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Does Posterior Fixation of Partially Unstable Open-Book Pelvic Ring Injuries Decrease Symphyseal Plate Failure? A Biomechanical Study

Metz, Rachelle, M., MD*; Bledsoe, J., Gary, PhD*,†; Moed, Berton, R., MD*

doi: 10.1097/BOT.0000000000001083
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Objective: Recent clinical study suggests an advantage to adding an iliosacral screw to the anterior fixation construct for anteroposterior compression type-2 (OTA/AO type 61-B1), partially unstable open-book pelvic ring injuries. Others have described stress examination to determine any required supplemental fixation. However, biomechanical studies investigating iliosacral fixation requirements for this injury are lacking. Our objective was to determine whether adding an iliosacral screw to symphyseal plate fixation decreases displacement in a well-defined open-book pelvic ring injury model.

Methods: An open-book pelvic ring injury was created in 10 human cadaveric pelves by unilaterally releasing the sacrospinous, sacrotuberous, and anterior sacroiliac ligaments plus transection of the pubic symphysis, approximating the classically described anteroposterior compression type-2 (APC-2) injury. Specimens were divided into 2 groups: (1) symphyseal plating and (2) plating plus an iliosacral screw. Using a standard bilateral stance model loaded at 550 N, displacement measurements were obtained at 210,000 and 500,000 cycles.

Results: Three specimens failed before 210,000 cycles because of technical errors and were excluded from analysis. For the remaining 7, there was no significant difference in displacement between the 2 groups, and none sustained implant failure. Post hoc analysis showed that a large sample size (45/group) would be required to detect any difference with 80% power, indicating a small effect size with limited clinical application.

Conclusions: Adding an iliosacral screw to the symphyseal plate fixation does not provide improved biomechanical outcome in classically described APC-2 injuries. Clinically, stress examination may be useful to determine the need for supplemental posterior fixation in APC-2 injuries.

*Department of Orthopaedic Surgery, Saint Louis University School of Medicine, St. Louis, MO; and

Department of Biomedical Engineering, Parks College of Engineering, Aviation and Technology, Saint Louis University, St. Louis, MO.

Reprints: Berton R. Moed, MD, Department of Orthopaedic Surgery, Saint Louis University School of Medicine, 3635 Vista Avenue, 7th Floor Desloge Towers, St. Louis, MO 63110 (e-mail: moedbr@gmail.com).

Supported by funding from an AOTrauma North America Resident Research Grant. Otherwise, no benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

The authors report no conflict of interest.

Accepted November 02, 2017

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INTRODUCTION

The pelvis is a ring-like structure made up of 2 innominate bones and the sacrum.1 These bones by themselves provide no inherent stability. The biomechanical stability of the pelvic ring is mainly due to its surrounding soft tissues, namely the pubic symphysis, the posterior sacroiliac complex, and the pelvic floor.2 The posterior sacroiliac complex consists of the iliolumbar, interosseous, posterior sacroiliac, and anterior sacroiliac ligaments.1–3 The main pelvic floor components are the sacrotuberous and sacrospinous ligaments.1–3 The anterior structures, including the pubic symphysis and the pubic rami, contribute approximately 40% to the stability of the pelvis. The remaining 60% of the stability of the pelvis is provided by the posterior structures, including the sacroiliac joints.2 Clinically, determining the magnitude of pelvic instability after injury is an important factor in deciding on appropriate patient management. Historically, pelvic injury classification systems have been instrumental in making this determination, serving as a general guide to treatment.2,4,5 Over time, a number of systems have been developed for the classification of pelvic ring injuries.2,5–8 They have been incorporated into a comprehensive classification system developed by the AO and the Orthopaedic Trauma Association.9 However, the classification system developed by Young and Burgess remains widely used.5,8

