Diagnosis of spondylolysis on MRI: Importance of recognition of hypoplastic L5 on MRI

ObjectiveTo perform a combined (retrospective and prospective) study to further characterize hypoplastic L5, its correlation with spondylolysis and other associated abnormalities on routine Computed tomography and magnetic resonance imaging.MethodsWe studied the Computed tomography and magnetic resonance imaging images of 29 patients with hypoplasia and posterior wedging of L5 with bilateral spondylolysis at L5. These cases were followed up retrospectively and prospectively. The anteroposterior diameter of L4, L5 and S1 was calculated and compared. The percentage of posterior wedging of L5 was calculated. Anterolisthesis, hypoplastic pedicle, facet joints, L4–5 and L5–S1 intervertebral discs, nerve roots and fragmentation of pars interarticularis were also studied.ResultsThe mean difference of anteroposterior diameter between L4 and L5 was 2.75 mm and of L5 and S1 was 3.78 mm. The mean percentage of posterior wedging was 31%. Grade I anterolisthesis was present in 13 patients and grade II in 2 patients. Facet joint arthropathy was seen in 20 patients. In 24 patients, there was hypoplastic pedicle. Pars fragmentation was seen in 7 patients. L5–S1 disc disease was seen in 21 patients out of whom 8 had exiting nerve root compression. L4–L5 disc disease was seen in 10 patients of whom 5 had exiting nerve root compression.ConclusionsHypoplastic L5 is a strong predictor of bilateral spondylolysis even in the absence of true anterior slippage. True anterior slippage and disproportionate adjacent disc disease result in varying degrees of exiting nerve root compression.Advances in knowledgeL5 hypoplasia can simulate anterolisthesis and can predict the bilateral spondylolysis. L5 hypoplasia can lead to early disc disease.