Open technique for supra-acetabular pin placement in pelvic external fixation: a cadaveric study

Standard techniques for identifying the entry point of a supra-acetabular pin using fluoroscopy have been widely used and reported [5,6,7], and, more recently, ultrasound-assisted entry was utilized in a cadaver study [13]. Due to the superficial structure of the AIIS, which can be palpated, the most accurate guidance tool we have is digital palpation. There is no need for fluoroscopy in unfamiliar views that may reduce the surgical field (especially during utilization of the obturator oblique view, which interferes with the surgeon’s position when applying the Schanz screw) [7, 8], lead to unnecessary radiation exposure, and require extra time.

LFCN injuries are unavoidable due to the inherent variation in this zone of pin placement under both open and fluoroscopic guidance techniques [11], but a mini-open blunt dissection with soft tissue protection technique can minimize these potential injuries [14]. Hip joint penetration is still a risk if pin placement is below the AIIS [8], which can lead to septic arthritis [9]. According to this cadaver study, the open technique without fluoroscopy may not increase the risk of hip joint or sciatic notch penetration compared to fluoroscopic guidance, since we found that one pin in the fluoroscopic guidance group penetrated the hip joint but we did not find any such occurrence in the open technique group. The hip capsule in our study inserted above the acetabulum edge 12 mm proximal to the joint on average (range, 8–25 mm), whereas Haidukewych et al. found that the hip capsule inserted up to 16 mm proximal to the joint on average (range 11–20 mm), and recommended half-pin placement at least 20 mm from the upper edge of the acetabulum [5]. Radiological landmarks related to the AIIS were investigated by Lidder et al. to avoid intra-capsular placement of the pins. They concluded that the pin should be placed in the upper half of the supra-acetabular bone tunnel, which broadly corresponds to the superior half of the AIIS [8]. In this study, we found that hip capsule distance ranged between 8 and 16 mm in 94%, which will not violate the hip capsule if the pin is placed at least 20 mm from the upper edge of the acetabulum, but one hip capsule distance extended up to 25 mm, which may possibly cause pin penetration into the hip joint, so it is reasonable that the pin should be placed in the superior half of the AIIS to avoid intra-capsular placement of the pin. In this study, we found that one pin penetrating into the hip joint was placed in the lower half of the AIIS (Table 1).

We did not encounter sciatic notch penetration, which can cause injury to the superior gluteal neurovascular bundle and lumbosacral trunk [10], because our pin medial inclination angle was not more than 45° in both groups. Pin penetration through the external cortex of the ilium was found in 4 hemipelves in the open technique group, for which the mean pin medial inclination angle was 19.8° (range 5–40°), which was less than in the percutaneous group: 30.4° (range 20–45°). In 2021, Krassnig et al. analyzed computed tomography (CT) scans in trauma patients who were examined with a whole-body CT and had an uninjured pelvic region, and they reported that the angle of insertion of the supra-acetabular pin in the transversal plane (the angle between the Schanz screw and the median sagittal plane) was 21.8° (range 11.7–31.8°) [15]. The pin medial inclination angle of 20° and a cephalad inclination of 10–20° seen in our study cannot be accurately obtained with every pin, just as in clinical practice; these pin angles may vary because of the shape of the AIIS, which causes the pin to tend to slide medially or laterally. Digital palpation of the inner cortex of the ilium to guide the angle may help with pin orientation to prevent inner cortex penetration, but it may not be a reliable reference due to its deep structure, and it cannot represent the proper orientation of the pin or its tendency to exit from the outer cortex of the ilium. Our study indicated that the pin angle itself is most reliable, even for a distorted bony anatomy, due to its large corridor and greater difficulty in exiting the outer cortex earlier, as with an iliac crest pin.

Thus, we recommend that the pin medial inclination should be at least 20° (the lower limit observed for the percutaneous group) to avoid lateral cortex penetration, and should not be more than 45° (the upper limit for both groups) to avoid sciatic notch penetration in the open technique, but these angles may be altered in unstable pelvic fracture patients with a distorted bony anatomy. The pin may extend about 5 cm into the bone, as all the threaded parts of self-tapping Schanz screws anchor inside the cancellous bone and are not long enough to exit from the bony ilium. Krassnig el al. stated that the mean length of the intraosseous part of the Schanz screw in supra-acetabular placement was 80.4 mm, but it was statistically significantly shorter in females, with an average of 74.1 mm (it was 82.7 mm in males) [15].

We do not recommend using an open technique in unstable pelvic fracture patients whose posterior ring stability must be restored and for whom intra-operative fluoroscopy is available, because the accuracy involved in inserting the longest pin toward the posterior inferior iliac spine is unpredictable. However, in cases where only anterior pelvic stabilization is needed, or where fluoroscopy cannot be utilized (such as when a radiolucent operating table is not available, as in a life-saving procedure for an unstable pelvic injury in the emergency room), this open technique takes only one-third of the time required for the percutaneous technique, and is a better way to get adequate stability due to its larger bony corridor than when the pin is placed in the iliac crest. We advocate using this useful technique, as it makes it easier to get adequate stability and, similar to when the pin is placed in the iliac crest, there is no need for fluoroscopic guidance.

There are several limitations of this study. Firstly, pin placement was performed by an experienced trauma surgeon. Secondly, elderly cadaver anatomy cannot be representative of young adult anatomy because of the distorted bony anatomy in pelvic ring injuries. Thirdly, the number of cadavers used in our study is low, but the results are in line with those previously described in the literature [5, 8]. However, all structures at risk should be recognized prior to using this technique, including the femoral artery, which can be palpated medially to the entry point in a patient with a pelvic ring injury.

Additional radiographic studies are necessary to define the proper pin angle to facilitate safe placement of the pin without fluoroscopy in unstable pelvic fracture patients.

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