Original Research

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

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We sought to determine whether computed tomography (CT) is an accurate tool for evaluation of reduction, prediction of neurologic deficit, and evaluation of need for revision surgery in unstable pelvic ring injuries treated with percutaneous sacroiliac (SI) screw fixation and whether any neural foramen penetration violation is safe. Using medical records and radiographic data, we retrospectively evaluated 46 patients with 51 fractures or widenings of the SI joint that were surgically treated with percutaneous SI screw fixation, either alone or associated with anterior fixation. Using the Young and Burgess classification, there were 3 vertical shear injuries, 13 lateral compression injuries, 17 anterior-posterior injuries, 7 sacral fractures, and 6 combination or unclassifiable pelvic injuries. Satisfactory reduction was obtained in all cases.

All patients had postoperative CT scans, and 23 of 51 screws had some foramen penetration with an average of 3.3 mm (range, 1.4-7.0 mm). After percutaneous screw fixation, 10 of 46 patients had postoperative neurologic deficit, 4 of which were unchanged from preoperative evaluation. Of the 6 patients with new or worsened neurologic deficit, CT showed neural foramen penetration of 2.1 and 7.0 mm in 2 patients. Both patients underwent screw revision, resulting in improved neurologic deficit. The remaining 4 patients did not have foramen penetration; their neurologic function improved, with full return at 6 weeks without screw removal. Neural foramen penetration documented with CT did not correlate with neurologic deficit unless the penetration was greater than 2.7 mm. Postoperative CT showing neural foramen penetration was the cause of revision surgery in 2 of 10 patients with postoperative neurologic deficit after percutaneous SI screw fixation.

Based on these findings, we recommend postoperative CT only in those cases where there is new neurologic deficit and screw removal if foramen penetration is greater than 2.1 mm. We also describe a new “safe zone” for screw insertion encompassing the superior 2 mm of the sacral foramen with adequate pelvic reduction.


 

References

Pelvic injuries account for 3% of all skeletal fractures.1 Injury to the sacroiliac (SI) joint is frequently associated with unstable pelvic ring fractures, which are potentially life-threatening injuries. Surgical fixation of these injuries is preferred to nonoperative treatment given the potential for improved reduction and early mobilization and weight-bearing, thereby decreasing perioperative morbidity and improving functional outcome.2

The classic method of surgical fixation of the SI joint consisted of open reduction and internal fixation. This method carried a substantial risk for large dissection, iatrogenic nerve injury, and increased blood loss to the already traumatized patient.3 Percutaneous fixation allows for a shorter operating time, decreased soft-tissue stripping, and decreased blood loss compared with a traditional open procedure.4 However, posterior pelvic anatomy is complex and variable, and reports have found screw misplacements as high as 24%5 and neurologic complication rates up to 18%.6-9 

Various imaging modalities, including fluoroscopy,5 computed tomography (CT),6-7 fluoroscopic CT, and computer-assisted techniques5,9 have been used to achieve proper screw placement. Conventional fluoroscopy is the standard for intraoperative screw placement. However, acceptable reduction of the SI joint and proper implantation of the screws without perforation of the neural foramina is challenging, especially when coupled with difficulties of fluoroscopic imaging and variations in pelvic anatomy. 

Sacral dysplasia has been reported to occur in up to 20% to 40% of the population and has significant implications in patients indicated for iliosacral screw placement.10 Incorrect placement of iliosacral screws may result in iatrogenic neurovascular complications.11-13 Malpositioned screws using fluoroscopic guidance have been reported in 2% to 15% of patients with an incidence of neurologic compromise between 0.5% and 7.7%. As little as 4° of misdirection can result in damage to neurovascular structures.14

At our institution, we routinely obtained postoperative CT to evaluate the placement of SI screws. The objective of this retrospective study is to evaluate the rate of revision surgery of percutaneous SI screw fixation, to determine whether CT is an accurate tool for evaluation of the reduction and the need for revision surgery, and to decide if any violation of the neural foramina is safe.

Materials and Methods

After institutional review board approval, we retrospectively reviewed and evaluated medical records and radiographs of all patients who sustained unstable pelvic ring fractures between July 1, 2005, and June 30, 2010. We identified all patients who were treated with closed reductions and percutaneous iliosacral screw fixation, according to the method described by Routt in 1995.4 We excluded all pelvic fractures in patients who underwent open reduction for the posterior injury or did not have percutaneous SI screws placed, those with spinal injury, and those without follow-up. Of the 46 patients who met the inclusion criteria were 26 men and 20 women with a mean age of 42 years (range, 16 to 73 years). Motor vehicle accidents accounted for 13 cases; 19 were crush injuries and 14 were falls from height. Seventeen patients (37%) met the radiographic criteria for sacral dysmorphism. Forty-two of the 46 patients were polytrauma patients with associated musculoskeletal injuries and/or abdominal, chest, or head injuries.

Six patients presented with some neurologic deficit at the time of injury; all fractures were closed. The initial imaging study included plain anteroposterior (AP), inlet, and outlet radiographs of the pelvis and a pelvic CT scan. Using the classification of Young and Burgess,15 there were 3 vertical shear injuries, 13 lateral compression–type injuries, 17 anterior-posterior–type injuries, 7 sacral fractures, and 6 combination- or unclassifiable-type pelvic injuries. Of the sacral fractures, there were 3 Denis zone 1, 3 Denis zone 2, and 1 Denis zone 3. 

The pelvic CT scan included the entire pelvis from the ilium to the ischial tuberosities. Each scan consisted of either a 5.0-mm or a 2.5-mm sequential axial image. A picture archiving and communication system (PACS) workstation using Centricity version 2.1 (GE Medical Systems, Waukesha, Wisconsin) was used to analyze each scan with a bone algorithm. On PACS, each initial displacement was characterized by the amount of SI joint widening at the level of the S1 and was measured using digital calipers.   

Surgery

Mean time to surgery was 4 days (range, 2 to 15 days) after the injury. A total of 51 SI screws were implanted in 46 patients. We achieved closed reduction of the posterior pelvic ring by various techniques, including compression with percutaneous partially threaded screw fixation. In the cases in which the posterior ring lesion was associated with a pure pubic symphysis disruption, the anterior pelvis was initially reduced and stabilized with small-fragment plate fixation (Synthes, Inc, Paoli, Pennsylvania). The posterior complex was stabilized with 1 screw in 41 patients, 2 cases required a transiliac screw, and 2 screws (S1 and S2) were placed in each of the remaining 3 cases. Definitive stabilization of the posterior pelvis was achieved with percutaneous, partially threaded 7.3- or 7.5-mm–diameter cannulated screws (Synthes, Inc, and Zimmer Inc, Warsaw, Indiana, respectively) in 42 fractures and 6.5-mm screws (Synthes, Inc) in 4 fractures. In 11 cases where the fracture was through the sacrum, fully threaded cannulated screws were used to avoid compression. Screw insertion was performed under fluoroscopic guidance with inlet, outlet, and lateral sacral views. One of 2 fellowship-trained trauma surgeons performed the surgeries. Rehabilitation plans were customized to each patient based on concomitant injuries. 

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