Identifying the correct lumbar spine level is very important for performing interventional procedures for back pain patients with radicular pain of the lower limb. Injection at the wrong level can produce unwanted or no therapeutic effect on the patient. Even though many pain physicians use fluoroscopy to identify the segment of the lumbar vertebrae, an anatomical deformity in the lumbar spine, such as lumbosacral transitional vertebra (LSTV [lumbarization or sacralization]) is often observed, with a reported incidence of 9–30% [1–3]. LSTV found in the fifth lumbar vertebra may be shown to be similar to the sacrum (sacralization), and those affecting the first sacral vertebra may be shown to the features of the lumbar spine (lumbarization) [1,2]. In these circumstances, many clinicians designate the L1 vertebra based upon the location of the 12th rib. Generally, the vertebra bearing the most caudal rib is identified as T12, and the vertebra just below T12 is identified as the first lumbar vertebra. However, this method is prone to errors because there may be a significant morphological variation in the thoracolumbar junction [1,3–5].

The thoracolumbar transitional vertebra (TLTV) is a relatively unknown congenital spinal anomaly in the thoracolumbar junction [2,6]. The TLTV may exist in the 19th or 20th vertebrae, and it may present with a hypoplastic rib, a transverse process or an accessory ossification center, either unilaterally or bilaterally [6]. However, the accurate identification of the TLTV is not easy in clinical practice when only a simple X-ray or fluoroscopic image is obtained during interventional procedures, because the morphology of TLTV can’t be easily figured out especially in the geriatric patients with severe osteoporosis. Furthermore, the existence of concurrent LSTV or abnormal rib count (11 or 13 ribs) may lead to be more difficult to distinguish the first lumbar vertebra from TLTV even with imaging the entire spine [6,7]. Therefore, it is helpful to understand the clinical characteristics of the patients with spinal variation such as TLTV and LSTV or abnormal rib count.

In the present study, we investigated the prevalence of TLTV and LSTV (lumbarization and sacralization) by evaluating whole spine spiral three-dimensional (3-D) computed tomography (CT) images. And the authors evaluated the relationships between the existence of TLTV and the existence of abnormal rib count as the primary outcome and the correlation between the existence of thoracolumbar and lumbosacral transitional vertebra as the secondary outcome.

This study retrospectively reviewed 1,553 consecutive adult patients, aged 18 to 80 years, who underwent whole spine spiral 3-D CT due to trauma or diseases from January 2018 to December 2018. Our Institutional Review Board approved this retrospective study (no. CUH 2019-03-028), and the requirement for informed consent was waived.

The following cases were excluded from the study: 1) Severe fracture of the body or transverse process of transitional vertebrae, 2) Advanced Ankylosing spondylitis, 3) Severe scoliosis, 4) Severe degenerative change, 5) Others where the distinction is ambiguous in the simple images or 3-D CT images.

A total of 1,340 patients, who were 857 males (64.0%) and 483 females (36.0%) were included in this study, and the consort diagram is described in Fig. 1. Patient demographics and spinal morphological features are described in Table 1. The results from the two expert physicians differed in distinguishing the type of vertebral segment at the thoracolumbar junction (n = 21) and at the lumbosacral junction (n = 16). There was a difference in the length of ribs at the thoracolumbar junction in six out of 21 cases. Therefore, after discussion, the six cases with a difference in rib lengths were regarded as non-TLTV. All of the cases (n = 16) in which the LSTV type differed were agreed to be LSTV.

The normal twelve ribs were observed in 1,228 of the 1,340 cases (91.6%). A case of thirteenth ribs (lumbar ribs) and lumbarization (LSTV) is described in Fig. 2. If the lowest rib is interpreted as the 12th ribs (white arrows), the lumbar spinal configuration might be identified as sacralization (Fig. 2A). However, by counting inferiorly from C1 using whole spine CT images, the lowest rib is confirmed as thirteenth ribs and the lowest incomplete-fused vertebra as S1 (lumbarization) (Fig. 2B). Meanwhile, a case of thirteenth ribs without LSTV is also described in Fig. 3. In this case, L1 was articulated at the facet joint with the thirteenth normal rib on axial CT image of L1 (black arrows) (Fig. 3B). Of the 96 patients with 11 ribs and TLTV of the 19th vertebra, sacralization was observed in 28 patients (28/96) (Table 1 and Fig. 4), meanwhile, of the 54 patients with 12 ribs and TLTV of the 20th vertebra, lumbarization was observed in 23 patients (23/54). The images of our cases showing the different types of TLTV with or without LSTV are described in Fig. 5.

The prevalence of TLTV was 11.2% (150 of 1,340 patients) and that of LSTV was 8.3% (111 of 1,340 cases). In detail, sacralization was observed in 68 of 1,340 cases (5.1%) and lumbarization in 43 of 1,340 cases (3.2%). Patients with TLTV have a higher rate of coexistent abnormal rib count (11 or 13) when compared with patients without TLTV (P < 0.001) (Table 1). The proportions of the patients with coexistent LSTV were 34.0% and 5.0% in the TLTV and no-TLTV group, respectively (P < 0.001). In patients with 11 ribs in the no-TLTV group, LSTV was observed in two of the patients (2/11), and all of them had sacralization. Of the 1,174 patients with 12 ribs and with no-TLTV, LSTV was observed in 55 patients (55/1,174), with 39 cases of sacralization, and 16 cases of lumbarization. In the no-TLTV patients, LSTV was observed in three of the patients with 13 ribs (3/5), all of which had lumbarization. There is a statistically significant probability that there is accompanying LSTV in subjects in the TLTV group (P < 0.001).

