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Twist pop and swell - it's an ACL (or is it)?

A twisting injury to a knee followed by a popping sensation and rapid swelling is a typical injury mechanism for an anterior cruciate ligament (ACL) injury. It is, however, important to recognise that not all patients with this injury mechanism have injured this ligament.  

Patients who experience a similar injury mechanism may also have subluxed or dislocated their patella. These patients often present with a very similar history to those who have injured their ACL.

In this x-ray image, we can see an osteochondral injury involving the medial patella (resulting from an avulsion of the medial patellofemoral ligament). This is much larger (and more rounded and chronic looking) than the more typical avulsion fracture. This injury can be better appreciated on MRI. On this modality, the ligament can be seen to be torn (or attenuated) while the medial femoral condyle and lateral tibia both typically have high signal from an impaction injury. The main value of an MRI, however, is to look for and define, a significant chondral or osteochondral injury. When this injury is present there may be a role for early surgical management. In most other situations rehabilitation is the treatment of choice.

Other causes of rapid swelling in a sporting setting include meniscal tears (usually only when the player is in their teens and still has a relatively vascular meniscus), a hyperextension injury with an associated subchondral fracture (bone bruise) or a fracture.

As an aside, it is interesting to note (and important to remember) that only 50% of patients who have injured their ACL report experiencing a pop sensation, and only a similar number experience rapid swelling. The vast majority, however, describe a strong sense of something going out of place. It is important not to be distracted by the absence of a pop and/or early swelling.

Any patient presenting with a painful and grossly swollen knee likely required a specialist review (and in most cases further imaging) to establish an accurate anatomical diagnosis. This type of patient can be reviewed early in our acute knee clinic.