In our study, we used the technique described by Matta and colleagues for placement of the screws and performed a postoperative CT to evaluate screw placement and to assess pelvic reduction.7 We had a high penetration rate using CT, which increased with better resolution, even though none of the radiographs showed any obvious evidence of misplacement of the screws. Ebraheim and colleagues6 described the relationship of the S1 nerve root in its neural foramen and found it to be approximately 8.7 mm inferior and 7.8 mm medial to the starting point for a pedicle screw. Given these numbers, it is possible that a large amount of skiving can be tolerated contingent on an adequate reduction of the SI joint.
Because of our high rates of skiving and low rates of neurologic deficit, a new “safe zone” for screw insertion can be expanded to include skiving of the S1 neural foramen up to 3 mm without fear of nerve root injury. However, drilling and screw insertion at higher speeds can also cause neurologic injury secondary to thermal injury or soft tissue being caught up in a rotating drill/screw.
Evaluation of placement of percutaneous SI screw placement in our study resulted in neural foramen penetration in 43% of SI screws, which is higher than other studies.14,19,20 Our study showed that screw penetration up to 2 mm does not correlate with neurologic deficit. Iatrogenic neurologic deficit secondary to perforation of the foramina occurred in only 1 patient. Penetration of the foramina in all cases was in the superior portion of the foramen. We propose that there is a safe zone within the S1 neural foramen, and small amounts of penetration in the superior one-third of the foramen on axial CT images do not correlate with neurologic deficit. This potential safe zone is predicated on adequate reduction of the SI joint.
Neural foramen penetration shown on postoperative CT does not necessarily correlate with neurologic deficit. A postoperative CT is not indicated unless there are findings of a postoperative nerve injury. Our ideal screw placement skives the superior S1 foramen allowing for a larger screw diameter in a safe zone.
CT-guided placement has been proposed; however, concerns about radiation exposure, cost, and feasibility with similar outcomes compared with fluoroscopic-guided screw placement has resulted in its falling out of favor.
Iatrogenic nerve injuries are reported to occur in 0% to 6% of all percutaneous SI screw placement.14,21 Risk factors for iatrogenic nerve injury while using fluoroscopic guidance include sacral morphologic abnormalities, presence of intestinal gas, or contrast.22 Although these may be minimized with proper use of fluoroscopy, obtaining anatomic reduction as well as a thorough understanding of the pelvic morphology, the surgeon must be prepared to obtain further studies, such as a CT scan, if there is postoperative neurologic deficit.
Based on our findings, we do not routinely obtain a postoperative CT for SI screw placement, unless there is concern for malreduction or there is neurologic deficit. We also believe that up to 2 mm of foramen penetration is safe and does not result in neurologic deficit.