Musculoskeletal · Sports Medicine
The facts most likely to be tested
The classic mechanism of injury is a non-contact deceleration or pivoting maneuver with the knee in valgus stress.
Patients frequently report an audible pop at the time of injury followed by immediate hemarthrosis and inability to continue activity.
The Lachman test is the most sensitive and reliable physical exam maneuver for detecting an ACL tear.
The anterior drawer test is less sensitive than the Lachman test and may be falsely negative due to hamstring guarding.
An MRI of the knee is the gold standard diagnostic imaging modality to confirm the diagnosis and evaluate for associated injuries.
The Segond fracture, an avulsion fracture of the lateral tibial plateau, is highly pathognomonic for an associated ACL tear.
The Unhappy Triad (or O'Donoghue's triad) consists of concurrent injury to the ACL, medial collateral ligament (MCL), and medial meniscus.
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A 22-year-old collegiate soccer player presents to the urgent care clinic after sustaining a knee injury during a game. She reports that while attempting to change direction rapidly, her foot became planted and she felt a loud pop followed by immediate pain and swelling. On physical examination, there is a large effusion present in the right knee. The Lachman test demonstrates increased anterior translation of the tibia compared to the contralateral side. Radiographs of the knee reveal a small avulsion fracture of the lateral tibial plateau.
Which of the following is the most likely diagnosis?
Anterior Cruciate Ligament (ACL) tear
The combination of a non-contact pivoting injury, a reported 'pop', and a Segond fracture (the lateral tibial plateau avulsion) is classic for an ACL tear, which is confirmed by the positive Lachman test.
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High yield triage
Etiology / Epidemiology
Common in non-contact pivoting sports (soccer, basketball). Female athletes are at significantly higher risk due to biomechanical factors.
Clinical Manifestations
Patients report a pop followed by immediate hemarthrosis and knee instability. The Lachman test is the most sensitive physical exam maneuver.
Diagnosis
The MRI is the gold standard for definitive diagnosis. Clinical diagnosis is often confirmed via the Lachman test.
Treatment
Initial management is RICE (Rest, Ice, Compression, Elevation). Surgical ACL reconstruction is indicated for active patients or those with persistent instability.
Prognosis
High risk of early-onset osteoarthritis regardless of surgical intervention. Return to sport typically requires 6-12 months of rehabilitation.
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Epidemiology & Etiology
Most common knee ligament injury, frequently occurring during deceleration or cutting maneuvers. Incidence is higher in female athletes due to increased Q-angle and hormonal influences. Often associated with the unhappy triad (ACL, MCL, and medial meniscus).
Pertinent Anatomy
The ACL originates from the lateral femoral condyle and inserts into the anterior intercondylar area of the tibia. It serves as the primary restraint to anterior tibial translation on the femur. It also provides rotational stability to the knee joint.
Pathophysiology
Injury typically occurs via valgus stress combined with internal rotation of the tibia on a fixed foot. The sudden tension causes ligamentous fiber disruption, leading to hemarthrosis within 1-2 hours. Chronic instability results from the loss of the primary mechanical restraint to anterior glide.
Clinical Manifestations
Patients classically describe a pop sensation followed by rapid swelling and a feeling of the knee giving way. Physical exam reveals a positive Lachman test (most sensitive) and anterior drawer test. Neurovascular compromise is rare but must be ruled out if the knee is dislocated.
Diagnosis
The Lachman test is the most sensitive clinical exam maneuver for acute tears. MRI is the gold standard imaging modality to confirm the tear and assess for associated meniscal or chondral injuries. Radiographs are primarily used to rule out Segond fracture (an avulsion of the lateral tibial plateau), which is pathognomonic for ACL injury.
Treatment
Acute management includes RICE and physical therapy to restore range of motion. ACL reconstruction using an autograft (e.g., patellar tendon or hamstring) is the standard for active individuals. Avoid premature return to sport to prevent graft failure and secondary meniscal damage.
Prognosis
Long-term outcomes include a high incidence of post-traumatic osteoarthritis. Successful return to high-impact sports requires a structured neuromuscular training program and objective strength testing.
Differential Diagnosis
MCL tear: localized tenderness over the medial joint line with positive valgus stress test
LCL tear: tenderness over the lateral joint line with positive varus stress test
Meniscal tear: joint line tenderness and mechanical locking or catching
PCL tear: positive posterior drawer test and sag sign
Patellar dislocation: visible deformity and tenderness over the medial patellofemoral ligament