Trans-obturator Cable Fixation for Disrupted Pubic Symphysis: A Reasonable Option Compared to Plating?

Methods Abstract Operative treatment of ruptured pubic symphysis by plating is often accompanied by complications. Trans-obturator cable xation might be a more reliable technique; however, have not yet been tested for stabilization of ruptured pubic symphysis. This study compares symphyseal trans-obturator cable xation versus plating through biomechanical testing and evaluates safety in a cadaver experiment. APC type II injuries were generated in synthetic pelvic models and subsequently separated into three different groups. The anterior pelvic ring was xed using a four-hole steel plate in Group A, a stainless steel cable in Group B, and a titan band in Group C. Biomechanical testing was conducted by a single-leg-stance model using a material testing machine under physiological load levels. A cadaver study was carried out to analyze the trans-obturator surgical approach. Peak-to-peak displacement, total displacement, plastic deformation and stiffness revealed a tendency for higher stability for trans-obturator cable/band xation but no statistical difference to plating was detected. The cadaver study revealed a safe zone for cable passage with sucient distance to the obturator canal. Trans-obturator cable xation has the potential to become an alternative for symphyseal xation with less complications.


Introduction
Disruption of the pubic symphysis is commonly seen in pelvic ring injuries of trauma patients [1,2]. The disruption often occurs in combination with a posterior pelvic ring impairment of variable severity. When diastasis of the disrupted symphysis pubis exceeds a certain displacement, stabilization of the anterior pelvic ring is recommended [3,4]. Adequate reduction and stable xation can restore pelvic ring alignment and allow early patient mobilization [5]. Currently, symphyseal plating represents the most common technique for anterior pelvic ring xation in such conditions [6]. This plate xation, however; represents a static xation of what is actually a dynamic junction. The pubic symphysis comprises a brocartilaginous disc between the articular surfaces of the pubic bones, encapsulated and reinforced by surrounding ligaments allowing limited movement [7]. From the healing perspective, the currently preferred treatment is not the ideal one as it does not provide a dynamic xation [8] and the plate is placed in an unfavorable rectangular position to the load vectors that affect the ruptured symphysis.
Other disadvantages of plating include hardware breakage or implant loosening, which lead to recurrent instability of the pubic diastasis. These implant failures are the main reasons for revision surgery and may be linked to the rigid character of the xation technique [9]. More dynamic xation techniques like a trans-obturator cable cerclage could bypass this problem but have not been taken up in daily practice.
Clinical and laboratory studies on pubic wiring are inconsistent and often small in size [10][11][12][13][14][15]. Recent studies only consider simple wiring instead of a more stable cable-system xation [16]. The aim of this study is to analyze dynamic trans-obturator cable xation as an alternative stabilization for the disrupted pubic symphysis. The hypothesis is that, compared to plating, trans-obturator cable xation provides an equal or even superior xation strength. If so, the cable xation could be a dynamic, less invasive alternative to symphyseal plating with less complications.

Specimen and Fracture Generation
Thirty synthetic pelvises (Pelvis Complete, Synbone, Art. No. 4060) were used. In this model, the sacroiliac joint and the symphysis are joined using exible plastic foam. There are no ligaments or muscles attached to the pelvis. An anterior-posterior compression injury (Young and Burgess APC II; OTA/AO: 61-B2.3d) was simulated, in which the pubic symphysis and sacroiliac joint connection on one side was disrupted. Preparation and xation of posterior instability was conducted in all specimens using the same technique. A 3.5-mm drill was used to open the cortical bone and create an osseous corridor to S1 and S2 while the pelvis was still intact. Final xation was done with two partially threaded 7.3-mmdiameter, 90-mm-long cannulated titan screws, including washers.

