- Email: [email protected]
Imaging of lumbosacral transitional vertebrae
Clinical Radiology (2004) 59, 984–991
PICTORIAL REVIEW
Imaging of lumbosacral transitional vertebrae R.J. Hughes, A. Saifuddin* Department of Radiology, The Royal National Orthopaedic Hospital NHS Trust, Stanmore, Middlesex, UK Received 18 December 2003; received in revised form 3 February 2004; accepted 6 February 2004
KEYWORDS Spine; Anatomy; Radiography; Magnetic resonance (MR)
Lumbosacral transitional vertebrae (LSTV) are a common finding in the general population. Their clinical significance is controversial with no consensus as to their relationship to low back pain or disc prolapse. However, on magnetic resonance imaging (MRI) they may be difficult to positively identify on sagittal sequences and can lead to confusion with respect to numbering of lumbar discs and vertebrae, with the consequent risk of surgical intervention at an inappropriate level. The imaging findings of LSTV on plain radiography and MRI are reviewed and their clinical significance discussed. q 2004 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction A lumbosacral transitional vertebra (LSTV) is one in which the last lumbar vertebra shows elongation of its transverse process, with varying degrees of fusion to the “first” sacral segment. The term LSTV is used to avoid having to decide whether the vertebra is a “sacralized L5” or a “lumbarized S1”. LSTV is a common finding in the general population with a reported prevalence of 4 – 21%.1 – 5 The presence of a LSTV is best identified on a true 308 cranially angled anteroposterior (AP) radiograph of the lumbosacral junction (Ferguson view) together with an AP view including the thoracolumbar junction to enable assessment of the vertebral level. In 1982, a classification system for LSTV was proposed by Castellvi et al.,1 based on plain radiographic appearances (Table 1; Fig. 1). Plain radiographic studies have shown that the lumbosacral intervertebral disc is of significantly reduced height compared with normal lumbosacral interval discs,6 and that on lateral radiographs the transitional vertebra has a “squared” appearance, with the ratio of the AP diameter of the superior *Guarantor and correspondent: A. Saifuddin, Department of Radiology, The Royal National Orthopaedic Hospital NHS Trust, Stanmore, Middlesex HA7 4LP, UK. Tel.: þ 44-20-8909-5443; fax: þ 44-20-8909-5281. E-mail address: [email protected]
vertebral end plate to the inferior end-plate being less than 1.377 (Fig. 2).
Clinical significance of LSTV For nearly a century the clinical significance of LSTV has been debated. In 1917, Bertolotti8 described the association between low back pain and assimilation of the fifth lumbar vertebrae into the sacrum, and this is sometimes referred to as “Bertolotti’s Syndrome”. This association has been disputed,2 and Tini et al.,4 in a series of 4000 individuals, demonstrated no relationship between low backache and the presence of a LSTV. However, other studies have shown a larger than expected proportion of patients with LSTV amongst those being imaged for back pain or surgery for a prolapsed disc.9 Type II LSTVs (pseudarthroses) appear to be related to pain in some series.10 Large series have shown no difference in the incidence of spondylolysis or spondylolisthesis between patients with LSTVs and controls.1,2,4 However, the degree of slip in patients with lytic spondylolisthesis is greater at L4/5 above an L5 transition than at L5/S1 above an S1 transition. This is postulated to be due to the restraining action of the iliolumbar ligament, which stabilizes the L5 vertebra.11
0009-9260/$ - see front matter q 2004 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2004.02.019
Imaging of lumbosacral transitional vertebrae
Table 1 Classification of LSTV after Castellvi et al.1 Type I: Dysplastic transverse process. Unilateral (Ia) or bilateral (Ib) with height .19 mm (Type I has not been considered a ‘true’ LSTV by some authors5 as there is no articulation with the sacrum) Type II: Incomplete lumbarisation/sacralisation Enlarged transverse process with unilateral (IIa) or bilateral (IIb) pseudarthrosis with the adjacent sacral ala Type III: Complete lumbarisation/sacralisation Enlarged transverse process, which has a unilateral (IIIa) or bilateral (IIIb) complete fusion with the adjacent sacral ala Type IV: Mixed Type IIa on one side and type IIIa on the other
