INTRODUCTION: The diagnosis of pseudoarthrosis is based on imaging and clinical exam findings. The standard for pseudarthrosis diagnosis remains post-operative observation through computer tomography (CT) and patient's symptoms. This can be further augmented by dynamic x-ray imaging or nuclear positron emission tomography (PET) CT to demonstrate an absence of fusion by showing a persistence of mobility. However, there is not a uniform diagnostic approach that is a standard of care amongst spine practioners. The aim of this study is to describe the timeline and diagnostic analysis for pseudoarthrosis between the initial surgery and follow-up procedure. METHODS: This is a single-center retrospective observational study. The aim was to enroll patients reoperated for pseudarthrosis after 1 or 2 level lumbar fusions, between August 1, 2008 and August 1, 2018.The exams were reviewed by one surgeon and one radiologist, defining a status either in favor of pseudarthrosis, or against it, or inconclusive, based on the radiological criteria mentioned below.We then investigated different combinations of exams and their specific chronology before a diagnosis was established. RESULTS: 44 patients were included, 70.5% male and with a mean age of 47.3 years. The median time between the 2 surgeries was 23.7 months. Plain X-rays supported the diagnosis in 38.7% of cases, dynamic X-rays showed hypermobility in 50% of cases. The CT scan demonstrated pseudarthrosis in 94,4% of cases. A MODIC 1 signal was observed in 87,2% of cases on MRI. SPECT-CT showed a tracer uptake in 70% of cases. CONCLUSION: Reducing the time to reintervention is a key objective for improving the management and clinical outcomes of these patients. We suggest that MRI is an additional tool in combination with CT in the assessment of suspected mechanical pseudarthrosis, in order to optimize the diagnosis and shorten the time to revision surgery. LEVEL OF EVIDENCE: IV.< Réduire