The Termination Level of the Dural Sac Relevant to Caudal Epidural

Background: Lumbosacral transitional vertebrae (LSTV) are a relatively common variant and have

been considered as one of the reasons for back pain. It is not unusual for clinicians to encounter

patients with LSTV who require caudal epidural block (CEB) for pain management.

Objective: We investigated the termination level of the dural sac (DS) and anatomical features

of the lumbosacral region relevant to CEB in patients with LSTV and compared these findings

between sacralization and lumbarization groups.

Study Design: A retrospective evaluation.

Setting: A university hospital with inpatient and outpatient LSTV cases presenting low back pain.

Methods: Four hundred ninety-four LSTV patients were included and categorized into sacralization

(n = 201) or lumbarization groups (n = 293). Magnetic resonance imaging (MRI) of all of the LSTV

patients were reviewed to determine the level of DS termination, the shortest distance between

the apex of the sacral hiatus and DS, and the presence and the caudal level of sacral perineural

cysts. Each lumbosacral vertebra column was divided into 3 equal portions (upper, middle, and

lower thirds). The MRI findings in both of the groups were compared and analyzed.

Results: The distribution frequency of the levels of DS termination demonstrated a significant

difference between the 2 groups. The mean caudal DS level in the lumbarization group was

significantly lower than the sacralization group (lower third of the S2 [131 {44.7%} of 293 patients]

vs. lower third of the S1 [78 {38.8%} of 201 patients]). The DS terminated at the S3 in more than

19% of the lumbarization group, whereas in only one case of the sacralization group. Although

the incidence of perineural cysts was not significantly different between the 2 groups, the mean

level of caudal margin of perineural cysts in the lumbarization group was significantly lower than

the sacralization group (middle third of the S3 [10 {35.7%} of 28 cases] vs. middle third of the S2

[11 {44%} of 25 cases]).

Limitations: This study reveals several limitations including the practical challenge of accurate

enumeration of the transitional segment and the constraints on generalizability posed by the

single-country study.

Conclusion: When planning CEB for patients with LSTV, pre-procedural MRI to check the

anatomical structures, including the level of DS termination and caudal margin of perineural cysts,

would be of great use for lowering the risk of unexpected dural puncture during the procedure,

especially in the lumbarization cases.

Key words: Termination of the dural sac, dural sac termination, lumbosacral transitional

vertebrae, transitional vertebra, caudal epidural block

Pain Physician 2018; 21:73-81

Retrospective Study

The Termination Level of the Dural Sac Relevant

to Caudal Epidural Block in Lumbosacral

Transitional Vertebrae: A Comparison between

Sacralization and Lumbarization Groups

From:

Department of Radiology,

Gachon University, Gil Medical

Center, Republic of Korea;

Department of Neurosurgery,

Gachon University, Gil Medical

Center, Republic of Korea

Address Correspondence:

Yu Mi Jeong, MD

Department of Radiology,

Gachon University,

Gil Medical Center

21, Namdong-daero 774 beon-gil,

Namdong-gu, Incheon, 21565,

Republic of Korea

E-mail:

[email protected]

Disclaimer: There was no

external funding in the

preparation of this manuscript.

Conflict of interest: Each author

certifies that he or she, or a

member of his or her immediate

family, has no commercial

association (i.e., consultancies,

stock ownership, equity interest,

patent/licensing arrangements,

etc.) that might pose a conflict of

interest in connection with the

submitted manuscript.

Manuscript received: 01-11-2017

Revised manuscript received:

03-29-2017

Accepted for publication:

04-16-2017

Free full manuscript:

www.painphysicianjournal.com

Ji Young Jeon, MD

, Yu Mi Jeong, MD

, Sheen-Woo Lee, MD

, Jeong Ho Kim, MD

Hye-Young Choi, MD

, and Yong Ahn, MD

www.painphysicianjournal.com

Pain Physician 2018; 21:73-81 • ISSN 1533-3159

Pain Physician: January/February 2018; 21:73-81

74 www.painphysicianjournal.com

Population

We electronically searched the radiologic reports

for MRIs of the lumbar-sacral spine that were per-

formed from January 2000 to December 2016. The 3

keywords for the computer search were “transitional

vertebra,” “lumbarization,” and “sacralization.” This

initial search selected 665 images for assessment of

possible causes of lower back pain and radiculopathy.

