Therefore, these studies are not useful as routine for differentiating mechanical failure from occult infection in painful loose total knee prostheses. Filippi and Schillaci [105] applied SPECT/CT using a hybrid camera to conventional planar Tc-99m-HMPAO- labeled leukocyte scintigraphy in patients with suspected infection. SPECT/CT was able to differentiate soft-tissue involvement from bone involvement. The authors argued that SPECT/CT might eliminate the necessity for a correlative bone scan with labeled leukocyte scans. WBC scans also have a decreased sensitivity with low-grade infection [66] and a limited neutrophilic component. Labeled leukocyte imaging may lead to a high false-positive rate because leukocytes accumulate in reactive bone marrow as well as in infection and it is not always possible to differentiate between the two [64,106]. The addition of Tc-99m-labeled sulfur colloid bone marrow scanning has been investigated to reduce this confusion. Palestro et al [107] reported that sequential combined leukocyte/marrow imaging was 95% accurate for diagnosing prosthetic knee infection and was superior to bone scans alone or to bone scans in combination with labeled leukocyte imaging. Joseph et al [106] found that low sensitivity and the potential for false-negative results made this combination of scans of limited utility for diagnosing prosthetic infection, and therefore it is no longer used at their institution. In that group of 22 total knee prostheses evaluated and later operated upon, there was a sensitivity of 66%, a specificity of 100%, a PPV of 100%, an NPV of 88%, and an accuracy of 91%. Blanc et al [108] did a retrospective review of 168 patients. They determined Tc-99m-HMPAO labeled leucocyte scintigraphy was more sensitive for knee (84%) than hip prosthesis (57%) but was less specific for knee (52% versus 75%).
Imaging After Total Knee Arthroplasty. Therefore, these studies are not useful as routine for differentiating mechanical failure from occult infection in painful loose total knee prostheses. Filippi and Schillaci [105] applied SPECT/CT using a hybrid camera to conventional planar Tc-99m-HMPAO- labeled leukocyte scintigraphy in patients with suspected infection. SPECT/CT was able to differentiate soft-tissue involvement from bone involvement. The authors argued that SPECT/CT might eliminate the necessity for a correlative bone scan with labeled leukocyte scans. WBC scans also have a decreased sensitivity with low-grade infection [66] and a limited neutrophilic component. Labeled leukocyte imaging may lead to a high false-positive rate because leukocytes accumulate in reactive bone marrow as well as in infection and it is not always possible to differentiate between the two [64,106]. The addition of Tc-99m-labeled sulfur colloid bone marrow scanning has been investigated to reduce this confusion. Palestro et al [107] reported that sequential combined leukocyte/marrow imaging was 95% accurate for diagnosing prosthetic knee infection and was superior to bone scans alone or to bone scans in combination with labeled leukocyte imaging. Joseph et al [106] found that low sensitivity and the potential for false-negative results made this combination of scans of limited utility for diagnosing prosthetic infection, and therefore it is no longer used at their institution. In that group of 22 total knee prostheses evaluated and later operated upon, there was a sensitivity of 66%, a specificity of 100%, a PPV of 100%, an NPV of 88%, and an accuracy of 91%. Blanc et al [108] did a retrospective review of 168 patients. They determined Tc-99m-HMPAO labeled leucocyte scintigraphy was more sensitive for knee (84%) than hip prosthesis (57%) but was less specific for knee (52% versus 75%).