What is the recommended assessment and workup for a knee injury?

Knee Injury Assessment and Workup

Plain radiographs (anteroposterior and lateral views) should be obtained as the first-line imaging study for any acute knee injury when Ottawa knee rule criteria are met, including focal tenderness, effusion, or inability to bear weight for 4 steps. 1, 2

Initial Clinical Assessment

History Taking

• Document the mechanism of injury (twisting, direct blow, hyperextension, motor vehicle accident) 3, 4
• Identify immediate symptoms: audible pop, immediate swelling (suggesting hemarthrosis), or delayed swelling (suggesting effusion) 4, 5
• Assess ability to bear weight immediately after injury and currently 1, 2
• Document mechanical symptoms such as locking, catching, or giving way 2, 6

Physical Examination Priorities

• Always examine the uninjured knee first for comparison 4
• Inspect for gross deformity, which mandates immediate radiographs regardless of clinical decision rules 3, 2
• Palpate for focal tenderness at the patella, fibular head, joint line, and femoral condyles 1, 2
• Assess for effusion by ballottement or bulge sign 1, 2
• Test range of motion, specifically ability to flex knee to 90 degrees 2
• Perform ligament stability testing: valgus/varus stress for collateral ligaments, Lachman test and pivot shift for anterior cruciate ligament, posterior drawer for posterior cruciate ligament 4, 6
• Evaluate for patellar stability and apprehension 7, 6
• Perform meniscal testing: McMurray's test, Apley's grind test, and bounce test 4, 6
• Palpate for tendon gaps suggesting rupture 7

Imaging Algorithm

When to Order Radiographs

Obtain radiographs immediately if ANY of the following are present: 3, 2

• Age >55 years 2
• Focal tenderness at patella or fibular head 2
• Inability to bear weight for 4 steps immediately after injury 2
• Inability to flex knee to 90 degrees 2
• Gross deformity 3, 2
• Palpable mass 3
• Penetrating injury 3
• Prosthetic hardware 3
• Altered mental status (head injury, intoxication, dementia) 3
• Neuropathy (paraplegia, diabetes) 3
• History suggesting increased fracture risk 3

Standard Radiographic Views

• Minimum two views: anteroposterior and lateral (with knee at 25-30 degrees flexion) 2
• Consider additional views based on clinical suspicion: patellofemoral view, cross-table lateral, internal/external oblique views 2

Advanced Imaging Indications

MRI without IV contrast is the next appropriate study after negative radiographs when: 3, 2

• Significant joint effusion persists 2
• Inability to fully bear weight after 5-7 days 2
• Mechanical symptoms suggesting meniscal injury (locking, catching) 2
• Joint instability suggesting ligamentous injury 2
• Suspected occult fracture in adults or skeletally mature children 3
• Suspected internal derangement (meniscal or ligamentous injury) 3, 2

CT is indicated for: 3, 8

• Better characterization of fractures identified on radiographs 3, 8
• Suspected occult fracture when MRI is contraindicated 2

CT angiography or MR angiography is indicated for: 3

• Knee dislocation with suspected vascular injury (approximately 30% risk with posterior dislocation) 3
• Abnormal vascular examination (diminished pulses, expanding hematoma) 3

Diagnostic Arthrocentesis

Perform arthrocentesis when effusion is present to: 1

• Rule out septic arthritis (orthopedic emergency requiring urgent intervention) 1
• Exclude crystal arthropathy precipitated by trauma 1
• Identify lipohemarthrosis indicating occult intra-articular fracture despite negative radiographs 1

Synovial Fluid Analysis Priorities

• Cell count with differential to assess for infection 1
• Gram stain and culture if infection suspected 1
• Crystal analysis to exclude gout or pseudogout 1
• Gross appearance assessment for lipohemarthrosis (fat globules floating on blood) 1

Common Pitfalls to Avoid

• Do not skip radiographs based solely on ability to bear weight—negative radiographs with weight-bearing ability rule out most fractures requiring surgery, but significant soft tissue injuries may still be present 2
• Do not rely on clinical examination alone in the first 48 hours—excessive swelling and pain can mask the diagnosis of tendon ruptures and ligamentous injuries 7
• Consider re-examination after 3-5 days if initial assessment is limited by pain and swelling, particularly for suspected partial tendon tears 7
• Physician judgment should supersede clinical decision rules when clinical suspicion is high 3, 2
• Do not routinely order CT, ultrasound, or CT angiography for initial evaluation of knee trauma with normal vital signs and no vascular compromise 1

Specific Injury Patterns

Significant Trauma (Motor Vehicle Accident, Knee Dislocation)

• Obtain radiographs first 3
• Assess for vascular injury clinically 3
• Order CTA if vascular injury suspected (less invasive than conventional angiography with similar accuracy) 3
• Consider MRI for comprehensive evaluation of soft tissue, osseous, and neural injuries 3

Fall or Twisting Injury with Focal Tenderness/Effusion

• Obtain radiographs first 3, 1
• If radiographs negative but clinical suspicion high, proceed to MRI without contrast 3, 2
• Consider arthrocentesis if significant effusion present 1

Suspected Tendon Rupture

• Obtain anteroposterior and lateral radiographs 7
• Perform at least two specific physical examination tests for the suspected tendon 7
• Re-examine after 3-5 days if initial assessment limited by swelling 7