Imaging Presentations of Spinal Instability

Low back pain is a common condition with a lifetime prevalence of up to 84% in the general population. In many cases it remains difficult to determine the exact cause of back pain, but among various other conditions, lumbar instability is important. Spinal instability is defined as an abnormal response to applied loads, characterized by movement in the motion segment beyond normal constraints. The etiologies of spinal instability can be classified as congenital (dysplastic), spondylolytic (isthmic), degenerative, traumatic, pathologic and iatrogenic (i.e., postoperative). The diagnosis of intervertebral instability is based on the direct and indirect radiological findings of an abnormal vertebral motion. A variety of imaging modalities are currently used to assess spinal instability with each having its merits and demerits. Plain radiographs are the baseline investigation but have uncertain diagnostic value as they show only indirect evidences of spinal instability. Functional radiography is promising; however, it has its limitations of inaccurate reproducibility and non-standardized techniques. Computed tomography offers a higher sensitivity than plain radiographs due to its superior contrast resolution and better delineation of bony structures. Functional CT illustrates abnormal motion between the facet joints and objectively establishes spinal instability. MR imaging is highly sensitive in detection of soft tissue abnormalities that contribute to spinal instability. Analysis of the various imaging modalities reveal that no single modality is complete by itself and needs to be complemented by other.