Investigations
1st investigations to order
x-rays
Test
Ottawa knee rules recommend x-rays in acute knee injury in an adult if any of the following features is present:[51][52]
Inability to bear weight at time of injury and at time of evaluation
Isolated tenderness at patella or fibular head
Active flexion of knee <90 degrees
Age >55.
Ottawa guidelines have excellent sensitivity (up to 100%) for detecting fractures around the knee and significantly decrease unnecessary use of x-rays. However, most acute knee injuries, including ACL tears, are not associated with fracture visible on x-ray.[58]
The absence of bony injury on x-ray does not imply ACL injury has not occurred.
Although x-rays are usually negative, they may reveal the lateral capsular sign/Segond fracture, a small capsular avulsion off the lateral aspect of the proximal tibia.[53] This is uncommon but virtually pathognomonic for ACL tear.
Result
impaction fracture of the lateral femoral condyle and fracture of the posterior aspect of the lateral tibial plateau (uncommon); radiographic drawer sign (anterior subluxation of tibia on femur) (uncommon); effusion (common); bony avulsion of ACL at intercondylar tubercle (mainly in skeletally immature patients)
Investigations to consider
MRI
Test
False-negative MRI may occur if ACL appears grossly intact but fibres have been overstretched and are unable to function properly.[Figure caption and citation for the preceding image starts]: T1-weighted MRI showing ACL tearFrom the personal collection of Philip H. Cohen [Citation ends].
[Figure caption and citation for the preceding image starts]: T2-weighted MRI showing bone bruise, effusion, and flattening of sulcus terminalisFrom the personal collection of Philip H. Cohen [Citation ends].
Result
ACL fibres appear disrupted, blurry on T1-weighted images, with abnormal high signal on T2-weighted images
arthroscopy
Test
Usually performed as part of reconstruction procedure, but occasionally torn ACL found during diagnostic arthroscopy. Allows direct visualisation and probing of ACL, as well as inspection of other intra-articular structures.
Result
torn ligament
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