In the study by Castellvi et al.,1 correlation of LSTV with myelographic findings suggested that type I was a “forme fruste” of LSTV with no clinical significance, and this is supported by Tini et al.’s finding of an increased incidence of type I morphology in families with a high incidence of LSTV.4 In type II LSTV an increased number of disc prolapses have been reported at the level above the transition.1 Elster,2 in a study of 2000 adults with back pain compared patients with LSTV ðn ¼ 140Þ with those with normal lumbosacral junctions. All had plain radiographs and either magnetic resonance imaging (MRI) or computed tomography (CT). He found the overall incidence of structural pathology (e.g. disc protrusion or spinal stenosis) no higher in patients with LSTV, but found a different distribution, with a significantly higher proportion of disc bulges or prolapses at the inter-space immediately above a transition. It has been postulated that hypermobility and altered stresses become concentrated at the level above a LSTV. Connolly et al.12 studied the scintigraphy of a series of 48 patients with LSTV and low back pain and found high uptake at the articulation between the transverse process of the LSTV and the sacrum in 81% (seen only on SPECT in 59%). In this series all the patients with increased uptake had “diarthroidial” joints (i.e. Castellvi type II, n ¼ 28) or were equivocal ðn ¼ 11Þ on plain radiography. In three patients with Castellvi type III LSTV, none demonstrated scintigraphic abnormalities at the transitional articulation. It is postulated that this indicates stress at the pseudarthrosis in type II articulations. Direct injection of anomalous lumbosacral articulations with local anaesthetic and steroid has been reported to produce a good response in 90% of patients13 (Fig. 3). Another group report successful fluoroscopically guided anaesthetic injection in nine of 10 patients with low back pain and LSTV. None of eight patients undergoing bone
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scintigraphy before injection had abnormal bone scans. In the same series, 11 patients underwent surgical resection of the transverse process with resolution of back pain in seven of 11 and improvement in a further two.14 LSTV have also been reported as a cause of extraforaminal stenosis causing nerve root entrapment and radiculopathy.15,16 Hashimoto et al.15 showed that the affected roots were compressed between the transverse segment of the transitional vertebra and the sacral ala. This was best demonstrated on coronal MRI. It has also been shown that patients with nerve root symptoms and a LSTV have a significantly greater incidence of disc prolapse than those without a LSTV and that the symptomatic disc is commonly directly above the LSTV. Similarly, spinal stenosis in the absence of spondylolisthesis is more likely to occur at the level above the LSTV.17 Brault et al.18 described a painful L6/S1 facet joint contralateral to a unilateral anomalous lumbosacral articulation in a young female, successfully diagnosed and treated by facet joint injection and eventually resection of the transitional articulation.
Identification and numbering of LSTV on MRI The correct identification of spinal segmentation is essential if operative errors are to be avoided, as most surgical errors have been shown to occur in patients with segmental variant spines, LSTVs or both.7 In many instances, lumbar spine MRI is reported without access to plain radiographs, either because these have not been performed or are unavailable. The identification of LSTV on MRI may be difficult in the absence of the radiographs, and if LSTV is suspected, then the decision as to whether it represents a “sacralized L5” or “lumbarized S1” must be made. Erroneous numbering of the level of disc prolapses will be associated with a risk of operative intervention at the wrong level. With regards the identification of LSTV, O’Driscoll et al.5 looked at 100 consecutive sagittal T1 and T2-weighted lumbar spine MRI studies and classified the lumbosacral junction into four types according to the appearance of the disc between what was considered to be the uppermost sacral segment and the remainder of the sacrum. Type 1 had no disc material present; type 2 had a small residual disc not extending for the whole AP diameter of the sacrum; type 3 had a well-formed disc extending for the whole AP diameter of the sacrum; and type 4
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Figure 1 Appearances LSTV based on Castellvi et al.1 (a) Type 1b with bilaterally enlarged L5 transverse processes but no articulation with the sacrum. (b) Type 2a with unilateral pseudarthrosis. (c) Type 2b with bilateral pseudarthroses. (d) Type 3a with unilateral fusion of the enlarged transverse process to the sacral ala. (e) Type 3b with bilateral fusion. (f) Type 4 appearance with fusion on the left side and a pseudarthrosis on the right.
