What should be included on the MRI requisition for a 20‑year‑old female undergoing left knee MRI who also experiences numbness from the hip to the toe when sitting cross‑legged (criss‑cross/applesauce)?

MRI Order for 20-Year-Old Female with Left Knee Pain and Hip-to-Toe Numbness

Order BOTH left knee MRI without contrast AND consider lumbar spine MRI, as the numbness from hip to toe with positional provocation (sitting cross-legged) strongly suggests lumbosacral nerve root or plexus pathology rather than isolated knee pathology.

Critical Clinical Context

The numbness distribution from hip to toe is a red flag that this is not simply a knee problem. This dermatomal pattern suggests:

• Lumbosacral radiculopathy (L4-S1 nerve roots)
• Sciatic nerve involvement
• Lumbosacral plexus pathology

The positional nature (provoked by sitting cross-legged) further supports nerve compression or entrapment rather than intra-articular knee pathology.

What to Include on the MRI Requisition

For the Left Knee MRI:

Imaging Protocol:

• MRI left knee without IV contrast 1
• Standard three orthogonal planes (sagittal, coronal, axial) 2
• T2-weighted fluid-sensitive sequences and T1-weighted images 2

Clinical Information to Document:

• Age: 20-year-old female
• Chief complaint: Left knee pain
• Key associated symptom: Numbness from hip to toe when sitting cross-legged
• Duration: [specify if chronic vs acute]
• Prior imaging: Document if recent knee radiographs were obtained (within past year) 1, 3

Specific Areas of Interest to Note:

• Evaluate for meniscal pathology
• Assess for ligamentous injury
• Rule out popliteal cyst (Baker's cyst) that could compress neurovascular structures 1
• Evaluate for soft tissue masses or ganglion cysts that could cause nerve compression 1

Critical Additional Consideration:

You should ALSO order or strongly consider:

• Lumbar spine MRI or MR neurography of the lumbosacral plexus/sciatic nerve

The ACR Appropriateness Criteria explicitly state that "referred pain from the lower back must be considered, especially if the knee radiographs are unremarkable and there is clinical evidence or concern for lumbar spine pathology" 1. Your patient's hip-to-toe numbness IS clinical evidence of lumbar spine or peripheral nerve pathology 4, 5.

Algorithmic Approach to This Case

Step 1: Obtain Plain Radiographs First

• Left knee radiographs (AP, lateral, tangential patellar view) should be obtained BEFORE MRI if not already done 1
• This is critical: 82% of inappropriate knee MRI requests occur because MRI is used as the initial imaging test 3
• Only 47% of patients have prior radiographs when MRI is ordered 3

Step 2: Determine Primary Pathology Location

Given the clinical presentation:

• If knee radiographs are normal AND numbness persists → the primary problem is likely NOT the knee
• The dermatomal distribution suggests lumbosacral origin 4, 5

Step 3: Order Appropriate Advanced Imaging

Primary recommendation:

• Lumbar spine MRI to evaluate for:
  • Disc herniation (L4-S1 levels)
  • Nerve root compression
  • Spinal stenosis

Secondary/concurrent:

• Left knee MRI without contrast to rule out:
  • Popliteal cyst with mass effect
  • Soft tissue mass causing nerve compression
  • Intra-articular pathology contributing to symptoms

Consider MR neurography if initial studies are unrevealing 6, 5:

• Evaluates sciatic nerve, femoral nerve, obturator nerve
• Can identify piriformis syndrome 4
• Detects peripheral nerve entrapment 4, 5

Common Pitfalls to Avoid

1. Ordering knee MRI alone without considering referred pain 1

   • The numbness pattern is NOT explained by knee pathology alone
   • Hip and lumbar spine pathology commonly refer pain to the knee

2. Skipping plain radiographs 3

   • 45% of knee MRI requests are inappropriate, primarily because radiographs weren't done first
   • Radiographs significantly reduce inappropriate MRI requests (OR = 0.05) 3

3. Not documenting the neurologic symptoms clearly

   • The radiologist needs to know about the numbness to look for nerve-related pathology
   • Without this information, they'll focus only on intra-articular structures

4. Assuming the knee is the problem because the patient complains of knee pain

   • Young patients (20 years old) rarely have significant intra-articular knee pathology without trauma
   • Dermatomal numbness is a neurologic finding requiring neurologic imaging

Specific Requisition Language

Write on the order: "20-year-old female with left knee pain and numbness from hip to toe provoked by sitting cross-legged. Please evaluate for intra-articular pathology, popliteal cyst, and any soft tissue masses that could cause nerve compression. Patient also has concurrent lumbar spine imaging ordered to evaluate for radiculopathy."

This documentation ensures:

• The radiologist understands the full clinical picture
• Appropriate sequences are used
• The report addresses nerve-related findings
• The study is deemed appropriate by insurance criteria 3