38 C.F.R. § 4.66
The common cause of disability in this region is arthritis, to be identified in the usual manner. The lumbosacral and sacroiliac joints should be considered as one anatomical segment for rating purposes. X-ray changes from arthritis in this location are decrease or obliteration of the joint space, with the appearance of increased bone density of the sacrum and ilium and sharpening of the margins of the joint. Disability is manifest from erector spinae spasm (not accounted for by other pathology), tenderness on deep palpation and percussion over these joints, loss of normal quickness of motion and resiliency, and postural defects often accompanied by limitation of flexion and extension of the hip. Traumatism is a rare cause of disability in this connection, except when superimposed upon congenital defect or upon an existent arthritis; to permit assumption of pure traumatic origin, objective evidence of damage to the joint, and history of trauma sufficiently severe to injure this extremely strong and practically immovable joint is required. There should be careful consideration of lumbosacral sprain, and the various symptoms of pain and paralysis attributable to disease affecting the lumbar vertebrae and the intervertebral disc.
38 C.F.R. §4.66