Injuries to the pelvic ring occur along a wide spectrum, ranging from physiologically stable to completely unstable. Similarly, treatment may range from nonoperative management to internal fixation of the anterior and posterior arches of the ring. The unilateral open-book injury in which the affected hemipelvis is externally rotated is characterized by disruption of the anterior arch of the pelvis, usually through the symphysis pubis.2 In unstable OTA/AO type 61-B1 open-book injuries, along with the pubic symphyseal injury, the pelvic floor (including its sacrospinous and sacrotuberous ligaments) is disrupted and the anterior sacroiliac ligaments rupture; however, the posterior ligaments remain intact.2,5,9 The hemipelvis is defined as being partially unstable, and translational displacement is not possible. This injury corresponds to the anteroposterior compression (APC) type-2 category of the Young and Burgess classification.5,8 Complete disruption of the posterior sacroiliac complex or arch renders the hemipelvis completely unstable and subject to translational displacement. This injury corresponds to OTA/AO type 61-C and Young and Burgess APC type-3 classification subgroups.

It has been generally accepted that the APC-2 (OTA/AO type 61-B1) open-book pelvic ring injury requires anterior fixation alone, usually with a pubic symphyseal plate.10–13 More recently, a need for supplemental posterior ring fixation with an iliosacral screw has been recognized.12,14–17 However, the specific clinical indicators for this supplemental fixation have not been identified.14,16–18 Therefore, the spectrum of pelvic ring disruption represented by the APC-2 group, along with its preferred fixation construct, remains ill-defined, constituted by a potentially wide range of injuries and awaits further study.12,14–18

In 2011, Sagi et al15 proposed a modification to the Young-Burgess classification of open-book injuries based on a dynamic stress examination with fluoroscopy under anesthesia (EUA). In this modification, APC-2 injuries are divided into 2 subgroups: (1) those requiring only anterior fixation (APC-2a) and (2) those requiring additional posterior fixation (APC-2b).15 Although there are numerous biomechanical studies evaluating pelvic ring stability, to our knowledge, there have been none to specifically compare anterior fixation versus anterior fixation with a supplemental iliosacral screw in a well-defined open-book pelvic ring injury model under cyclic loading to fatigue failure. Our objective was to determine whether adding an iliosacral screw to the symphyseal plate fixation using this construct with intact interosseous and posterior sacroiliac ligaments, which could be the biomechanical approximate of the APC-2a physiologic injury, decreases hemipelvic displacement. We hypothesized that iliosacral screw augmentation would decrease the amount of diastasis at the pubic symphysis and displacement of the hemipelvis, with the null hypothesis being that there is no difference.

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MATERIALS AND METHODS

Twelve whole embalmed human pelvic specimens were obtained with intact ligamentous structures, including the pubic symphysis, posterior sacroiliac complex (iliolumbar, interosseous, posterior sacroiliac, and anterior sacroiliac ligaments), and pelvic floor with its sacrospinous and sacrotuberous ligaments. The specimens were dissected including the removal of soft tissues, internal pelvic contents, and dislocation/removal of the proximal femora while taking care to avoid damaging the ligamentous structures. The specimens were visually inspected to ensure that none had any fractures or defects. An APC-2 (OTA/AO type 61-B1) pelvic ring injury was created with a scalpel by performing complete transection of the pubic symphysis, with excision of the pelvic floor and release of the left sacrospinous, sacrotuberous, and anterior sacroiliac ligaments, taking care to leave the remainder of the posterior sacroiliac complex intact (Fig. 1). After sectioning of the ligaments, with minimal manual force, each pelvis was able to be opened to greater than 3 cm to document successful creation of the desired pelvic injury.

