Figure 1.

Renshaw’s classification of sacral agenesis. (A, B) Type I: either partial or total unilateral sacral agenesis; (C, D) Type II: partial sacral agenesis with a partial but bilaterally symmetrical defect and a stable articulation between the ilia and a normal or hypoplastic first sacral vertebra; (E, F) Type III: variable lumbar and total sacral agenesis with the ilia articulating with the sides of the lowest vertebra present; (G, H) Type IV: variable lumbar and total sacral agenesis, the caudal end-plate of the lowest vertebra resting above either fused ilia or an iliac amphiarthrosis.