromise and reduction must be attended to emergently. Osgood-Schlatter is apophysitis of the tibial tubercle – where the patellar ligament inserts into the tibia.
CT scans can help better define the injury and asset Ortho in making operative decisions. Below you will find a gif of a 3D reconstruction of the above Xray which demonstrates how much displacement and comminution this fracture involves.
Ultimately, with a case like this one I’d like you to remember the following:
- Straightforward patellar dislocations almost always leave the knee flexed with the patella displaced laterally
- If the knee is straight, and you think the patella is dislocated/subluxed then there is likely a patellar tendon tear or avulsion fracture
