What are the potential causes and treatment options for a patient with a history of joint disease or trauma, who recently underwent hip replacement surgery, and is now experiencing pain above the incision site and radiating into the buttock region?

Post-Hip Replacement Pain Above Incision Radiating to Buttock

The most likely cause is nerve injury (ilioinguinal, iliohypogastric, or lateral femoral cutaneous nerve entrapment), and you should immediately perform a diagnostic nerve block at the site where these nerves exit the internal oblique muscle to confirm the diagnosis before considering surgical neurectomy. 1

Differential Diagnosis Algorithm

Primary Considerations (in order of likelihood):

1. Peripheral Nerve Entrapment (Most Common)

• The diagnostic triad includes: burning or lancinating pain near the incision radiating to the nerve distribution, impaired sensory perception in the nerve territory, and pain relief with local anesthetic infiltration at the nerve exit point 1
• Ilioinguinal and iliohypogastric nerves are most commonly affected after lower abdominal/hip procedures 1
• Lateral femoral cutaneous nerve can cause pain radiating to the buttock region 2
• This occurs in approximately 3.3% of major noncardiac surgeries and results in significant functional impairment 3

2. Lumbosacral Junction Pathology

• If the patient has an elongated L5 transverse process articulating with the sacral ala (Bertolotti's syndrome), this can cause upper buttock pain that mimics surgical complications 4
• Pain is typically unilateral and localized to the upper buttock region 4

3. Inadequate Multimodal Analgesia

• Before assuming pathology, verify the patient is receiving scheduled (not PRN) paracetamol 1g every 6 hours plus NSAIDs or COX-2 inhibitors 2, 5
• Opioid-only regimens are inadequate and should never be used as monotherapy 5

4. Infection/Inflammatory Process

• Less likely if pain is above the incision, but must be excluded 5
• Check for fever, erythema, wound drainage, elevated inflammatory markers 2

Immediate Diagnostic Steps

Physical Examination Findings to Assess:

• Map the exact pain distribution and compare to dermatome patterns 1
• Test sensory perception in the ilioinguinal (upper medial thigh), iliohypogastric (suprapubic), and lateral femoral cutaneous (lateral thigh) distributions 1
• Palpate along the incision for neuromas or trigger points 1
• Assess hip internal rotation—if this reproduces the pain, consider intra-articular pathology instead 2, 6

Diagnostic Nerve Block (Gold Standard):

• Infiltrate local anesthetic at the point where the ilioinguinal and iliohypogastric nerves exit the internal oblique muscle (typically 2-3 cm medial and superior to the anterior superior iliac spine) 1
• Complete pain relief during the anesthetic duration confirms nerve entrapment 1
• This is both diagnostic and temporarily therapeutic 1

Imaging Protocol

First-Line:

• Plain radiographs (AP pelvis and lateral hip) to exclude hardware complications, heterotopic ossification, or lumbosacral transitional anatomy 2, 6

Second-Line (if nerve block is negative or imaging concerns exist):

• MRI with metal artifact reduction sequences (MARS-MRI) to assess soft tissues, pseudocapsule, and neurovascular structures around the prosthesis 5
• This excludes infection, hematoma, or prosthetic complications 5

Treatment Algorithm

If Nerve Block Provides Complete Relief:

1. Initial Conservative Management (4-6 weeks):

   • Scheduled paracetamol 1g every 6 hours 2, 5
   • NSAIDs or COX-2 inhibitors (unless contraindicated) 2, 5
   • Avoid opioids except for rescue analgesia 5
   • Consider gabapentinoids for neuropathic component 2

2. If Conservative Management Fails:

   • Radiofrequency ablation of the affected nerve(s) can provide 12-16 months of relief 4
   • Surgical neurectomy with scar revision is definitive treatment, with 70% achieving complete symptom resolution 1

If Nerve Block Provides No Relief:

• Reassess for lumbosacral pathology (consider fluoroscopically guided injection into L5-S1 pseudarthrosis if Bertolotti's syndrome suspected) 4
• Evaluate for prosthetic complications with MARS-MRI 5
• Consider referred pain from intra-articular hip pathology 2

Critical Risk Factors for Persistent Pain

Patient-Specific Factors Associated with Worse Outcomes:

• Female sex, Asian ethnicity, history of chronic pain, coronary artery disease, tobacco use 3
• These patients require more aggressive multimodal analgesia and closer follow-up 3

Perioperative Factors That Increase Risk:

• Withholding NSAIDs or COX-2 inhibitors on the day of surgery significantly increases persistent pain risk 3
• Inadequate multimodal analgesia in the immediate postoperative period 3

Common Pitfalls to Avoid

Do not assume treatment failure without proper medication optimization - scheduled paracetamol plus NSAIDs should be the foundation, not opioids 5

Do not delay diagnostic nerve block - if the clinical triad is present (burning pain radiating from incision, sensory changes, localized tenderness), perform the block immediately rather than waiting weeks for "conservative management" to fail 1

Do not order MRI before plain radiographs - radiographs must be obtained first to identify hardware complications and guide further workup 6

Do not confuse nerve entrapment with surgical site infection - nerve pain is typically burning/lancinating with sensory changes, while infection presents with erythema, warmth, and systemic signs 2, 1

Do not perform neurectomy without confirming diagnosis with nerve block - surgical intervention should only proceed after demonstrating complete pain relief with local anesthetic 1