Confusable diagnoses · PANCE / PANRE

Meniscus Tear vs Anterior Cruciate Ligament (ACL) Tear

Written & clinically reviewed by Ryan Busch, PA-C · Updated August 2026 · Report an error

Meniscus Tear and Anterior Cruciate Ligament (ACL) Tear are easy to mix up on the boards. Here's a side-by-side comparison — presentation, workup, imaging, and first-line treatment — drawn from our full outlines.

Meniscus Tear vs Anterior Cruciate Ligament (ACL) Tear at a glance

  • Meniscus Tear: Acute or degenerative tear of the medial or lateral meniscus causing joint-line pain, effusion, and mechanical symptoms.
  • Anterior Cruciate Ligament (ACL) Tear: Non-contact pivoting injury with audible pop, immediate large effusion, and knee instability; reconstruction in active patients.

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Question 1MusculoskeletalMedium
A 24-year-old man twisted his left knee while pivoting during a basketball game 3 days ago. He has medial-sided knee pain and difficulty squatting, and he has continued to walk with a mild limp. On examination there is a small effusion, terminal extension is limited by about 5 degrees, and the knee is guarded so that Lachman and drawer testing are equivocal. Which additional history finding would most strongly support a medial meniscal tear over an anterior cruciate ligament rupture?
  • ACatching that resolves after straightening the leg
  • BSwelling of the knee within 1 hour of the injury
  • CImmediate inability to continue playing the game
  • DA loud pop heard at the moment the knee twisted
Reveal answer & full explanation
Correct answer: A — Catching that resolves after straightening the leg
  • ACatching that resolves after straightening the leg✓
  • BSwelling of the knee within 1 hour of the injury
  • CImmediate inability to continue playing the game
  • DA loud pop heard at the moment the knee twisted

Why Catching that resolves after straightening the leg is correct

  • When a guarded, swollen knee makes ligament testing unreliable, the history carries the diagnostic weight, and the meniscus and the ACL are separated mainly by the tempo of swelling and by mechanical symptoms.
  • A torn meniscal fragment intermittently displaces into the joint and then reduces, producing catching, clicking, or transient locking that the patient relieves by wiggling or straightening the leg.
  • Catching that resolves after straightening the leg — a mechanical symptom specific to an unstable meniscal fragment and the finding that best supports meniscal injury here.

Why the others are wrong

  • Swelling of the knee within 1 hour of the injury — describes an acute hemarthrosis; the meniscus is largely avascular except at the peripheral red zone, so meniscal effusions accumulate over 12 to 24 hours, while an effusion within the first 2 hours predicts ACL rupture in roughly 70 percent of cases.
  • A loud pop heard at the moment the knee twisted — classic for ACL rupture and reported by about half of those patients, whereas meniscal tears more often produce a subtle tearing sensation.
  • Immediate inability to continue playing the game — reflects the gross instability of a ruptured ACL; patients with isolated meniscal tears usually finish the activity and stiffen later.

Terminal extension loss can occur with either injury and does not discriminate, though a true springy block to extension suggests a displaced bucket-handle tear. Joint line tenderness and the Thessaly and McMurray maneuvers should be checked once guarding settles, and MRI is used for confirmation.

Question 2MusculoskeletalEasy
A 20-year-old female collegiate soccer player felt a pop in her right knee 3 hours ago when she planted her foot and pivoted to change direction. The knee swelled within the first hour and she could not continue playing. She has no prior knee injury and did not receive a direct blow. The knee is held in slight flexion with a large tense effusion, and both flexion and extension are limited by pain and guarding. Which examination maneuver would best evaluate the suspected ligament injury?
  • APosterior drawer with the knee at 90 degrees
  • BLachman test with the knee at 20 degrees
  • CValgus stress with the knee at 30 degrees
  • DAnterior drawer with the knee at 90 degrees
Reveal answer & full explanation
Correct answer: B — Lachman test with the knee at 20 degrees
  • APosterior drawer with the knee at 90 degrees
  • BLachman test with the knee at 20 degrees✓
  • CValgus stress with the knee at 30 degrees
  • DAnterior drawer with the knee at 90 degrees

