The effect of patient positioning during radiographs on the resulting Cobb angle measurements

Abstract

BackgroundStanding spinal radiographs have been the primarymethod of spinal deformity evaluation in patients withscoliosis. During periods of patient surveillance, the clin-ician compares radiographs over a period of time toassess the progression of the deformity [1-3]. One of thepotential problems in comparing one radiograph toanother is difference in positioning [4-6]. The goal ofthis study is to quantify the effect of trunk rotation onCobb angle measurements, and provide an algorithm todescribe this relationship.Material and methodsCT scans of three patients with Adolescent Idiopathic Sco-liosis were used retrospectively. Three-dimensional recon-structions of the images were created by CT scan software.Cobb angles were drawn for scoliosis curves in the anteriorplane. The 3-D image was then rotated two degrees to theright, and Cobb angle measurements were repeated. Thisprocedure was repeated through 14 degrees of right rota-tion, and then subsequently through 14 degrees of leftrotation.ResultsThe effect of trunk rotation on Cobb angle measure-ments is directly related to the location of the scoliosiscurves, the magnitude of those curves, and the magnitudeof lumbar lordosis and thoracic kyphosis. In general how-ever, a two degree rotation of the patient’s trunk whilepositioning results in a one degree change in themeasured Cobb angle (in patients with larger scoliosiscurves, and in the first six degrees of trunk rotation).ConclusionsPatient positioning can have a significant effect on thecalculation of scoliosis measurements, and this needs tobe considered when evaluating the progression of spinaldeformity.