Which bone scan is used to rule out hardware loosening after a hemiarthroplasty?

Bone Scan for Hardware Loosening After Hemiarthroplasty

For suspected hardware loosening after hemiarthroplasty, a Tc-99m 3-phase bone scan with SPECT or SPECT/CT is the recommended nuclear medicine study, though it has high sensitivity but low specificity and cannot reliably differentiate loosening from infection. 1

Understanding the Imaging Approach

Why Bone Scans Are Used

Tc-99m bone scintigraphy is highly sensitive for detecting hardware loosening when radiographs are normal or equivocal. However, the critical limitation is poor specificity—increased uptake can represent:

• Normal postoperative bone remodeling (up to 1-2 years post-surgery)
• Aseptic loosening
• Periprosthetic infection
• Periprosthetic fracture 1

Single-Phase vs. 3-Phase Bone Scan

Single-phase bone scan (Tc-99m MDP):

• Sensitive for loosening detection
• Low specificity
• Cannot differentiate between loosening, infection, or normal remodeling 1

3-phase bone scan with SPECT or SPECT/CT (preferred):

• Provides better anatomic localization of active bone remodeling 1
• Still nonspecific but offers more information
• Positive uptake on delayed images (but not blood-pool phase) suggests loosening rather than infection 2, 3
• SPECT/CT improves diagnostic accuracy by combining metabolic and anatomic data 4

Critical Timing Considerations

The specificity of bone scans increases significantly in older prostheses (>2 years post-surgery) once normal postoperative remodeling has stabilized. 1

• Normal postoperative uptake persists up to 1-2 years after surgery
• Aseptic loosening tends to show progressive uptake over time on serial scans
• Normal postoperative changes decrease over time 1

When Bone Scans Are Most Useful

High negative predictive value: A normal bone scan effectively excludes loosening or infection 2

Caveat: False negatives can occur if loosening is at the cement-prosthetic interface without new bone formation 2

Alternative and Complementary Imaging

CT with Metal Artifact Reduction

CT is actually superior to bone scans for evaluating hardware loosening when infection has been excluded:

• Better demonstrates periprosthetic lucency, osteolysis, hardware malposition, and component migration 1
• Evaluates bone stock and component alignment
• Modern metal reduction software has dramatically improved image quality 1
• Does not require IV or intra-articular contrast 1

If Infection Cannot Be Excluded

When bone scan is positive and infection remains a concern, proceed to:

• In-111 WBC scan combined with Tc-99m sulfur colloid bone marrow imaging to differentiate infection from aseptic loosening 1
• Spatially congruent WBC and marrow activity = aseptic loosening (reactive marrow)
• WBC accumulation with marrow photopenia = infection 1

Practical Algorithm

1. Start with radiographs (always first-line)
2. If radiographs normal/equivocal and loosening suspected:
   • If prosthesis >2 years old: 3-phase bone scan with SPECT/CT has better specificity
   • If prosthesis <2 years old: Consider CT with metal artifact reduction first (avoids false positives from normal remodeling)
3. If bone scan positive: Cannot differentiate loosening from infection—proceed to WBC/sulfur colloid imaging if infection not excluded clinically
4. If infection excluded: CT with metal artifact reduction provides superior anatomic detail for surgical planning 1

Common Pitfalls

• Don't order bone scans in early postoperative period (<1 year) without understanding that positive results may reflect normal healing
• Don't rely on bone scans alone to differentiate loosening from infection—they cannot do this reliably 1, 3
• Don't forget serial imaging: Progressive uptake over time favors loosening; decreasing uptake favors normal postoperative change 1
• Recent evidence suggests SPECT/CT without arthrography is sufficient—adding arthrography provides minimal benefit and adds infection risk 5