FIGURE 1

FIGURE 1

Next, reduction of the pubic symphysis was performed. After adequate reduction, the pelves were fixed with a Zimmer Biomet (Warsaw, IN) 6-hole 3.5-mm pelvic reconstruction plate using 6 nonlocking screws. To ensure uniformity in screw placement among all specimens, a drill guide was specially fabricated to maintain constant predetermined angles for each screw. The drill guide was secured to the pelvis through 4 contact points (Fig. 2). Then, the specimens were divided into 2 groups of 6 each, and dual-energy x-ray absorptiometry scans were performed on each specimen to ensure uniformity between the 2 groups. Group 1 consisted of pelves with symphyseal plating alone; in group 2 the symphyseal plate was supplemented by a single 7.0-mm diameter partially (32 mm) threaded cannulated iliosacral screw (Zimmer Biomet) inserted through a washer and into the S1 vertebral body along a line approximately perpendicular to the plane of the left sacroiliac joint. Fluoroscopic lateral, inlet, and outlet views were used to determine the appropriate screw placement and length (Fig. 3).

FIGURE 2

FIGURE 2

FIGURE 3

FIGURE 3

One specimen from each group was selected as a sample to obtain preliminary failure data and to identify an acceptable load for the planned bilateral stance pelvic testing model19,20 to be used in this study. Based on a previous investigation by Grimshaw et al21 comparing locked versus unlocked pubic symphysis plating using the same bilateral stance pelvic model, 440 N proved to be a sufficient load to induce maximal pubic symphyseal mobility. However, this study did not show any significant difference in pubic symphysis diastasis between groups at one million cycles. A load of 440 N would roughly simulate the vertical force vector of a 200-pound (90 kg) patient or a 100-pound normal compressive force in the 2-legged stance model. As our objective was to examine failure, the 2 pilot specimens were run at 660 N, which was 1.5 times greater than the aforementioned study. Both specimens failed before completion of 210,000 cycles and showed pubic symphysis diastasis greater than 15 mm with concomitant anterior sacroiliac joint widening. Therefore, to reach the minimum predetermined number of cycles, we elected to use 550 N for cyclic loading of each specimen to ensure sufficient force while maintaining physiologic parameters.

The 10 remaining pelvises were mounted on a material testing machine, MTS 858 Mini Bionix (MTS Systems, Inc, Eden Prairie, MN), using a bilateral stance pelvic testing apparatus, as used in Grimshaw et al, originally developed by Hearn et al and described by Varga et al (Figs. 4, 5).19–21 This testing apparatus was developed to provide a laboratory model representing pelvic motion and forces that occur in the ambulating patient, creating distraction at the pubic symphysis and shear through the sacroiliac joint.19,20 The axial load was applied through a male ball bearing that freely articulated with a matched plate firmly fixed atop the fifth lumbar spinal vertebrae. Distally, bipolar modular hip prostheses articulated with the acetabula to allow for motion in all planes. The modular stems were potted and recessed in ball bearing side plates on a track to allow coronal plane movement that would occur when the axial load was distributed across the pelvis. Each pelvis was stressed at 550 N for a total of 500,000 cycles at 2 cycles per second (2 Hz) or until gross failure of the specimen. While maintaining the 550 N load, measurements were taken at 210,000 and 500,000 cycles, representing 6 weeks and 3 months of daily walking, respectively, in the average adult.22 Measurements were taken at the superior pubic symphysis and inferior pubic symphysis using an electronic LCD Digital Vernier Caliper accurate to 0.05 mm. These 2 measurements were made to allow for the detection of any asymmetric symphyseal gapping. The average of 3 separate measurements was used to minimize investigator measurement error. Failure was defined as the point on the load displacement curve when force measurement essentially declines rapidly toward zero, with no further change in displacement or obvious visual failure of the fixation.

FIGURE 4

FIGURE 4

FIGURE 5

FIGURE 5

Because of the great variability in the data of the previously published studies of this type,20,21,23–25 an a priori sample size power analysis was determined to be of no value. However, a post hoc sample size power analysis was planned. Appropriate for the number of specimens, nonparametric statistics were used to compare the 2 fixation method groups. The Mann–Whitney U test was used to compare the 2 groups of cadaver pelvic specimens for all statistical evaluations with the exception of sex, for which the Fisher exact test was used. The level of statistical significance was defined as P < 0.05.