Among them, 84 images were excluded from our study

owing to postoperative spine (n = 33), presence of

severe degenerative changes (n = 31), such as severe

scoliosis or central canal stenosis, metastasis in the

lumbosacral area (n = 2), and in patents where identi-

fication of the DS was difficult (n = 18). Patients under

the age of 16 years (n = 12) or those that did not get

lumbar radiographic series—anteroposterior (AP) and

lateral and with or without oblique views (n = 39) were

excluded. Patients who did not have available plain ra-

diographs or computed tomography (CT) images, such

as radiographs of the entire spine, rib series, thoracic

radiographs, or chest CT image, that enabled counting

of the number of thoracic vertebrae and correct identi-

fication of the L1 vertebral body, were further filtered

(n = 36) to determine the correct numeric assignment

of the LSTV. Finally, a total of 494 LSTV patients (mean

age of 55.05 years, range of 17–89 years; 236 men and

258 women), composed of sacralization (n = 201) and

lumbarization (n = 293) groups, were included in the

study (Fig. 1).

Image Analysis

All images were reviewed on picture archiving

and communication system workstations (G3, Infinitt,

Korea). All of the 494 lumbar radiographic series were

independently read by 2 radiologists (J.Y.J., 5 years of

experience and Y.M.J, 8 years of experience). All LSTV

patients were categorized into sacralization or lum-

barization groups. Since depending on the number of

thoracic vertebrae, the transitional vertebra can be in-

terpreted as a sacralized L5 segment (sacralization) or

a lumbarized S1 segment (lumbarization); we assume

the number of thoracic vertebrae to be 12 in all of the

patients, to gain consistency and reliability in determin-

ing the level of lumbar or sacral segment which locates

the caudal tip of the DS. This would be especially help-

ful in confusing situations when hypoplastic ribs from

transverse processes at the thoracolumbar junction or

11 and 13 pairs of ribs are seen.

Then, LSTV patients were classified according to

Castellvi et al (10) (Fig. 2) into one of 4 types. Type I

umbosacral transitional vertebrae (LSTV) are a

relatively common variant and can be seen in 25%

(range 15–35%) of the general population (1-3).

Morphologically, LSTV have intermediate characteristics

between the sacral and the lumbar vertebrae, and

the transitional segment shows an elongation of its

transverse process, with varying degrees of fusion.

LSTV can be defined as either sacralized L5, which

means “sacralization” of the lowest lumbar segment,

or lumbarized S1, which means “lumbarization” of

the most superior sacral segment of the spine (2,4).

Back and buttock pain can be associated with LSTV, as

shown by Lorenzo et al (2). Appreciation of anatomical

variations due to LSTV may impact safe performance of

caudal epidural steroid injection.

Caudal epidural block (CEB) is a relatively safe pro-

cedure which is used in a wide range of clinical settings

(5-8). It is used for intra and postoperative analgesia in

a variety of operations and has become one of the most

commonly performed interventions in pain practice for

those with low back pain and radiculopathy. Although

it is rare, the potential complication of dural puncture

during CEB could occur. Therefore, determining the

anatomical location of the sacral hiatus, the sacrococcy-

geal ligament, and the level of termination of the dural

sac (DS) are essential before performing CEB, to prevent

dural puncture (7-9). In this context, the questions could

arise whether the termination level of the DS of LSTV

patients would be significantly different from that of

others with similar clinical symptoms and furthermore,

within the LSTV group, the caudal level of the DS would

be different based on the type of transitional seg-

ment—sacralization and lumbarization.

Although there have been many studies regarding

the mean level of termination of the DS in patients with

low back pain and⁄or sciatica, there is a lack of publication

that evaluated the DS termination separately for LSTV

patients. Therefore, the purpose of the present study was

to investigate the level of DS termination in relation to the

spine in patients with LSTV, by using magnetic resonance

imaging (MRI), and to compare this position between the

sacralization and lumbarizaion groups. In addition, the

distance between the apex of the sacral hiatus and the

DS and the presence and caudal level of sacral perineural

cysts were also evaluated in our LSTV patients.

