What is the appropriate workup and differential diagnosis for a patient presenting with unilateral hip pain?

Workup of Unilateral Hip Pain

Start with plain radiographs (AP pelvis and lateral femoral head-neck views) as the first imaging test in nearly all cases of unilateral hip pain, combined with a targeted history and physical examination to localize pain and guide subsequent testing. 1

Initial Clinical Assessment

Pain Localization

Determine the anatomic location of pain, as this narrows the differential significantly:

• Anterior hip/groin pain: Suggests intra-articular pathology including osteoarthritis, labral tears, femoroacetabular impingement (FAI), or hip flexor injuries 2, 3
• Lateral hip pain: Most commonly greater trochanteric pain syndrome (gluteus medius tendinopathy, trochanteric bursitis, iliotibial band friction) 3
• Posterior hip pain: Consider lumbar radiculopathy, sacroiliac joint dysfunction, piriformis syndrome, or hamstring tendinopathy 2, 3

Key History Elements

• Onset: Sudden (trauma, fracture) vs. insidious (degenerative, inflammatory) 4
• Aggravating factors: Pain with specific motions, weight-bearing, or positions 4
• Night pain: Suggests tumor or advanced osteonecrosis 1
• Mechanical symptoms: Locking or snapping suggests labral pathology or snapping hip syndrome 1

Physical Examination

• FADIR test (Flexion-Adduction-Internal Rotation): A negative test helps rule out hip-related pain in young and middle-aged adults, though sensitivity is limited 1
• Range of motion assessment: Restricted internal rotation suggests intra-articular pathology 2
• Palpation: Assess for trochanteric tenderness (lateral pain syndrome) or anterior tenderness (hip flexor pathology) 3

Critical caveat: Most clinical special tests have limited diagnostic accuracy and should not be used in isolation to confirm diagnoses 1

Initial Imaging: Plain Radiographs

Obtain AP pelvis and lateral femoral head-neck views (Dunn, frog-leg, or cross-table) as the first imaging study. 1

Rationale

• Excellent screening tool for common disorders (osteoarthritis, fractures, tumors, dysplasia, FAI morphology) 1
• Provides essential baseline for comparison with advanced imaging 1
• For osteoarthritis specifically, physical examination plus radiographs may be superior to MRI 1

What to Look For

• Osteoarthritis: Joint space narrowing, osteophytes, subchondral sclerosis 1
• FAI morphology: Cam deformity (increased alpha angle), pincer deformity 1
• Dysplasia: Decreased center-edge angle, shallow acetabulum 1
• Fractures: Acute traumatic or stress fractures 2
• Tumors: Bone lesions, periosteal reaction 1

Advanced Imaging: When Radiographs Are Negative or Equivocal

MRI Without Contrast (First-Line Advanced Imaging)

MRI should be the first advanced imaging technique after negative or non-diagnostic radiographs. 1

Indications

• Suspected soft tissue pathology (tendinopathy, bursitis, muscle injuries) 1
• Occult fractures or stress fractures 2
• Osteonecrosis of the femoral head 2
• Inflammatory conditions 1
• Tumors 1

What MRI Detects

• Extra-articular soft tissue: Iliopsoas bursitis, trochanteric bursitis, abductor tendinosis/tears, hamstring injuries, athletic pubalgia 1
• Intra-articular pathology: Labral tears, cartilage defects, joint effusions, synovitis 1
• Referred pain sources: Lumbar spine or knee pathology 1

Important limitation: Imaging alone has limited diagnostic utility with only small to moderate shifts in post-test probability; diagnosis should never be made on imaging alone 1

MRI/MRA or CT Arthrography (For Intra-Articular Detail)

Use when three-dimensional morphological assessment is needed or to evaluate specific intra-articular structures (labrum, cartilage, ligamentum teres). 1

• MR arthrography: Diagnostic test of choice for labral tears 2
• CT arthrography: Alternative when MRI is contraindicated 1

Caution: Incidental intra-articular findings are common in asymptomatic individuals and must be interpreted with clinical context 1

Ultrasound (Targeted Use)

• Superficial structures: Abductor tendons, trochanteric bursitis, calcific tendonitis 1
• Dynamic assessment: Snapping hip syndrome (iliopsoas tendon) 1
• Procedure guidance: Aspiration of fluid collections, diagnostic/therapeutic injections 1

Nuclear Medicine (Limited Role)

Bone scan (Tc-99m or 18F-fluoride PET) may be considered for occult fractures or tumors when other imaging is non-diagnostic, though this is rarely needed in current practice. 1

Diagnostic Injections

Image-guided diagnostic injections with anesthetic (±corticosteroid) can help localize the pain source after a suspected diagnosis is established. 1

• Intra-articular injection: Pain relief suggests intra-articular pathology 1
• Trochanteric injection: Confirms greater trochanteric pain syndrome 1
• Iliopsoas injection: Confirms iliopsoas pathology 1

Critical point: Injections should be image-guided and performed only after initial imaging, not as a first-line diagnostic tool 1

Differential Diagnosis by Category

Intra-Articular Causes

• Osteoarthritis 1
• Labral tears 1
• FAI syndrome (cam or pincer morphology) 1
• Acetabular dysplasia/hip instability 1
• Chondral defects 1
• Ligamentum teres tears 1
• Osteonecrosis 2
• Inflammatory arthropathy 1
• Septic arthritis (requires urgent aspiration) 5

Extra-Articular Causes

• Greater trochanteric pain syndrome (gluteus medius tendinopathy, trochanteric bursitis) 1, 3
• Iliopsoas tendinopathy/bursitis 1
• Hip flexor strains 3
• Hamstring tendinopathy 1, 3
• Athletic pubalgia 1
• Snapping hip syndrome 1

Referred Pain

• Lumbar radiculopathy 1, 2
• Sacroiliac joint dysfunction 2
• Knee pathology 1

Red Flags Requiring Urgent Evaluation

• Suspected septic arthritis: Joint aspiration mandatory 5
• Hemarthrosis: Check coagulation studies (PT, PTT, von Willebrand panel) before invasive procedures 5
• Inflammatory arthropathy with systemic symptoms: ESR, CRP, ANA, anti-dsDNA, rheumatoid factor 5

Algorithmic Approach Summary

1. History and physical examination → Localize pain (anterior/lateral/posterior) and identify red flags 4, 2
2. Plain radiographs (AP pelvis + lateral hip) → Screen for common pathology 1
3. If radiographs diagnostic → Treat accordingly (e.g., osteoarthritis management)
4. If radiographs negative/equivocal → MRI without contrast for soft tissue and occult osseous pathology 1
5. If intra-articular pathology suspected → Consider MRA or CT arthrography for labral/cartilage detail 1, 2
6. If diagnosis remains uncertain → Image-guided diagnostic injection to localize pain source 1
7. Consider ultrasound → For superficial soft tissue pathology or dynamic assessment 1

Never diagnose based on imaging alone—always integrate symptoms, clinical signs, and imaging findings. 1