The sacrum exhibits a complex anatomy that is critical for treating sacral fractures. This is especially true in percutaneous minimally invasive osteosynthesis using sacroiliac screws or trans-sacral implants,[2, 3] the latter being increasingly used in the treatment of sacral insufficiency fractures. These fractures, occurring predominantly in osteoporotic patients, are isolated to the sacrum or a part of fragility fractures of the pelvic ring and are typically located in the paraforaminal lateral region of the sacral ala. Complex anatomy, reduced bone mass, and limited intraoperative visibility make adequate fixation difficult to achieve.
Trans-sacral implants must be placed through safe intraosseous pathways, also termed trans-sacral corridors. They extend laterally from the ilium, traversing the sacroiliac joint, passing through the vertebral body on level S1 or S2 to reach the contralateral side of the sacrum and the ilium. These pathways are bordered anteriorly by the cortex of the anterior sacrum, posteriorly by the vertebral canal, and superiorly and inferiorly by the adjacent neural foramen. In S1, the superior border is formed by the sacral ala. The entrance and exit points are located on the outer surface of the iliac bone. In contrast to safe pathways for sacroiliac screws reaching the vertebral body, trans-sacral corridors are more limited in their critical diameter,[8, 9] exhibiting an oval shape. Their 3D volume was previously computed in an automatic process. The upper sacral anatomy was highly variable with up to 35% of the sacra called “dysmorphic” providing only limited space to position implants on level S1. Surgical fracture fixation is further complicated by areas of different bone mass, especially in the osteoporotic, where screw anchorage is reduced due to decreased bone mass. An area of decreased bone mass (the “alar void”) is located in the paraforaminal lateral region of the sacrum (the sacral ala), whereas in the vertebral bodies, bone mass is comparably higher.
Trans-sacral safe pathways cover distinct anatomical volumes allowing dedicated implants to be placed safely. However, these volumes may display large inter-individual variation regarding size, shape, and available bone mass. These variables can significantly affect surgical decision making and hence the operative procedure.
We used innovative computed tomography (CT) based 3D statistical models to study the anatomy of the human sacrum, addressing the variability in size and shape and the impact on trans-sacral corridors. To assess bone mass distribution, we adopted methods of averaging CT gray values given in Hounsfield Units (HU).
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The sacrum is a complex bone, formed by the fusion of 4–6 vertebrae. In many cases, the fusion site potentially permits trans-sacral implant positioning within safe pathways to fix sacral fractures. Such surgical procedures are complicated by the variation in sacral anatomy and by reduced bone mass in osteoporotic individuals. Here, we used 3D models of the sacrum, the trans-sacral corridor S1, and the mean bone mass by applying advanced computational techniques to provide an understanding of sacral anatomy.
3D statistical modeling is a valid technique to analyze bony anatomy with its statistical variations. PCA is a method in multivariate statistics clustering the 3D anatomical variability representing the largest variability in the 1st PC, the second most important variation in the 2nd PC and further PC having a gradually diminishing influence on the variability. Whitmarsh et al. published a 3D statistical model generated from 80 CT's of the proximal femur and confirmed main variations in the first three PC's to be the size of the bone, the neck-shaft-angle, and the neck length neck. Averaged bone mineral density (BMD) values were calculated within the mean shape, and a PCA was calculated demonstrating bone density variation. Daruwalla et al. described a method using cylindrical parameterization in 21 CT scans of clavicles. They reported main variation in the 1st PC being length, width and thickness of the clavicles, and in the 2nd PC the anatomy varied in the lateral angle and depth. van de Giessen et al. described a statistical shape model of the human lunate and scaphoid from 50 CT scans. The variations were less pronounced in the 1st and 2nd PC with ∼60% of the variation being covered by the first 5 PC's. Kamer et al. studied the anatomy of the calvaria by calculating a statistical model using 80 CT scans. The 1st PC correlated significantly with the size, the 2nd and 3rd PC each with a spot of high variability in the temporal and the occipital region. Using a TPS transformation to parameterize, Noser et al. and Kamer et al. reported scaled and unscaled evaluations of a fuzzy area typically affected in orbital fractures. In our study, TPS transformation was an appropriate method to parameterize the inside and outside surfaces of the sacrum. Semitransparent visualization allowed us to study the influence of the anatomy on the trans-sacral corridors by applying PCA. Due to the tube-like aspect of the trans-sacral corridors lacking distinct anatomical criteria, we adopted a cylindrical parameterization to calculate a 3D statistical model.
We consider PCA of the sacrum helpful in understanding size and shape variabilities and analyzing the impact on the trans-sacral corridors. The configuration of the sacral alae and the vertical position of the sacroiliac joints varied significantly. These were important anatomical structures narrowing the trans-sacral corridor S1, while trans-sacral corridor S2 was less influenced in the critical diameter. This was confirmed by manual measurements; however, our series did not contain a sacrum not allowing placement of a trans-sacral implant with a 7.3 mm diameter. Sacra offering only limited space for implant positioning in the upper sacrum were described as “dysmorphic,” consisting of 14–35%[10, 25-27] in different series applying implants of 5–7.3 mm. A trans-sacral corridor was more consistently available on level S2 in these “dysmorphic” sacra[25-28] depending less on the variable 3D anatomy of the upper sacrum. The relationship between the upper sacral anatomy and the trans-sacral corridors was visualized in the PCA of the sacrum.
The 3D statistical model of the trans-sacral corridor S1 revealed length to be the main variable parameter as shown in 1st PC. The 2nd PC varied mainly in size and shape of the corridor's oval cross-section, hence affecting the dimensions to safely place trans-sacral implants. The knowledge about the previously described oval cross-section, demonstrated by PCA to be relatively consistent in its shape, helps to position a trans-sacral implant using a radial safety zone to the limiting cortical structures.
The averaged gray value model of the sacrum revealed a distinct bone mass distribution. The trabecular bone displayed higher HU in the vertebral bodies and lower HU values located in the sacral ala (paraforaminal lateral) on the levels S1 and S2, correlating to findings with measurements in quantitative CT scans of 13 sacra. Further, values in S2 were lower than in S1. The lower bone mass in the sacral ala impairs implant anchorage; hence, better purchase is given in the vertebral body.
The use of routine clinical CT's to assess bone mass distribution represents a limitation in our study as there was no calibration to BMD values. However, in previous studies a good correspondence of HU in clinical CT's to BMD in dual-energy X-Ray absorption was demonstrated in the spine.[30-33] Further, due to limited spatial resolution and the high content of yellow bone marrow, bone density may be underestimated. The innominate bones, possibly also limiting trans-sacral corridors, were not incorporated into the statistical model of the sacrum. This study compromised a limited number of predominantly male sacra. For future work, increased information about sacral anatomy and the surgically important trans-sacral pathways could be provided by including more individuals into the statistical model and by investigating the influence of other factors, such as ethnicity, age and gender.
We conclude that 3D statistical modeling of the sacrum is a valuable approach to study the surgical anatomy relevant for trans-sacral fixation. The critical safe pathways are influenced by sacral anatomy and hence surgical decision making depends on analysis of the underlying sacral anatomy including the distinct bone mass. Trans-sacral implant positioning is often possible; however, screw purchase is best in the vertebral body and on level S1.