Biomechanical analysis of a new minimally invasive system for osteosynthesis of pubis symphysis disruption

Injury, Int. J. Care Injured (2012) 43(S2), S20–S27

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Biomechanical analysis of a new minimally invasive system for osteosynthesis of pubis symphysis disruption 1 , J. Mart´ınez-Reina 2 , F.J. Serrano-Escalante 1 , P. Cano-Luis 1, *, M.A. Giraldez-Sanchez ´ 2 1 , J. Garc´ıa-Rodr´ıguez 2 , A. Navarro 2 C. Galleguillos-Rioboo , A. Lazaro-Gonz ´ alvez ´ 1 2

Clinical Orthopaedics, Trauma Surgery and Reumatology Management Unit, Hospital Universitario Virgen del Roc´ıo, Seville, Spain Department of Mechanical Engineering, University of Seville. Escuela Superior de Ingenieros, Seville, Spain

ARTICLE INFO Keywords Minimally invasive fixation Biomechanics Pelvis Open-book fracture Tile B fracture

ABSTRACT Introduction: We analysed the effectiveness of a new percutaneous osteosynthesis system for the treatment of pelvis fractures with rotational instability. Methods: A pre-clinical cross-sectional experimental study wherein Tile type B1 injuries (open-book fractures) were produced in 10 specimens of fresh human cadavers, including the L4–5 vertebrae, pelvic ring, and proximal third of the femur, keeping intact the capsular and ligamentous structures, is presented in this paper. The physiological mobility of the intact pelvis in a standing position post-injury was compared to that following the performance of a minimally invasive osteosynthesis of the symphysis with two cannulated screws. A specially designed test rig capable of applying loads simulating different weights, coupled with a photogrammetry system, was employed to determine the 3D displacements and rotations in three test cases: intact, injured and fixed. Results: After applying an axial load of 300 N, no differences were observed in the average displacement (mm) of the facet joints of the intact pubic symphysis in comparison to those treated with screws (p >0.7). A statistical difference was observed between the average displacements of the sacroiliac facet joints and pelvises with symphyseal fractures treated with screws after the application of a load (p <0.05). Conclusion: The symphyseal setting with two crossed screws appears to be an effective alternative to osteosynthesis in pelvic fractures with rotational instability. © 2012 Elsevier Ltd. All rights reserved.

Introduction Open reduction and internal fixation (ORIF) treatment has become the standard of care for displaced fractures of the pelvic ring as it facilitates appropriate access for anatomical reduction, effective fixation and early mobilisation.1–14 However, it requires exposure of the deep structures of the pelvis, which may lead to damage to important neurovascular structures, extensive soft tissue scarring, and increased incidence of infection.15–24 In order to avoid the above complications, lately percutaneous techniques for stabilisation of the pelvic ring have gained great popularity. Such techniques allow stabilisation of fractures in polytrauma patients who, due to their general status, do not tolerate decubitus or prolonged anaesthesia, in elderly patients with compromised biological reserve, in patients with extensive soft tissue involvement, open fractures or who have wounds associated with the pins placed for external fixation which have become infected, and in patients who * Corresponding author at: Trauma Surgery and Reumatology Management Unit, HU Virgen del Roc´ıo, Avd. Manuel Siurot s/n, E-41020 Seville, Spain. Tel.: +34 955012622; fax: +34 955012612. E-mail addresses: [email protected] (P. Cano-Luis); [email protected] (M.A. Giraldez-Sanchez). ´ 0020-1383$ – see front matter © 2012 Elsevier Ltd. All rights reserved.

may display sequelae from previous emergency surgeries (urological, gynaecological, or abdominal).25–27 A percutaneous osteosynthesis technique employing cannulated screws for the pubic symphysis has been used in our institution to treat patients with Tile type B1 and B3 fractures where open surgery was not indicated. This technique is controlled using conventional X-ray equipment without the need for navigation or CT techniques. The clinical and associated initial stability data in the intermediate term have been favourable among these patients, motivating the conception and design of the present study. The primary objective of this study was to compare displacement under a load of 300 N in the pubic symphysis and sacroiliac joint in intact pelvic rings, pelvic rings with Tile type B1 fractures, and pelvic rings submitted to osteosynthesis using 6.5 mm cannulated symphyseal screws. The secondary objective of this study was to compare the mobility (rotation around the three axes of space) of the iliac bones with respect to the sacrum across the same three aforementioned test cases.

