[Geometrical and mechanical analysis of lumbar lordosis in an asymptomatic population: proposed classification]

The main objective of this study was to describe the morphology and the mechanism underlying the organization of lumbar lordosis in terms of position and shape of the pelvis. A classification of lumbar lordosis was proposed based on the orientation of the sacral plane.

One hundred sixty asymptomatic young adult volunteers were x-rayed in a standardized standing position. A dedicated software was used for analysis of the spine and pelvis. The pelvic parameters were: pelvic incidence, sacral slope, pelvic tilt. The point separating thoracic kyphosis and lumbar lordosis was called the inflexion point. Lumbar lordosis was bounded by the sacral plate and the inflexion point. At the apex, the lumbar curve was divided into two tangent arcs of circle, quantified by an angle and the number of included vertebrae. The lower arc was geometrically equal to the sacral slope. Regarding the vertical line, a lordosis tilt angle was drawn between the inflexion point and the frontal limit of the sacral plate.

The value of the lumbar lordosis was very variable. The best correlation was between lumbar lordosis and sacral slope, then between sacral slope and pelvic incidence. The upper arc of a circle remained constant while the lower arc changed with sacral slope. Good correlations were found between the sacral slope and the position of the apex and between sacral slope and lordosis tilt angle.

Regarding sacral slope, lumbar lordosis can be classified into four types. When the sacral slope is low, lumbar lordosis can either be both short and curved with a low apex and a backward tilt (type 1) or both long and flat with a higher position of the apex (type 2). When the sacral slope increases, lumbar lordosis increases in angle and number of vertebrae with an upper apex, with a progressively forward tilt (types 3 and 4). Depending on the shape and position of the pelvis, and because of the relation between sacral slope and pelvic tilt, the morphology of lumbar lordosis could be the main mechanical cause of degenerative diseases of the lumbar spine.