Experimental Groups
Group A, the control group, received traditional plate xation (N = 10). Here, pubic disruption was xed using a four-hole stainless steel symphysis plate (Symphyseal Plate 3.5 with coaxial combi-holes, DePuy Synthes; Art. No. 02.100.004) and 3.5-mm self-tapping, non-locking cortex screws. The medial and lateral screws were 55 mm and 60 mm long, respectively. Medial screws were placed with lateral inclination and lateral screws were tilted towards the symphysis [17].
Pubic symphysis disruption in Group B (N = 10) was xed with a trans-obturator cable, using a 1.7-mmdiameter interwoven stainless steel cable and cable crimp (cable with crimp, DePuy Synthes, Art. No. 298.800.01S). A medium size cable passer was used to tunnel the cable twice through the obturator foramen and embrace the pubic symphysis. The cable tensioner in combination with the provisional tensioning device and attachment bit pre-tensioned the cable wire up to 40 kg. Care was taken that the cable did not fully cut into the cortex. Final xation was conducted with the cable crimper. The cable was shortened near the tip with an adequate cutter.
A titanium cerclage band was used in Group C (N = 10). The cerclage band was 5.8 mm wide and 240 mm long (Titanium-Cerclage Band according to Thabe, Link, Art. No. L63-4300/02), and was used with a cerclage band guide of the appropriate size. The cerclage was passed through the obturator foramen around the pubic symphysis, ensuring broad contact of the cable band to the bone. A cable tensioner was used for repositioning and tightening. The cable was locked using a hexagon screwdriver and shortened using a cutter (Figs. 1 and 2).

Biomechanical Test Setup
A single-leg-stance model was established and testing was performed using a universal testing machine (Z020; Zwick/Roell) and testXpert II software (Version 3.6; Zwick/Roell) [18,19]. Pelvic samples were attached at the sacrum to the testing machine at a physiological 45° tilt using a custom-made aluminum device. A hemiarthroplasty prosthesis was used for articulation with the acetabulum at 15% adduction on one side to simulate unilateral axial load. The femoral stem was wedged in a steel quiver attached to the bottom of the machine. Photographic documentation of the pubic symphysis was conducted throughout the testing. In contrast to previous studies by the authors, a cable-pulley system simulated the abductor muscles to increase maximum load levels [18][19][20]. A series of pretests were performed to establish the protocol and summarize the data for power analysis. The pretests encompassed load levels of 50 to 1,000 N and test cycles of 500 to 3,000 repetitions. All pretests indicated that 200, 400, and 600 N were valuable load levels, and that test cycles of 500/1000/1500 repetitions were adequate to show differences between the experimental groups. The decision to use such load levels was based on in vivo measurements, the literature and previous work by the authors [18-23] (Fig. 3). The main tests were started with 10 setting cycles at 0-to 10-N loads at 50 mm/min frequency. A load-displacement curve was generated during the testing. Outcome measurements were peak-to-peak displacement at 200, 400, and 600 N, total displacement, plastic deformation, and stiffness.

Surgical Approach
A cadaver study was conducted to understand the relationship of the trans-obturator cerclage to surrounding structures. Fresh frozen male and female cadavers (one of each) were placed in a supine position. Using a Pfannenstiel approach, a slightly curved 15-cm-long horizontal incision was made, centered about 2 cm cranial to the pubic symphysis. The anterior portion of the rectus sheath was prepared and the rectus sheath was divided to identify the rectus abdominis and the pyramidalis muscles. The muscle was separated from the attachment at the pubic bone. For better exposure, dissection was continued laterally to the external inguinal ring, the spermatic cord/round ligament, and the vascular and muscular lacuna. Dissection of the obturator foramen was conducted, including the obturator canal and membrane. A 1.7-mm cable wire was passed through the medial part of the obturator foramen and xed. The distance to the surrounding anatomical structures was evaluated.

Statistical Analysis
A power analysis performed using a power of 80% and a signi cance level of 5% showed that the sample size was adequate. The results are presented as mean values with standard deviation. All data underwent statistical analysis for normal distribution using the Shapiro-Wilk test. Analysis of variance was used to compare the means. A p value < 0.05 was considered to indicate signi cance. As no statistical differences were found, further tests were not conducted.

Results
Peak-to-Peak Displacement at 200, 400, and 600 N Peak-to-peak displacement was measured for the entire pelvic ring. Under 200 N, there was a mean peakto-peak displacement of 0.27 ± 0.10 mm in group A, 0.24 ± 0.98 mm in group B, and 0.20 ± 0.06 in group C. Under 400 N, the mean peak-to-peak displacement was 0.57 ± 0.21 mm in group A, 0.50 ± 0.16 mm in group B, and 0.45 ± 0.09 in group C. Under a 600-N load, the values were 1.16 ± 0.71 mm in group A, 1.34 ± 1.0 in group B, and 0.78 ± 0.15 in group C. There were no signi cant differences between the groups (p = 0.29; p = 0.27; p = 0.25; Fig. 4).