In multiple logistic regression analysis, there was a significant relationship between the existence of TLTV and the number of ribs and LSTV (P < 0.001) (Table 2). Abnormal rib count and existence of LSTV had respective ORs of 117.26 (95% CI: 60.77–226.27) and 7.38 (95% CI: 3.99–13.63). Age and sex had no statistically significant associations on the existence of TLTV.

The current study using whole-spine spiral 3D-CT images revealed a strong association between the existence of TLTV and the number of ribs and the LSTV. If TLTV was present, it is highly likely that there is a coexistent abnormality in the number of ribs and LSTV. Therefore, if a LSTV is present in fluoroscopic images during interventional procedures of the lumbar spine, pain physicians should concern that there is a strong possibility of the presence of TLTV. And in patients with TLTV at thoracolumbar junction, there is a higher probability of an abnormal rib count compared to the patients without TLTV. In these cases, whole spine imaging should be used to identify the accurate vertebral structure. To the best of our knowledge, this is the first investigation that demonstrates these associations.

Even though the cause of vertebral abnormality has not been clearly determined, genetic factors are thought to play a certain role in the segmental development of the spine [14]. The number of cervical vertebrae is extremely stable at seven. However, the number of thoracic vertebrae and lumbar vertebrae may range from 11 to 13 and from 4 to 6, respectively [8]. Variations in the thoracolumbar junction can have the potential to promote morphological shifts in the lumbosacral junction because the thoracic, lumbar and sacral spine develop craniocaudally in early fetal life [15]. If TLTV forms on T12 (19th vertebrae), then L5 (24th vertebra) might fuse with S1, resulting in sacralization. Meanwhile, if TLTV forms on L1 (20th vertebrae), then S1 (25th vertebra) might separate from S2, resulting in lumbarization. For this reason, if TLTV is present, it is highly likely that there is a coexistent abnormality such as LSTV, as shown in the current study.

Many pain physicians use fluoroscopy to identify the segment of the lumbar vertebrae. If there is an anatomical deformity in the lumbar spine, such as LSTV (lumbarization or sacralization), the pain physician is more likely to map the lumbar segment by counting down from the T12 vertebra. Meanwhile, considering the high frequency of back pain in patients with LSTV, the incidence of LSTV may be higher in patients who visit pain clinics than others [16,17]. The lumbar spinal level is easily defined on radiographs by counting inferiorly from T12, which is defined as the lowest vertebra with ribs. As our study results demonstrated in Fig. 2, this method is prone to errors because there may be a significant variation in the number and morphology of the most caudal ribs and the possibility of the presence of TLTV. Instead of counting inferiorly from T12, if possible, whole spine imaging should be used to identify the accurate vertebral structure. In addition, the facet gap can be used as a meaningful marker to judge the vertebra as TLTV as described in Fig. 4. In 143 cases of the 150 patients with TLTV (95.3%) in the current study, the facet gap was observed even in simple images (chest anteroposterior [AP], T-spine AP, and rib AP).

We used the criteria to distinguish the deformation of the most caudal ribs [9]. The criteria include measuring the length of separated bones, and determining the presence of joints with separated bones. In the current study, we also used this method to determine whether vertebrae segments in the thoracolumbar junction were TLTV or not, and the frequency of TLTV (11.2%) was similar to that observed by Park’s et al. [6]. Meanwhile, Wigh [8] suggested that segments with short ribs on thoracolumbar junction can be considered thoracic-type TLTV, while segments with accessory ossification centers can be considered lumber-type TLTV. They also suggested that showing that Type 1 vertebrae is thoracic TLTV and Type 4 vertebrae is lumbar TLTV. The consensus may be obtained through the expert discussion or the further studies.

The study by Nakajima et al. [2], in which counting was done from C1 or C2 inferiorly on a whole-spine image, revealed that the incidence of lumbarization and of sacralization was respectively 6.2% and 2.7%. However, in the present study, the incidence of lumbarization was 3.2% and that of sacralization was 5.1%. Widely variable incidences of LSTV have been reported in the literature, ranging from 4% to 35.9% [1,9,11,14,18,19]. This variation may be explained by differences in diagnostic criteria, imaging techniques and confounding factors among the investigated populations.

Hughes and Saifuddin [19] reported another technique to identify LSTV by locating the iliolumbar ligaments. Sacralization was determined by the lack of the iliolumbar ligament at the level above the sacrum. When an iliolumbar ligament was identified above the LSTV, the vertebra could be considered L5 and the LSTV was termed lumbarization. However, this technique assumes that there are always 7 cervical, 12 thoracic and 5 lumbar vertebrae. They suggested that the most ideal scenario would be to acquire whole spine images in every case to avoid any potential mistake in vertebral counting. Therefore, caution in numbering the lumbosacral vertebrae in patients with LSTV is of the utmost importance in spinal interventional procedures and surgeries because the presence of LSTV and rib anomalies can lead to inaccurate identification of vertebral levels.

Our study had a few limitations. First, we could not rule out the possibility of selection bias because most of the patients included in our study were those who were treated for trauma or disease. The second limitation is that rib length measurements are prone to errors. Although we used 3D-CT images to reduce such errors, inaccuracies in rib length measurements still remain unavoidable.

In conclusion, our results suggested that patients with TLTV were more likely to have abnormal rib counts or LSTV. If TLTV or LSTV was present on the fluoroscopic images, the order of the lumbar segment needed to be well understood. However, there is no proper way to identify accurately the number of the transitional segment without imaging of the entire spine. Only a whole spine image allows correct numbering of the lumbar vertebra in all cases and remains, therefore, the gold standard. Another alternative method, such as the thoracolumbar scout MR images, should be used and counting done caudally from T1.