What is the likely diagnosis and first‑line management for a patient with a one‑week history of dull aching lateral knee and popliteal pain that worsens with standing and walking, no swelling, and a normal knee X‑ray?

Likely Diagnosis and Management of Lateral Knee and Popliteal Pain

Most Likely Diagnosis

The most likely diagnosis is a lateral meniscus tear or posterolateral corner soft-tissue injury, and the first-line management is MRI without IV contrast to identify the specific internal derangement. 1

Clinical Reasoning

The presentation of dull aching pain in the popliteal and lateral knee region that worsens with weight-bearing activities (standing, walking) in the absence of swelling or bony abnormality on X-ray strongly suggests soft-tissue pathology rather than osseous injury. 1

Key Diagnostic Considerations

In patients with normal knee radiographs but persistent symptoms, internal derangement—including meniscal tears, ligamentous injuries, or bone marrow contusions—is the primary concern. 1

• The lateral location of pain specifically raises suspicion for lateral meniscus pathology, posterolateral corner injury (including popliteus tendon, fibular collateral ligament, or lateral capsule), or iliotibial band syndrome. 2, 3

• The popliteal region pain with weight-bearing could indicate popliteus tendinopathy, lateral meniscus tear with posterior horn involvement, or Baker's cyst, though the absence of swelling makes cyst less likely. 4, 3

Recommended Diagnostic Algorithm

Step 1: Immediate Next Imaging Study

Order MRI of the knee without IV contrast as the next imaging study after negative radiographs. 1

• MRI demonstrates 96% sensitivity and 97% specificity for detecting meniscal tears when correlated with arthroscopy. 1

• MRI accurately identifies bone marrow contusions (which may be present despite normal X-rays), meniscal tears, ligamentous injuries, and posterolateral corner pathology. 1

• Both 1.5T and 3T MRI protocols show similar high accuracy for meniscal and ligament evaluation; 3T does not significantly improve diagnostic accuracy over 1.5T for routine knee assessment. 1

Step 2: Specific MRI Findings to Evaluate

The radiologist should specifically assess for:

• Lateral meniscus tears (particularly posterior horn tears, which correlate with popliteal region pain). 1

• Posterolateral corner structures including popliteus tendon, fibular collateral ligament, popliteofibular ligament, and lateral capsule. 2

• Bone marrow edema patterns on the lateral tibial plateau or lateral femoral condyle, which predict associated soft-tissue injuries even without visible fracture. 1

• Iliotibial band abnormalities or lateral recess pathology. 3

Important Vascular Differential (Less Likely But Critical)

When to Consider Popliteal Artery Entrapment Syndrome (PAES)

If the patient is a young athlete with exertional symptoms that resolve at rest, consider PAES as a differential diagnosis. 5, 4, 6

• PAES presents with exercise-induced calf claudication, paresthesia, and swelling in young adults without atherosclerotic risk factors. 5, 4

• Initial vascular screening includes checking dorsalis pedis and posterior tibial pulses at rest and after provocative maneuvers (plantar flexion). 4, 6

• If PAES is suspected, duplex ultrasound with dynamic maneuvers is the initial vascular test, followed by MRA for confirmation. 5

When to Consider Popliteal Artery Aneurysm

In older patients or those with a palpable popliteal mass, perform duplex ultrasonography to exclude popliteal artery aneurysm. 1, 4

• Popliteal aneurysms ≥2.0 cm require surgical repair to prevent thromboembolic complications and limb loss. 1

• Absence of pedal pulses or asymmetric pulses between legs suggests vascular pathology requiring urgent evaluation. 4

First-Line Conservative Management (Pending MRI Results)

While awaiting MRI, initiate conservative management:

• Activity modification: reduce weight-bearing activities that provoke symptoms (standing, walking). 1

• NSAIDs for pain control if not contraindicated. 1

• Physical therapy focusing on quadriceps strengthening and range-of-motion exercises may begin if pain allows. 1

Common Pitfalls to Avoid

Do not assume the diagnosis is simply "knee strain" or delay MRI in patients with persistent symptoms despite normal radiographs. 1

• In patients under 40 years old with acute knee injury, knee effusion >10 mm on lateral radiograph should prompt immediate MRI consideration to decrease delayed diagnosis and improve outcomes. 1

• Do not order MRI with IV contrast or MR arthrography for initial evaluation of suspected internal derangement; non-contrast MRI is sufficient and appropriate. 1

• Do not overlook vascular causes in young athletic patients with exertional symptoms, as PAES is the most common surgically correctable vascular disorder in this population. 5, 4

Definitive Management Based on MRI Findings

If MRI confirms meniscal tear: Consider arthroscopic partial meniscectomy or repair depending on tear pattern, location, and patient age. 1

If MRI shows posterolateral corner injury: Surgical reconstruction may be necessary for grade III injuries to prevent chronic instability. 2

If MRI demonstrates bone marrow contusion without structural tear: Continue conservative management, as contusions predict higher risk of focal osteoarthritis at 1 year but typically resolve with activity modification. 1