What is the appropriate evaluation and management for a knee that locks?

Evaluation and Management of a Locked Knee

For a knee that locks, obtain plain radiographs (AP, lateral, and patellofemoral views) as the initial imaging study, followed by MRI if radiographs are negative or show only effusion, as MRI is the gold standard for diagnosing meniscal tears and other internal derangement that cause true mechanical locking. 1, 2

Initial Clinical Assessment

A true locked knee presents with three key clinical features that predict a mechanical cause requiring intervention: 3

• History of definite injury (acute trauma or twisting mechanism)
• Localized joint line tenderness on examination
• Presence of joint effusion

When all three features are present, a mechanical cause (meniscal tear, ACL rupture, or loose body) can be identified in approximately 90% of cases. 3 The McMurray test (61% sensitivity, 84% specificity) and joint line tenderness (83% sensitivity, 83% specificity) assist in diagnosing meniscal tears specifically. 4

Important distinction: True mechanical locking (fixed block to full extension) must be differentiated from pseudo-locking (pain-related limitation or catching without fixed block). 5

Imaging Algorithm

Step 1: Plain Radiographs (Initial Study)

Obtain minimum two views (AP and lateral) of the affected knee, with the lateral view at 25-30 degrees of flexion to evaluate for joint effusion and lipohemarthrosis. 1 Add patellofemoral views if patellar pathology is suspected. 1

Radiographs should be obtained when Ottawa Rule criteria are met (age >55 years, focal tenderness, inability to bear weight, or inability to flex to 90 degrees) or when clinical judgment suggests mechanical pathology. 1

Step 2: MRI (When Radiographs Are Negative or Show Only Effusion)

MRI without contrast is the next indicated examination and the gold standard for diagnosing meniscal tears and internal derangement. 2 The American Academy of Orthopaedic Surgeons provides strong evidence that MRI is the preferred imaging modality for acute meniscal pathology, with the greatest diagnostic accuracy. 2

Key advantages of MRI in locked knee: 1

• 96% sensitivity and 97% specificity for meniscal tears
• Can change management from surgical to conservative in up to 48% of patients presenting with locked knee
• Detects meniscal subluxation/dislocation (rare cause of locking)
• Identifies ACL tears, loose bodies, and bone marrow contusions
• Both 1.5T and 3T MRI protocols show similar high accuracy

Critical caveat: A "normal" MRI does not exclude hypermobile lateral meniscus with peripheral meniscocapsular tears, which can cause recurrent locking episodes. 6 This diagnosis requires high clinical suspicion and arthroscopic confirmation.

Management Based on Etiology

Meniscal Tears (Most Common Cause)

The goal is to preserve as much functional meniscal tissue as possible to mitigate osteoarthritis risk. 2

• Traumatic bucket-handle tears with displaced tissue: Surgical intervention (arthroscopic repair or partial meniscectomy) is required 4
• Degenerative tears: Conservative management with exercise therapy for 4-6 weeks is first-line, even with mechanical symptoms 4
• Acute traumatic tears in younger patients (<40 years): Consider arthroscopic repair when feasible 2

ACL Rupture with Associated Pathology

MRI detects ACL tears with high accuracy and identifies associated meniscal tears, bone contusions, and anterolateral ligament injuries that may contribute to mechanical symptoms. 1

Loose Bodies

CT may be performed if radiographically occult fractures or loose bodies are suspected, showing 80% sensitivity and 98% specificity for bony avulsion fractures. 1

Hypermobile Lateral Meniscus

This rare cause presents with recurrent locking episodes despite "normal" MRI. 6, 7 Diagnosis requires:

• High clinical suspicion with history of recurrent locking in flexion
• MRI performed during acute locked episode may show anterior dislocation of posterior horn
• Arthroscopic confirmation and capsular repair

Common Pitfalls

Do not bypass radiographs and proceed directly to MRI, as approximately 20% of patients with chronic knee pain undergo MRI without recent radiographs, leading to unnecessary costs. 8 However, in acute locked knee with high clinical suspicion for internal derangement, expedited MRI is appropriate after initial radiographs.

Do not assume all "locking" requires surgery: MRI can prevent unnecessary arthroscopy in up to 48% of cases by identifying pathology amenable to conservative management. 1

Do not dismiss recurrent locking with "normal" MRI: Consider hypermobile lateral meniscus and refer to orthopedics for arthroscopic evaluation. 6

Urgent Referral Indications

Immediate orthopedic referral is warranted for: 5, 3

• True mechanical block to extension with inability to unlock
• Suspected bucket-handle meniscal tear
• Knee dislocation (requires vascular assessment)
• Locked knee with neurovascular compromise