What causes a knee to lock?

What Causes a Knee to Lock

A locked knee is most commonly caused by a displaced meniscal tear (particularly bucket-handle tears), followed by loose bodies in the joint, anterior cruciate ligament tears, and less commonly by pigmented villonodular synovitis, cruciate ligament cysts, or superior patellar dislocation. 1, 2

Primary Mechanical Causes

Meniscal Tears

• Bucket-handle meniscal tears are the predominant cause of true mechanical locking, where a displaced fragment physically blocks full knee extension 1, 2
• The term "locked knee" specifically refers to a knee with fixed flexion or a mechanical block preventing complete extension, though some degree of active or passive extension may still be achievable 1
• MRI can change management from surgical to conservative in up to 48% of patients presenting with a locked knee, which is typically considered an indication for arthroscopic intervention 3

Loose Bodies

• Osteochondral loose bodies from trauma, degeneration, or osteochondritis dissecans can cause mechanical obstruction 4, 1
• These fragments physically interfere with normal knee kinematics and block extension 2

Ligamentous Injuries

• Ruptured anterior cruciate ligament can contribute to locking symptoms 4, 1
• Cysts of the cruciate ligaments represent an unusual but documented cause of mechanical blocking 2

Less Common Causes

Synovial Pathology

• Pigmented villonodular synovitis can present as knee locking, appearing as yellowish lumps on meniscal surfaces that mechanically obstruct joint motion 4, 2
• Focal synovial lesions may not be visible on MRI and require diagnostic arthroscopy for definitive diagnosis 4

Patellar Abnormalities

• Superior dislocation of the patella is a rare traumatic cause with fewer than 20 reported cases 5
• Interlocking osteophytes between the medial femoral condyle and inferior pole of the patella can cause locking even without trauma 5

Diagnostic Approach

Imaging Strategy

• MRI is the gold standard for diagnostic imaging of the locked knee, facilitating detection of meniscal tears, loose bodies, and soft tissue pathology 1
• MRI can identify anterolateral ligament injuries and posterolateral corner injuries that may be associated with ACL ruptures and contribute to instability 3
• When MRI findings are inconclusive but clinical suspicion remains high, diagnostic arthroscopy is considered the gold standard for management and can reveal pathology not visible on imaging 4, 1

Critical Clinical Distinctions

True Mechanical Locking vs. Pseudo-Locking

• True mechanical locking requires a physical obstruction preventing full extension, distinguishing it from pain-related limitation or "catching" sensations 6, 1
• The presence of clicking, catching, or intermittent "locking" sensations in degenerative knee disease does NOT indicate true mechanical obstruction and responds equally well to conservative treatment 6
• Only a small subset of patients with truly obstructing displaced meniscus tears causing mechanical symptoms benefit from surgical intervention 6

Age-Related Considerations

• In patients over 35 years with degenerative changes, mechanical symptoms alone should not automatically trigger surgical intervention, as these often respond to conservative management 6
• Young patients with traumatic bucket-handle tears represent a different clinical scenario requiring more urgent surgical consideration compared to older patients with degenerative tears 7

Common Pitfalls to Avoid

• Do not assume all mechanical symptoms require surgical intervention - many respond to conservative treatment 6
• Do not rush to surgery based on MRI findings alone, as meniscal tears are common incidental findings in older patients 7
• Be aware that diagnostic arthroscopy may be necessary when clinical presentation suggests true locking but MRI is negative or inconclusive 4
• Remember that interference with normal knee kinematics is non-specific regarding diagnosis, requiring careful evaluation for less common causes beyond meniscal pathology 2