Lumbosacral Transitional Vertebrae 

                                     Dr. KS Dhillon

Introduction

Lumbosacral transitional vertebrae (LSTV) are congenital spinal anomalies that are defined as either sacralization of the lowest lumbar segment or lumbarization of the most superior sacral segment of the spine. LSTVs are common in the general population. The reported prevalence of LSTV is between 4%–30% [1–15].

The degree of morphologic variation of these segments varies from L5 vertebrae with broadened elongated transverse processes to complete fusion to the sacrum. Conversely, the S1 vertebral segment can also show varying degrees of lumbarization, such as the formation of an anomalous articulation rather than fusion to the remainder of the sacrum. There can be well-formed lumbar-type facet joints, a more squared appearance in the sagittal plane, as well as a well-formed full-sized disc, rather than the smaller-sized disc typically seen between S1 and S2.

In 1984, Castellvi et al [16] described a radiographic classification system identifying 4 types of LSTVs on the basis of morphologic characteristics.

The Castellvi classification (Fig 1) is used for lumbosacral transitional vertebrae (LSTV):

Type I: enlarged and dysplastic transverse process (at least 19 mm)

Ia: unilateral

Ib: bilateral

Type II: pseudoarticulation of the transverse process and sacrum with incomplete lumbarisation/sacralization; enlargement of the transverse process with pseudoarthrosis

IIa: unilateral

IIb: bilateral

Type III: transverse process fuses with the sacrum and there is complete lumbarisation or sacralization, enlarged transverse process with complete fusion

IIIa: unilateral

IIIb: bilateral

Type IV: type IIa on one side and type IIIa on the contralateral side

Lumbosacral transitional vertebrae have been classically identified by using lateral and Ferguson AP radiographs. Although useful for characterizing the relationship between the transitional segment and the level above or below, this classification system does not provide information relevant to the accurate enumeration of the involved segments.

Other morphologic characteristics of transitional vertebrae include squaring of the upper sacral segment when it is lumbarized and wedging of the lowest lumbar segment when it is sacralized [17.] These morphologic changes represent cranial and caudal shifts of the spine, respectively. This results in either a greater or lesser number of motion segments. Wigh and Anthony [17] described the squared appearance of transitional vertebrae on lateral radiographs as the ratio of the AP diameter of the superior vertebral endplate to that of the inferior vertebral endplate as equal to or less than 1.37. This squaring and wedging represent a spectrum of vertebral body morphologic change and cannot be reliably used to definitively identify an LSTV.

Nicholson et al [18] described a decrease in the height of the disc on radiographs between a lumbar transitional segment and the sacrum compared with the normal disc height between L5 and S1. It has been observed that when a lumbarized S1 is present, the disc space between S1 and S2 is larger than the rudimentary disc that is most often seen in spines without transitions. O'Driscoll et al [11] developed a 4-type classification system of S1–2 disc morphology by using sagittal MR images. Type 1 shows no disk material and is seen in patients without transitional segments. Type 2 consists of a small residual disc with an AP length less than that of the sacrum. This type is most often seen in patients without transitional segments. Type 3 is a well-formed disc extending the entire AP length of the sacrum and can be seen in normal spines as well as in those with LSTVs. Type 4 is similar to type 3 but with the addition of squaring

Figure 1. Castellvi classification.

of the upper sacral segment. A good correlation has been found between a type 4 S1–2 disc and an S1 LSTV Castellvi type III or IV.

Fig 2--O'Driscoll classification system of S1–2 disk morphology.

LSTVs with anomalous diarthrodial articulations of the transverse processes with the sacrum (Castellvi type II) and some Castellvi type III vertebrae are not reliably identified by using this sagittally disc-based classification system. They should be evaluated with axial or coronal MR images. Without using other techniques or additional imaging, one can have difficulty determining what is actually S1 and, therefore, reliably identifying the S1–2 disk.

A final morphologic observation is that the facet joints between a transitional L5 and the sacrum are typically hypoplastic or even nonexistent in patients with complete osseous fusion of L5 to S1. Conversely, with a lumbarized S1, facet joints are often identified between S1 and S2, where there is osseous fusion.