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RESULTS

Three specimens failed before the first time point (210,00 cycles) because of technical errors and were not included in the data analysis. Of these remaining 7 specimens, 4 were in group 1 (pubic symphyseal plate alone) and 3 were in group 2 (pubic symphyseal plate plus iliosacral screw). The characteristics of the analyzed specimens were not significantly different between these 2 groups (Table 1). Pelvis dual-energy x-ray absorptiometry scan T-scores ranged from −2.9 to 0.7 overall with no significant difference between the fixation groups (P = 0.84).

TABLE 1

TABLE 1

None of the specimens had any evidence of implant failure, by either visual inspection or biomechanical parameters. In addition, no significant difference in any of the measurements was found between the 2 groups at 210,000 or 500,000 cycles (Table 2). The mean superior and inferior pubic symphysis displacement in group 1 at 210,000 cycles was 2.38 and 5.22 mm, respectively, compared with 2.74 and 5.08 mm in group 2 (P = 1.0 and 1.0). The mean superior and inferior pubic symphysis displacement in group 1 at 500,000 cycles was 3.60 and 7.01 mm, respectively, compared with 3.77 and 6.92 mm in group 2 (P = 1.0 and 0.86).

TABLE 2

TABLE 2

Because of the small sample size, although typical of similar biomechanical investigations, a post hoc statistical power analysis was performed. This analysis showed only 15% power to detect a significant difference at the P < 0.05 level. However, it also showed that a large sample size (45–60 specimens within each group) would be required to detect any difference with 80% power. These data indicate that there is a small effect size, having limited clinical application.

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DISCUSSION

For the treatment of APC-2 (OTA/AO 61-B1) open-book disruptions of the pelvic ring, an anterior plate and screw construct has been the preferred fixation method.10–13,22 More recently, this has become a controversial clinical topic.18

In a 2008 clinical study, Sagi and Papp12 compared 2-hole and multi-hole symphyseal plates in the treatment of a variety of pelvic ring injuries. Although it was not the focus of their study, they noted that most of their patients who required a reoperation or had anterior fixation failure resulting in pelvic malunion had the APC-2 injury pattern associated with anterior sacroiliac joint widening and no posterior fixation.12 The authors questioned whether anterior fixation in these injuries is adequate to control the sagittal plane rotation of the hemipelvis and prevent malunion and suggested further study. In 2009, Collinge et al26 reported on a consecutive series of 20 male patients who had “saddle-horn” open-book pelvic injuries with disruption through the pubic symphysis after being bucked from a horse. Percutaneous iliosacral screw fixation was added to the anterior plate fixation in 13 of the patients at the discretion of the operating surgeon. Although there were no specific indications noted for the supplemental iliosacral screw, the patients treated this way tended to be larger and have a wide symphyseal diastasis.26 At the final follow-up, a radiographic partial loss of reduction was observed in 18 of the 20 patients, with a loss of screw fixation in 16 and plate failure in 2; however, 19 of the 20 injuries healed with acceptable alignment, and none required additional surgical treatment. Unfortunately, the authors did not compare the failure rate for the patients treated with and those treated without posterior fixation.

In 2011, Van Loon et al14 reported on 37 patients with OTA/AO 61-B1 injuries; in 9 of whom an iliosacral screw was inserted. They did not report any functional or radiologic advantage to posterior fixation; however, without any supporting data, they conjectured that external rotation and accompanying inferior displacement of the ipsilateral hemipelvis may be a sign of partial lesion of the posterior sacroiliac complex, causing a residual instability after symphyseal stabilization without causing a true vertical instability. They went on to advocate an additional sacroiliac screw fixation in this situation. Also in 2011, Sagi et al15 introduced the concept of modification of the pelvic injury classification system to reflect the dynamic component of pelvic ring instability not evident on preoperative radiographs and computed tomography scans but recognized on EUA as follows: APC-2a for those injuries requiring only anterior fixation and APC-2b for those injuries that may require treatment with anterior and posterior fixation. In this clinical study, the EUA consisted of a resting static film followed by internal rotation, external rotation, and push–pull maneuvers of both lower extremities. Each of these maneuvers was in conjunction with using the anteroposterior, inlet, and outlet fluoroscopic imaging. If any flexion or extension sagittal plane rotational instability (one pubic body displacing to greater than 1 cm above or below the contralateral pubic body) was noted on push–pull examination, an iliosacral screw was added to augment the anterior fixation.15 Of the 23 injuries initially categorized as being APC-2, 13 were diagnosed as APC-2a and treated with anterior fixation alone, 9 were found to be APC-2b and treated with supplemental iliosacral screw fixation, and 1 was identified as an APC-1 injury and treated nonoperatively. Unfortunately, the authors did not report any clinical results of their treatments and therefore, their findings and treatment algorithm could not be correlated with actual radiologic or functional outcome.