Methods

This retrospective study was approved by the

institutional review board and the requirement for in-

formed consent was waived.

www.painphysicianjournal.com 75

Termination Level of Dural Sac Relevant to CEB in LSTV

Fig. 1. A flow chart of the patient selection.

Fig. 2. A schematic illustration of the Castellvi classification (10).

Pain Physician: January/February 2018; 21:73-81

76 www.painphysicianjournal.com

exhibits dysplastic transverse processes, measuring at

least 19 mm in height (craniocaudal dimension). Type II

includes incomplete lumbarization or sacralization with

the enlarged transverse process(es) that has pseudoar-

throsis with the adjacent sacral ala. Type III describes

complete lumbarization or sacralization with the en-

larged transverse process(es), which has complete fu-

sion with the adjacent sacral ala. Type IV involves type II

on one side and type III on the other side. To minimize

the number of categories, we classified the cases into 4

basic types regardless of unilateral or bilateral findings

(2). Any discrepancies were settled by mutual consensus

between the authors. If there were additional available

CT scans covering the entire lumbosacral junction and

sacrum, the CT images were used to arbitrate a dis-

agreement between the readers (Fig. 3).

Fig. 3. Appearances of LSTV based on the Castellvi

classification (10). The white asterisks show enlarged

transverse processes in type I LSTV. The black asterisks show

pseudoarthrosis, whereas the black stars show osseous fusion.

As shown in types III and IV, additional CT images could be

used for confirmation of the classification of LSTV.

The anatomic level of the DS tip was determined

by using conventional lumbar MR images. Lumbar MR

images were acquired with 3.0 Tesla (T) MRI machines

(Skyra and Verio, Siemens Healthcare, Erlangen, Ger-

many) or 1.5-T scanners (Avanto, Siemens Healthcare,

Erlangen, Germany) using phased array spine coils in a

supine position. Sagittal images were acquired using a

slice thickness of 3.0 mm and a 0.3 mm interslice gap.

Routine scans consisted of T1-weighted sagittal and

axial images (TE range 3 – 13ms, TR range 450 – 704ms)

and T2-weighted sagittal and axial images (TE range 91

– 95ms, TR range 3200 – 5910ms). In some MRI studies,

sagittal short-tau inversion recovery (STIR) sequence (TE

range 55 – 81ms, TR range 2500 – 4310ms) of the whole

spine was available. The midsagittal sections for each

subject were used when possible to improve the reli-

ability for the measurement of the caudal end of DS.

Two radiologists independently reviewed the MR

images. To establish the caudal tip of the DS in rela-

tion to the lumbosacral spine, the L5 and each sacral

vertebra column was divided into 3 equal portions (up-

per, middle, and lower thirds [u, m, and l, respectively])

in considering the method of Saifuddin et al (11) (Fig.

4). By extending a line perpendicular to the long axis

of the dura at the most inferior convergence point of

the DS, the point where the perpendicular line crosses

the adjacent vertebrae was determined to be the most

caudal level of the DS. For statistical evaluation, each

lumbosacral vertebral regional level of the caudal DS

tip was assigned as numbers from one (lower third of

the L5) to 10 (lower third of the S3). The shortest dis-

tance between the upper margin of the sacrococcygeal

membrane and the DS was measured to estimate the

safe length of advancing the needle for CEB (Fig. 5).

The presence of a perineural cyst (Tarlov cyst) and its

caudal level were also investigated (8). The level of cau-

dal margin of perineural cysts were assigned as num-

bers in the same manner as described above. In cases

where there was a disagreement between the readers,

further discussion had been made to achieve consensus.

Statistical Analysis

All statistical analysis was performed using com-

mercial software SPSS Version 20.0 (IBM Corporation,

Armonk, NY). Descriptive statistics for baseline demo-

graphic data were calculated. Continuous variables are

presented as mean ± standard deviation and categori-

cal variables as frequency (percentages). For quantita-

tive and qualitative comparisons between the 2 LSTV

groups with lumbarization and sacralization, continu-

www.painphysicianjournal.com 77

Termination Level of Dural Sac Relevant to CEB in LSTV

Fig. 4. The midsagittal section of a MR image of the

lumbosacral spine demonstrating our method for determining

the position of termination of the DS in relation to the

lumbosacral vertebrae (u: upper third, m: middle third, l:

lower third).

