When can a bone scan be ordered after a total knee arthroplasty (TKA)?

From the Guidelines

A bone scan can be ordered after a total knee arthroplasty (TKA) when there is suspicion of implant loosening, infection, or periprosthetic fracture that cannot be adequately evaluated with standard radiographs, typically after at least 1-2 years have passed since surgery. This recommendation is based on the most recent evidence from the Journal of the American College of Radiology, which suggests that bone scintigraphy may be helpful in diagnosing loosening, especially when obtained many years after surgery 1. The delay in maximum utility is due to the observation that positive bone scans are noted in 20% of asymptomatic knees 1 year after surgery and in 12.5% of individuals 2 years after surgery 1.

Key Considerations

• Bone scans are most useful when patients present with persistent pain without clear etiology on plain radiographs.
• Three-phase bone scans using technetium-99m are most commonly used, with increased uptake in all three phases suggesting infection, while increased uptake only in the delayed phase may indicate aseptic loosening.
• For improved specificity, bone scans are often combined with other imaging modalities like indium-111 labeled leukocyte scans or SPECT/CT.
• The rationale for waiting is that normal bone remodeling after surgery can take several months to stabilize, and earlier scans may not reliably distinguish between normal healing and pathological processes.

Imaging Modalities

• Tc-99m 3-phase bone scan knee is rated as a 6, indicating that it may be appropriate for diagnosing aseptic loosening, with SPECT/CT being optional and dependent on the findings of the 3-phase bone scan 1.
• Other imaging modalities, such as CT knee without IV contrast, fluoroscopy knee, and MRI knee without IV contrast, have lower ratings and are not recommended as the first line of imaging for diagnosing aseptic loosening after TKA.

Clinical Implications

• The high negative predictive value (NPV) of bone scans makes them useful as a screening test, with a high NPV indicating that loosening or infection is unlikely if the scan is normal 1.
• However, a potential false negative may occur if there is loosening at the cement-prosthetic interface that does not incite new bone formation 1.

From the Research

Timing of Bone Scan after Total Knee Arthroplasty (TKA)

• A bone scan can be ordered 1 year postoperatively after TKA, but may have positive findings due to normal physiologic bone remodeling 2.
• Nuclear studies may be falsely positive for up to 2 years after TKA 2.
• Bone scans immediately postoperative and at three months demonstrated increased uptake, which gradually decreased to baseline levels at ten to 12 months 3.

Diagnostic Accuracy of Bone Scans

• The sensitivity and specificity of bone scans for detection of loosening and patellofemoral problems after TKA can be high, especially when using SPECT/CT 4.
• A negative bone scintigram is reassuring and makes loosening or infection unlikely 5.
• The pattern of isotope uptake in abnormal studies may not be specific enough to reliably differentiate aseptic from septic loosening 5.

Factors Affecting Bone Scan Results

• The prosthesis type (TKA versus total hip arthroplasty) and the use of cement can affect radiotracer uptake patterns in scintigraphy 2.
• Cemented and cementless TKA have comparable radionuclide activity, but individual patient variability is high 3.
• Early onset of pain after TKA can be correlated with tibiofemoral stress pattern in SPECT/CT 4.