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Materials and methods The anatomical specimens used in this study were ten fresh cadavers, aged 75±13 (range: 55–92) years, one male and 9 females. The anatomical samples were dissected to obtain the specimens formed by L4–5 vertebra, the pelvis and the proximal third of both femurs. All of the capsular and ligamentous elements from the pubic symphysis were maintained, as were the sacroiliac joints, sacrospinous ligaments, sacrotuberous ligaments, soft parts of the lumbar vertebra, and both hip joints. Two pelvises showed evident signs of sacroiliac osteoarthritis. None of the specimens had histories of previous fractures, pelvic surgeries, tumours, or metabolic bone diseases. After preparation, the specimens were frozen at −20°C before the experiment. It has been demonstrated that freeze–thaw cycles do not affect the biomechanical properties of ligaments.28 The specimens were kept until the experiment, which was performed at the facilities of the CATEC (Advanced Centre of Aerospace Technology of Sevilla) according to protocols for the manipulation of donor bones. A registered and patented system was used to experimentally analyse the mechanical behaviour of fractured pelvises (Fig. 1). The model was designed to permit the simulation of an upright standing position, such that the anterior–superior iliac spines and the pubic symphyseal tubercles are aligned in the coronal plane and the femurs can be positioned at a 15° anteversion and 10° valgus. Thus, the direction of the axial load passes through the pubic symphysis and provides a biomechanical model for studying the effects of anterior fixation in the symphysis29,30 (Fig. 2). The versatility of this system allows the specimens to be correctly positioned and adjusted despite having distinct separation between the femurs. These objectives were achieved by using a proximal bearing, which corrects angles and

Fig. 3. System setup for the assay and positions of the sensors on the iliac bones and the symphysis.

allows for the correct pelvic inclination, as well as a lower linear motion rolling guide system, which allows the distance between the femurs to be adjusted. After the simulation setup for the test was completed, the specimen was fixed to the loading rig; thereafter, only vertical displacements of the upper frame were permitted. This was generated by a controlled hydraulic system that applies a physiological load to the pelvis. Upon the application of this load, the entire system readapts, and the corresponding movements are inferred from the recorded displacements of a set of discrete points where reflective adhesive markers are placed (Fig. 3). The PONTOS 5M® system was used to visualise the markers (GOM mbH, Braunschweig, Germany) using photogrammetry to calculate the positions of these markers with a resolution of 2448×2050 pixels and a precision error of 0.005 mm. A Zwick/Roell Z100 (BT1-FB100TN, Zwick GmbH & Co. KG, Ulm, Germany) uniaxial hydraulic testing machine was used for the mechanical experiment, and TestXpert II software was used to control the applied load. Experimental setup

Fig. 1. Diagram of the anchor system.

Centre of rotation of the head of the femur

Anterior superior iliac spine

Line of the load

Pubic spine

10º

Anatomical axis of the femur Knee joint

Fig. 2. Position selected for the assays: (a) Lateral view of the pelvis. (b) Anterior– posterior view of the femur.

The specimens were thawed. Bone and ligament dehydrationassociated changes were minimised by keeping the specimens in water for 16–20 hours at room temperature and by keeping them moist before and after the experiments.31 Markers were adhered to the heads of 3×16-mm steel screws that were anchored to the bone and placed on all of the pelvises as follows: 3 markers were aligned on the internal side of the sacroiliac (SI) joint and spaced 2 cm apart at a distance of 1 cm from the joint line; 4 markers were located in a lateral position with respect to the SI joint in a diamond pattern; 2 markers were positioned close (1 cm away) to the joint line; and 2 markers were situated externally with variable locations depending on the individual morphology. Finally, markers were placed bilaterally in the symphysis in the upper cortical area of the superior pubic ramus at a 1-cm distance from the joint, and a second pair of markers was placed on the bone vertex of the conjunction of the superior pubic ramus with the ischiopubic ramus (Fig. 3). The specimens were placed in the universal testing machine using a plate angled at 130°, which has a rod that is inserted in the machine. The plate was fixed to the specimen with 4-mm screws in the vertebra and 6-mm screw-bolts in the sacrum. Moreover, PMMA cement and

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Fig. 4. Proximal anchorage of the angled plate.

bi-component acrylic cement was used to glue the surface of the plate to the sacrum (Palacos LV® ) (Fig. 4). The femurs were set distally within the sliders, at a 15° anteversion and 10° valgus, using fast-curing bi-component polyurethane resin (Feropur® PR55-E55) to glue each femur to the corresponding slider.

Experimental protocol Once each pelvis was fixed to the machine, a compression load of 300 N was applied at a rate of 20 mm/min to each specimen over the lumbar spine (L4–5) and the sacrum in the axial direction so as to simulate the equivalent of half of the weight of a 60-kg person. This force was established as the load limit for the experiments in all of the configurations. The test consisted of three sequential phases, as follows: • Test A: First, each intact pelvis was exposed to a progressively higher load until reaching the load limit, and the displacements and rotations of each marker on the pelvic ring were recorded. • Test B: A Tile type B lesion was simulated by sectioning off the pubic symphysis and the sacrotuberous, sacrospinous, anterior sacroiliac, and right interosseous ligaments and by producing anterior sacroiliac diastasis (an injury caused by Young–Burgess type II anterior–posterior compression) until a lower displacement of the right hemipelvis ipsilateral to the sacroiliac injury was obtained32 while keeping the posterior sacroiliac ligaments intact (Fig. 5). Once this injury was created, a load of 300 N was applied, and the displacement and rotation of the system were recorded. • Test C: The damaged symphysis was fixed using 6.5-mm-diameter cannulated titanium screws. Therein, a standard position was defined as being perpendicular to the symphyseal plane with an anterior inclination of 45°, such that one screw had a point of entrance at the lower base of the pubic tubercle and the other in the

