Journal of Orthopaedic Diseases and Traumatology

: 2020  |  Volume : 3  |  Issue : 1  |  Page : 8--10

Pinhole effect and manual bowel gas displacement: Simple two tricks for better fluoroscopy imaging in iliosacral screw fixation

Sandeep Patel, Mandeep S Dhillon, Saurabh Vashisht, Vishal Kumar 
 Department of Orthopaedics, PGIMER, Chandigarh, India

Correspondence Address:
Saurabh Vashisht
Department of Orthopaedics, PGIMER, Nehru Hospital, 15 Ground Floor, Chandigarh - 160 012


Introduction: Presently written abstract. Material and Methods: Total 50 out of 75 patients with pelvic fractures were managed with ilio-sacral screws. Results: At mean followup of 18 months non had iatrogenic injuries and ilio sacral screws were well within bony corridors. Conclusion: The use of pin hole effect and manual displacement of bowel gas shadows can reduce the margin of error while passing percutaneous iliosacral screws.

How to cite this article:
Patel S, Dhillon MS, Vashisht S, Kumar V. Pinhole effect and manual bowel gas displacement: Simple two tricks for better fluoroscopy imaging in iliosacral screw fixation.J Orthop Dis Traumatol 2020;3:8-10

How to cite this URL:
Patel S, Dhillon MS, Vashisht S, Kumar V. Pinhole effect and manual bowel gas displacement: Simple two tricks for better fluoroscopy imaging in iliosacral screw fixation. J Orthop Dis Traumatol [serial online] 2020 [cited 2020 Jul 15 ];3:8-10
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Full Text


Iliosacral screws have emerged as the foremost modality in managing injuries of the posterior pelvic ring.[1],[2] They have established their biomechanical superiority over other methods,[3] but some issues with ease of placement still remain.

The basic prerequisite for iliosacral screw fixation is a good understanding of the anatomy and understanding of safe bony corridors. Computer navigation and intraoperative computed tomography (CT) facilities have ensured that screws are placed well within the bony tunnels, and neural foramina are not violated.[4],[5] Unfortunately, these facilities may not be available in all the centers, where C-arm imaging remains the mainstay of intraoperative imaging for most surgeons. Many times surgeons are frustrated by the fact that an inadequately prepared bowel, with a lot of gas shadows, causes artifacts and obscures adequate viewing of the sacral foramina. The problems caused by gas shadows artifacts in imaging are commonly encountered by pelvic and spine surgeons; however, available literature on solutions for addressing this is not available. To counteract this, we have devised two techniques, which can improve visualization of intraoperative fluoroscopic images during percutaneous iliosacral fixation of the posterior pelvic ring injury. The technique and our experience are presented.

 Technique 1-Pin Hole Effect

Principle of pinhole effect – when an object is viewed, there are multiple point sources of light emanating from it, which lead to the formation of the retinal image because of sharply focused rays on the retina. A blurred image is formed in case of poorly focused rays, but the pinhole effect reduces the number of rays coming from a point object. The size of pinhole can be reduced to such a level that only the principal rays will be projected on the retina. This reduces the blurring circles and will lead to the formation of sharper images.[6]

The authors have routinely applied this pinhole principle for the interpretation of fluoroscopic images for better appreciation of the sacral foramina. The phenomenon of the pinhole effect can be achieved by squeezing the eyes and then focusing on the C-arm image. For illustration purpose of the effect of pinhole effect on image, the authors have tried to demonstrate the effect [Figure 1] using the portrait mode available in the latest smartphones. The principle behind portrait mode is similar to the pinhole effect.{Figure 1}

 Technique 2-Manual Displacement of Bowel Shadows

Principle behind the technique – gas shadows are seen both in the small intestine suspended by the omentum as well as in the large intestine. The gas shadows keep changing their position due to the peristaltic movements of the intestines. Gentle palpation of the abdomen and pelvis in the area of gas shadow (similar to gentle massage movements) tends to shift the shadow from the existent position to a new position. This maneuver helps in locating the sacral neural foramina on the outlet view, which was previously obscured by gas shadows [Figure 2].{Figure 2}

 Indications for the above Two Techniques

These two techniques can be of use in percutaneous iliosacral screw insertion for the management of posterior ring injuries (sacroiliac joint disruptions and sacral fractures) done under fluoroscopic guidance. The scenarios where it truly comes to help are as follows:

Associated abdominal injuries where routine bowel preparation methods cannot be followedPelvic fractures operated in an acute scenario as part of ongoing acute hemorrhage control.