Total Displacement
The mean total displacement was 3.96 ± 0.42 mm in Group A, 3.58 ± 0.54 mm in group B, and 3.70 ± 0.43 mm in Group C. There were no signi cant differences among the groups (p = 0.20).

Plastic Deformation
Plastic deformation was measured using a load-displacement curve. It represents the irreversible deformation of the whole pelvic ring at 200-N load. The mean plastic deformation was 0.63 ± 0.20 mm in group A, 0.55 ± 0.17 mm in group B, and 0.56 ± 0.19 mm in group C. There was no signi cant difference among the groups (p = 0.58) Stiffness Stiffness was measured using the slope of the load-displacement curve at 400 N. The mean stiffness was 105 ± 13 N/mm in group A, 118 ± 18 N/mm in group B, and 107 ± 16 N/mm in group C. There was no signi cant difference among the groups (p = 0.21; Fig. 5).

Surgical Approach
A cadaver study was conducted to understand the relationship of the trans-obturator cable to surrounding structures (Figs. 6 and 7). The obturator foramen is closed by a brous membrane. A cable can be passed through the weak medial part of the membrane. The distance between the cable and the obturator canal was 3.0-3.5 cm. Interference with the spermatic cord/round ligament of the femoral vessels should be considered [24].

Discussion
Disruption of the pubic symphysis of more than 2.5 cm (1.8-4.5 cm) is a severe injury for which many orthopedic trauma surgeons would agree that operative stabilization is the preferred treatment [25]. In case of minor diastasis, conservative treatment is an option but was abandoned in severe cases due to disadvantages caused by longtime immobilization [26]. Wiring of the pubic symphysis was performed in the past [27] but the available clinical data is limited, outdated, and only consists of small case series [10,13,28]. Beside the above mentioned clinical reports different biomechanical studies explore the use of a simple wiring technique: Tile et al. xed the pubic symphysis in one group with double plating (4.5-mm plates), in the second group with four trans-osseous wire loops, and in the third group with absorbable suture material. Wiring resulted in signi cantly less symphyseal motion than the other methods and they concluded that symphyseal wiring can oppose the tensile load caused during bilateral stance loading at the symphysis [12]. Hofmann compared pubic reconstruction by plate to wiring using cadavers (N = 83). While plate stabilization was stiffer; he mentioned no other difference compared with the wiring technique [11]. Meißner et al. performed a biomechanical study that compared wiring and plating in a cadaver model (N = 24) under physiological conditions. Cut-out of the wires was seen in many specimens and highest stability was provided by plating of the pubic symphysis, what raises concerns about stability of the wiring technique [14]. This are some of the reasons why treatment ultimately evolved to plate xation over the years [5,29,30]. Unfortunately plating is also a procedure which carries a signi cant risk for complications. Most frequently subclinical implant loosening (31-80%) is noted in follow-up X-rays [9,[31][32][33][34]. In case of complete failure or reoccurrence of the diastasis (> 10mm), revision surgery is required (3-21%) [9,[32][33][34]. Because clinical and biomechanical studies just focus on simple symphyseal wiring, the role of modern cable-systems for this purpose was unclear. Lenz et al. con rmed a superior stability of cable-systems over wiring when xing long bones. In their biomechanical study a double loop cable cerclage, as used in our group B, has a load-to-failure of mean 2734 N (±330 N) compared with a single loop wire that only bears 606 N (±109N) [16]. Our study con rms high stability of double loop cable xation of the pubic symphysis; however, we did not compare it to simple wiring.
While passing a cable through the obturator foramen, knowing the location of the obturator canal is important for safety. The obturator canal is a pathway through the obturator membrane containing an artery, vein, and nerve and can be found at the cranio-lateral margin. The obturator artery originates in most cases of the internal iliac vessel (62%) [35], sometimes forming a connection to the external iliac vessel system, called corona mortis [36]. After passing the obturator canal, the obturator artery divides into an medial and lateral branch to form a vascular circle [37] or splints into small tributaries without forming such circle [38]. The obturator nerve runs from the obturator canal inferomedially between the adductor longus and brevis muscles and divides into an anterior and posterior division that mainly innervates adductor muscles [37]. Jo et al. conducted a morphometric study of the nerve exit zone in the obturator foramen and showed a median distance to the pubic tubercle of 30 mm [39]. This makes it possible to identify a safe zone (Fig. 6). Trans-obturatorial surgical procedures in the safe zone are also known from other disciplines like vascular surgery (bypass) [40], general surgery (obturator hernia repair) [37] and gynecology (midurethral sling procedure) [41,42].
Beside plating or trans-obturatorial cable xation other innovative stabilitation techniques for the pubic symphysis were described in the past. Chen et al. published an article about 45 patients with pubic disruption, treated using percutaneous screw xation. The authors recommend this technique because of lower blood loss and a better functional outcome [43] The technique was con rmed by biomechanical ndings [44].

