Normative values for the L5 incidence in a subgroup of transitional anomalies extracted from 147 asymptomatic subjects

Pelvic incidence angle is not always measurable due to lumbosacral transitional vertebrae (LSV). The fifth lumbar vertebra (L5) is rarely abnormal. The purpose of this study was to quantify from full-body standing X-rays, the L5 incidence angle (L5I) in a normal asymptomatic population and to correlate it with standard spino-pelvic parameters taking the sacrum (S1) as a reference. One hundred and forty seven asymptomatic volunteers were enrolled. The ethics committee approved the study protocol. Subjects underwent a low-dose full spine X-ray. 3D reconstructions were obtained and L5I was measured using the upper L5 endplate as the reference instead of the S1 endplate. A group of subjects with LSV was identified and subdivided in two subgroups. Standard spino-pelvic parameters and normative values for the L5 parameters were obtained. Statistical correlations were calculated between the standard and L5 parameters as well as L5I with L1-L5 lordosis in both subgroups. Twenty two (14.96 %) subjects with LSV were found. Ten of these had an unidentifiable S1 endplate due to a sacralisation of L5. Mean values for the L5I, L5 tilt, L5 slope and L1-L5 lordosis were, respectively, 22.43, 4.65, 17.73, and 45.51 for normal subjects (N = 137) and 32.75, 6.63, 26.38, and 55.02 for sacralisation of L5 subjects (N = 10). Mathematical relationship found: L5I = 0.7641 * PI - 17.725 (R = 0.83) and L1-L5 = 0.67 * L5I + 30.7 (R = 0.64). This prospective study is first to provide normative spino-pelvic values at the L5 level in an asymptomatic population, particularly in case of (LSV) sacralisation of L5 (N = 10) where L5I and L1-L5 lordosis appears to be 10A degrees more important than in normal population. We propose L5I as a new spino-pelvic parameter to restore in case of L5-S1 disk disease. These normative values will help to control peri-operatively the adequate lordosis restoration, in the presence of LSV.