Painful Prosthesis: Loosening vs Infection⁹⁹ᵐTc-MDP · ⁹⁹ᵐTc/¹¹¹In-WBC · ⁹⁹ᵐTc-SC

Quick Answer

The painful joint arthroplasty asks one central question: aseptic loosening vs periprosthetic infection? A ⁹⁹ᵐTc bone scan is sensitive but nonspecific — it stays positive for months to a year after implantation and cannot reliably separate loosening from infection by itself. The most specific nuclear test is combined labeled-WBC + ⁹⁹ᵐTc-sulfur-colloid marrow imaging: infection is called when WBC activity is spatially incongruent with marrow (WBC uptake without corresponding marrow uptake). Loosening typically shows focal uptake at the prosthesis tip/lesser trochanter (hip) on the bone scan without a WBC/marrow mismatch. FDG-PET is an alternative but is less specific because uptake also occurs with aseptic inflammation and normal early post-op remodeling.

Why the bone scan alone is not enough

After arthroplasty, normal periprosthetic remodeling keeps the bone scan positive for up to ~12 months (cementless components longer), so a hot scan is expected and cannot distinguish loosening from infection. What the bone scan does well is act as a sensitive gatekeeper: a truly normal scan makes either complication unlikely. Certain patterns hint at loosening — focal uptake at the femoral stem tip and lesser trochanter for a hip, or at the tibial component for a knee.

The definitive test: WBC + marrow imaging

The reference nuclear approach for suspected periprosthetic joint infection is dual labeled-WBC + ⁹⁹ᵐTc-sulfur-colloid marrow imaging. Leukocytes accumulate in both infection and normal/displaced marrow, so marrow imaging provides the map of where marrow actually is. The read:

This spatial-incongruence rule is what gives the combined study its high accuracy (reported specificity for PJI is high). ⁹⁹ᵐTc-HMPAO- or ¹¹¹In-labeled leukocytes are both used; the paired marrow scan is essential because periprosthetic marrow is frequently altered.

Where FDG fits

FDG-PET is convenient (single session, high resolution) and sensitive, but specificity is limited: aseptic loosening, particle disease, and normal early post-op change all take up FDG, and there is no universally agreed uptake-pattern threshold. It is best viewed as an adjunct or alternative when labeled-WBC imaging is impractical; periprosthetic (bone-implant interface) uptake is more concerning than uptake limited to synovium or the stem tip.

High-Yield Pearls

• The bone scan stays positive up to ~12 months post-op — it cannot separate loosening from infection alone.
• WBC + sulfur-colloid marrow imaging is the specific test: WBC without matching marrow = infection (incongruent).
• Congruent WBC/marrow uptake = displaced/normal marrow, not infection — this avoids a false-positive.
• Loosening on bone scan = focal uptake at the stem tip / lesser trochanter (hip) without WBC/marrow mismatch.
• FDG is sensitive but less specific; interface uptake matters more than stem-tip or synovial uptake.

Common Pitfalls

• Calling a hot bone scan "infection" in the first year after arthroplasty (expected remodeling).
• Reading labeled-WBC uptake as infection without the paired marrow scan — displaced marrow is a classic false-positive.
• Over-calling FDG stem-tip or synovial uptake as infection.

Related Pages

• Protocol: Bone scintigraphy, Infection imaging; disease: Prosthetic joint infection.

Differential Diagnosis

• Aseptic loosening — bone-scan focal uptake at the stem tip and lesser trochanter (hip) or the tibial component (knee), without WBC/marrow incongruence.
• Periprosthetic infection — labeled-WBC uptake spatially incongruent with the sulfur-colloid marrow map (WBC without matching marrow).
• Particle (wear-debris) disease — periprosthetic uptake with osteolysis/lucencies on radiographs; no WBC/marrow mismatch, but can mimic both loosening and infection.
• Heterotopic ossification — increased uptake in periarticular soft tissue; serial bone-scan activity gauges maturity before any resection.
• Normal postoperative remodeling (< ~12 months, longer for cementless components) — diffuse periprosthetic uptake that is expected, not pathologic.
• Periprosthetic fracture / stress reaction — focal uptake localizing to the fracture site.

Reporting Checklist

• State the time since arthroplasty and component type (cemented vs cementless), which define the expected remodeling window.
• Describe the bone-scan uptake pattern and location (stem tip, lesser trochanter, tibial component, or diffuse).
• For WBC + marrow imaging, explicitly report congruent vs incongruent — call infection only on spatial incongruence.
• Correlate with radiographs for osteolysis/particle disease and component position.
• Give a clear conclusion — loosening vs infection vs indeterminate — and recommend correlation (joint aspiration, CRP/ESR) as appropriate.

Self-Check

Key References

• SNMMI/EANM guidelines on labeled-leukocyte and combined WBC/marrow imaging for prosthetic joint infection.
• Reviews comparing bone scan, WBC/marrow, and FDG-PET for the painful arthroplasty.