Imaging of lumbosacral transitional vertebrae
Figure 2 Coned lateral radiograph of the lumbosacral junction showing typical appearances of a LSTV, with “squaring” of the transitional vertebral body and reduced height of the transitional disc.
was the same as type 3 but with in addition, an abnormal upper sacral outline analogous to the “squaring” appearance described on lateral radiographs (Fig. 4). This study demonstrated a good
Figure 3 Therapeutic pseudarthrosis.
injection
of
a
left-sided
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Figure 4 Sagittal T1-weighted MRI of the lumbar spine demonstrating a type 4 lumbosacral junction according to O’Driscoll et al.5 Note the “squaring” of the transitional vertebra.
correlation between the presence of a fused LSTV (Castellvi type 3 or 4) and a type 4 MRI appearance. Patients with a pseudarthrosis (Castellvi type IIa or IIb) were not identified with this method. Another study identified two types of disc abnormality on sagittal T2-weighted spin-echo MRI in patients with LSTV.19 A type 1 “transitional disc” appeared smaller than the disc at the adjacent mobile segment, maintained T2-weighted signal intensity (SI), lacked an intra-nuclear cleft and showed no evidence of fusion between the anterior vertebral body endplates (Fig. 5(a)). This was associated with a pseudarthrosis of the transverse process of the LSTV to the sacrum (Fig. 5(b)). However, we have noted cases of pseudarthrosis where the transitional disc appears normal (Fig. 6). A type 2 “transitional” disc was a rudimentary disc that was smaller than the type 1 “transitional” disc, maintained its SI and lacked an intra-nuclear cleft. In addition there was evidence of anterior endplate
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Figure 6 Sagittal T2-weighted MRI showing a transitional disc associated with Castellvi type 2 LSTV. The intra-nuclear cleft is clearly seen.
Figure 5 (a) Sagittal T2-weighted MRI showing a type 1 transitional disc. Nuclear SI is maintained and the intranuclear cleft is absent. (b) Sagittal T2-weighted MRI showing a type 2 transitional disc. The LSTV is indicated by the arrow.
fusion and concave endplates to the disc (Fig. 5(b)). These were associated with complete fusion of the LSTV to the sacrum. However, this survey did not take account of the common finding of residual disc material at the S1/2 level, seen in 58% of cases,5 which would lead to over diagnosis of LSTV using these criteria. Identification of whether a LSTV is a “sacralized L5” or “lumbarized S1” remains a perplexing problem in the absence of plain radiographs. This is compounded by the fact that there may be a concomitant thoracolumbar transition7 (Fig. 7). The true nature of the lower vertebral segmentation can only be established on plain radiography including the thoracolumbar junction so that hypoplastic true ribs may be differentiated from large transverse processes allowing correct identification of the L1 vertebral body.9 Once this has been correctly established the LSTV level can be confirmed.
Imaging of lumbosacral transitional vertebrae
Figure 7 Plain radiograph of the lumbar spine in a patient with a lumbosacral transition and associated thoracolumbar transition.
Peh et al.20 investigated the ability of radiologists to correctly identify lumbar levels on sagittal and coronal MRI T1 localizer images. When compared with a method of counting caudally with additional sagittal cervico-thoracic localizer
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Figure 8 Sagittal T2-weighted MRI showing the right renal artery (arrow). Using this technique, the closest intervertebral disc is labelled L1/2 and the transitional vertebra as L5.
Figure 9 The iliolumbar ligaments (large arrows) on T1-weighted axial MRI in a patient with a type 3b LSTV. They are identified at the level above the LSTV, identifying this level as L5. The LSTV is then designated S1 (small arrow).