Numbering Technique

Identification of an LSTV is important. Accurate numeric identification of the vertebral segments on MR imaging is essential before surgery can be carried out. Inaccurate numbering can lead to an interventional procedure or surgery at an unintended level. Establishing whether an LSTV is a lumbarized S1 or a sacralized L5 on MR imaging can often be problematic. Conventional spine radiographs are often unavailable at the time of imaging. Cervicothoracic localizers may not be routinely obtained. Radiographs of the entire spine allow the radiologist to count from C2 inferiorly. It also helps to differentiate hypoplastic ribs from lumbar transverse processes, therefore enabling the counting of the number of thoracic segments and correct identification of the L1 vertebral body. After the vertebral body is correctly identified, determining the correct numeric assignment of the LSTV is possible. It is rare to have radiographs of the entire spine. Most often lumbar spine radiographs alone are available. In such cases, correct enumeration can often be achieved, but there remain cases in which it is difficult to differentiate hypoplastic ribs from transverse processes at the thoracolumbar junction. The presence of thoracolumbar transitions as well as segmentation anomalies complicates the evaluation of these patients.

Hahn et al [6] were the first to describe the use of a sagittal cervicothoracic MR localizer to better evaluate transitional vertebrae. With the sagittal MR localizer, the vertebrae can be counted in a caudad direction from C2 rather than cephalad from L5. Using a sagittal cervicothoracic MR localizer alone assumes 7 cervical and 12 thoracic vertebrae and it does not account for thoracolumbar transitions or allow differentiation of dysplastic ribs from lumbar transverse processes. The coronal MR cervicothoracic localizer increases the accuracy of enumerating lumbosacral transitional vertebrae because it allows better differentiation at the thoracolumbar junction [13]. Most radiologists, however, do not routinely obtain an MR localizer inclusive of the cervical and thoracic spine when imaging patients with low back pain.

To correctly number an LSTV another technique can be used. It is by locating the iliolumbar ligaments because they reliably arise from the L5 transverse processes [19]. These ligaments' function is to restrain flexion, extension, axial rotation, and lateral bending of L5 on S1. On T1- and T2-weighted MR images it is seen as a single or double band extending from the transverse process of L5 to the posteromedial iliac crest [19-21]. Hughes and Saifuddin [19] labeled an LSTV as L5 when no iliolumbar ligament was identified at the level above. When an iliolumbar ligament was seen above the LSTV, then the vertebral body with the iliolumbar ligament was labeled L5 and the LSTV, as S1. This technique has limitations because it assumes that there are always 12 thoracic, 

7 cervical, and 5 lumbar vertebrae. Various segmentation anomalies can occur along with thoracolumbar transitional vertebrae, and in these cases, identification of the iliolumbar ligament is not sufficient to accurately identify the L5 vertebral body [22].

The use of anatomic markers, including the aortic bifurcation, right renal artery, and conus medullaris is not very useful. Lee et al [23] reported that the position of the aortic bifurcation and right renal artery are reliable landmarks for determining the lumbar vertebral segments on MR imaging and CT. These anatomic markers are widely believed to be less than satisfactory [19]. Although the right renal artery is usually located at the L1–2 disk space level, 25% of the time it is either not imaged or is present at another location [19]. Variability can be seen in the position of the aortic bifurcation as it has been found at the L4 level in 83% of patients [23]. Lee et al [23] have also reported that the conus medullaris should not be used as a landmark because its position is quite variable.

Without high-quality imaging of the entirety of the spine, there is no foolproof method for accurately numbering a transitional segment. 

Clinical Significance

Bertolotti Syndrome

Mario Bertolotti first described the morphologic characteristics of LTSV and its association with low back pain (LBP) in 1917, and this association has been termed Bertolotti syndrome [24]. The relationship between LSTV and LBP has been described in several studies but the relationship remains uncertain. Although Tini et al [25] suggested that LSTV was not associated with LBP, the findings of other studies indicated an association of LBP with LSTV [24,26,27,28]. 