Fig. 5. The midsagittal section of a MR image of the

lumbosacral spine showing how the distance (double arrow)

between the upper margin of the SCM and the DS was

measured.

ous variables were compared by using the analysis of an

independent samples t-test, and categorical variables

were compared by using a chi-square test. For all tests,

P < 0.05 was considered to be indicative of a statistically

significant difference.

Results

Demographic data and cross-tabulation of imaging

findings, other than the level of DS termination of all

LSTV patients (n = 494), sacralization group (n = 201),

and lumbarization group (n = 293), are outlined in

Table 1.

No significant difference was found in age be-

tween the sacralization and lumbarization groups (P

= .364). There were significantly more women (133 of

201) than men (68 of 201) in the sacralization group,

whereas there were more men (168 of 293) than

women (125 of 293) in the lumbarization group (P <

.001). The 494 patients with LSTV were placed into 4

Castellvi classification subgroups: 101 patients (20.4%)

had LSTV type I, 194 patients (39.3%) had LSTV type II,

141 patients (28.5%) had LSTV type III, and 58 patients

(11.7%) had LSTV type IV. LSTV type I was significantly

more common in the lumbarization group than in the

sacralization group (92 [31.4%] of 293 patients vs. 9

[4.5%] of 201 patients, P < .001), whereas LSTV type III

and IV were significantly more common in the sacraliza-

tion group (84 [41.8%] of 201 patients for type III and

36 [17.9%] of 201 for type IV) than in the lumbarization

group (57 [19.5%] of 293 patients for type III and 22

[7.5%] of 293 for type IV) (P ≤ .001).

In our study population, 53 cases (25 [12.4%] of the

sacralization group and 28 [9.6%] of the lumbarization

group) of sacral perineural cysts were seen. The inci-

dence of perineural cysts was not significantly different

between the 2 groups (P = .375) (Table 1).

The mean shortest linear distance between the up-

per margin of the sacrococcygeal membrane (SCM) and

the DS showed no significant difference between the

sacralization (31.1 mm) and lumbarization groups (27.9

mm) (P = .232) (Table 1).

The level of DS termination was extending from

the lower third of the L5 to the lower third of the S3.

The level of termination of the DS in all of the LSTV

patients was most commonly the lower one-third of

the S2 (in 26.9% of the total population). In comparing

by groups, however, the distribution frequency of the

levels of DS termination on MR images demonstrated

a significant difference between the sacralization and

lumbarization groups. The mean caudal DS level in the

Pain Physician: January/February 2018; 21:73-81

78 www.painphysicianjournal.com

Table 1. Demographic data and results of cross-tabulation of findings

Total

(n = 494)

Sacralization Group

(n = 201)

Lumbarization Group

(n = 293)

P-value

Age (years)*

55 ± 17.8 53.7 ± 18.2 55.9 ± 17.6 0.364

Gender

< 0.001

Male 236 68 168

Female 258 133 125

Subtype

LSTV type I 101 (20.4) 9 (4.5) 92 (31.4) < 0.001

LSTV type II 194 (39.3) 72 (35.8) 122 (41.6) 0.223

LSTV type III 141 (28.5) 84 (41.8) 57 (19.5) < 0.001

LSTV type IV 58 (11.7) 36 (17.9) 22 (7.5) 0.001

Perineural Cyst

0.375

Presence 53 25 28

Absence 441 176 265

Shortest Distance Between SCM

and DS (mm) *

29.1 ± 24.5 31.1 ± 24.5 27.9 ± 24.4 0.232

LSTV = lumbosacral transitional vertebrae, SCM = sacrococcygeal membrane, DS = dural sac

*Data are the mean ± standard deviation

Table 2. Incidence of caudal level of the DS in LSTV patients and comparison between the sacralization and lumbarization groups

Caudal Level of DS Total (n = 494)

Number of LSTV Patients (%)