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Figure 10 The iliolumbar ligaments (large arrows) on T1-weighted axial MRI in a patient with a type 2b LSTV. As the iliolumbar ligaments arise at this level, the LSTV can be assigned L5.
images, there was only concordance in 80% of patients. It has also been proposed that the position of the right renal artery (RRA) may be used to identify lumbar levels on sagittal MRI.21 However, this method appears insensitive. Although the RRA is commonly seen to lie adjacent to the L1/2 disc (Fig. 8), this criterion cannot be used in approximately 25% of cases, either because the artery cannot be identified or because it is situated midway between disc spaces.5,13 It would therefore appear that this technique is not sufficiently sensitive for numbering lumbar levels.22 In this institution, we identify the iliolumbar ligament on the axial T1 MRI through the lumbosacral junction and designate the vertebral body from which it arises as L5. This is based on anatomical literature, which indicates that the iliolumbar ligament arises from the transverse process of L5.23,24 The iliolumbar ligament is clearly identified on MRI as a single or double hypointense band extending from the transverse process of L5 to the posteromedial iliac crest.24,25 Using this technique, if the iliolumbar ligament is identified at the level above the LSTV, the LSTV is designated as S1 (Fig. 9). If the iliolumbar ligament is identified at the level of the LSTV, in the case of a unilateral pseudarthrosis, the LSTV is designated as L5 (Fig. 10). Finally, if the normal iliolumbar ligament is not identified, either at the level above the LSTV or at the level of the LSTV, the LSTV is again designated as L5. In this situation, it is presumed that the enlargement of the transverse processes results in development of smaller ligaments, which resemble the sacroiliac portion of the iliolumbar ligaments (Fig. 11).26 In practice, the most serious consequence of
Figure 11 (a) AP radiograph demonstrating at type 3b LSTV. (b) T1-weighted axial MRI through the LSTV demonstrates small ligaments, consistent with the sacroiliac portion of the iliolumbar ligament (arrows). Normal iliolumbar ligaments were not identified at the level above. The LSTV is therefore designated L5.
reporting MRI studies in patients with LSTV is that an operation is performed at an incorrect level. However, this can be avoided by designating the LSTV as either L5 or S1 and marking the level on both the sagittal MRI study and the lateral plain radiograph (which we believe should be obtained), to which the surgeon can refer at the time of operation.
References 1. Castellvi AE, Goldstein LA, Chan DPK. Lumbosacral transitional vertebrae and their relationship with lumbar extradural defects. Spine 1984;9:493—5.
Imaging of lumbosacral transitional vertebrae
2. Elster AD. Bertolotti’ syndrome revisited. Transitional vertebrae of the lumbar spine. Spine 1989;14:1373—7. 3. Hahn PY, Strobel JJ, Hahn FJ. Verification of lumbosacral segments on MR images: identification of transitional vertebrae. Radiology 1992;182:580—1. 4. Tini PG, Wieser C, Zinn WM. The transitional vertebra of the lumbosacral spine: its radiological classification, incidence, prevalence, and clinical significance. Rheumatol Rehabil 1977;16:180—5. 5. O’Driscoll CM, Irwin A, Saifuddin A. Variations in morphology of the lumbosacral junction on sagittal MRI: correlation with plain radiography. Skeletal Radiol 1996;25:225—30. 6. Nicholson AA, Roberts GM, Williams LA. The measured height of the lumbosacral disc in patients with and without transitional vertebrae. Br J Radiol 1988;61:454—5. 7. Wigh RE. The thoracolumbar and lumbosacral transitional junctions. Spine 1980;3:215—22. 8. Bertolotti M. Contributo alla conoscenza dei vizi di differenzazione regionale del rachide con speciale riguardo all assimilazione sacrale della v. lombare. Radiol Med 1917;4: 113—44. 9. Wigh RE, Anthony HF. Transitional lumbosacral discs: probability of herniation. Spine 1981;6:168—71. 10. Dai L. Lumbosacral transitional vertebrae and low back pain. Bull Hosp Jt Dis 1999;58:191—3. 11. Kim NH, Suk KS. The role of transitional vertebrae in spondylolysis and spondylolytic spondylolisthesis. Bull Hosp Jt Dis 1997;56:161—6. 12. Connolly LP, d’Hemecourt PA, Connolly SA, et al. Skeletal scintigraphy of young patients with low back pain and a lumbosacral transitional vertebra. J Nucl Med 2003;44: 909—14. 13. Marks RC, Thulbourne T. Infiltration of anomalous lumbosacral articulations. Steroid and anesthetic injections in 10 back pain patients. Acta Orthop Scand 1991;62:139—41. 14. Jonsson B, Stromqvist B, Egund M. Anomalous lumbosacral
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16.
