Quantification of the Upper and Second Sacral Segment Safe Zones in Normal and Dysmorphic Sacra

Abstract

Sacral dysmorphism occurred in 44% of patients in this consecutive series. Many anatomic differences were consistently found between the two morphologies with clinical relevance to iliosacral screw placement. Specifically, the dysmorphic upper sacral segment safe zone is significantly smaller and more obliquely oriented but is still large enough to accommodate an iliosacral screw in nearly all patients. The second sacral segment safe zone is approximately transversely oriented in both sacral types but is more than twice as large in dysmorphic sacra. This segment may be a primary fixation opportunity in patients with sacral dysmorphism.

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

“…A compensatory larger transsacral osseous S2 corridor in dysmorphic versus nondysmorphic sacra [7,8,11], was confirmed in our study, as shown by a weak negative correlation of the S1 and S2 transsacral corridor diameters. Furthermore, the larger range of maximum corridor diameters for S1 compared with the smaller more-sigmoidal distribution for S2 (Fig.…”

“…4). 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].…”

“…A triangle ratio of 1.5 (calculated by the quotient of the anterior height and width of the first sacral body) represents the boundary ratio for the prevalence of sacral dysmorphism, defined as transsacral corridors with a diameter less than 7.3 mm, unsuitable for screw placement. In contrast to these studies, we defined sacral dysmorphism as the absence of a transsacral corridor at the level of the S1 vertebra, as a high sacral anatomic variability [4] and different corridor diameter cutoffs (5, 9, or 10 mm) exist for transsacral implant placement based on the implant types used (screws, bars, or nails), implant sizes (5-8 mm diameters), and insertion technique (fluoroscopic-controlled versus computer-guided navigation) [2,5,8,11,21,24,37]. Our study provides additional information regarding transsacral corridor-size distributions and proportions of pelves based on their individually set cutoffs in clinical settings (Fig.…”

“…If no transsacral osseous corridor was prevalent, such pelves were defined as having sacral dysmorphism. The rationale for such CT-based designation of sacral dysmorphism was the continuum of reported radiographicbased criteria for sacral dysmorphism [8]. The Ryan procedure with the Welch test (for heterogeneous variances) or the t-test (homogeneous variances) was used for analyses of corridor diameter differences.…”

“…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].…”