Role of 99m TC HMPAO WBC scan in the localization of infection in patients with orthopedic implants

Objectives: Patients presenting with pain after implantation of orthopedic devices pose major diagnostic problem with regards to loosening vs. infection. Three phase bone scan is less specific technique in this regard.99mTcHMPAO labeled leukocyte accumulation has been observed not only in the sites of skeletal infection but also in the soft tissue infection sites as well. We undertook a study to assess the role of 99m HMPAO labeled leukocyte scan in this context. Method: Totally 27 patients suspected of infection were subjected to 99mTc HMPAO leukocyte scan. Whole body and spot images of the region of interest were acquired at 30 minutes and 3 hours after the injection 555 MBq 99mTc labeled leucocytes. The focal area of abnormal accumulation of leucocytes was considered as infection while the mild and diffuse uptake pattern was considered in favor of inflammation. The scan findings were compared with surgery, cytopathological (13), bacteriological (9) and clinical follow-up up to 1 year. Results: Of the 27 patients who underwent leukocyte scan 19 patients showed evidence of infection on the scan. There were totally 33 foci of infection of which 31 were found to be true positive while two were false positive for infection. In the true positive group 25 foci were confined to the implants region while 7 foci were found to be in the soft tissue compartment. One focus was involving both osseous and non-osseous region. One false negative patient was found to have chronic osteomyelitis on surgery. All these findings were well correlated with surgical, cytopathological and the follow-up data. The overall sensitivity of the technique for both osseous and non-osseous lesion was 96% and specificity 82% with a positive predictive value of 94%. Conclusion: We conclude that 99mTc HMPAO leukocyte scan is a sensitive and specific tool for the detection and localization of the infection in osseous and non- osseous sites in patients with orthopedic devices