The above techniques are routinely used by the team of pelvi-acetabular surgeons at our institute for the past 4 years. A total of 75 patients presented with pelvic ring fractures between January 2016 and December 2017. Fifty cases among these required surgical management, of which iliosacral screws were inserted in 22 patients. The patients were followed up for a mean of 18 months. Part of the above study has been published as an epidemiology study.[7] All the patients had postoperative X-rays and CT scans. None of the patients had iatrogenic injuries and the sacroiliac screws were well as within the safe bony corridors.

The second study by the same authors between January 2017 and February 2018 evaluated the role of early administration of deep-vein thrombosis prophylaxis in pelvi-acetabular fractures. We had 181 cases of the acetabulum and pelvic fractures, of which 79 patients were pelvic ring injuries. Forty patients of the 79 were operated; and iliosacral screw fixation was done in 20 patients. Postoperatively, contrast venography and CT were done as part of deep venous thrombosis study. Iliosacral screws were within the bony tunnels in safe corridors. There was no violation of sacral foramina in any case.

In the 42 cases operated as part of other studies, these two simple techniques used by the authors lead to improved visualization of intraoperative fluoroscopic images during surgery. Although above two tricks may appear easy and simple, these have never been reported before.

The limitations of the above two tricks are that they are subjective, and hence, there are chances of interobserver variability, and it needs to be evaluated in future.


Injuries of the pelvis are often life-threatening injuries; modern orthopedic philosophy is to aggressively stabilize these, and minimize the risk of mortality. Most surgeons, even in the developing world, are now trained in fixation techniques but are often hindered by less than adequate imaging modalities, which could delay surgery or even lead to referral to other centers with the consequent problems of delay. Some problems that are exclusive to imaging leading to poor surgery are unique to the developing world. Surgeons in these areas thus have to learn some tricks, or even master some unconventional methods, to be able to deliver appropriate care in the emergency setting.

Percutaneous iliosacral screws have emerged as the gold standard in the management of posterior pelvic ring injuries.[8] The biomechanical superiority and ease of application have made this technique popular.[9] This procedure has also been used in acute stages of injury, along with anterior external fixators, as a part of damage control surgery to reduce the pelvis volume and control internal bleed by the tamponade effect.[10] There are thus multiple occasions when pelvic injury patients are directly shifted to the operation theater, and there may not be ample time for good bowel preparation; gas shadows are usually present in this scenario. The above two tricks can be of use in such a situation.

Most spine and pelvic surgeons, who need intraoperative C-arm for evaluation and placement of fixation devices, routinely rely on adequate bowel preparation; unfortunately, there is scarce literature discussing this.[11] Even in our institute, it is a routine practice to use one of the standard techniques of bowel preparations, as all surgeons are aware that gas shadows can cause difficulties in the intraoperative assessment of C-arm images. However, many times an adequate bowel preparation may not be possible, either because of an associated bowel injury or because the patient was taken for an emergency procedure immediately. In such circumstances, the two simple tricks and techniques described by the principal author can be used to facilitate good intraoperative visualization of the C-arm images.

The development of newer techniques of navigation and intraoperative CT further highlights the importance of recognizing the neural foramina and identifying the safe corridors in preventing iatrogenic injuries. However, such state of the art facilities are still not available for all orthopedic surgeons, and there are still many centers in the developing world that rely on conventional C-arm imaging for sacroiliac screws. In conclusion, the above techniques suggested by us can be of significant use for these orthopedic surgeons, and the apprehension about lack of proper visualization during surgery for the posterior pelvic injuries could be reduced.

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Conflicts of interest

There are no conflicts of interest.


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