It is interesting to note that multiple investigators have shown that because of the orientation of the sacroiliac joint, external rotation of the hemipelvis in open-book injury will show inferior vertical and horizontal displacement on the anteroposterior (AP) radiograph, despite the interosseous and posterior sacroiliac ligaments being intact (Fig. 1).27 In addition, in 2012, 2 large clinical series documented that symphyseal plating shows frequent radiographic signs of failure without evidence of clinical relevance.28,29 Fixation failure was not found to be related to the presence or absence of posterior pelvic fixation.29 In contradistinction to these studies, in 2016, in a retrospective series of 134 patients with APC-2 injuries (92 treated with anterior and posterior fixation and 42 with anterior fixation alone), the investigators found that the use of an anterior plate and a supplemental posterior screw for fixation of APC-2 pelvic ring injuries significantly decreased the rate of anterior plate failure and malunion compared with the use of an anterior plate alone.16 However, indications for supplemental posterior fixation could not be defined. Furthermore, although the rate of anterior plate failure in this series was high, the clinical relevance remains to be determined, given that there were no reoperations to address malunion.16 Despite the radiologic improvement, there is little evidence of clinical functional importance. In addition, none of the clinical studies have reported malunions with translational displacement. Therefore, the routine use of supplemental iliosacral screw fixation has not been universally advocated.14,16 Those who have done so continue to report radiographic failures.17 Consequently, the controversy remains.

Biomechanical studies have similarly differed in their results. Simonian et al25 in 1994 examined the stability of various combinations of fixation techniques, concluding that combined anterior and posterior fixation was optimal for open-book injuries. Limitations in their study include the use of multiple chains to stabilize the pelvis, which may have restrained motions in the fracture planes and perhaps most importantly, their protocol of releasing the interosseous sacroiliac ligament in conjunction with the release of the anterior sacroiliac ligament. Dujardin et al23 reported decreased micromotion at the sacroiliac joint with combined anterior plate and posterior iliosacral screw fixation as compared to anterior plate fixation alone. However, there were a number of limitations in their study, including the sequential nature of their testing, the small number of specimens, and the fact that there was no direct comparison of APC-2 injuries with and without supplemental posterior fixation. Interestingly, they did find that sectioning of the interosseous ligament after transection of the pubic symphysis and release of the sacrospinous, sacrotuberous, and anterior sacroiliac ligaments resulted in a significant increase in micromotion despite symphyseal plating.23 Van den Bosch et al24 performed a cadaver study directly comparing the stability of open-book injuries with anterior plate and posterior iliosacral screw fixation to plate fixation alone. They concluded that the addition of a sacroiliac screw does not provide significant additional stability in the translation and rotation stiffness in an open-book pelvic lesion. Limitations of this study include the sequential testing method, the small number of specimens, incomplete descriptions of the ligaments sectioned, incomplete description of the model used, and inability to complete load to failure testing. Direct comparison of these studies is not possible because of the very different testing methods used. None of these studies used cyclic loading to fatigue failure in a standardized biomechanical model specific for the evaluation of the open-book injury.

