A non-contact pivoting injury with a "pop" is characteristic of an ACL injury. This type of injury is very common in female basketball players, and research to determine risk factors in this population is underway. Up to 75% of patients with acute knee effusions will have an ACL injury. The key diagnostic examination is the Lachman test, performed with the knee in 30 degrees of flexion. Radiographs of the knee typically show normal results, but more than 50% of patients with have an associated "bone bruise" on magnetic resonance imaging (MRI) studies.