Tests
1st tests 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:[55][56]
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 anterior cruciate ligament (ACL) tears, are not associated with fracture visible on x-ray.[65]
The absence of bony injury on x-ray does not exclude ACL injury.
Result
typically normal but may show: effusion (common); 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); bony avulsion of ACL at intercondylar tubercle (mainly in skeletally immature patients); mild depression on the lateral anterior femoral condyle (sulcus terminalis) contour on lateral radiograph (uncommon); lateral capsular sign (Segond fracture), a small capsular avulsion off the lateral aspect of the proximal tibia visible on anteroposterior radiographs (uncommon but virtually pathognomic for ACL tear)
MRI
Test
MRI is the imaging modality of choice to assess for anterior cruciate ligament (ACL) injury.[59] MRI has excellent sensitivity and specificity for ACL tears, and can reveal associated injuries, such as meniscal injuries that may require additional preoperative planning and repair. False-negative MRI may occur if ACL appears grossly intact but fibers 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 lateral compartment femoral condyle and tibial plateau impaction fracture, effusion, and flattening of sulcus terminalis which occurs with pivot mechanism with an ACL injury.From the personal collection of Philip H. Cohen [Citation ends].
If the patient is a child, do not order MRI until all appropriate clinical, laboratory, and plain radiographic exams have been completed. MRI is costly, frequently requires sedation in the young child and, if needed, is best ordered by the specialist who will be treating the patient.[62]
Result
ACL fibers appear disrupted, loss of structural detail on T1-weighted images, with abnormal high signal on T2-weighted images
Investigations to avoid
CT
Recommendations
CT is usually not appropriate unless there has been significant trauma to the knee, or a fall or acute trauma to the knee and no fracture or tibial plateau fracture is seen on radiographs, or suspected occult fracture or suspected additional bone or soft tissue injury.[56]
If the patient is a child, do not order CT for these disorders until all appropriate clinical, laboratory, and plain radiographic exams have been completed.[62]
Rationale
CT involves significant levels of radiation.
History, physical exam, and appropriate radiographs are sufficient for diagnosing most pediatric musculoskeletal conditions.[62]
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