Anterior cruciate ligament (ACL) tear is one of the most common and important ligament injuries of the knee joint. The ACL extends from the lateral femoral condyle to the tibial eminence and prevents anterior tibial translation and internal rotation. Annual incidence is 68.6 per 100,000 and is most common in athletes aged 15-25. Women tear 2-8 times more frequently than men (anatomic, hormonal, and neuromuscular factors). Injury mechanism is usually non-contact pivot/deceleration — sudden direction change with knee in slight flexion and valgus stress. MRI is the gold standard for diagnosis (95% sensitivity). Primary findings include ACL fiber discontinuity and signal increase. Secondary findings include pivot-shift bone contusion (posterolateral tibial plateau + anterolateral femoral condyle), anterior tibial translation, lateral meniscus tear, and Segond fracture. Treatment is usually surgical with ACL reconstruction, especially in young and active patients.
Age Range
15-45
Peak Age
25
Gender
Male predominant
Prevalence
Common
The ACL consists of two main bundles: anteromedial bundle (AM — taut in flexion, controls anterior translation) and posterolateral bundle (PL — taut in extension, provides rotational stability). The ACL's primary vascular source is the middle genicular artery, supplying through the synovial sheath around the ligament. ACL tear usually occurs through non-contact mechanism: with knee at 10-30 degrees flexion, under tibial rotation and valgus stress, the tibia is forced anteriorly and into internal rotation on the femur — this combination exceeds ACL's tolerance. At the moment of tear, the lateral femoral condyle impacts the posterolateral tibial plateau ('pivot-shift' mechanism) creating bone contusion on both surfaces — this 'pivot-shift contusion pattern' appears as bone marrow edema on MRI. Segond fracture is an avulsion fracture of the lateral capsular ligament at its tibial attachment and is pathognomonic for ACL tear (75-100% association). Increased anterior tibial translation (>7 mm measured on sagittal images) reflects anterior tibial subluxation resulting from ACL functional loss. Spontaneous healing of ACL after tear is extremely rare because synovial fluid in the intra-articular environment washes away the fibrin clot eliminating the healing scaffold.
Subchondral bone marrow edema in the posterolateral tibial plateau and anterolateral-to-mid lateral femoral condyle on T2 fat-suppressed/STIR MRI. This specific localization reflects the pivot-shift mechanism during ACL tear: impaction of the lateral femoral condyle against the posterolateral tibial plateau during anterior tibial subluxation creates bone contusion on both surfaces. This pattern has >90% specificity for ACL tear and guides diagnosis even when direct ACL visualization is inadequate.
On sagittal PD fat-suppressed or T2 fat-suppressed images, ACL fibers appear discontinuous, thickened, and show diffuse hyperintense signal. In complete tear, fiber continuity is completely lost and replaced by hyperintense edema/hemorrhage. Best evaluated on sagittal oblique sections following the ACL's normal oblique course. In partial tear, some fibers remain intact while others show signal increase and thickening. In acute tear, the ligament is diffusely edematous with indistinct margins, while in chronic tear, the ligament may be thinned or completely resorbed.
Report Sentence
ACL fibers are discontinuous and diffusely hyperintense, consistent with complete tear.
On T2 fat-suppressed or STIR sequences, bone marrow edema is seen in the posterolateral tibial plateau and anterolateral-to-mid lateral femoral condyle. This 'pivot-shift contusion pattern' results from impaction of the lateral femoral condyle against the posterolateral tibial plateau during ACL tear. Bone edema is usually subchondral and overlying articular cartilage may be damaged. This contusion pattern has >90% specificity for ACL tear and strongly predicts ACL tear even when the ligament cannot be directly visualized.
Report Sentence
Subchondral bone marrow edema consistent with pivot-shift contusion pattern is seen in the posterolateral tibial plateau and anterolateral femoral condyle.
On sagittal PD images, the anterior position of the tibial posterior margin is measured using the lateral femoral condyle posterior line as reference. >7 mm anterior tibial translation indicates ACL insufficiency. This measurement is the MRI equivalent of the 'anterior drawer' test. In acute tears, subluxation may increase due to hydraulic effect of hemarthrosis. In chronic ACL insufficiency, persistent anterior translation leads to secondary meniscal and chondral damage.
Report Sentence
Approximately ___ mm anterior tibial translation is seen on sagittal images, consistent with ACL insufficiency.
A thin avulsion fracture fragment (Segond fracture) is seen at the anterolateral margin of the lateral tibial plateau. This fracture is avulsion of the lateral capsular ligament/anterolateral ligament from the tibial attachment and is pathognomonic for ACL tear (75-100% association). Best seen on CT but can also be detected on MRI as bone marrow edema and cortical irregularity. The fragment is usually small (few mm) and can be missed on conventional radiographs.
Report Sentence
A thin avulsion fragment (Segond fracture) is seen at the anterolateral margin of the lateral tibial plateau, pathognomonic for ACL tear.
On T2 fat-suppressed sequences, prominent joint effusion and hemarthrosis (hemorrhagic effusion) is seen. Hemarthrosis is present in >95% of acute ACL tears. Hemarthrosis may show more heterogeneous signal than synovial fluid — fluid-fluid level and subacute blood methemoglobin signal on T1 may be seen. Prominent effusion distends the joint capsule and accumulates in the suprapatellar recess and posterior compartment.
Report Sentence
Prominent joint effusion with fluid-fluid level is seen, consistent with hemarthrosis and suggesting acute ligament injury.
Criteria
Both bundles (AM + PL) completely torn — no fiber continuity
Distinct Features
Most common type (85-90%), significant instability, surgical reconstruction indication, diffuse hyperintense signal and fiber loss
Criteria
One bundle (usually AM) torn, other bundle (PL) intact — partial fiber continuity present
Distinct Features
10-15% of all tears, partial instability, conservative treatment or augmentation surgery, ligament thickened but some fibers intact
Criteria
Ligament resorbed or atrophic after old ACL tear — secondary degenerative changes
Distinct Features
ACL fibers thin/invisible, attachment to PCL (PCL impingement), anterior tibial translation, secondary meniscal and chondral damage, O'Donoghue triad sequelae
Distinguishing Feature
PCL tear affects the posterior ligament with dashboard injury mechanism; ACL tear is anterior ligament with pivot/deceleration mechanism — different localization and contusion pattern
Distinguishing Feature
MCL injury is medial extra-articular ligament pathology characterized by periligamentous edema; ACL is intra-articular ligament tear — both can coexist (O'Donoghue triad)
Distinguishing Feature
Isolated bone contusion can occur without ligament tear; however pivot-shift contusion pattern specifically predicts ACL tear
Urgency
urgentManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralThe main treatment decision in ACL tear is between surgery (ACL reconstruction) and conservative treatment. ACL reconstruction is standard treatment in young (<40 years), active, sports-playing patients — autograft (hamstring, patellar tendon, quadriceps) or allograft is used. Conservative treatment (physiotherapy, quadriceps strengthening, bracing) may be applied in sedentary elderly patients or low-demand profiles. Concomitant meniscal tear is repaired or partial meniscectomy is performed during ACL reconstruction. Segond fracture requires no specific treatment — heals with ACL reconstruction. Surgical timing is important: surgery is performed after the acute phase (first 2-3 weeks) subsides, when inflammation decreases and knee range of motion is regained (usually 4-6 weeks). Early reconstruction increases arthrofibrosis risk.
ACL tear is the main cause of knee instability and requires surgical reconstruction in active athletes and young patients. Associated meniscal tear, bone contusion, and lateral structure injury should be evaluated. Untreated tears increase the risk of early osteoarthritis.