17.
18.
19.
20.
21. 22. 23. 24. 25.
26.
articulations and low back pain. Evaluation and treatment. Spine 1989;14:831—4. Hashimoto M, Watanabe O, Hirano H. Extraforaminal stenosis in the lumbosacral spine. Efficacy of MR imaging in the coronal plane. Acta Radiol 1996;37:610—3. Abe E, Sato K, Shimada Y, Okada K, Yan K, Mizutani Y. Anterior decompression of foraminal stenosis below a lumbosacral transitional vertebra. A case report. Spine 1997;22:823—6. Otani K, Konno S, Kikuchi S. Lumbosacral transitional vertebrae and nerve-root symptoms. J Bone Joint Surg Br 2001;83:1137—40. Brault JS, Smith J, Currier BL. Partial lumbosacral transitional vertebra resection for contralateral facetogenic pain. Spine 2001;26:226—9. Desmond RM, Buirski G. Magnetic resonance appearances of developmental disc anomalies in the lumbar spine. Australas Radiol 1993;37:21—9. Peh WCG, Siu TH, Chan JHM. Determining the lumbar vertebral segments on magnetic resonance imaging. Spine 1998;17:1852—5. Ralston MD, Dykes TA, Applebaum BI. Verification of lumbar vertebra levels. Radiology 1992;185:615—6. Hahn PY, Strobel JJ, Hahn FJ. Verification of lumbar vertebra levels. Radiology 1992;185:616. Hanson P, Sonesson B. The anatomy of the iliolumbar ligament. Arch Phys Med Rehabil 1994;75:1245—6. Basadonna PT, Gasparini D, Rucco V. Iliolumbar ligament insertions. In vivo anatomic study. Spine 1996;21:2313—6. Rucco V, Basadonna PT, Gasparini D. Anatomy of the iliolumbar ligament. A review of its anatomy and a magnetic resonance study. Am J Phys Med Rehabil 1996;75:451—5. Pool-Goudzwaard AL, Kleinrensink GJ, Snijders CJ, Entius C, Stoeckart R. The sacroiliac part of the iliolumbar ligament. J Anat 2001;199:457—63.
PICTORIAL REVIEW
Imaging of lumbosacral transitional vertebrae R.J. Hughes, A. Saifuddin* Department of Radiology, The Royal National Orthopaedic Hospital NHS Trust, Stanmore, Middlesex, UK Received 18 December 2003; received in revised form 3 February 2004; accepted 6 February 2004
KEYWORDS Spine; Anatomy; Radiography; Magnetic resonance (MR)
Lumbosacral transitional vertebrae (LSTV) are a common finding in the general population. Their clinical significance is controversial with no consensus as to their relationship to low back pain or disc prolapse. However, on magnetic resonance imaging (MRI) they may be difficult to positively identify on sagittal sequences and can lead to confusion with respect to numbering of lumbar discs and vertebrae, with the consequent risk of surgical intervention at an inappropriate level. The imaging findings of LSTV on plain radiography and MRI are reviewed and their clinical significance discussed. q 2004 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction A lumbosacral transitional vertebra (LSTV) is one in which the last lumbar vertebra shows elongation of its transverse process, with varying degrees of fusion to the “first” sacral segment. The term LSTV is used to avoid having to decide whether the vertebra is a “sacralized L5” or a “lumbarized S1”. LSTV is a common finding in the general population with a reported prevalence of 4 – 21%.1 – 5 The presence of a LSTV is best identified on a true 308 cranially angled anteroposterior (AP) radiograph of the lumbosacral junction (Ferguson view) together with an AP view including the thoracolumbar junction to enable assessment of the vertebral level. In 1982, a classification system for LSTV was proposed by Castellvi et al.,1 based on plain radiographic appearances (Table 1; Fig. 1). Plain radiographic studies have shown that the lumbosacral intervertebral disc is of significantly reduced height compared with normal lumbosacral interval discs,6 and that on lateral radiographs the transitional vertebra has a “squared” appearance, with the ratio of the AP diameter of the superior *Guarantor and correspondent: A. Saifuddin, Department of Radiology, The Royal National Orthopaedic Hospital NHS Trust, Stanmore, Middlesex HA7 4LP, UK. Tel.: þ 44-20-8909-5443; fax: þ 44-20-8909-5281. E-mail address: [email protected]
vertebral end plate to the inferior end-plate being less than 1.377 (Fig. 2).
