Image-Based Techniques for Percutaneous Iliosacral Screw Start-Site Localization
Despite the routine use of iliosacral screws for the treatment of a variety of pelvic fractures, the technique is demanding, and complications are well described. This article describes a novel image-based technique for accurately identifying and reproducing the appropriate placement of iliosacral screws. Using the stab-incision technique presented here allows for more accurate landmark identification and safer placement of implants.
The same process is repeated using an outlet view to create outlet lines 1 and 2 (Figures 4A, 4B). At this point there are 4 lines drawn on the patient (Figure 2). A stab incision is made at the intersection of the 2 lines drawn on the lateral buttock; this represents the skin start point, labeled “start incision” (Figure 2). The procedure continues in standard fashion.
The 4 external reference lines serve multiple purposes. First, the lines mark the true lateral start point for the pin at the level of the skin. This contrasts with the standard technique in which bony landmarks are marked on the skin and the surgeon must estimate a point on the skin that will provide an appropriate trajectory to the bony start point on the ilium. Further, the lines can also be used to reorient the cartesian plane so that adjustments can be isolated to a single plane, ensuring movements only alter the position on a single radiographic view (Figure 5).
Discussion
Despite the widespread use of percutaneous screw placement for posterior pelvic ring injuries, this remains a technically demanding surgery. Recent data suggest patient pelvic anatomy is extremely varied, especially the sacrum.7 Further, screw trajectories vary depending on surgical goals, fracture pattern, and number of screws. Taken together, this implies that there is no perfect universal starting site along the external ilium. Therefore, while classic teaching states to begin screw insertion within the vicinity of the intersection of the greater trochanter and the ASIS, it is our experience that this location is often not ideal.
The inlet, outlet, and lateral radiographs are all vital to assess correct trajectory of the guide pin and drill prior to final screw insertion, but the start site remains a critical step to assure a successful surgical outcome. We present 2 techniques, used together or separately, that allow the surgeon to place the initial guide pin more accurately for percutanous iliosacral screws. Though not specifically examined in this study, we think technique 2 has the potential to save operative time and use less fluoroscopic imaging because a lateral image is not required until later in the case. Technique 2 identifies the start point at the level of the skin. This is in contrast to technique 1, which identifies the desired sacral target and requires a surgeon to select a skin start site that will provide an optimal trajectory towards the desired target. Judging trajectory can be difficult, particularly in obese patients, and technique 2 eliminates this extra variable.
It is also important to consider that criteria-based nonorthogonal imaging is required for percutaneous screw placement. In these cases, it is more difficult to judge trajectory corrections because the fluoroscopic beam cannot guide perpendicular corrections as it can in operations that use orthogonal imaging. Adjustments made perpendicular to the fluoroscopic beam will change trajectory in multiple planes.8 Moreover, because the standard cartesian frame of reference is rotated, understanding the location of the sacrum in space can be especially challenging. When using the first technique, sacral landmarks are delineated, and a virtual sacrum drawn on the patient’s exterior helps with orientation. In the second technique, the ideal pin placement is mapped, and the external reference lines guide uniplanar changes. For example, the line drawn co-planar with the inlet view is essentially marking the sacral slope. Therefore, by following this line, uniplanar changes in the cranial and caudal direction are achieved on the outlet view (Figure 5). Because this line is also in reference to the already known ideal pin placement, ideal pin placement can be maintained in 1 radiographic projection while changing the start site in the appropriate direction. In a similar fashion, the co-planar line identified on the inlet view can be used on the outlet image to affect uniplanar changes in the anteroposterior direction. This technique effectively minimizes disorientation when placing percutaneous SI screws. This can be particularly beneficial when placing screws in the prone position.
Conclusion
We have shown 2 techniques that are routinely used at our institution to help identify an accurate starting position for percutaneous screw placement in posterior pelvic ring injuries. Even experienced traumatologists can more quickly and accurately identify the correct stab incisions leading to more confidently placed screws. Further, we believe understanding the usage of fluoroscopy and the concepts involved in drawing the lines enhance trainees’ comprehension of the complex anatomy of the sacrum.