Limitations
The use of synthetic pelvic models instead of cadaver specimens is controversial in orthopaedic research but there are some reasons why synthetic bones are increasingly used. They are affordable at a justi able budget, have similar biomechanical properties than cadaver specimens, are easily available without logistical problems, have almost no variability between specimens and are ethically uncritical [48]. In contrast, there is a high variability in biomechanical properties between cadaver specimens and the specimens themselves represent disproportionally the elderly population [49]. Therefore, cadaver specimens often do not adequately simulate the biomechanical behavior of orthopaedic implants. Especially polyurethane foam based bone models were intensely tested and con rmed for biomechanical studies [50]. Still, the synthetic pelvic model used here has no ligaments or muscular attachments. The load vectors are therefore expected to differ in cadaver models. In summary, a synthetic pelvis model is useful for a comprehensive statistically preliminary testing but cadaver studies should be considered before rst-in-human study. Further, the monoaxial, single leg load applied in the biomechanical testing does not simulate bilateral strain. No implant loosening was observed in the plating group what may be a result of the low number of test cycles, despite a high number used in our pretests. Moreover, loosening of the cable xation was not detected, making it di cult to verify the cable to be a more dynamic xation. Placing the cerclage in the desired position could be more di cult in real surgery than it is in a synthetic pelvic model. The Pfannenstiel approach only allows for easy dissecting of the cranial corner of the foramen through which to pass the wire. Further studies are required to assess the ideal surgical approach [51]. Another concern is loosening of the cable-system leading to displacement of pubic bones in the axial and sagittal planes caused by continuous movement in the vertical and horizontal axis of the pelvis (Fig. 8).

Conclusion
Our study con rms adequate stability of symphyseal plating and demonstrates similar biomechanical properties of trans-obturator cable xation. Passing a cable at the medial margin of obturator foramen seems possible and relatively safe.

Declarations Acknowledgment
We thank Dr. R. Wagner for sharing his clinical experience with pubic symphysis wiring and A. Stow for editing service.

Con ict of Interest Statement:
No author has association of nancial involvement (i.e. consultancies/advisory board, stock ownerships/options, equity interest, patents received or pending, royalties/honorary) with any organization or commercial entity having a nancial interest in or nancial con ict with the subject matter or research presented in the manuscript. Figures Figure 1 Different groups tested. A) Stabilization with a four-hole 3.5-mm stainless steel plate. B) Trans-obturator wire using a 1.7-mm cable system. C) Trans-obturator xation using a broad titan band. Ten specimens were tested in each group.

Figure 2
Stepwise application of a trans-obturator xation. A) Instruments and titan band B) Tensioner C) Fixation of the clamp using a screw driver B) Final position.   Total displacement represents the nal change of the probe after all load levels were applied. Plastic deformation represents the permanent and irreversible strain of the hole construct. The plate and cable systems showed only minimal amounts of deformation. Even though group B had the highest stiffness, no statistical difference was found.  Schematic illustration of the obturator foramen. The obturator artery divides into medial and lateral branches as it emerges from the obturator canal. An acetabular branch rises from the lateral part and runs towards the hip joint. The obturator nerve divides into anterior (pectineus m., Adductor longus m., Adductor brevis m. Gracilis m.) and posterior divisions (Obturator externus m., Adductor magnus m.). At the medial border, a safe zone can be identi ed in which the cable wire can be passed with limited risk.