Clinical significance of LSTV For nearly a century the clinical significance of LSTV has been debated. In 1917, Bertolotti8 described the association between low back pain and assimilation of the fifth lumbar vertebrae into the sacrum, and this is sometimes referred to as “Bertolotti’s Syndrome”. This association has been disputed,2 and Tini et al.,4 in a series of 4000 individuals, demonstrated no relationship between low backache and the presence of a LSTV. However, other studies have shown a larger than expected proportion of patients with LSTV amongst those being imaged for back pain or surgery for a prolapsed disc.9 Type II LSTVs (pseudarthroses) appear to be related to pain in some series.10 Large series have shown no difference in the incidence of spondylolysis or spondylolisthesis between patients with LSTVs and controls.1,2,4 However, the degree of slip in patients with lytic spondylolisthesis is greater at L4/5 above an L5 transition than at L5/S1 above an S1 transition. This is postulated to be due to the restraining action of the iliolumbar ligament, which stabilizes the L5 vertebra.11
0009-9260/$ - see front matter q 2004 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2004.02.019
Imaging of lumbosacral transitional vertebrae
Table 1 Classification of LSTV after Castellvi et al.1 Type I: Dysplastic transverse process. Unilateral (Ia) or bilateral (Ib) with height .19 mm (Type I has not been considered a ‘true’ LSTV by some authors5 as there is no articulation with the sacrum) Type II: Incomplete lumbarisation/sacralisation Enlarged transverse process with unilateral (IIa) or bilateral (IIb) pseudarthrosis with the adjacent sacral ala Type III: Complete lumbarisation/sacralisation Enlarged transverse process, which has a unilateral (IIIa) or bilateral (IIIb) complete fusion with the adjacent sacral ala Type IV: Mixed Type IIa on one side and type IIIa on the other
In the study by Castellvi et al.,1 correlation of LSTV with myelographic findings suggested that type I was a “forme fruste” of LSTV with no clinical significance, and this is supported by Tini et al.’s finding of an increased incidence of type I morphology in families with a high incidence of LSTV.4 In type II LSTV an increased number of disc prolapses have been reported at the level above the transition.1 Elster,2 in a study of 2000 adults with back pain compared patients with LSTV ðn ¼ 140Þ with those with normal lumbosacral junctions. All had plain radiographs and either magnetic resonance imaging (MRI) or computed tomography (CT). He found the overall incidence of structural pathology (e.g. disc protrusion or spinal stenosis) no higher in patients with LSTV, but found a different distribution, with a significantly higher proportion of disc bulges or prolapses at the inter-space immediately above a transition. It has been postulated that hypermobility and altered stresses become concentrated at the level above a LSTV. Connolly et al.12 studied the scintigraphy of a series of 48 patients with LSTV and low back pain and found high uptake at the articulation between the transverse process of the LSTV and the sacrum in 81% (seen only on SPECT in 59%). In this series all the patients with increased uptake had “diarthroidial” joints (i.e. Castellvi type II, n ¼ 28) or were equivocal ðn ¼ 11Þ on plain radiography. In three patients with Castellvi type III LSTV, none demonstrated scintigraphic abnormalities at the transitional articulation. It is postulated that this indicates stress at the pseudarthrosis in type II articulations. Direct injection of anomalous lumbosacral articulations with local anaesthetic and steroid has been reported to produce a good response in 90% of patients13 (Fig. 3). Another group report successful fluoroscopically guided anaesthetic injection in nine of 10 patients with low back pain and LSTV. None of eight patients undergoing bone
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scintigraphy before injection had abnormal bone scans. In the same series, 11 patients underwent surgical resection of the transverse process with resolution of back pain in seven of 11 and improvement in a further two.14 LSTV have also been reported as a cause of extraforaminal stenosis causing nerve root entrapment and radiculopathy.15,16 Hashimoto et al.15 showed that the affected roots were compressed between the transverse segment of the transitional vertebra and the sacral ala. This was best demonstrated on coronal MRI. It has also been shown that patients with nerve root symptoms and a LSTV have a significantly greater incidence of disc prolapse than those without a LSTV and that the symptomatic disc is commonly directly above the LSTV. Similarly, spinal stenosis in the absence of spondylolisthesis is more likely to occur at the level above the LSTV.17 Brault et al.18 described a painful L6/S1 facet joint contralateral to a unilateral anomalous lumbosacral articulation in a young female, successfully diagnosed and treated by facet joint injection and eventually resection of the transitional articulation.