Why Lachman test with the knee at 20 degrees is correct

  • A non-contact pivoting injury with an audible pop and a hemarthrosis that appears within the first few hours is an anterior cruciate ligament (ACL) rupture until proven otherwise, so the examination must target abnormal anterior translation of the tibia on the femur.
  • The Lachman test is done with the knee flexed only 20 to 30 degrees while the examiner stabilizes the distal femur with one hand and pulls the proximal tibia anteriorly with the other, grading both the amount of translation and the quality of the endpoint.
  • It is the single most sensitive physical test for ACL rupture, roughly 85 percent, and it is the right choice in an acutely swollen knee because it requires minimal flexion and is not defeated by hamstring guarding.
  • Lachman test with the knee at 20 degrees — the correct maneuver in this acute, guarded, effused knee.

Why the others are wrong

  • Anterior drawer with the knee at 90 degrees — tests the same ligament, but 90 degrees of flexion is poorly tolerated acutely, and hamstring spasm plus a posterior horn meniscus acting as a wedge blunt translation, dropping acute sensitivity to roughly 50 percent.
  • Posterior drawer with the knee at 90 degrees — the reference maneuver for the posterior cruciate ligament, which is torn by a posteriorly directed force on a flexed tibia such as a dashboard blow, not by a pivot.
  • Valgus stress with the knee at 30 degrees — isolates the superficial medial collateral ligament and is the maneuver to use after a blow to the lateral side of the knee.

The pivot shift test is highly specific for ACL rupture but is usually falsely negative in an awake, guarded patient. MRI confirms the tear, but the bedside examination makes the diagnosis.

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Side-by-side comparison

FeatureMeniscus TearAnterior Cruciate Ligament (ACL) Tear
At a glanceAcute or degenerative tear of the medial or lateral meniscus causing joint-line pain, effusion, and mechanical symptoms.Non-contact pivoting injury with audible pop, immediate large effusion, and knee instability; reconstruction in active patients.
Classic presentationAcute tear: twisting injury, often with a popping sensation, followed by pain along the joint line; Effusion developing over 12-24 hours (slower than ACL effusion); Mechanical symptoms: catching, locking, giving way; Degenerative tear: insidious medial joint-line pain, may follow squatting or rising from chair, often with coexisting OA…Non-contact pivoting or deceleration injury with audible/sensed 'pop'; Immediate large effusion (within 1-2 hours) — hemarthrosis; Inability to continue activity; Knee instability ('giving way') with subsequent pivots; Pain often diffuse; Large effusion; Lachman test — most sensitive (knee 20-30° flexion, anterior translation of tibia,…
Workup / key labsNot indicated unless concern for inflammatory or septic etiologyNot indicated
ImagingWeight-bearing knee X-rays — evaluate for OA, fracture, alignment; MRI — gold standard for confirming meniscal tear and characterizing morphology; high sensitivity and specificity; Arthroscopy — diagnostic and therapeutic in selected casesKnee X-rays — evaluate for fractures, including Segond fracture (avulsion of lateral capsule from the lateral tibial plateau — pathognomonic for ACL tear); MRI — gold standard; confirms diagnosis, characterizes associated injuries (meniscus, MCL, bone bruises in classic 'kissing contusion' pattern at lateral femoral condyle and…
First-line treatmentActivity modification, avoidance of pivoting and deep squatting; RICE (rest, ice, compression, elevation) acutely; NSAIDs — ibuprofen, naproxen, meloxicam — for pain and effusion; Acetaminophen as adjunct; Physical therapy — quadriceps and core strengthening, range of motion; Intra-articular corticosteroid injection — short-term adjunct…Acute management: RICE — rest, ice, compression, elevation; Acute management: Knee immobilizer or hinged brace for comfort; Acute management: NSAIDs — ibuprofen, naproxen, meloxicam — for pain and effusion; Acute management: Crutches with weight-bearing as tolerated; Acute management: Early range of motion to prevent stiffness —…

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Full outlines

Meniscus Tear Anterior Cruciate Ligament (ACL) Tear All 514 diagnoses

Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA or PAEA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants; End of Rotation™ is a program of the Physician Assistant Education Association.