Meniscal Tear
The most likely diagnosis is a meniscal tear, specifically a degenerative meniscal lesion given the several-week history of mechanical symptoms (locking) and pain elicited by weight-bearing rotation (positive Thessaly test). 1, 2
Clinical Reasoning
The clinical presentation strongly suggests a meniscal tear based on three key features:
- Knee locking is an independent diagnostic factor for meniscal tears with a specificity of 96% and positive predictive value of 98.8% when present 1
- Pain with weight-bearing knee flexion and torso rotation describes a positive Thessaly test, which when combined with other clinical findings increases diagnostic accuracy for meniscal tears 3
- Several-week duration suggests either a degenerative process or a non-acute traumatic tear that has persisted 2
Diagnostic Approach
Obtain standing knee radiographs first (AP, lateral, tunnel, and patellar views) to exclude osteoarthritis and other bony pathology, as these are the appropriate initial imaging study for chronic knee pain 4, 5. This is critical because degenerative meniscal tears frequently coexist with early osteoarthritis 2.
MRI without contrast should follow if radiographs are normal or non-diagnostic, as MRI has 96% sensitivity for detecting meniscal tears and can change management from surgical to conservative in up to 48% of patients presenting with a locked knee 6, 1. MRI demonstrates 96% sensitivity and 97% specificity for meniscal tear detection 6.
Critical Clinical Pearls
Joint line tenderness has superior diagnostic accuracy (81% for medial meniscus, 90% for lateral meniscus) compared to other physical examination maneuvers 3. The absence of joint line tenderness in your patient with an otherwise unremarkable exam is notable but does not exclude a meniscal tear.
Combining clinical findings increases diagnostic certainty: When locking and the Thessaly test (your weight-bearing rotation maneuver) are combined with MRI, the predicted correct percentage for meniscal tears reaches 91.6% 1.
Important Caveats
Degenerative meniscal tears are common incidental findings and may not be the pain source, particularly in patients over 40 years 6, 2. The British Association for Surgery of the Knee consensus defines three symptom categories, with "locking" classified as strongly suggestive of a treatable meniscal lesion 2.
The medial meniscus is more vulnerable to injury due to its intimate attachment to the medial collateral ligament, making it the more likely site if this is a traumatic tear 7. However, the lateral meniscus is more commonly involved in degenerative tears.
Arthroscopic surgery should not be reflexively pursued even with confirmed meniscal tear on MRI, as the BMJ strongly recommends against arthroscopy for degenerative knee disease given lack of sustained benefit beyond one year 6. Surgery is only appropriate for patients with persistent, severe mechanical symptoms who fail conservative management 6.