What is the diagnostic approach for testing knee tears?

Diagnostic Approach for Knee Tears

Initial Imaging: Plain Radiographs First

Start with anteroposterior and lateral knee radiographs as the initial test for any patient with acute knee trauma who meets Ottawa Knee Rule criteria, including focal tenderness, effusion, or inability to bear weight. 1, 2

When to Order Initial X-rays

Apply the Ottawa Knee Rule for patients ≥18 years—order radiographs if ANY of the following are present: 1, 2

• Age ≥55 years
• Isolated patellar tenderness
• Tenderness at fibular head
• Inability to flex knee to 90 degrees
• Inability to bear weight immediately after injury or take 4 steps in the emergency department

Override clinical decision rules and obtain radiographs regardless if the patient has: 1, 2

• Gross deformity
• Palpable mass
• Penetrating injury
• Prosthetic hardware
• Altered mental status (head injury, intoxication, dementia)
• Neuropathy (paraplegia, diabetes)
• Multiple injuries affecting reliability of examination

Optimal Radiographic Views

Obtain a minimum of two views: 1, 2

• Anteroposterior view
• Lateral view with knee at 25-30 degrees flexion (demonstrates patella in profile and allows evaluation for joint effusion)

Add supplemental views based on clinical suspicion: 1, 2

• Cross-table lateral with horizontal beam (visualizes lipohemarthrosis from intra-articular fractures)
• Patellofemoral view (for suspected patellar fracture, subluxation, or dislocation)
• Internal and external oblique views

Advanced Imaging When Radiographs Are Negative

MRI Without Contrast: The Gold Standard for Soft Tissue Injuries

If radiographs are negative but clinical suspicion remains high for meniscal or ligamentous injury, MRI without contrast is the appropriate next study. 2, 3

Order MRI when any of these are present after negative X-rays: 2

• Significant joint effusion
• Inability to fully bear weight after 5-7 days
• Mechanical symptoms (locking, catching) suggesting meniscal injury
• Joint instability on examination suggesting ligamentous injury

Physical examination findings that warrant MRI: 3, 4

• McMurray test positive (knee rotation with extension): 61% sensitive, 84% specific for meniscal tears
• Joint line tenderness: 83% sensitive, 83% specific for meniscal tears
• Anterior knee pain during squat: 91% sensitive for patellofemoral pain
• Ligament instability on stress testing

CT for Occult Fractures

Use CT as the next study if you suspect a radiographically occult fracture rather than soft tissue injury. 1, 2

• CT shows 100% sensitivity versus 83% for radiographs in detecting tibial plateau fractures 1
• CT demonstrates 80% sensitivity and 98% specificity for bony avulsion fractures 1
• Superior for fracture characterization and surgical planning 1

Critical Clinical Pitfalls to Avoid

The Asymptomatic Meniscal Tear Problem

Horizontal or oblique meniscal tears on MRI are frequently found in asymptomatic knees and may not be the source of symptoms. 5

• In patients with symptomatic meniscal tears, 63% had tears in the contralateral asymptomatic knee 5
• Radial, vertical, complex, or displaced tears are clinically meaningful—these are found almost exclusively on the symptomatic side 5
• Collateral ligament abnormalities, pericapsular soft tissue changes, and bone marrow edema are more specific for symptomatic pathology 5

Timing of Physical Examination

Re-examine patients 3-5 days after injury if initial examination is limited by swelling and pain. 6

• Excessive swelling can mask complete tendon ruptures in the first 48 hours 6
• Partial tears may be difficult to distinguish from complete ruptures acutely 6

When Conservative Management Trumps Imaging

For degenerative meniscal tears in patients ≥40 years with gradual onset symptoms, exercise therapy is first-line treatment—surgery is not indicated even with mechanical symptoms. 3

• Only severe traumatic tears (bucket-handle with displaced tissue) require surgical referral 3
• MRI may be deferred if clinical presentation clearly suggests degenerative pathology in older patients 3

Modalities NOT Recommended for Initial Evaluation

Do not use these as initial imaging studies for acute knee trauma: 1

• MRI (not initial—use after negative radiographs with persistent suspicion)
• MR arthrography
• CT (unless specifically for occult fracture characterization)
• Ultrasound
• Bone scan with SPECT/CT