Image-guided sensory block and radiofrequency ablation of the nerves innervating the sacro-iliac joint require readily identifiable bony landmarks for accurate needle/electrode placement. Understanding the relative locations of the transverse sacral tubercles along the lateral sacral crest is important for ultrasound guidance, as they demarcate the position of the posterior sacral network (S1–S3 ± L5/S4) innervating the posterior sacro-iliac joint. No studies were found that investigated the spatial relationships of these bony landmarks. The purpose of this study was to visualize and quantify the interrelationships of the transverse sacral tubercles and posterior sacral foramina to inform image-guided block and radiofrequency ablation of the sacro-iliac joint. The posterior and lateral surfaces of 30 dry sacra (15 M/15 F) were digitized and modeled in 3D and the distances between bony landmarks quantified. The relationships of bony landmarks (S1–S4) were not uniform. The mean intertubercular and interforaminal distances decreased from S1 to S4, whereas the distance from the lateral margin of the posterior sacral foramina to the transverse sacral tubercles increased from S1 to S3. The mean intertubercular distance from S1 to S3 was significantly (
Radiofrequency ablation (RFA) of the lateral branches of the posterior sacral rami has been increasingly used to treat chronic, refractory sacroiliac joint (SIJ) complex pain [
In a recent anatomical study related to RFA, carried out in our laboratory, the frequency and course of the lateral branches innervating the posterior aspect of the SIJ were documented using digitization and three-dimensional (3D) modeling [
Ultrasound-guided sacro-iliac joint (SIJ) radiofrequency ablation (RFA): posterior sacral network (PSN) lateral crest technique. (a) Dissected specimen of the PSN innervating the posterior aspect of the SIJ demonstrating the location of the RFA strip lesion (white line between the first (TST1) and third (TST3) transverse sacral tubercles). The position of the ultrasound probe is indicated by the black rectangle. PSF, posterior sacral foramen; P, posterior superior iliac spine. (b) Transverse ultrasound scan showing sacral bony landmarks at the level of the third transverse sacral tubercle (TST3). Note that the increments of the scale bar on the left margin are 1 cm. AT, articular tubercle.
No literature was found describing the geometry and spatial relationships of the TSTs and posterior sacral foramina. Previous studies focused on quantification of the dimensions and distances between the posterior sacral foramina (Table
Studies quantifying posterior sacral foramina and interforaminal distance.
Cadaveric/imaging studies
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Specimens | Method | Posterior sacral foramen | Interforaminal distance (mm) | |
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McGrath and Stringer 2011 [ |
27 | Dry sacra | Photo scale bar | X | S2: |
S1-S2: |
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Arman et al. 2009 [ |
100 | Dry sacra | Vernier caliper | S1: |
S1: |
S1-S2: |
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Ebraheim et al. 1998 [ |
21 | X-Ray dry sacra | Ruler on X-ray | S1: |
X | S1-S2: |
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Ebraheim et al. 1997 [ |
20 | Cadaveric pelvis | Caliper | X | X | S1-S2: |
Sex comparison studies
Authors | Posterior sacral foramen | Interforaminal distance (mm) | ||||
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Height (mm) | Width (mm) | |||||
Male | Female | Male | Female | Male | Female | |
McGrath and Stringer 2011 [ |
X | X | S2: |
S2: |
S1-S2: |
S1-S2: |
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Male |
Female |
Male | Female | Male | Female | |
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Ebraheim et al. 1998 [ |
S1: |
S1: |
X | X | S1-S2: |
S1-S2: |
Significant (
Image-guided SIJ RFA techniques require readily identifiable bony landmarks for accurate needle/electrode placement. Understanding the relative locations of the TSTs is important when using ultrasound guidance, as the TSTs demarcate the position of the PSN innervating the posterior aspect of the SIJ [
Thirty dry adult sacra (15 M/15 F) from the teaching collection were used in this study. These sacra were selected as they were the only specimens that were completely intact without bone chipping and/or visible pathology. Ethics approval was received from the University of Toronto Health Sciences Research Ethics Board (#27210).
