Upper sacral segment dysplasia refers to a sacral phenotype in which the size and orientation of the upper sacral segment doesn’t allow safe passage of a trans-iliac, trans-sacral screw and characterized by one or more of the following features/criteria:1
| Criteria | Projection | Frequency |
| Acute alar slope (qualitative criteria without cut-off value) | Outlet | 97% |
| Mamillary process | Outlet | 79% |
| Oval shaped neuroforamen | Outlet | 76% |
| Collinearity (upper segment of sacrum not recessed in pelvis) | Outlet | 50% |
| Residual S1/S2 disc space | True lateral | 47% |
| Tongue-in-groove morphology | Axial CT | 29% |
Lateral view: (4) S1/S2 residual disc space
Axial CT view: (6) Tongue in groove appearance of iliosacral articulation
Quantitatively, sacral dysmorphism is defined by a sacral dysmorphism score >70.
Sacral dysmorphism score = S1 coronal angle + 2 X S1 axial angle
- Coronal angle: measured by an angle formed by a line drawn along the axis of the osseous corridor and a line connecting the top of the iliac crests
- Axial angle: measured as the angle formed by a line drawn on the axis of the osseous corridor and a line connecting the posterior iliac spines
In a dysplastic sacrum, the upper sacral safe zone for iliosacral screw insertion is 36% smaller and more oblique. A modified S1 trajectory with 30° caudal to cranial obliquity on the pelvic outlet view and 15° posterior to anterior on the pelvic inlet view or a S2 screw is recommended in these cases.2
References:
- Wendt H, Gottschling H, Schröder M, Marintschev I, Hofmann G, Burgkart R, Gras F. Recommendations for iliosacral screw placement in dysmorphic sacrum based on modified in-out-in cor ridors. J Orthop Res. 2018;37(1):689–96. https://doi.org/10.1002/ jor.24199.
- Laux, C.J., Weigelt, L., Osterhoff, G. et al. Feasibility of iliosacral screw placement in patients with upper sacral dysplasia. J Orthop Surg Res 14, 418 (2019). https://doi.org/10.1186/s13018-019-1472-7
He is the section editor of Orthopedics in Epomedicine. He searches for and share simpler ways to make complicated medical topics simple. He also loves writing poetry, listening and playing music. He is currently pursuing Fellowship in Hip, Pelvi-acetabulum and Arthroplasty at B&B Hospital.