Identification and numbering of LSTV on MRI The correct identification of spinal segmentation is essential if operative errors are to be avoided, as most surgical errors have been shown to occur in patients with segmental variant spines, LSTVs or both.7 In many instances, lumbar spine MRI is reported without access to plain radiographs, either because these have not been performed or are unavailable. The identification of LSTV on MRI may be difficult in the absence of the radiographs, and if LSTV is suspected, then the decision as to whether it represents a “sacralized L5” or “lumbarized S1” must be made. Erroneous numbering of the level of disc prolapses will be associated with a risk of operative intervention at the wrong level. With regards the identification of LSTV, O’Driscoll et al.5 looked at 100 consecutive sagittal T1 and T2-weighted lumbar spine MRI studies and classified the lumbosacral junction into four types according to the appearance of the disc between what was considered to be the uppermost sacral segment and the remainder of the sacrum. Type 1 had no disc material present; type 2 had a small residual disc not extending for the whole AP diameter of the sacrum; type 3 had a well-formed disc extending for the whole AP diameter of the sacrum; and type 4
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Figure 1 Appearances LSTV based on Castellvi et al.1 (a) Type 1b with bilaterally enlarged L5 transverse processes but no articulation with the sacrum. (b) Type 2a with unilateral pseudarthrosis. (c) Type 2b with bilateral pseudarthroses. (d) Type 3a with unilateral fusion of the enlarged transverse process to the sacral ala. (e) Type 3b with bilateral fusion. (f) Type 4 appearance with fusion on the left side and a pseudarthrosis on the right.
Imaging of lumbosacral transitional vertebrae
Figure 2 Coned lateral radiograph of the lumbosacral junction showing typical appearances of a LSTV, with “squaring” of the transitional vertebral body and reduced height of the transitional disc.
was the same as type 3 but with in addition, an abnormal upper sacral outline analogous to the “squaring” appearance described on lateral radiographs (Fig. 4). This study demonstrated a good
Figure 3 Therapeutic pseudarthrosis.
injection
of
a
left-sided
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Figure 4 Sagittal T1-weighted MRI of the lumbar spine demonstrating a type 4 lumbosacral junction according to O’Driscoll et al.5 Note the “squaring” of the transitional vertebra.
correlation between the presence of a fused LSTV (Castellvi type 3 or 4) and a type 4 MRI appearance. Patients with a pseudarthrosis (Castellvi type IIa or IIb) were not identified with this method. Another study identified two types of disc abnormality on sagittal T2-weighted spin-echo MRI in patients with LSTV.19 A type 1 “transitional disc” appeared smaller than the disc at the adjacent mobile segment, maintained T2-weighted signal intensity (SI), lacked an intra-nuclear cleft and showed no evidence of fusion between the anterior vertebral body endplates (Fig. 5(a)). This was associated with a pseudarthrosis of the transverse process of the LSTV to the sacrum (Fig. 5(b)). However, we have noted cases of pseudarthrosis where the transitional disc appears normal (Fig. 6). A type 2 “transitional” disc was a rudimentary disc that was smaller than the type 1 “transitional” disc, maintained its SI and lacked an intra-nuclear cleft. In addition there was evidence of anterior endplate
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Figure 6 Sagittal T2-weighted MRI showing a transitional disc associated with Castellvi type 2 LSTV. The intra-nuclear cleft is clearly seen.
