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

Abstract

OBJECTIVE. Spondylolysis is reported in up to 7% of the population. An uncommon process that can accompany a pars interarticularis defect is a fibrocartilaginous mass, which can cause impression on the nerve roots and thecal sac. Recognition is important so that the presence may be addressed at the time of surgery. This report describes the MR imaging appearance and clinical significance of a fibrocartilaginous mass in association with spondylolysis. MATERIALS AND METHODS. We reviewed data regarding 336 patients who had lumbar spine imaging at our institution during a 12-month period. Contiguous axial and sagittal MR imaging using T1-weighted and fast spin-echo T2-weighted sequences was used. Images were evaluated for a mass of tissue surrounding the pars defect with MR characteristics of cartilaginous and fibrous low signal intensity on T1-weighted images and low to intermediate signal intensity on T2-weighted images. The position of the fibrocartilaginous mass and its relation to the thecal sac were noted. Surgical correlation between those patients with a fibrocartilaginous mass and those without was examined. RESULTS. Twenty-nine (8.6%) of 336 patients were identified as having a pars 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 thecal sac. Of the remaining 23 patients, 20 had a fibrocartilaginous mass external to the thecal sac, and three did not show any fibrocartilaginous mass. Histologic analysis in one patient confirmed the fibrous and cartilaginous nature of the lesion at the pars defect. CONCLUSION. A fibrocartilaginous mass was present in 90% of the patients with spondylolysis. Mass effect on the thecal sac that required surgery was identified 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.