Transsacral screw safe zone size by sacral segmentation variations

Abstract

Variations in sacral segmentation may preclude safe placement of transsacral screws for posterior pelvis fixation. We developed a novel automated 3D technique to determine the safe zone size for transsacral screws in the upper two sacral segments in 526 adult pelvis computed tomography scans. Safe zone sizes were then compared by gender and sacral segmentation variations (number of neuroforamen and the presence/absence of lumbosacral transitional vertebrae, AELSTV). Ten millimeters was used as the safety threshold for a large screw. 3 (0.6%), 366 (70%), and 157 (30%) sacra had 3, 4, or 5 neuroforamen, respectively. Eighty-eight (17%) were þLSTV. Safe zone size depended on gender, number of neuroforamen in ÀLSTV sacra and presence of LSTV (p < 0.001) but not on the uni-or bilateral nature of the LSTV. 17% of ÀLSTV sacra were below the safety threshold in S1, 27% in S2, whereas 3% of þLSTV sacra were below in S1, 74% in S2. Of ÀLSTV sacra that cannot take an S1 screw safely, 77% can do so in S2, leaving only 4% of sacra that cannot accommodate a screw safely in either upper segment. The results demonstrate a predictable pattern of safe zone size based on gender and sacral segmentation variations. Keywords: lumbosacral transitional vertebrae; sacral dysmorphism; sacral safe zone; sacral segmentation; transsacral screw Transsacral screws have become a popular method of fixation of the posterior pelvis as they can be placed percutaneously, avoiding posterior dissection and potential wound complications, with minimal blood loss and with the patient supine or prone. 1,2 Transsacral screws are technically challenging in that they span both foraminal regions of the sacrum within an intraosseous corridor that is dependent on sacral morphology. [3][4][5][6][7][8][9][10][11][12] With advances in the technique of fluoroscopically guided percutaneous screw placement and anatomic understanding of the sacrum and surrounding neurovascular structures, there has been a low incidence of neurovascular injury. 3,[13][14][15][16][17] The lumbosacral junction is the most variable portion of the spine with variations in sacral segmentation, thought to be related to its load-related fusion, seen in both number of segments, typically four to six, and lumbosacral transitional vertebrae (LSTV) defined as either sacralization of the lowest lumbar segment or lumbarization of the upper sacral segment. [18][19][20][21][22] LSTV are estimated to be present in 4-30% of the population and have been described since at least the early 20th century. [23][24][25] Castellvi et al. identified four types based on morphology in their classification system for LSTV. 24,25 Type I includes unilateral (Ia) or bilateral (Ib) dysplastic transverse processes; type II, incomplete unilateral (IIa) or bilateral (IIb) lumbarization/sacralization with an enlarged transverse process that has a diarthrodial joint between itself and the sacrum; type III, unilateral (IIIa) or bilateral (IIIb) lumbarization/ sacralization with complete osseous fusion of the tran...

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

“…Using the largest 10-mm diameter cutoff, as reported by Gardner et al [8] and Moed and Geer [24], transsacral osseous corridors at the S1 and S2 vertebrae would have been too narrow in 36% and 26% of pelves compared with 32% and 12% when using the 9-mm cutoff. Measured cylindrical diameters in our study (S1, 13 ± 0.3 mm; S2, 12 ± 2 mm) are similar to those reported by Lee et al [19] (S1: 14 ± 4 mm; S2 11 ± 3 mm) and larger than those reported by Vanderschot et al [37]. (S1, 8 ± 0.9 mm; S2, 7 ± 3 mm).…”

Simple mathematical model of sacroiliac screws safe-zone-Easy to implement by pelvic inlet and outlet views

“…Sacral dysmorphism has been a concern when inserting iliosacral screws owing to the reduced volume within the sacral ala for screw placement. Anatomic studies by Gardner et al and Lee et al have shown that there is up to 1 cm of bone available for screw insertion in 96% of patients with sacral dysmorphism . The results presented here support these conclusions as sacral dsymorphism was not associated with mal‐positioned screws.…”

Critical dimensions of trans‐sacral corridors assessed by 3D CT models: Relevance for implant positioning in fractures of the sacrum

“…In contrast to previous studies, we aligned the 3D model of the pelvis manually in the lateral view to assess the maximal possible diameter. Other studies have conducted measurements on CT images with the problem of CT reconstruction addressed above, created 3D models with automatic segmentation, risking incomplete or imprecise segmentation of cortical bone (especially in elderly patients), or aligned the pelves automatically . Due to some pelvic asymmetry, in our opinion, the latter often results in a possible underestimation of the corridor diameter.…”

“…A “dysmorphic” phenotype of the sacrum has been defined, which offers limited space for implant positioning within the upper sacrum. The percentage of sacra not allowing S1 trans‐sacral implants has been reported to be 11–53%, and is more often present in females and certain ethnicities . In contrast, pelves with “dysmorphic” sacra or small trans‐sacral corridor S1 offer more safe space in S2, with one fourth of these pelves presenting a trans‐sacral corridor in S3 .…”