Figure 5 (a) Sagittal T2-weighted MRI showing a type 1 transitional disc. Nuclear SI is maintained and the intranuclear cleft is absent. (b) Sagittal T2-weighted MRI showing a type 2 transitional disc. The LSTV is indicated by the arrow.
fusion and concave endplates to the disc (Fig. 5(b)). These were associated with complete fusion of the LSTV to the sacrum. However, this survey did not take account of the common finding of residual disc material at the S1/2 level, seen in 58% of cases,5 which would lead to over diagnosis of LSTV using these criteria. Identification of whether a LSTV is a “sacralized L5” or “lumbarized S1” remains a perplexing problem in the absence of plain radiographs. This is compounded by the fact that there may be a concomitant thoracolumbar transition7 (Fig. 7). The true nature of the lower vertebral segmentation can only be established on plain radiography including the thoracolumbar junction so that hypoplastic true ribs may be differentiated from large transverse processes allowing correct identification of the L1 vertebral body.9 Once this has been correctly established the LSTV level can be confirmed.
Imaging of lumbosacral transitional vertebrae
Figure 7 Plain radiograph of the lumbar spine in a patient with a lumbosacral transition and associated thoracolumbar transition.
Peh et al.20 investigated the ability of radiologists to correctly identify lumbar levels on sagittal and coronal MRI T1 localizer images. When compared with a method of counting caudally with additional sagittal cervico-thoracic localizer
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Figure 8 Sagittal T2-weighted MRI showing the right renal artery (arrow). Using this technique, the closest intervertebral disc is labelled L1/2 and the transitional vertebra as L5.
Figure 9 The iliolumbar ligaments (large arrows) on T1-weighted axial MRI in a patient with a type 3b LSTV. They are identified at the level above the LSTV, identifying this level as L5. The LSTV is then designated S1 (small arrow).
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R.J. Hughes, A. Saifuddin
Figure 10 The iliolumbar ligaments (large arrows) on T1-weighted axial MRI in a patient with a type 2b LSTV. As the iliolumbar ligaments arise at this level, the LSTV can be assigned L5.
images, there was only concordance in 80% of patients. It has also been proposed that the position of the right renal artery (RRA) may be used to identify lumbar levels on sagittal MRI.21 However, this method appears insensitive. Although the RRA is commonly seen to lie adjacent to the L1/2 disc (Fig. 8), this criterion cannot be used in approximately 25% of cases, either because the artery cannot be identified or because it is situated midway between disc spaces.5,13 It would therefore appear that this technique is not sufficiently sensitive for numbering lumbar levels.22 In this institution, we identify the iliolumbar ligament on the axial T1 MRI through the lumbosacral junction and designate the vertebral body from which it arises as L5. This is based on anatomical literature, which indicates that the iliolumbar ligament arises from the transverse process of L5.23,24 The iliolumbar ligament is clearly identified on MRI as a single or double hypointense band extending from the transverse process of L5 to the posteromedial iliac crest.24,25 Using this technique, if the iliolumbar ligament is identified at the level above the LSTV, the LSTV is designated as S1 (Fig. 9). If the iliolumbar ligament is identified at the level of the LSTV, in the case of a unilateral pseudarthrosis, the LSTV is designated as L5 (Fig. 10). Finally, if the normal iliolumbar ligament is not identified, either at the level above the LSTV or at the level of the LSTV, the LSTV is again designated as L5. In this situation, it is presumed that the enlargement of the transverse processes results in development of smaller ligaments, which resemble the sacroiliac portion of the iliolumbar ligaments (Fig. 11).26 In practice, the most serious consequence of
Figure 11 (a) AP radiograph demonstrating at type 3b LSTV. (b) T1-weighted axial MRI through the LSTV demonstrates small ligaments, consistent with the sacroiliac portion of the iliolumbar ligament (arrows). Normal iliolumbar ligaments were not identified at the level above. The LSTV is therefore designated L5.
reporting MRI studies in patients with LSTV is that an operation is performed at an incorrect level. However, this can be avoided by designating the LSTV as either L5 or S1 and marking the level on both the sagittal MRI study and the lateral plain radiograph (which we believe should be obtained), to which the surgeon can refer at the time of operation.
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