What are the differential diagnoses for a warm, swollen gluteal region in an older adult 3.5 months post total hip joint replacement via posterior approach?

Differential Diagnoses for Warm, Swollen Gluteal Region 3.5 Months Post-THA via Posterior Approach

The most critical differential to exclude is late periprosthetic joint infection, as this represents the highest morbidity and mortality risk and requires urgent intervention. At 3.5 months post-operatively, this falls into the "delayed infection" category (defined as ≥3 months post-op), which can present with subtle findings including localized warmth and swelling without systemic symptoms 1.

Primary Differential Diagnoses (in order of clinical priority):

1. Late Periprosthetic Joint Infection

• Most serious complication requiring immediate exclusion 1
• Delayed infections (≥3 months post-op) may present with pain alone or localized findings like warmth and swelling, often without fever 1
• Staphylococcus aureus and coagulase-negative Staphylococcus species are the most common organisms 1
• Key clinical features to assess:
  • Night pain or pain at rest (characteristic of infection vs. mechanical pain on weight-bearing) 1
  • Persistent pain >6 months post-operatively warrants infection exclusion 1
  • Risk factors: prior hip infection, superficial surgical site infection, operative time >2.5 hours, immunosuppression 1

Diagnostic approach for infection:

• Obtain ESR and CRP immediately; when both are negative, infection is unlikely 1
• CRP has 73-91% sensitivity and 81-86% specificity (cutoff 13.5 mg/L) 1
• CRP should normalize within 2 months post-operatively; persistent elevation is concerning 1
• Image-guided joint aspiration with synovial fluid analysis is the most useful confirmatory test 1
• Plain radiographs should be obtained first to assess for loosening/osteolysis 1

2. Gluteus Medius/Minimus Tendon Injury or Avulsion

• Highly relevant given posterior approach which can damage gluteal structures 2
• The posterior approach causes partial denervation of gluteus medius in 53.3% and gluteus maximus in 71.4% of cases 2
• Presents with lateral hip/gluteal pain, warmth from inflammation, and potential Trendelenburg gait 1
• Differentiation from bursitis can be difficult as these conditions frequently coexist 1, 3

Diagnostic approach:

• Assess for Trendelenburg sign on physical examination 1
• Ultrasound can identify tendinopathy, partial tears, and complete tears/avulsions of gluteus medius tendon 1
• MRI without contrast is superior for comprehensive assessment of peritrochanteric structures including gluteus minimus/medius muscles, abductor tendons, and trochanteric bursa 1
• MRI findings of tendon defects and fatty atrophy of gluteus medius/posterior gluteus minimus are uncommon in asymptomatic patients 1

3. Trochanteric Bursitis

• Can present with warmth and swelling over the greater trochanter/gluteal region 1
• Often coexists with gluteus medius tendinosis, making differentiation difficult 1, 3
• Ultrasound can detect trochanteric bursitis 1
• Joint distension and decompression of synovitis into the greater trochanteric bursa can occur with ARMD (adverse reaction to metal debris), which could be misinterpreted as isolated bursitis 1

4. Hematoma or Seroma

• Can present as warm, swollen area in the gluteal region
• More common in early post-operative period but can persist or develop later
• Ultrasound or MRI can differentiate fluid collections 1

5. Adverse Reaction to Metal Debris (ARMD) - if metal-on-metal prosthesis

• Presents with pseudotumors (solid or cystic), joint effusions, bursal collections, capsular thickening, and synovitis 1
• Ultrasound can detect ARMD pseudotumors and associated findings 1
• MARS-MRI (metal artifact reduction sequence) is superior for comprehensive evaluation 1

Critical Clinical Pitfalls to Avoid:

• Never assume infection is excluded based on absence of fever or normal peripheral leukocyte count - these are often normal in prosthetic infections 1
• Do not rely on single inflammatory markers - use combination of ESR and CRP; abnormal results in ≥2 of 3 tests (CRP, ESR, fibrinogen) provides 93% sensitivity and 100% specificity 1
• Recognize that gluteal muscle/tendon pathology and bursitis frequently coexist and may require MRI for definitive characterization 1, 3
• Posterior approach specifically increases risk of gluteal muscle denervation and injury compared to other approaches 2

Recommended Diagnostic Algorithm:

1. Immediate laboratory testing: ESR, CRP (and consider fibrinogen) 1
2. Plain radiographs of the hip to assess for loosening, osteolysis, or other bony abnormalities 1
3. If infection not excluded (elevated inflammatory markers, clinical suspicion): proceed to image-guided joint aspiration with synovial fluid analysis 1
4. If infection excluded and soft tissue pathology suspected: ultrasound for initial evaluation of gluteal tendons and bursae 1
5. If ultrasound equivocal or surgical planning needed: MRI without contrast for comprehensive soft tissue assessment 1