What is the most likely diagnosis for a 56-year-old male presenting with knee pain, locking sensation, and tenderness over the lateral joint line, with pain aggravation upon torso rotation while standing on the affected leg at 20° flexion?

Lateral Meniscus Tear

The most likely diagnosis is a lateral meniscus tear, given the combination of lateral joint line tenderness, locking sensation, and pain provocation with torso rotation on a weight-bearing flexed knee (positive Thessaly test).

Clinical Reasoning

The clinical presentation strongly points to a lateral meniscal injury based on several key features:

• Lateral joint line tenderness is highly specific (97%) and accurate (96%) for lateral meniscal tears, with sensitivity of 89% 1
• Locking sensation is a classic mechanical symptom indicating internal derangement, most commonly from meniscal tears or loose bodies 2
• The provocative maneuver described (torso rotation while standing on the affected leg at 20° flexion) represents a Thessaly test, which stresses the meniscus through rotational forces on a weight-bearing, partially flexed knee 3

Age-Related Considerations

At 56 years old, this patient falls into a demographic where meniscal tears are common:

• Meniscal tears in middle-aged patients often occur without specific trauma due to degenerative processes 3
• However, the majority of people over 70 years have asymptomatic meniscal tears, and the likelihood of a meniscal tear being present in either a painful or asymptomatic knee is not significantly different in patients 45 to 55 years of age 4
• The key distinguishing feature here is the mechanical symptoms (locking) combined with focal tenderness, which suggests a symptomatic tear rather than an incidental degenerative finding 5

Lateral vs. Medial Meniscus

The lateral location is critical:

• The lateral meniscus is more mobile and less prone to tear than the medial meniscus except when associated with ACL injury 3
• Joint line tenderness has higher accuracy for lateral meniscal tears (96%) compared to medial tears, with better specificity (97% vs lower rates for medial side) 1
• The lateral meniscus can cause locking through subluxation or bucket-handle tears 6

Diagnostic Approach

Initial imaging should be plain radiographs (AP, lateral, tunnel, and tangential patellar views) to rule out other pathology such as osteochondritis dissecans, loose bodies, or fractures 5, 7:

• Radiographs are recommended as the initial imaging modality when popping, locking, or mechanical symptoms are present 5

MRI without contrast is the next step if radiographs are normal or non-diagnostic and symptoms persist 4:

• MRI is the gold standard for detecting meniscal tears with pooled sensitivity of 88% and specificity of 90% 4
• MRI accurately depicts meniscal abnormalities, articular cartilage damage, and associated bone marrow lesions 4

Important Caveats

• ACL injury must be considered, as the lateral meniscus is more vulnerable to tear when the ACL is injured 3
• Accuracy of joint line tenderness decreases in the presence of ACL tears or chondromalacia patella 1
• Arthroscopy remains the gold standard for both diagnosis and treatment of locked knee 2
• Other rare causes of locking include lateral meniscal subluxation without tear, loose bodies, or pigmented villonodular synovitis, though these are less common 8, 6