To determine how long after injury a single photon emission computed tomography (SPECT) scan may remain positive in cases of symptomatic posterior element lumbar stress injuries. SPECT scans can identify posterior element lumbar stress injuries earlier than other imaging modalities. As these lesions evolve and the spondylolysis becomes chronic, the SPECT scan tends to revert to normal even though healing of the defect has not occurred. The aim of this study was to determine how long after initial injury a SPECT scan might remain positive. One hundred and sixty-five patients (85 male, 80 female) between the ages of 8 and 38 years with suspicion of lumbar spondylolysis or posterior element lumbar stress injuries were investigated. All patients underwent plain radiographs, planar bone scintigraphy and SPECT imaging. The duration of symptoms at clinical assessment was recorded. The age, sex, symptom reproduction on flexion or extension, level of sporting activity, and the Oswestry Disability Index both pre- and post-treatment were also recorded. SPECT positive images (hot scans) were depicted as cases and SPECT negative images as controls. Univariate and multivariate analysis was performed. Eighty-five patients (63 male, 22 female) had positive SPECT scans (cases); eighty had negative scans (controls). The mean age at onset of symptoms was 20.2 years for cases and 17.4 years for controls. Bilateral increased uptake on SPECT scan was more common than unilateral. The commonest site for increased uptake was the posterior elements of the fifth lumbar vertebra. Low back pain in extension was more common in SPECT positive cases. The mean time from injury / onset of symptoms to a positive SPECT scan was 7.1 months (range 5.2–9.2 months) and to a negative SPECT scan was 22.5 months (range 16.8–28.4 months). Intense scintigraphic activity in the posterior elements of the lumbar spine was associated with a more recent injury and was concordant with the patient’s history and physical examination. Chronic, un-united spondylolysis was often scintigraphically occult. There was a window of approximately six months from the onset of symptoms to investigation after which the sensitivity of SPECT imaging diminished.