P-value

Sacralization Group

(n = 201)

Lumbarization Group

(n = 293)

Lower third of L5 5 (1) 5 (2.5) 0 (0) 0.011

Upper third of S1 22 (4.5) 22 (10.9) 0 (0) < 0.001

Middle third of S1 48 (9.7) 47 (23.4) 1 (0.3) < 0.001

Lower third of S1 83 (16.8) 78 (38.8) 5 (1.7) < 0.001

Upper third of S2 75 (15.2) 38 (18.9) 37 (12.6) 0.073

Middle third of S2 71 (14.4) 8 (4.0) 63 (21.5) < 0.001

Lower third of S2 133 (26.9) 2 (1.0) 131 (44.7) < 0.001

Upper third of S3 43 (8.7) 1 (0.5) 42 (14.3) < 0.001

Middle third of S3 12 (2.4) 0 (0) 12 (4.1) 0.002

Lower third of S3 2 (0.4) 0 (0) 2 (0.7) 0.516

LSTV = lumbosacral transitional vertebrae, DS = dural sac

lumbarization group was significantly lower than the

sacralization group (lower third of the S2 [131 {44.7%}

of 293 patients] vs. lower third of the S1 [78 {38.8%}

of 201 patients], P < .001). The caudal end of the DS

terminated above the S2 vertebra in only 2% of the

lumbarization group (6 of 293 patients), while in more

than 75% of the sacralization group (152 [75.6%] of

201 patients). The DS was found to terminate at the

S3 in more than 19% of the lumbarization group (56

[19.1%] of 293 patients), whereas in only one case of

the sacralization group (one [0.5%] of 201 patients)

(Table 2) (Fig. 6).

The mean level of caudal margin of perineural cysts

in the lumbarization group was significantly lower than

in the sacralization group (middle third of the S3 [10

{35.7%} of 28 cases] vs. middle third of the S2 [11 {44%}

of 25 cases], P ≤ .001) (Table 3).

discussion

It has been known that back pain correlates with

LSTV (2,12-15). Therefore, it is not uncommon to en-

counter patients with LSTV in the process of back and

buttock pain management such as epidural steroid

injection or epidural adhesiolysis. Although the cau-

www.painphysicianjournal.com 79

Termination Level of Dural Sac Relevant to CEB in LSTV

Table 3. The vertebral level of caudal margin of perineural cysts, seen in 53 of 494

LSTV patients

Caudal Margin

of Perineural

Cysts

Number of Patients with Perineural Cysts (%)

P-value

LSTV Group

Sacralization Group

(n = 25)

Lumbarization Group

(n = 28)

Lower third of S1 1 (4.0) 0 (0) 0.285

Upper third of S2 9 (36.0) 2 (7.1) 0.016

Middle third of S2 11 (44.0) 0 (0) < 0.001

Lower third of S2 3 (12.0) 4 (14.3) 1.000

Upper third of S3 1 (4.0) 6 (21.4) 0.104

Middle third of S3 0 (0) 10 (35.7) 0.001

Lower third of S3 0 (0) 6 (21.4) 0.016

LSTV = lumbosacral transitional vertebrae

Fig. 6. A bar chart of the information in Table 2; the comparison of caudal

levels of the DS between the 2 LSTV groups.

dal approach to the epidural space is a

relatively safe procedure, extension of

the dura to a lower-than-expected sacral

level as a simple anatomic variant could

increase the risk of dural puncture or

inadvertent spinal anesthesia during

procedure (6,8,16). Hence, our study

investigated the level of termination of

the DS in a group of patients with LSTV

and demonstrated that the mean level

DS termination was significantly lower

in the lumbarization group than in the

sacralization group.

We found that the level of DS ter-

mination in all of the LSTV patients was

most commonly the lower third of the S2.

This finding is in agreement with previous

cadaveric studies (17,18) and MRI studies

with the living population, revealing that

the mean level of the DS tip in adults lies

at the lower border of the S2 (19,20). In

our study, the caudal tip of the DS was

found at the S3 in about 19% of cases of

the lumbarization group, with a higher

rate than earlier reports by Aggarwal et

al (9), Cilliers et al (21), and Senoglu et al

(8). It has been reported that the termi-

nation of the DS at the S3 might result

in a higher rate of dural puncture than

at the S2 (6,8,9). Therefore, more caution

should be taken to the LSTV patients

with lumbarization when using the cau-

dal approach.