Although not initially described, the low back pain of this syndrome is currently thought to be of varying etiologies. It arises from different locations such as:

 1) Disc, spinal canal, and posterior element pathology at the level above a transition [5,8,9,12,24,29,30]. 

2) Degeneration of the anomalous articulation between an LSTV and the sacrum. 

3) Facet joint arthrosis contralateral to a unilateral fused or articulating LSTV [31,32] 

4) Extraforaminal stenosis secondary to the presence of a broadened transverse process [12,30,31,33,34].

Most of the literature that supports Bertolotti syndrome, the implicated transitional segments are Castellvi types II-IV. Castellvi [2] states that type I LSTVs are of no clinical significance and are a forme fruste and therefore has no relation to what was initially described as Bertolotti syndrome.  Aihara et al [35,36] determined that short and broad iliolumbar ligaments lend a protective effect to the L5-S1 disk space and potentially destabilize the L4-L5 level. There can be an association of such iliolumbar ligament morphology with broadened long transverse processes (Castellvi type I). 

Tini et al [10] in a series of 4000 patients reported no correlation between low back pain and transitional vertebrae. Elster [5] found that the incidence of structural pathology such as disk pathology, spinal and foraminal stenosis did not differ in patients with LSTV compared with those without transitional vertebrae.

However, the distribution of pathology was markedly different in lesions that occurred at the intervertebral disc space above the level of the transitional vertebra. Taskaynatan et al [9] did not find an increased incidence of pathology in patients with LSTV. They reported increased severity of low back pain in patients with LSTV and an associated increase in nerve root symptoms.

There are several studies of patients being imaged for low back pain or surgery for disc pathology that demonstrated a greater than expected number of transitional vertebrae [2,3,4,8,15]. There are several studies that have shown an increased incidence of disc pathology above LSTVs [2,5,30,36]. Luoma et al [8] reported an increased risk of early degeneration of the upper disc in young patients, but this change was obscured by age-related changes in the middle-aged population. Epstein et al [37] showed the presence of increased disc herniation in adolescents with spinal anomalies, including LSTV.

Transitional vertebrae do affect the normal biomechanics of the lumbar spine. The lack of mobility at a fused transitional level or the decreased mobility at a partially fused or anomalously articulating vertebra provides stability to this level. There is a decreased prevalence of disc pathology in the disc below the transitional vertebral body [5,8,15]. This may be due to altered biomechanics from the aberrant joints between the LSTV and sacrum. First, there is restricted motion between the transitional vertebra and sacrum due to the anomalous articulation and/or bony fusion [5]. The load is absorbed by the fused transverse process or the aberrant joint decreasing motion and relieving stress on the intervertebral disc. This results in the preservation of disc integrity seen on MR imaging [11,38].

The increased stability between the sacrum and an LSTV can potentially lead to hypermobility above the transitional segment, at the ipsilateral anomalous articulation [5,34,36,39,40], and/or at the contralateral facet joint [31]. Similar hypermobility occurs at the disc level above and below postsurgical fusion masses or a block vertebra [5]. Hypermobility and abnormal torque moments at the intervertebral disc place the disc and facet joints at increased risk of accelerated degeneration [5,30,36,41]. Additionally, Aihara et al [36] found that the iliolumbar ligaments above an LSTV were thinner and weaker, potentially further predisposing this level to hypermobility and premature degeneration. There has been no difference in the incidence of spondylolysis or spondylolisthesis between patients with LSTVs and controls [2,5,10,42]. It has been observed that in patients with lytic spondylolisthesis, there is a greater degree of slip at the L4–5 level above an L5 transition compared with the L5-S1 level above an S1 transition [43].

Connolly et al [44] in a series of 48 patients with low back pain and an LSTV, showed increased uptake on skeletal scintigraphy at the anomalous articulation between the transverse process of the LSTV and the sacrum in 81% of patients. All the symptomatic patients had diarthrodial joints (Castellvi type II LSTV). Increased uptake was not seen in patients with osseous fusion of the transverse process to the sacral ala. 

