Types of sacral agenesis. A, Type I, total or partial unilateral sacral agenesis. B, Type II, partial sacral agenesis with partial, bilateral symmetric defects in stable articulation between ilia and normal or hypoplastic S1 vertebra. C, Type III, variable lumbar and total sacral agenesis; ilia articulate with lowest vertebra. D, Type IV, variable lumbar and total sacral agenesis; caudal endplate of lowest vertebra rests above fused ilia or iliac amphiarthrosis.