Approach

The key to diagnosis of an ACL injury is a thorough history and physical exam.

History

Key historical features indicating an ACL injury include a high-risk mechanism (contact or noncontact), an audible pop at the time of injury, rapid development of hemarthrosis, inability to return to play, pain, and a feeling that the knee is unstable or buckles with attempted weight bearing.[42][45]

History of prior ACL injury and female sex should also heighten suspicion.​[11][12][13][14][15][28]​​ Patients are often adolescents and young adults, or middle-aged individuals who are involved in extensive exercise.

Physical examination

Physical exam usually reveals a hemarthrosis if the injury was recent. Chronic ACL tears may or may not have an associated effusion. Acutely, range of motion is usually impaired, especially in flexion. This may be due to a combination of pain and stiffness from hemarthrosis, as well as associated bone bruise, meniscal tears, or articular cartilage injury.[46]Tenderness is found at the lateral femoral condyle and lateral tibial plateau.[Figure caption and citation for the preceding image starts]: T1-weighted MRI showing ACL tearFrom the personal collection of Philip H. Cohen [Citation ends].com.bmj.content.model.Caption@145b719

It is crucial to perform tests gently and smoothly, and to have the patient relax; otherwise the patient's discomfort and fear may lead to guarding, tensing of the hamstrings, and false-negative or inconclusive results.

Lachman test

The Lachman test is the most accurate maneuver for detecting an acute ACL tear, whereas the pivot shift test performs better in chronic tears or under anesthesia.[42][47][48] The Lachman test is performed with the patient in a supine position. The patient’s knee is positioned at 20 to 30 degrees of flexion. One hand is placed on the patient’s thigh and the other behind the tibia, with the clinician's thumb on the tibial tuberosity. The tibia is then pulled anteriorly. If the ACL is intact, a firm end point is felt. If this does not occur, and there is >2 mm of anterior movement compared with the uninjured knee, the test is positive, suggesting a torn ACL. [Figure caption and citation for the preceding image starts]: Lachman maneuverFrom the personal collection of Philip H. Cohen [Citation ends].com.bmj.content.model.Caption@55527a71

Anterior drawer test

The anterior drawer test will often be positive, but is less sensitive and specific.[49] This test involves placing the patient in a supine position and flexing the hips to 45 degrees, with the knees at 90 degrees and the patient's feet on the table.[Figure caption and citation for the preceding image starts]: Anterior and posterior drawer test starting positionFrom the personal collection of Philip H. Cohen [Citation ends].com.bmj.content.model.Caption@5c81511 Sitting on the patient's feet, the clinician takes hold of the tibia and pulls it anteriorly. If the tibia moves more than usual, the test is positive.

Varus/valgus stress test

A standard part of the physical exam.[42]​ Although not specific for identifying ACL tears, this test is necessary to evaluate for other or concomitant injuries.[Figure caption and citation for the preceding image starts]: Varus and valgus stability testingFrom the personal collection of Philip H. Cohen [Citation ends].com.bmj.content.model.Caption@bcae9a2

Pivot shift maneuver

Usually performed under anesthesia in the operating room, before or after reconstruction surgery. It is technically difficult, but the best test for dynamic rotatory instability.[50]

Arthrocentesis

Usually not indicated, but sometimes done for symptom relief if tense hemarthrosis is present, or to assist with the diagnosis in confusing cases.

Imaging

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 years.

​​

X-rays are usually negative, but 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.

Magnetic resonance imaging (MRI)

MRI has excellent sensitivity and specificity for ACL tears, and can reveal associated injuries. However, MRI is not necessary if the clinical diagnosis is clear.[54][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].com.bmj.content.model.Caption@7294f6d7

MRI may be useful diagnostically when the clinical exam is impaired by factors such as patient guarding, tense effusion, or a locked joint (intra-articular block from displaced meniscal tear, etc.). MRI can also be helpful when evaluating the possibility of a torn ACL graft.[55]

MRI is often ordered as an initial test, especially in high-level athletes. However, this practice is generally unnecessary and is not supported by evidence-based research. In particular, 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.[56]

From a surgical perspective, there is some evidence that preoperative MRI may be helpful in determining the suitability of a given patient's patellar tendon for use in an ACL bone-patellar tendon-bone autograft.[57]

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.[52]​ However, if the patient is a child, do not order CT until all appropriate clinical, laboratory, and plain radiographic exams have been completed in order to limit radiation exposure.[56]​

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