Diagnosis of collateral ligament injury is made after the clinician takes the history of the injury and examines the knee.

Imaging methods may be used to confirm the diagnosis.

The 'history' may be one of contact or non-contact injury, but in either there is usually a sudden excessive varus or valgus force (forcing the knee into a sideways bend).

If the collateral is torn, rather than just sprained, an audible 'pop' may be heard. Pain is immediate, and experienced over the site of the ligament, but the patient may be able to continue to bear weight on that leg, albeit with bent knee, with the pain tending to increase with time.

The examination findings will differ depending on the nature and extent of the injury. With a sprain, there is tenderness over the ligament, but the joint line does not open up when the surgeon applies stress to try and bend it sideways (varus or valgus stress). In a partial tear the joint opens slightly, but in a complete tear there may be more than 10mm of opening when the varus or valgus stress is applied.

Quite often there is bruising over the ligament. In contact injury, the force may actually have been applied at the opposite side of the knee to the area of pain and bruising. Local swelling is common.

If the whole joint has swollen up, it is likely that the anterior cruciate has gone, too, and that there is blood in the joint. A knee surgeon's advice should be sought, and the postero-lateral corner carefully evaluated - as other important structures at the back of the knee may have been damaged.