MR imaging of fibrocartilaginous masses arising on the margins of spondylolysis defects

OBJECTIVE. Spondylolysis is reported in up to 7% of the population. An unco mmon pro cess that can accompany a pars interarticularis defect is a fibroc artilaginous mass, which can cause impression on the nerve roots and thecal sac. Recognition is important so that the presence may he addressed at the time of surgery. This report describes the MR imaging appearance and clini cal significance of a fibrocartilaginous mass in association with spondylol ysis. MATERIALS AND METHODS. We reviewed data regarding 336 patients who had lumb ar spine imaging at our institution during a 12-month period. Contiguous ax ial and sagittal MR imagine using T1-wejghted and fast spin-echo T2-weighte d sequences was used. Images were evaluated for a mass of tissue surroundin g the pars defect with MR characteristics of cartilaginous and fibrous low signal intensity on T1-weighted images and low to intermediate signal inten sity on T2-weighted images. The position of the fibrocartilaginous mass and its relation to the thecal sac were noted. Surgical correlation between th ose patients with a fibro cartilaginous mass and those without was examined . RESULTS. Twenty-nine (8.6%) of 336 patients were identified as having a par s interarticularis defect. A fibrocartilaginous mass was present in 26 (90% ) of 29 patients. Six (21%) of the 29 patients had a mass effect on the the cal sac. Of the remaining 23 patients, 20 had a fibrocartilaginous mass ext ernal to the thecal sac, and three did not show any fibrocartilaginous mass . Histologic analysis in one patient confirmed the fibrous and cartilaginou s nature of the lesion at the pars defect. CONCLUSION. A fibrocartilaginous mass was present in 90% of the patients wi th spondylolysis. Mass effect on the thecal sac that required surgery was i dentified in 21% of the patients with spondylolysis. Awareness of this mass and proper imaging protocols will enable the radiologist to preoperatively alert the surgeon to its presence.