Optimum Vertebral Level of Castellvi Type III or Higher Lumbosacral Transitional Vertebrae When Measuring Spinopelvic Parameters

Abstract

Background: Castellvi type III and IV lumbosacral transitional vertebrae (LSTVs) are fused to the sacrum. In these cases, the pelvic incidence (PI) and pelvic tilt (PT) may vary according to the selected “S1.” This study aimed to determine the optimum vertebral level of these LSTVs when measuring PI and PT. Methods: PI and PT were measured twice in 56 patients with type III and IV LSTVs with a balanced spine, with LSTV considered as the lowest lumbar vertebra (LLV) or S1. PI and PT measured with LSTV as LLV were denoted as LLV_PI and LLV_PT, and those measured as S1 were denoted as S_PI and S_PT. Reference ranges (mean −2 SD to +2 SD) of PI and PT were derived from 183 non-LSTV patients with a balanced spine as 35.5° to 68.8° (PI) and 2.5° to 29.6° (PT). If S_PI, S_PT, or both were below the reference range, the LSTV was interpreted as LLV. If LLV_PI, LLV_PT, or both were above the reference range, it was interpreted as S1. If all parameters were within the respective reference range, it was interpreted as an intermediate type. Results: The optimum vertebral level of LSTV was S1 (n = 29, 51.8%), most frequently due to high LLV_PT (35.4°±4.7), followed by LLV (n = 14, 25%) due to low S_PI (31.5°±5.2) and intermediate type (n = 13, 23.2%). Conclusions: If PI is too small or PT is too large to represent the actual sagittal alignment in patients with Castellvi type III and IV LSTVs, the selected S1 should be reconsidered. Clinical Relevance: PI and PT measurements can be used to determine whether the optimum vertebral level of Castellvi type III and IV LSTV should be considered LLV or S1.