ACL impaction injury

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This young football player presented with a painful, swollen knee following an innocuous twisting injury. She described a feeling of “something going out of place”, a snapping sensation and immediate pain. She was unable to continue. 

The player had a restricted ROM and a moderate-large effusion. She had pronounced tenderness about the lateral joint line and gross laxity with Lachman’s test.

This lateral x-ray image shows an impaction injury involving the lateral femoral condyle. This is further demonstrated on a sagittal MRI image where there is pronounced increased signal involving the lateral femoral condyle and posterolateral tibia. The MRI images also show a large effusion.

The patient was managed with an ACL reconstruction. This was done using hamstrings autograft. The player returned to football nine months following the surgery.

An impaction injury is a common MRI finding after an ACL injury. As the ligament ruptures the knee transiently ‘dislocates’ allowing the posterolateral tibial to strike the lateral femoral condyle. While a degree of increased signal is almost always seen on MRI following ACL injuries it is less common to see this on x-ray. On a normal x-ray the lateral femoral condyle has a notch. Following an ACL injury the notch is deeper than 2mm or has a significant change in the contour. While the impaction injury itself requires no specific treatment, the associated injury often requires surgical management. 

When an x-ray image shows a prominent sulcus affecting the lateral femoral condyle it is important to consider whether a player has had an injury to their ACL. Other x-ray findings consistent with ACL injuries are a Segond fracture, a lipohaemarthrosis or an avulsion from the tibial spines.

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