First metatarsophalangeal joint prosthesis loosening without infection
Presentation
Two year history of moderate pain in the right first metatarsophalangeal joint on the background of a joint replacement at the site 10 years prior.
Patient Data
Technetium scintigraphy (bone scan) was performed with IV injection of 800 MBq of Tc-99m MDP was performed followed by gallium scintigraphy with IV injection of 100 MBq of Ga-67 Citrate.
The technetium bone scan images demonstrate increased osteoblastic activity at the site of the bone surrounding the first metatarsophalangeal joint prosthesis. To differentiate possible infection from loosening, direct comparison is made to the gallium scintigraphy images. The site of faint increased gallium tracer uptake spatially corresponds well with the bone scan images, however the intensity (signal compared to background noise) is much fainter for gallium compared to technetium. This indicates that the increased osteoblastic activity on the technetium bone scan is consistent with prosthetic loosening, and underlying infection is unlikely.
Case Discussion
Combined technetium (bone scan) and gallium scintigraphy can be used to help differentiate between osteomyelitis and increased bone turnover from another pathology (e.g. from prosthetic loosening) 1,2. Since Tc-99m is preferentially taken up by osteoblasts and Ga-67 Citrate by inflammatory cells, interpretation of the comparative studies can be summarized as:
- If the gallium images demonstrate no increased uptake, the study is negative for infection (regardless of the bone scan images)
- If the spatial distribution of increased uptake is discordant between technetium and gallium, the study is positive for infection
- If the intensity (signal to background noise ratio) of gallium uptake exceeds that of technetium in a spatially concordant area of uptake, the study is positive for infection
- If the intensity and distribution of gallium and technetium are concordant, study is equivocal for infection
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