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

Type Ia A unilateral TP height greater than or equal to 19 mm Type Ib Both processes heights greater than or equal to 19 mm Type IIa Presence of unilateral articulation between the TP and the sacrum Type IIb Presence of bilateral articulation between the TP and the sacrum Type IIIa Unilateral fusion of the TP and the sacrum Type IIIb Bilateral fusion of the TP and the sacrum Type IV Unilateral type II transition (articulation) with a type III (fusion) on the contralateral side

TP: lowest lumbar transverse process.