Review Question - QID 214049

The S1 corridor is most commonly narrower than the S2 corridor and is not a feature unique to sacral dysmorphism.

Sacral dysmorphism is an anatomic variant that is seen in up to 40% of adults. Radiographic signs of sacral dysmorphism include an anterior up-sloping upper sacral ala, an irregular, and non-circular sacral nerve root tunnels, residual S1 disk, presence of mamillary processes, and a tongue-and-groove SI joint. Moreover, if the osseous pathways in the upper sacral segment are narrower and more oblique, then the safe placement of transiliac-transssacral screws at the S1 level cannot be achieved. Therefore, the much larger S2 corridor allows for the placement of a transiliac-transsacral screw in dysmorphic patients.

Wendt et al. reviewed iliosacral osseous corridor diameters in sacral dysmorphism. Utilizing CT imaging, the iliosacral corridors of sacral dysmorphism were determined by measuring trajectories, diameters, and lengths. They report that the prevalence of transsacral osseous corridors with diameters of <7.5 mm in axial CT images correlates with qualitative and quantitative criteria of sacral dysmorphism. They demonstrated that enlarging the osseous corridor diameters by penetration of the posterior iliosacral recesses increases the safe corridor diameters. They conclude that the dysmorphic sacra can be reliably detected on standard axial CT slice images and modified in-out-in corridors on the level of S1-vertebra allow screw placement in all patients but is still demanding compared to non-dysmorphic sacra, due to the oblique corridor axis.

Conflitti et al. retrospectively quantified upper sacral dysmorphic osseous anatomy and assess its impact on second sacral segment iliosacral screw insertion in 24 patients with unstable posterior pelvic ring disruptions and sacral dysmorphism. They report that the dysmorphic S1 and S2 width available for screw insertion averaged 13.2 mm and 15.2 mm, respectively. The maximum potential screw length for the dysmorphic S1 and S2 averaged 100.8 mm and 151.9 mm, respectively. They concluded that dysmorphic S1 segments are anatomically competent for routine screw fixation, but the S2 segment provides a larger osseous site for screw insertion than S1 in a dysmorphic sacrum.

Gardner et al. reviewed the obliquity and dimensions of the upper and second sacral segment iliosacral screw safe zones and to determine the differences between normal and dysmorphic sacral morphology. They report with sacral dysmorphism, the upper sacral segment safe zone cross-section was 36% smaller than in normal sacra and no transverse screws could be placed. Conversely, a transverse screw could be placed in 75% of normal sacra. They concluded that in the second segment safe zone, however, the cross-sectional area was more than twice as large in dysmorphic sacra compared to normal sacra and a transverse screw could be placed at this level in 95% of those with dysmorphic sacra and in only 50% of normal sacra.

Illustration A depicts the narrower corridor in a dysmorphic sacrum. Illustration B depicts the anterior up-sloping upper sacral ala in a dysmorphic sacrum. Illustration C depicts the acute angle for an S1 screw precluding transiliac-transsacral screw through the S1 corridor. Illustration D is demonstrates the presence of mamillary bodies. Illustration E depicts irregular, non-circular, sacral foramen. Illustration F depicts the residual S1/S2 disc.

Incorrect Answers:
Answers 1,2,4,5: These are radiographic markers for sacral dysmorphism