3D statistical modeling techniques to investigate the anatomy of the sacrum, its bone mass distribution, and the trans‐sacral corridors

Abstract

The complex anatomy of the sacrum makes surgical fracture fixation challenging. We developed statistical models to investigate sacral anatomy with special regard to trans-sacral implant fixation. We used computed tomographies of 20 intact adult pelves to establish 3D statistical models: a surface model of the sacrum and the trans-sacral corridor S1, including principal component analysis (PCA), and an averaged gray value model of the sacrum given in Hounsfield Units. PCA demonstrated large variability in sacral anatomy markedly affecting the diameters of the trans-sacral corridors. The configuration of the sacral alae and the vertical position of the auricular surfaces were important determinants of the trans-sacral corridor dimension on level S1. The statistical model of trans-sacral corridor S1 including the adjacent parts of the iliac bones showed main variation in length; however, the diameter was the main criterion for the surgically available corridor. The averaged gray value model revealed a distinct pattern of bone mass distribution with lower density particularly in the sacral alae. These advanced 3D statistical models provide a thorough anatomical understanding demonstrating the impact of sacral anatomy on positioning trans-sacral implants. 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 1 or trans-sacral implants, 2,3 the latter being increasingly used in the treatment of sacral insufficiency fractures. 4 These fractures, occurring predominantly in osteoporotic patients, are isolated to the sacrum or a part of fragility fractures of the pelvic ring 5 and are typically located in the paraforaminal lateral region of the sacral ala. 6 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. 7 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. 7 Their 3D volume was previously computed in an automatic process. 7 The upper sacral anatomy was highly variable with up to 35% of the sacra called "dysmorphic" 10 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, whe...

Transsacral Osseous Corridor Anatomy Is More Amenable To Screw Insertion In Males: A Biomorphometric Analysis of 280 Pelves

“…Furthermore, the larger range of maximum corridor diameters for S1 compared with the smaller more-sigmoidal distribution for S2 (Fig. 4) shows the greater variability of the S1 osseous corridor shape and confirms the results of Wagner et al [39] based on their investigation of 20 healthy pelves.…”

“…Compared with the oblique sacroiliac screw fixation described by Matta and Saucedo [20], additional fracture pattern like central sacral fractures (Denis Zone III) and bilateral posterior pelvic lesions can be addressed through a unilateral approach, whereas alternative sacroiliac screw fixation from both sides was accompanied with an increased risk of screw misplacement [9,12]. Furthermore, in biomechanical studies, increased fixation strength for transsacral screw placement was reported owing to the screw placement in three additional cortices on the contralateral side instead of placing the screw tip in the weaker cancellous bone of the sacral ala and S1 vertebral body [18,33,39]. This seems particularly important in the increasing number of insufficiency fractures of the pelvic ring caused by osteoporotic bone quality [14].…”

“…The same is true for transsacral screw placement. Using a cylindrical-shaped volume for automatic measurement of the osseous corridor diameters does not respect the ovoidshaped osseous corridor at the level of the S1 vertebra and therefore represents only the maximum osseous corridor height, but not necessarily the larger corridor width [11,22,39], which could facilitate placement of an additional second screw in the same corridor [22].…”

“…Furthermore there is limited ability to see all relevant bony structures on the intraoperative two-dimensional fluoroscopic images [7,22,29,31,40]. The corridor for the oblique sacroiliac screw placement in the first vertebra has been investigated in different biomorphometric studies [2,7,8,16], but data for transverse transsacral screw placement at the level of S1 and S2 are limited owing to the small size of study groups, different exclusion criteria, or measurement techniques [11,22,23,37,39].…”

Percutaneous screw fixation of the iliosacral joint: Optimal screw pathways are frequently not completely intraosseous

“…However, pathways have been recognised to be influenced by sacral anatomy [9]. Thus, the objective of the presented study was to (1) describe the dimensions of the pathway for potential cannulated screw fixation, ensuring secure and optimal fixation of the iliosacral joint (e.g.…”