Each sacrum was stabilized with a clamping system, leaving the entire posterior surface accessible for digitization. A MicroScribe™ G2X Digitizer (Immersion Corporation, San Jose, CA; accuracy: ±0.23 mm) was used to collect 3,000–4,300 data points per sacrum. The posterior and lateral surfaces of the sacrum, including the TSTs along the lateral sacral crest, auricular surfaces, and the circumferences of the posterior sacral foramina, were digitized. The data were imported into Autodesk® Maya® 2013 (Autodesk Inc., San Rafael, CA) and reconstructed into fully manipulable 3D models using custom plug-ins developed in our laboratory. All parameters were quantified bilaterally from the 3D models using the measurement tool in Maya®. To quantify the heights and interforaminal distances of the posterior sacral foramina, a vertical reference line through the center of each foramen, extending from the superior margin of the first posterior sacral foramen to the inferior margin of the fourth posterior sacral foramen, was used (Figure Height of the first, second, third, and fourth posterior sacral foramina was measured as the distance between the superior and inferior margins of each foramen at the reference line (H1–H4, Figure Interforaminal distance was measured between adjacent posterior sacral foramina at the reference line (D1-2, D2-3, and D3-4, Figure Distance from the superior margin of the first posterior sacral foramen to the inferior margin of the third posterior sacral foramen at the reference line (TD1–3, Figure Distance from the midpoint of the lateral margin of each posterior sacral foramen to the center of the ipsilateral TST of the same number (DT1, DT2, DT3 and DT4, Figure Intertubercular distance was measured between the centers of TST1 and TST2 (IT1-2), TST2 and TST3 (IT2-3), and TST3 and TST4 (IT3-4). See Figure Intertubercular distance was measured between the centers of TST1 and TST3 (IT1–3, Figure
The relationships of the transverse sacral tubercles to each other and to the posterior sacral foramina were quantified using the following parameters:
The relationship of the most inferior point of the auricular surface of the sacrum (i.e., the most inferior extent of the synovial part of the SIJ) to TST2 was quantified using a horizontal reference line drawn from the center of TST2 to the margin of the auricular surface. The distance between the most inferior point of the auricular surface of the sacrum and the reference line (IMAS) was measured (Figure
Quantification of sacral landmarks. (a) Distance from lateral margin of foramen to adjacent transverse sacral tubercle (DT), foraminal height (H), interforaminal distance (D), total distance from S1–S3 foramina (TD1–3), and transverse sacral tubercle (
Descriptive statistics were used to summarize each parameter: (1) for all sacra and (2) by sex. The sex of each sacrum was determined using Flander’s index-2 formula [
The shapes and sizes of the posterior sacral foramina were variable (Figures
Morphology of male and female sacra. Photograph (a) and model (b) of a male sacrum; photograph (c) and model (d) of a female sacrum. DT, distance from lateral margin of foramen to adjacent transverse sacral tubercle; TD1–3, total distance from S1–S3 foramina; IT1–3, intertubercular distance from TST1 to TST3;
Variation in the shape and size of the posterior sacral foramina. (a) and (b) photographs of male sacra; (c) and (d) photographs of female sacra.
The mean heights of the posterior sacral foramina decreased in all specimens from S1 (
Quantification of parameters of posterior sacral foramina.
PSF | Mean foraminal height ± SD (mm) | Mean distance from lateral margin of PSF to TST ± SD (mm) | ||||
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All (M + F) | Male | Female | All (M + F) | Male | Female | |
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PSF, posterior sacral foramen (S1–S4); TST, transverse sacral tubercle;
In all specimens, the mean interforaminal distance decreased from S1-S2 to S3-S4 by approximately 2 mm (Table
Quantification of interforaminal and intertubercular distances.
PSF | Mean interforaminal distance ± SD (mm) | Mean intertubercular distance ± SD (mm) | ||||
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All (M + F) | Male | Female | All (M + F) | Male | Female | |
S1-S2 |
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S2-S3 |
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S3-S4 |
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S1–S3 |
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PSF, posterior sacral foramen (S1–S4);
In all sacra, the mean distance from the lateral margin of the posterior sacral foramen to the adjacent TST increased from S1 (
The mean intertubercular distance decreased from superior to inferior by approximately 6 mm (Table
The inferior margin of the auricular surface of the sacrum was located inferior to TST2 (
Knowledge of the relationships of the TSTs and the posterior sacral foramina provides an anatomical basis for sensory blockade of the SIJ and for estimating the length and width of strip lesions for US and fluoroscopically guided SIJ RFA procedures.
In the two previous studies investigating the heights of the posterior sacral foramina, Arman et al. [
The interforaminal distances reported in the current study and in the literature decreased from S1 to S4 (Tables
The S2 posterior sacral foramen has been used as a reference point to demarcate the inferior limit of the SIJ for intra-articular injections [
The intertubercular distance from TST1 to TST3 is clinically important as it demarcates the superior and inferior extent of the PSN innervating the posterior aspect of the SIJ [
The distance between the lateral margin of the posterior sacral foramen and the medial margin of a strip lesion along the lateral sacral crest would increase from S1 to S3 (Table
Distance between posterior sacral foramen and the medial margin of a strip lesion from TST1 to TST3.
PSF | Mean DT (mm) | Mean distance: medial margin of lesion to lateral margin of PSF (mm) | ||||||||
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8 mm lesion diameter | 9 mm lesion diameter | 10 mm lesion diameter | ||||||||
M & F | M | F | M & F | M | F | M & F | M | F | ||
S1 | 6.34 | 2.34 | 1.92 | 2.75 | 1.84 | 1.42 | 2.25 | 1.34 | 0.92 | 1.75 |
S2 | 7.14 | 3.14 | 2.71 | 3.21 | 2.64 | 2.21 | 2.71 | 2.14 | 1.71 | 2.21 |
S3 | 9.45 | 5.45 | 5.68 | 5.22 | 4.95 | 5.18 | 4.72 | 4.45 | 4.68 | 4.22 |
DT, distance from lateral margin of foramen to transverse sacral tubercle; PSF, posterior sacral foramen; TST, transverse sacral tubercle.
The interrelationships of the TSTs and posterior sacral foramina were quantified to inform image-guided block and RFA of the SIJ. To ablate the PSN, the nerve plexus innervating the posterior aspect of the SIJ, the block/RFA strip lesion should extend from TST1 to TST3 (mean distance:
The authors declare that they have no conflict of interests.
The authors wish to thank William Wood for his valuable technical assistance, Tanya Robinson for her professional expertise in preparation of the figures, and the members of the Parametric Human Project for their discussions and insights. The authors also wish to thank the individuals who donate their bodies and tissue for the advancement of education and research.