Photo by speedyeNovember is football season in America, and physicians will see a surfeit of gridiron-related knee mishaps in this month. The usual culprits are injuries to the anterior cruciate ligament, medial collateral ligament, and the menisci, but occasionally one will see rarer injuries.
In this case, a 16 year-old high school football player felt a "pop" over the lateral aspect of his knee, and presented with pain and swelling, but had a stable knee on exam.
Sagittal T2 fatsat image reveals a hemarthrosis with a fluid-fluid level (red arrow) and moderate edema within the popliteus muscle (yellow arrow).
The popliteus muscle originates from the posterior shaft of the tibia, and inserts on the lateral femoral condyle:
Axial images from (A) the patient and (B) a normal individual for comparison reveal an irregular popliteus tendon (red arrows) in the injured football player. Compare with a normal popliteus tendon (green arrow):
Coronal intermediate images from (A,B) the patient and (C,D) a normal comparison individual delineate the nature of the injury:
There is an avulsion fracture involving the popliteus (red arrows = abnormal popliteus; green arrows = normal comparison popliteus). Note the hypointense bone avulsion fragment (blue arrows), which was confirmed on conventional radiographs (not shown)
More coronal images, posterior to anterior:
Red arrows = popliteus tendon; black arrows = fibular collateral ligament; blue arrows = avulsed bone fragments; pink arrow = combination of torn tendon and bone.
The patient had no other injuries, with normal menisci, collateral ligaments and cruciate ligaments. Isolated avulsion of the popliteus tendon is a rare injury. It was originally described by Naver and Aalberg in 1985, and this injury was also reported in two adolescent patients by Garth et al. in 1992 (JBJS 74:130, 1992). They stated that isolated avulsion of popliteus should be considered in the setting of a traumatic hemarthrosis. One may see a small flake of bone next to the lateral femoral condyle on radiographs, a clue to the diagnosis. In some patients, the tendon may retract out of the joint.
Thus, in the setting of lateral pain, traumatic hemarthrosis and a stable knee, one should examine the popliteus tendon carefully for signs of an avulsion injury.
Vic David MD