Sacral foramina: view at CT

A computed tomographic anatomical study of the upper sacrum. Application for a user guide of pelvic fixation with iliosacral screws in adult spinal deformity

PURPOSE

Widely used in traumatic pelvic ring fractures, the iliosacral (IS) screw technique for spino-pelvic fixation remains anecdotal in adult spinal deformity. The objective of this study was to assess anatomical variability of the adult upper sacrum and to provide a user guide of spino-pelvic fixation with IS screws in adult spinal deformity.

METHODS

Anatomical variability of the upper sacrum according to age, gender, height and weight was sought on 30 consecutive pelvic CT-scans. Thus, a user guide of spino-pelvic fixation with IS screws was modeled and assessed on ten CT-scans as described below. Two invariable landmarks usable during the surgical procedure were defined: point A (corresponding to the connector binding the IS screw to the spinal rod), equidistant from the first posterior sacral hole and the base of the S1 articular facet and 10 mm-embedded into the sacrum; point B (corresponding to the tip of the IS screw) located at the junction of the anterior third and middle third of the sacral endplate in the sagittal plane and at the middle of the endplate in the coronal plane. Point C corresponded to the intersection between the A-B direction and the external facet of the iliac wing. Three-dimensional reconstructions modeling the IS screw optimal direction according to the A-B-C straight line were assessed.

RESULTS

Age had no effect on the anatomy of the upper sacrum. The distance between the base of the S1 superior articular facet and the top of the first posterior sacral hole was correlated with weight (r = 0.6; 95% CI [0.6-0.9]); p < 0.001). Sacral end-plate thickness increased for male patients (p < 0.001) and was strongly correlated with height (r = 0.6; 95% CI [0.29-0.75]); p < 0.001) and weight (r = 0.8; 95% CI [0.6-0.9]); p < 0.001). The thickness of the inferior part of the S1 vertebral body increased in male patients (p < 0.001). Other measured parameters slightly varied according to gender, height and weight. Simulating the described technique of pelvic fixation, no misplaced IS screw was found whatever the age, gender and morphologic parameters.

CONCLUSION

This user guide of spinopelvic fixation with IS screws seems to be reliable and reproducible independently of age, gender and morphologic characteristics but needs clinical assessment.

LEVEL OF EVIDENCE

Level IV.

Closed posterior superior iliac spine impeding pediculocorporeal S-1 screw insertion

OBJECT

Placement of pedicle screws into S-1 is difficult. In cases in which there is a closed posterior superior iliac spine (PSIS), its medial situation prevents lateral oblique placement of the screw inserter sleeve and directing the screw to the anteromedial aspect of S-1. In the present study, the authors discuss anatomical variations of the PSIS and sacrum, and they describe a safe and effective S-1 screw insertion technique.

METHODS

The relation of 50 PSISs obtained from 25 dry pelvises (15 male and 10 female cadavers) was examined. The distance from the inferolateral aspect of the S-1 superior articular facet to its promontory was estimated. The relation between the point of anterior penetration of the "screw line" and "safe zone" was analyzed. Penetration of screw lines into the S-1 body was also measured. (An illustrative case of closed PSIS is presented with pre- and postoperative computerized tomography [CT] scan findings.) The authors found that that PSIS was situated in 28% of the specimens. When screws were directed anteromedially, the screw lines failed to penetrate the S-1 body in 24% of the male and in 15% of female specimens. The screw lines deviated from the safe zone anteriorly in 34% of the male and in 20% of the female specimens. When the PSIS was medial to the line that connects the inferolateral aspect of the S-1 superior articular facet to the promontory, a classification of closed PSIS was assigned.

CONCLUSIONS

The accuracy of the placement of the screws and their pullout strength are increased when using the present technique. Preoperative CT scanning should be performed to determine the presence of a closed PSIS; in cases in which a closed PSIS is found, the ilium should be resected to enable a greater anteromedial trajectory for placement of S-1 pedicle screws.

Current concepts in imaging of the pelvis and hip

Over the past decade, imaging evaluation of orthopedic conditions of the pelvis and hips has become increasingly complex. Although the conventional radiograph remains the initial examination of choice, the decision between secondary tests, such as computed tomography, MR imaging, or scintigraphy is far from clear. A thorough understanding of current imaging technology is necessary for the clinician to choose the most appropriate examination for a given situation. This article reviews the current status of orthopedic imaging in the areas of acute trauma, stress injuries, osteonecrosis, arthropathies, tumors, and interventional imaging.