What is the appropriate evaluation for knee instability in an adult patient who reports giving way, pain, swelling, or a sensation of looseness during activity?

Evaluation of Knee Instability in Adults

Begin with the Lachman test performed at 20-30 degrees of knee flexion—it is the most sensitive (74%) and specific (95%) clinical examination for ACL injury, and if positive, proceed directly to MRI without contrast for surgical planning. 1, 2

Initial Physical Examination Sequence

Critical Red Flags Requiring Immediate Evaluation

• Palpable gap in tendon 2
• Gross deformity 2
• Complete inability to bear weight 2
• Fever with joint effusion (requires urgent aspiration to rule out septic arthritis) 2

Specific Instability Tests to Perform

Anterior Cruciate Ligament Assessment:

• Lachman test (primary test): Stabilize the thigh, flex knee to 20-30 degrees, assess anterior tibial translation 1, 2
• Pivot shift test (secondary confirmation) 3
• Anterior drawer test 3

Posterior Cruciate Ligament Assessment:

• Posterior drawer test 3
• Tibial sag test 3

Collateral Ligament Assessment:

• Valgus stress testing (medial collateral ligament) 3
• Varus stress testing (lateral collateral ligament and posterolateral corner—19.7% of ACL ruptures have posterolateral corner involvement) 1, 3

Meniscal Injury Assessment:

• Bounce test for mechanical block to extension 2
• McMurray's test 3
• Apley's grind test 3
• Joint line tenderness on palpation 2

Examination Timing Considerations

Excessive swelling and pain can limit examination accuracy for up to 48 hours after acute injury—re-examine after 3-5 days if initial assessment is limited or partial tears are suspected. 2 Always examine the uninjured knee first for comparison. 3

Imaging Algorithm

Initial Radiographic Evaluation

Obtain weight-bearing radiographs first if any Ottawa Knee Rule criteria are present: 2, 4

• Age ≥55 years
• Isolated patellar tenderness
• Fibular head tenderness
• Inability to flex knee to 90 degrees
• Inability to bear weight for 4 steps immediately after injury or in examination room

Standard radiographic views for instability assessment: 5, 4

• Standing anteroposterior (AP) view
• Lateral view (knee at 25-30 degrees flexion)
• Tangential axial view of patellofemoral joint

Specialized stress radiographs for instability quantification: 5

• Extension-flexion position views
• Varus-valgus stress views
• Anterior and posterior drawer maneuver views

Advanced Imaging When Indicated

MRI Without Contrast (Usually Appropriate): Order MRI when clinical examination suggests ligamentous or meniscal injury with negative or non-diagnostic radiographs—MRI demonstrates 97% diagnostic accuracy for ACL tears. 1, 4 Indications include: 4

• Positive Lachman or other instability tests
• Significant joint effusion
• Inability to fully bear weight after 5-7 days
• Mechanical symptoms suggesting meniscal injury
• Joint instability on examination

MRI allows direct visualization of ligaments and tendons when effective metal suppression techniques are used. 5 Evaluate for concomitant meniscal tears and bone bruises (commonly on posterolateral tibial plateau and anterior lateral femoral condyle). 1

CT Imaging: CT is appropriate for assessing rotational positioning of prosthesis components in post-arthroplasty patients or for characterizing occult fractures, but has low sensitivity for soft-tissue injuries. 5, 4 IV contrast is not required for CT assessment of instability. 5

Common Pitfalls to Avoid

• Do not skip radiographs and proceed directly to MRI—plain films must precede MRI to exclude fracture and comply with evidence-based practice 4
• Do not order MRI with IV contrast for acute instability—it adds unnecessary cost and risk without improving diagnostic yield 4
• Do not use ultrasound as the primary assessment tool—while it can visualize collateral ligaments in post-arthroplasty patients, it is not typically used for instability assessment 5
• Do not rely on bone scan or nuclear medicine studies—these are not used for instability evaluation 5
• Avoid examining only the symptomatic knee—always compare with the contralateral uninjured knee 3

Special Considerations for Post-Arthroplasty Patients

Instability represents the third most common cause of total knee arthroplasty failure (7.5% of cases). 5 Assessment requires: 5

• Weight-bearing and stress view radiographs
• CT for rotational component positioning assessment
• Full-length standing radiographs to evaluate malalignment
• Fluoroscopy for dynamic assessment under stress 5