Original Research

The Role of Computed Tomography for Postoperative Evaluation of Percutaneous Sacroiliac Screw Fixation and Description of a “Safe Zone”

Am J Orthop. 2014 November;43(11):513-516

References

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Postoperative Assessment

AP, lateral sacral, and inlet and outlet postoperative radiographs were taken in all cases within 24 hours after surgery. Pelvic CT was also obtained within 24 hours of surgery to review reduction and screw placement.

Using the measurement tool on the PACS system, we measured the penetration of the screw into the foramen. Screws were graded as intraosseous (completely contained within the sacral bone), skived (less than 2 mm of partial penetration into the S1 foramen), or extruded (the screw not contained by the bone). Screw penetration of the S1 was evaluated on the radiographic images as well as the axial images of the CT scans.

After surgery, the senior orthopedic resident and attending surgeon performed and documented detailed neurologic evaluations. They reviewed the medical record for neurologic deficit following surgical fixation.

Results

The mean follow-up time was 12 months (range, 8 months to 2 years). Two patients expired secondary to associated injuries. There were no early deaths related to the pelvic surgery. Stable fixation, including bone or ligamentous healing, as well as full weight-bearing status, was noted in every case. No case exhibited loss of reduction or implant failure or infection.

According to Matta’s criteria of anatomic reduction within 1 cm, all patients were found to have satisfactory reductions.⁷ Six of 46 patients had documented preoperative neurologic deficits. After percutaneous screw fixation, 10 of 46 patients had postoperative neurologic deficit, 2 of which were unchanged from preoperative evaluation. Of the 8 patients with new/altered postoperative neurologic deficit, CT showed neural foramen penetration greater than 2.1 mm in only 2 patients. Both patients underwent screw revision, resulting in improved neurologic deficit. The remaining 4 patients did not have foramen penetration and improved their neurologic function over the course of 2 weeks with return to presurgical status by 6 weeks without necessitating screw removal.

Twenty-three of the 51 screws (45%) had some violation of the S1 foramen on the CT. There were 17 patients with dysmorphic sacrums in which 21 S1 screws were placed. Eleven of 21 (52%) screws showed some penetration of the S1 foramen on CT. There were 29 patients with normal sacral morphology in which 30 S1 screws were placed. Twelve of 30 (40%) screws penetrated the S1 foramen. All violations were in the superior one-third position of the foramen. Two of 46 (4%; 1 with dysmorphism, 1 without) had a new neurologic deficit associated with the surgery (Table). CT showed sacral foramen penetration, and both screws were revised with a better neurologic examination.

High-resolution CTs were obtained in 32 patients, while 14 patients underwent the standard 5.0-mm–cut CTs. Of the 32 patients in which a 2.5-mm high-resolution CT was obtained, 20 (62.5%) had evidence of screw penetration (Figures 1, 2). All violations of the S1 neural foramen were in the superior portion of the foramen.

When compared with patients who had a 5.0-mm CT, the patients who underwent a high-resolution CT were more likely to show neural foramen penetration (P = .3). The average screw penetration into the S1 neural foramen measured 3.3 mm (range, 1.6-5.7 mm) in dysmorphic sacrum and 2.7 mm (range, 1.4-7 mm) in normal sacrum. However, in our study, any foramen penetration of less than 2.1 mm on CT did not result in neurologic deficit.

Discussion

Pelvic fractures are fairly common and represent approximately 5% of all trauma admissions and 3% of all skeletal fractures nationwide.¹The current treatment for SI disruption is either nonoperative or operative. Surgical fixation is technically demanding and surgeons often need a long learning curve to acquire the demanding technique because of the limitations of radiographic visualization of the relevant landmarks.¹⁶

Letournel¹⁷ developed the technique for iliosacral screw fixation for the treatment of posterior pelvic ring injuries, where 1 or 2 large screws (6.5-7.3 mm in diameter) are inserted under fluoroscopic guidance through the ilium, across the SI articulation, and into the superior sacral vertebral bodies using percutaneous techniques. Currently, the standard procedure to accomplish the percutaneous placement of iliosacral screws derives mainly from the technique described by Matta with the C-arm fluoroscopy visualizing the pelvis in 3 views: strict AP, inlet, and outlet views.⁷

Routt and colleagues⁴ recommend a strict lateral view of the sacrum, particularly when crossing the narrow zone of the sacral alar. They reported high union rates and accurate placement of the screws.⁴ There are limitations to the use of biplanar fluoroscopy because the intraoperative images are not orthogonal, with the average arc (67º) between the ideal inlet and outlet. However, because of the variability in sacral anatomy, CT guidance was recommended by others.^2,6,8,18Operating in a CT suite had other complications. Misinterpretation of CT led to “in-out-in” screws, which resulted in neurapraxia.