A simple method for the selection of valid spinopelvic parameters and lumbar lordosis in patients with transitional lumbosacral vertebrae

Purpose Transitional lumbosacral vertebrae (TLSV) are a congenital anomaly of the lumbosacral region that is characterized by the presence of a vertebra with morphological properties of both the lumbar and sacral vertebrae, with a prevalence of up to 36% in asymptomatic patients and 20% in adolescent idiopathic scoliosis patients. In patients with TLSV, because of these morphological changes and the different numbers of lumbar vertebrae, there are two optional reference sacral endplates that can be selected intently or inadvertently to measure the spinopelvic parameters: upper and lower endplates. The spinopelvic parameters measured using the upper and lower endplates are significantly different from each other as well as from the normative values. Therefore, the selection of a reference endplate changes the spinopelvic parameters, lumbar lordosis (LL), and surgical goals, which can result in surgical over- or under-correction. Because there is no consensus on the selection of sacral endplate among these patients, it is unclear as to which of these parameters should be used in diagnosis or surgical planning. The present study describes a standardization method for measuring the spinopelvic parameters and LL in patients with TLSV. Methods Upper and lower endplate spinopelvic parameters (i.e., pelvic incidence [PI], sacral slope [SS], and pelvic tilt) and LL of 108 patients with TLSV were measured by computed tomography. In addition, these parameters were measured for randomly selected subjects without TLSV. The PI value in the TLSV group, which was closer to the mean PI value of the control group, was accepted as valid and then used to create an optimum PI (OPI) group. Finally, the spinopelvic parameters and LL of the OPI and control groups were compared. Results Except for SS, all spinopelvic parameters and LL were comparable between the OPI and control groups. In the OPI group, 60% of the patients showed valid upper endplate parameters, and 40% showed valid lower endplate parameters. No difference was noted in the frequency of valid upper or lower endplates between the sacralization and lumbarization groups. Both the OPI and control groups showed nearly comparable correlations between their individual spinopelvic parameters and LL, except for PI and LL in the former. Conclusions Because PI is unique for every individual, the endplate whose PI value is closer to the normative value should be selected as the reference sacral endplate in patients with TLSV.