Can the sagittal lumbar curvature be closely approximated by an ellipse?

For the sagittal lumbar curvature, existing spinal models are based only on the anthropomorphic radiographic characteristics of one individual, or, at best, of only a few individuals. This raises questions of applicability of the modeling results to clinical situations. Because spinal coupling and l oads on spinal tissues have been shown to be functions of the initial stati c posture, a rigorously derived neutral lumbar lordosis would be important for clinicians and spine researchers. This study presents modeling of the s agittal lumbar spine in the shape of an ellipse. Vertebral body and disc he ights, derived from digitized lateral lumbar radiographs of 50 normal subje cts, were used to create an ellipse along the posterior body margins from t he inferior of T12 to the superior sacral base. Additional data to create a n elliptical lumbar model were determined from a least-squares analysis of passing ellipses through the digitized posterior body points. This confirme d that an elliptical model closely fit the lumbar curvature with a least-sq uares error of 1.2 mm per digitized point. The elliptical model is approxim ately an 85 degrees portion of a quadrant. The semi-major and semi-minor ax es, a and b, are parallel to the posterior body margin of T12 and parallel to the inferior body endplate of T12, respectively, with a semi-minor to se mi-major radio of b/a = 0.39. The elliptic model has a height-to-length rat io of H/L = 0.963, where height is the vertical distance from inferior T12 to superior S1 and length is the are length along George's line (along the posterior longitudinal ligament) from T12 to S1.