A normal spine, when viewed from behind appears straight. A spine affected by kyphosis, however, shows evidence of a forward curvature of the back bones (vertebrae) in the upper back area, giving a hunchback and curved spine appearance.
The Scoliosis Research Society defines kyphosis as a curvature of the spine measuring 45 degrees or greater on an X-ray. The normal spine typically has 20 to 45 degrees of forward curvature in the upper back area. Kyphosis, also known as Scheuerman's disease, is a type of spinal deformity and should not be confused with poor posture.