In a 20-year-old woman with three weeks of dull intermittent posterior right knee pain without trauma, what physical examination maneuvers and imaging studies are indicated to establish a diagnosis?

Evaluation of Posterior Knee Pain in a 20-Year-Old Woman

Start with plain radiographs of the knee (AP, lateral, and tangential patellar views), followed by MRI without contrast if radiographs are normal or show only an effusion, as this is the standard algorithmic approach for chronic knee pain evaluation 1.

Physical Examination Approach

For posterior knee pain specifically, your physical exam should focus on:

Palpation Points

• Popliteal fossa: Check for popliteal (Baker's) cyst, which presents as a palpable mass behind the knee
• Posterior joint line: Assess for posterior meniscal pathology
• Hamstring tendons: Evaluate for tendinopathy at their insertions
• Gastrocnemius origin: Rule out muscle strain

Specialized Maneuvers

• Posterior drawer test: With knee flexed to 90°, push tibia posteriorly to assess PCL integrity (though less likely without trauma) 2
• Posterior sag test: Observe for posterior tibial translation with hip and knee flexed to 90°
• McMurray test: Assess for meniscal tears, particularly posterior horn involvement
• Range of motion testing: Both active and passive, noting any mechanical blocks or pain with flexion

These maneuvers are most accurate when performed in combination rather than relying on a single test 3.

Imaging Algorithm

Step 1: Plain Radiographs (Initial Study)

Obtain knee radiographs with at least one frontal projection (AP, Rosenberg, or tunnel view), tangential patellar view, and lateral view 1. This is the appropriate initial imaging regardless of the absence of trauma, as approximately 20% of patients with chronic knee pain inappropriately receive MRI without recent radiographs 1.

Step 2: MRI Without Contrast (If Radiographs Normal or Show Effusion)

If pain persists and radiographs are normal or demonstrate only a joint effusion, proceed directly to MRI without IV contrast 1. This is the "usually appropriate" next step per ACR guidelines.

MRI is particularly valuable for posterior knee pain as it:

• Accurately depicts popliteal cysts and their potential rupture 1
• Detects meniscal tears (especially posterior horn tears)
• Identifies subchondral bone marrow lesions
• Visualizes PCL integrity 2
• Assesses for less common causes like ganglion cysts or masses 1

Ultrasound Consideration

Ultrasound may be appropriate specifically to confirm a suspected popliteal cyst and is as accurate as MRI for this diagnosis 1. It can also guide aspiration if needed. However, it's not useful as a comprehensive screening examination 1.

Common Pitfalls to Avoid

1. Don't skip radiographs: Even without trauma, plain films are the required first step and may reveal unexpected findings like OCD, loose bodies, or early degenerative changes 1

2. Consider referred pain: In a 20-year-old with unremarkable knee findings, consider hip pathology (though less common at this age) or lumbar spine issues that can refer to the posterior knee 1

3. Don't order MRI with contrast initially: Contrast is not indicated for initial evaluation of chronic knee pain without specific suspicion for synovitis, infection, or tumor 1

4. Recognize age-appropriate pathology: In a 20-year-old, consider popliteal cyst, meniscal tear (particularly discoid meniscus), hamstring tendinopathy, or early patellofemoral issues rather than degenerative arthritis

Clinical Decision Point

If there's a palpable mass in the popliteal fossa on exam, you could reasonably proceed directly to ultrasound for confirmation and potential aspiration guidance, but radiographs should still be obtained first to exclude bony pathology 1.