Unilateral Hip Pain Radiating to Ankle: Diagnosis and Management
This presentation strongly suggests femoroacetabular impingement (FAI) syndrome with probable labral tear, and you should immediately obtain AP pelvis and lateral hip radiographs, followed by MRI if radiographs show FAI morphology or are equivocal. 1
Initial Diagnostic Approach
First-Line Imaging
- Obtain AP pelvis and lateral hip radiographs (Dunn, frog-leg, or cross-table views) as the mandatory initial test to evaluate for FAI morphology (cam or pincer lesions), increased alpha angle and center-edge angle, acetabular dysplasia, and joint space narrowing 1, 2
- Radiographs may reveal osteochondromas, degenerative arthritis, developmental hip dysplasia, or heterotopic ossification that can confound clinical evaluation 3
Key Clinical Examination Findings
- Test for FADIR (flexion-adduction-internal rotation) - a positive test is the key clinical finding in FAI and strongly supports this diagnosis 1
- Assess for "catching" or mechanical blockage during hip movement, which is pathognomonic for FAI with labral tear 1
- Evaluate for snapping hip (coxa saltans), which can be assessed with ultrasound in real-time during movement 1
Advanced Imaging When Indicated
MRI Protocol
- If radiographs show FAI morphology and FADIR is positive, obtain MRI or arthro-MRI before orthopedic referral 1
- MRI without IV contrast or direct arthro-MRI with diluted gadolinium (1:200) is highly sensitive and specific for detecting labral tears, cartilage damage, ligamentum teres pathology, and periarticular soft tissues 1, 2
- MRI is the imaging modality that globally evaluates all anatomic structures including ligaments, tendons, cartilage, and bone 2
Diagnostic Confirmation
Image-Guided Injection
- Perform image-guided intra-articular injection of anesthetic ± corticosteroid to confirm the hip as the pain source 1, 2
- Complete pain relief after injection definitively confirms intra-articular origin 1, 2
- This diagnostic procedure is safe and useful for confirming labral tear etiology 2
Addressing the Radiating Pain Pattern
Evaluate for Referred Pain Sources
- Pain radiating from hip to ankle requires evaluation of the lumbar spine and sciatic nerve pathway 4
- Assess for lumbar spinal pathology, deep gluteal syndrome with sciatic nerve entrapment, or ischiofemoral impingement as these cause posterior hip pain with radiation 4
- Evaluate the thoracolumbar junction for pain and mobility restriction, as it can refer pain to the hip/groin area 1
When to Image the Spine
- If lumbar pain is non-specific without radiculopathy, stenosis, or red flags, no initial spinal imaging is required 1
- However, persistent radiating pain down the leg warrants consideration of lumbar pathology as a concurrent or alternative diagnosis 4
Critical Management Pathway
Immediate Next Steps
- Obtain plain radiographs of the hip (AP pelvis and lateral views) 1, 2
- Perform FADIR test during physical examination 1
- If radiographs show FAI morphology or are equivocal, proceed to MRI 1, 2
- Consider diagnostic intra-articular injection to confirm hip as pain source 1, 2
Surgical Referral Considerations
- Early orthopedic referral is warranted because FAI, labral tears, and associated pathology typically have good surgical outcomes 4
- Advanced imaging should be completed before surgical consultation to optimize treatment planning 1
Common Pitfalls to Avoid
- Do not assume all hip-to-ankle pain is radicular from the spine - intra-articular hip pathology with FAI can cause referred pain down the leg 1, 4
- Do not skip radiographs and proceed directly to MRI - radiographs are essential for identifying FAI morphology and guiding subsequent imaging 1, 2
- Do not overlook the diagnostic value of intra-articular injection - this is the definitive test to confirm the hip as the pain generator 1, 2
- Do not delay referral if FAI with labral tear is confirmed - these conditions have favorable surgical outcomes when treated appropriately 4