We believe that we are presenting the first biomechanical study to specifically compare anterior fixation versus anterior fixation with a supplemental iliosacral screw in a well-defined open-book pelvic ring injury model using the important parameter of cyclic loading to fatigue failure. The testing apparatus used in this study is a proven model for biomechanical evaluation of pubic symphyseal fixation. It was initially developed by Hearn et al at the biomechanics laboratory at Sunnybrook Health Sciences Centre in Toronto.19,21 It has been used in numerous studies to evaluate the effectiveness and biomechanics of pelvic fixation.20,21 Specifically, the 2-legged stance apparatus is believed to be more appropriate than a single-leg stance model for the testing of open-book injury because it produces distraction at the pubic symphysis.19,20 Using this model, we found no significant difference in displacement of the superior or inferior pubic symphysis between the 2 fixation groups at 210,000 or 500,000 cycles. There were no specimens that experienced broken plates or screws. Ultimately, the addition of an iliosacral screw into the S1 body did not afford any improved biomechanical outcomes in our specific injury model.

This study had a number of limitations. First, 3 of 10 specimens failed before 210,000 cycles and were not included in the data analysis. This left a total of 4 specimens in the pubic symphysis plate only group and 3 specimens in the pubic symphysis plate plus sacroiliac screw group for final analysis. In the failed specimens, the left hemipelvis externally rotated and flexed causing the pelvis to become dislodged from the MTS machine and signal the displacement of the actuator to stop. We attribute the failure of these specimens to poor fit of the bipolar prosthesis into the ipsilateral acetabulum. Second, despite the use of nonparametric statistics, our failure to show a difference between the 2 groups may be a type 2 error because of the relatively small sample size and the lack of sufficient statistical power. This is a common issue with cadaveric biomechanical studies; the number of specimens in this study is very similar to that used by other investigators.20,21,23–25,27 However, our finding that a very large sample size (45–60 specimens within each group) would be required to detect any difference at the P < 0.05 level with 80% power suggests that the effect size is small with limited clinical relevance. Furthermore, it is not possible to conduct a biomechanical pelvis study with this number of specimens. Last, the cadaver specimens used for this study were embalmed and not fresh-frozen. However, Comstock et al30 used embalmed cadaver specimens in a biomechanical evaluation of fixation of the posterior pelvic ring and found comparative results to studies performed with fresh-frozen specimens. More recently, van Zwienen et al31 found embalmed pelvic specimens to be satisfactory for the biomechanical evaluation of unstable pelvic ring injuries.

There is no doubt that injury patterns classified under the APC-2 category actually represent a spectrum of ligamentous injury.16 Furthermore, in the evaluation of the injured trauma patient, initial radiographs and computed tomography scans are static images and may not show the full extent of the pelvic ring instability. Sagi et al15 described the technique of stress examination with fluoroscopy in an attempt to better determine the stability of the pelvis and the type of operative stabilization: APC-2a requiring only anterior fixation, and APC-2b requiring supplemental posterior fixation. However, this technique has not yet been shown to define the injuries actually clinically benefitting from supplemental posterior fixation nor has any other method. It is quite possible that our model (in which care was taken to avoid injury to the posterior sacroiliac complex other than sectioning of the anterior sacroiliac ligament) defines the APC-2a situation and that continuing injury to involve the interosseous ligament causes progression to APC-2b instability. What is certain is that the treatments and the outcomes of APC-2 injuries require further clinical and biomechanical investigation.

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CONCLUSIONS

Adding an iliosacral screw to symphyseal plate fixation does not provide improved biomechanical outcome in the classically described APC-2 (AO/OTA type 61-B1) pelvic ring injury. Clinically, stress examination may be useful to determine the need for supplemental posterior fixation in APC-2 injuries.

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ACKNOWLEDGMENTS

The authors thank Heidi Israel, PhD, for assistance with the statistical analysis.

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Keywords:

APC-2 pelvic injuries; open-book pelvic injuries; pelvic ring fixation

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