Presentation
Veteran with a long-standing history of shrapnel wound to the knee with recurrent cellulitis and a draining sinus tract.
Patient Data
Frontal and lateral views of the left knee show lucency in the tibial metaphysis with surrounding sclerosis.
Three-phase bone scan demonstrates:
increase flow to the region of the proximal left tibia
increase radiotracer uptake at the same site on the blood pool image
delayed phase whole-body images also show intense radiotracer uptake in the proximal left tibia. Additional less intense uptake is present in the shoulders, wrists, right knee, ankles and feet due to osteoarthritis
Dual isotope imaging of the left knee with Tc99m sulfur colloid and In-111 WBCs.
intense accumulation of WBCs in the proximal left tibia corresponding to the clinical area of concern and abnormality on prior bone scan
low-grade sulfur colloid accumulation in a similar location and surrounding marrow
uptake of the two tracers is incongruent and consistent with osteomyelitis
Case Discussion
The sensitivity of a bone scan for diagnosing osteomyelitis is quite high, so a negative bone scan effectively rules out osteomyelitis. It is also relatively specific in the setting of otherwise normal bone, but the specificity drops markedly in the setting of trauma (as in this case) or hardware since all of these will induce osteoblastic activity and increased radiotracer uptake.
Dual isotope imaging with In-111 WBCs and Tc99m sulfur colloid is helpful in this scenario given its high specificity for infection. Both sulfur colloid and WBCs accumulate in the bone marrow, but only WBCs accumulate in sites of infection. With osteomyelitis, WBCs uptake is increased but sulfur colloid uptake is suppressed (as in this case), a situation which is termed incongruent. The incongruence can be either from intensity (SBC uptake >> sulfur colloid) or spacial (WBC uptake location different than sulfur colloid).
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