Contralateral facetogenic pain can be seen in patients with unilateral anomalous articulations or osseous fusion (Castellvi type IIa or IIIa). In these unilateral cases, torque forces are distributed across to the contralateral facet joint. Facet arthrosis can be seen on CT or radiographs with the typical findings of osteoarthrosis i.e joint space narrowing, cartilage loss, and osteophyte formation. MR imaging and nuclear medicine can more reliably identify the facet as the discrete source of pain when bone marrow edema signal intensity or increased tracer uptake is identified respectively [45].

There can be extraforaminal stenosis with nerve root entrapment and radiculopathy in patients with an LSTV [33,46]. On coronal MR imaging, the nerve root can be seen compressed between the hyperplastic transverse process of the LSTV and the adjacent sacral ala [33]. Patients with nerve root symptoms and an LSTV are more likely to have disc prolapse at the level above the LSTV than those without an LSTV [33]. In the absence of spondylolisthesis, spinal stenosis is more likely to occur at the disc level above the LSTV [12]. Assessing nerve root symptoms in patients with an LSTV can be complicated by the fact that there is associated variation of lumbosacral myotomes  [40]. When a sacralized L5 vertebral body is present, the L4 nerve root serves the usual function of the L5 nerve root.   When a lumbarized S1 is present, the S1 nerve root functions as the L5 nerve root [3,47]. McCulloch and Waddell [48] showed that the functional L5 nerve root always originates at the lowest mobile level of the lumbosacral spine. This knowledge of variations can help to explain confounding radicular symptoms.

Wrong-Level Spine Surgery

Accurate assessment of spinal segmentation is important to eliminate surgical and procedural errors because most wrong-level spine surgery occurs in patients with variant spine anatomy, including LSTVs [49]. Surgical errors often occur when MR imaging of the lumbar spine is reported without accompanying conventional radiographs [11] or cervicothoracic MR localizers. Intraoperative radiographs are used during spinal surgery for confirmation of disk level. It is important to correlate prior MR imaging with these radiographs. It is also important to obtain high-quality intraoperative lateral radiographs. Lack of correlation by the surgeon of the intraoperative radiograph with the preoperative sagittal MR imaging can lead to the serious consequence of wrong-level spine surgery. 

Treatment

Several treatment strategies have been advocated although there is no consensus on the clinical significance of LSTVs. These treatment strategies include conservative nonsurgical management with local injection of anesthetic and corticosteroids within the pseudoarticulation or contralateral facet joint, radio-frequency ablation and surgical management with partial transverse process resection, and/or posterior spinal fusion. In patients where surgery is contemplated, it is suggested that local anesthetic injection be part of the diagnostic work-up [40]. Direct local anesthetic and steroid injection and surgical resection of the anomalous or contralateral facet joint have produced successful pain relief [31,34,40,50-53].

Operative treatment is necessary in some patients who have failed conservative treatment. Resection of the transverse process may be beneficial for those who demonstrate pain emanating from a transitional joint and have failed conservative treatment. If the source of pain is from a degenerated disk above a transitional level, posterior fusion is an option [40]. Brault et al [31] in a case report described successful treatment of contralateral facetogenic pain by resection of the ipsilateral anomalous articulation. Jonsson et al [34] reported pain relief in 9 of 11 patients following surgical resection of a unilateral LSTV pseudoarticulation. Ugokwe et al [51] and Almeida et al [52] similarly describe successful treatment following surgical resection. 

Santavirta et al [40] in a case series of 8 patients who underwent surgical resection of the unilateral anomalous articulation and 8 patients who underwent posterolateral fusion of the LSTV, reported improvement in pain in 10 of 16 patients at 9-year follow-up without a difference between the fusion or resection groups. Radio-frequency denervation is another possible treatment option. It provided temporary relief of pain due to an anomalous articulation in a case report by Almeida et al [52].

Conclusion

LSTVs are common anomalies of the spine. It is necessary to accurately identify and number the affected segment. There is fairly convincing evidence of an association of low back pain with LSTV. Knowledge of the biomechanical alterations within the spine caused by LSTVs will aid in understanding and recognizing the imaging findings seen in patients with low back pain and a transitional segment. A thorough understanding of the importance of both accurate enumeration of LSTV and communication to the clinician will help to avoid such dreaded complications as wrong-level spine surgery.

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