Table 1: Castellvi radiographic classification system of sacralization [7].

Type IaA unilateral TP height greater than or equal to 19 mm
Type IbBoth processes heights greater than or equal to 19 mm
Type IIaPresence of unilateral articulation between the TP and the sacrum
Type IIbPresence of bilateral articulation between the TP and the sacrum
Type IIIaUnilateral fusion of the TP and the sacrum
Type IIIbBilateral fusion of the TP and the sacrum
Type IVUnilateral type II transition (articulation) with a type III (fusion) on the contralateral side

TP: lowest lumbar transverse process.