A novel diagnostic parameter, foraminal stenotic ratio using 3D-MRI, as a discriminator for surgery in symptomatic lumbar foraminal stenosis

Abstract Background Context No previous studies have reported the radiological features of patients requiring surgery in symptomatic lumbar foraminal stenosis. Purpose To investigate the diagnostic accuracy of a novel technique, foraminal stenotic ratio (FSR), using three-dimensional magnetic resonance imaging (3D-MRI) for lumbar foraminal stenosis at L5-S by comparing patients requiring surgery, those with successful conservative treatment, and asymptomatic patients. Study Design Retrospective radiologic comparative study. Patient Sample We assessed the MRIs of 84 patients (168 L5-S foramina) aged ≥ 40 years without L4-5 lumbar spinal stenosis. Foramina were divided into three groups following standardized treatment: stenosis requiring surgery (20 foramina), stenosis with successful conservative treatment (26 foramina), and asymptomatic stenotic foramen (122 foramina). Outcome Measures The FSR was defined as the ratio of the length of the stenosis to the length of the foramen on the reconstructed oblique coronal image, referring to perineural fat obliterations in whole oblique sagittal images. We also evaluated foraminal nerve angle and minimum nerve diameter on reconstructed images, and Lee classification on conventional T1 images. Methods The differences in each MRI parameter between the groups were investigated. Receiver operating characteristic (ROC) curves were plotted after calculating the area under the ROC curve, to predict patients requiring surgery. Results FSR showed a stepwise increase when comparing asymptomatic, conservative, and surgical groups (mean, 8.6%, 38.5%, 54.9%, respectively). Only FSR was significantly different between the surgical and conservative group (p = 0.002), whereas all parameters were significantly different comparing the symptomatic and asymptomatic groups. The ROC curve showed that the area under the curve for FSR was 0.742, and the optimal cutoff value for FSR for predicting a surgical requirement in symptomatic patients was 50% (sensitivity, 75%; specificity, 80.7%). Conclusions FSR determined lumbar foraminal stenosis requiring surgery among symptomatic patients, with moderate accuracy. Foramina occupied ≥ 50% by fat obliteration were likely to fail conservative treatment, with a positive predictive value of 75%.