Since cystic structures such as pos-

terior sacral meningoceles or perineural

cysts extended to the lower sacrum

could cause unexpected dural puncture

(5,6,22), we also investigated the preva-

lence of these cystic structures in our LSTV

population. Perineural cysts were seen in

about 10% of all patients and no cases of

posterior sacral meningoceles were seen.

The higher prevalence of perineural cysts

in our study (10%) compared to previous

studies (1.3 – 4.6%) (6,8,22) may be partly

explained by a difference in the patient

population selected for the studies, see-

ing as this study was designed based on

LSTV cases. As expected, the mean level

of caudal margin of perineural cysts in

the lumbarization group was significantly lower than in the sacraliza-

tion group, since the DS terminal was located more distally in the for-

mer group. However, unlike previous reports, perineural cysts below

the S3 were not found in any group of LSTV patients (6,8).

With regards to the type of LSTV, the incidence of each subtype of

LSTV was significantly different between groups. In particular, the ma-

jority of type 1 LSTV patients was found in the lumbarization group,

therefore we could assume that sacralization of transitional vertebra

is more inclined to form pseudoarthrosis or complete osseous fusion

with the sacral ala.

During the caudal approach, it is also necessary to know the dis-

tance between the apex of the hiatus and the end of the DS for deter-

Pain Physician: January/February 2018; 21:73-81

80 www.painphysicianjournal.com

mining the length of the needle to be introduced into

the canal (5,7,8). Aggarwal et al (9) found the mean

distance between the apex of the hiatus and the end of

the DS to be 31.6 ± 12 mm (range 5.76 – 60.0 mm). We

found a similar mean distance of 29.1 mm, considering

measurement error, with a similarly large range of dis-

tances 3.7 – 85 mm. Since the minimum value was less

than 5 mm, the needle should be advanced carefully

from the sacrococcygeal ligament during a caudal ap-

proach to prevent dural puncture in patients with LSTV,

as applied to other patients.

However, there are several limitations to the cur-

rent study. First, it is possible that some cases of LSTV

could be missed because we had performed text

searches of radiologic reports of lumbosacral MRI for

the initial patient selection. These missing data might

have influenced the results. Second, for accurate

numbering of a transitional segment, we selected all

LSTV cases that were possible to identify the entire tho-

racic segments and to correctly assign the L1 vertebral

body— for example, patients who had radiographs of

the whole spine or thoracic spine. However, in local clin-

ics, as it is more common to have lumbar spine radio-

graphs alone without other available imaging, accurate

enumeration of the transitional segments could not

always be possible. Third, since this was a single-center,

single-country study, it may have limited the generaliz-

ability. Multi-center, multi-country studies with a larger

number of LSTV cases are warranted to consolidate our

findings. Fourth, in the subtype classifications, we did

not make a distinction between unilateral and bilateral

pseudoarthrosis and fusion. Since it is beyond our scope

to compare the difference of LSTV subtypes seen in the

sacralization and lumbarization groups, it was unneces-

sary to make the model complicated by increasing the

4-level outcome to an 8-level outcome. Finally, mea-

sures of interrater agreement were not obtained about

imaging data; however, analysis of the raw data did not

reveal a big difference between the readers.

conclusion

Consequently, as the main result, our study demon-

strated a difference in the DS termination level between

groups of transitional vertebra with lumbarizaion and

sacralization, which means that the position of the DS

tip in the lumbarization group was significantly lower

than in the sacralization group, and in the lumbariza-

tion group, the proportion of cases that the DS tip was

located at the S3 was greater than that in the previously

reported general population studies. Therefore, when

planning caudal procedures for pain management

in patients with LSTV, especially in the lumbarization

cases, pre-procedural MR imaging to check the termina-

tion level of the DS, the distance between the upper

margin of the SCM and the end of the DS, and the pres-

ence of incidental cystic structures would be of great

use for lowering the risk of unexpected dural puncture

during the procedure.

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