Percutaneously placed transsacral, transiliac (TSTI) screws are an increasingly common approach for treatment of unstable posterior pelvic ring injuries.1–3 Safe and effective placement of these screws relies on extensive preoperative planning and scrutiny of the preoperative images. Malpositioning of the implants risks injury to the surrounding structures, namely the L5 and S1 nerve roots.3
We present the case of a 33-year-old man who was hit by a car. He sustained a combined mechanism pelvic ring injury with associated right hip dislocation and greater trochanter fracture (see Figure 1, Supplemental Digital Content 1, http://links.lww.com/JOT/A365). He was initially managed with closed reduction of his hip and closed reduction and external fixation of his pelvic ring injury. Imaging revealed persistent right-sided sacroiliac joint widening, and he was offered operative stabilization with TSTI screws.
It is crucial to evaluate for sacral dysmorphism in all patients undergoing TSTI screw placement. This is typically best appreciated on outlet views of the pelvis. Findings suggestive of pelvic dysmorphism include mammillary processes, down-sloping sacral alae, residual disc space between S1 and S2, iliac wings parallel to the lumbosacral junction, and noncircular S1 foramen (see Figure 2A, Supplemental Digital Content 2, http://links.lww.com/JOT/A366). 1,4 In addition, recessed anterior alar cortices and tongue-in-groove sacroiliac articulations1,4 can be appreciated on axial computed tomography (CT) cuts (see Figure 2B and 2C, Supplemental Digital Content 2, http://links.lww.com/JOT/A366). Patients with sacral dysmorphism typically cannot accommodate safe screw placement in the upper sacral segment because of oblique, narrow bony corridors.1,5,6 Conversely, they often have a large bony corridor in the second sacral segment that can accommodate 1 or 2 TSTI screws.1,5,6
Next, it is critical to review axial CT imaging for osseous fixation pathways in the upper sacral segments. The pathways should remain anterior to the sacral neuroforamina and posterior to the anterior sacral cortex. To determine if an adequate corridor exists, measure the anterior to posterior depth between those landmarks at the appropriate level. This should measure at least 7–8 mm to safely accommodate a screw (see Figure 3, Supplemental Digital Content 3, http://links.lww.com/JOT/A367). 7
It is then critical to confirm the presence of adequate bony corridors by templating a starting point on sagittal CT images. In a nondysmorphic sacrum, the starting point is typically located in the caudal-anterior aspect of the first sacral segment (see Figure 4, Supplemental Digital Content 4, http://links.lww.com/JOT/A368). 1 For screws placed in the second sacral segment, the starting point is typically more centrally located. To ensure a safe osseous pathway, a line approximately the size of the cursor arrow should be drawn and the magnification adjusted so that the line measures approximately the size of the screw being used.7
The cursor arrow is then placed at the anticipated starting point. From there, scan laterally in both directions ensuring that the cursor arrow remains completely within a bony corridor, not breaching the sacral cortex in any direction until it has exited the outer table of the ilium. This process is repeated as necessary for any additionally planned screws. These pretemplated starting points correlate with intraoperative lateral fluoroscopic views and help obtain an accurate starting point for your guide pin (see Figure 5, Supplemental Digital Content 5, http://links.lww.com/JOT/A369).
It is important to estimate the anticipated screw length for each planned screw. Knowing the approximate screw length can corroborate accurate guide wire placement intraoperatively. If the guide wire is misplaced obliquely within the corridor, it will measure longer than the templated plan and may indicate the need for adjustment. To do this, the distance between the right and left lateral iliac cortical bones is measured at the appropriate level on the axial view (see Figure 6, Supplemental Digital Content 6, http://links.lww.com/JOT/A370). This process should be repeated for each level for which TSTI screw placement is anticipated.
Last, it can be helpful to estimate approximate angles for inlet and outlet views for intraoperative fluoroscopy using preoperative CT scans. To approximate the inlet angle, measure the angle between a horizontal line and a line tangential to the anterior sacral cortex (see Figure 7A, Supplemental Digital Content 7, http://links.lww.com/JOT/A371). To estimate the outlet angle, measure the angle between a horizontal line and a line drawn along the upper end plate of the appropriate sacral segment (see Figure 7B, Supplemental Digital Content 7, http://links.lww.com/JOT/A371). These will provide approximations for adequate intraoperative imaging, but may need to be adjusted slightly.
Other pitfalls to consider when templating for percutaneous TSTI screw placement include patient position within the CT gantry, misinterpretation of intraoperative fluoroscopy, and inadequate reduction of displaced sacral fractures.7 If a patient is positioned askew within the CT gantry, straight scroll through the sagittal scans cannot be used to template osseous fixation pathways. Ensure that the symphysis pubis is in line with the lumbar spinous processes on the anterior-posterior scout films or that they both appear at approximately the same time when scrolling through the sagittal cuts of the CT scan.7 Misinterpretation of intraoperative fluoroscopy can lead to cortical penetration and iatrogenic nerve injury despite adequate preoperative planning.3,7 Ensure that hip joints and sciatic notches are superimposed on the lateral view, and the anterior cortices of the upper sacral segments are superimposed on the inlet view.1 Inadequate reduction can lead to reduced cross-sectional area and screw penetration. If adequate reduction cannot be obtained by closed methods, an open approach may be required.
On review of our patients imaging, there were several findings suggestive of dysmorphism. When evaluating for adequate bony corridors in the upper sacral segments, it was noted that there was inadequate space for TSTI screws in the upper sacral segment but sufficient space for at least 1 screw in the second sacral segment. With the guidance provided by our preoperative templating, we were able to stabilize our patient's pelvic ring injury by safely placing 2 TSTI screws within his second sacral segment (see Figure 8, Supplemental Digital Content 8, http://links.lww.com/JOT/A372). Before percutaneous screw placement, however, he did require open reduction of his right sacroiliac joint to achieve adequate reduction after several failed attempts at closed reduction (see Video, Supplemental Digital Content 9, http://links.lww.com/JOT/A386).
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