Fig. 6. (a) Inlet and (b) outlet views of the pelvis. Directions of both symphyseal screws.

opposite direction, caudally parallel, and approximately 1 cm away from the obturator foramen (Fig. 6). Then, the load of 300 N was applied and data were recorded. It is widely known that ligamentous structures exhibit viscoelastic behaviour.33 To ensure that the stresses in these structures were relaxed, a rest interval of 10 min was scheduled between successive tests. Similarly, to ensure that the test up to 300 N with the injured pelvis (B) did not cause further damage in the ligamentous structures, a load of 80 N was applied after tests A and B (during the so-called A' and B' tests). A comparison of the apparent stiffness of the specimen in test A' and B' allowed for the determination of whether any damage had occurred. Therefore, the final test sequence was as follows: A (up to 300 N with an intact pelvis), A' (up to 80 N with an intact pelvis), B (up to 300 N with an injured pelvis), B' (up to 80 N with an injured pelvis), and C (up to 300 N with a pelvis with screws). Data analysis Independent variables were the applied load, which was established as 300 N; injury status of the pelvis; intact pelvis and ligaments; ruptured and injured ligaments; and synthesised pelvis fixed with symphyseal screws were measured. Dependent variables were the relative displacements among the bone markers (mm) and iliac rotations with respect to the sacrum, which were determined with five pairs of sensors along three axes, measured in degrees. Based on the positions of the markers recorded during each test, the displacements and the relative rotations of the different markers were calculated. For data acquisition, a frequency of 4 images/second was used. A standardised time of 30 seconds was used for all data acquisitions in all tests. Thus, 120 images were obtained in each test (1 image every 0.25 seconds). A paired analysis of categories was performed to compare the aforementioned continuous quantitative variables in each configuration (intact vs. injured, and intact vs. synthesised with screws). A Wilcoxon signed-rank test was used for this analysis. Results

Fig. 5. Young–Burgess II injury of the right hemipelvis.

Tables 1, 2 and 3 show the variations in marker position for the different specimens in each test relative to the initial position with no load in an intact pelvis, an injured pelvis, and an osteosynthesed injured pelvis. Negative values indicate that the corresponding markers moved closer together upon application of the load. The analysis of the intact pelvises shows almost all negative results (moving closer together) in the upper part of the symphysis and almost all positive results (moving apart) in the lower part (Table 1). These results demonstrate how the iliac bones tend to move closer to the sacrum near the anterior part of the sacroiliac joints (negative displacement) (Fig. 7). The greatest difference between the intact and the injured pelvises occurred near the pubic symphysis. In all of the investigated cases,

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Table 1 Variations in distance (mm) between markers in the upper and lower pubic symphyses for each specimen and for each type of test at an applied load of 300 N Specimen 1 2 3 4 5 6 7 8 9 10 Mean±SD

Upper symphysis A −0.072 −0.137 −0.064 −0.114 −0.050 −0.069 −0.048 −0.054 −0.122 0.009 3.288±3.445

B 7.925 3.410 0.783 10.992 2.674 2.285 1.254 1.473 1.674 0.411 3.288±3.445

C −0.153 −0.028 −0.049 −0.114 −0.117 −0.083 −0.033 −0.107 −0.066 −0.037 −0.079±0.042

Lower symphysis A 0.120 0.000 0.050 0.075 0.073 −0.029 0.010 0.157 0.415 −0.007 0.086±0.129

B 7.479 3.400 0.971 5.344 2.641 2.476 1.632 1.957 2.485 0.628 2.901±2.081

C 0.098 −0.021 0.030 0.247 0.076 −0.106 0.043 0.155 0.166 0.046 0.093±0.082

Table 2 Variations in distance (mm) between markers in the upper right SIJ and lower right SIJ for each specimen and for each type of test at an applied load of 300 N Specimen 1 2 3 4 5 6 7 8 9 10 Mean±SD

Upper right SIJ A −0.289 −0.013 −0.011 −0.182 −0.103 −0.112 0.002 −0.100 −0.267 0.012 −0.106±0.110

B −0.536 0.052 −0.052 −0.142 −0.256 −0.447 −0.111 −0.347 −0.413 0.037 −0.222±0.208

C −0.502 −0.012 −0.035 −0.480 −0.137 −0.257 −0.087 −0.203 −0.240 0.025 −0.193±0.183

Lower right SIJ A −0.202 −0.037 −0.022 −0.059 0.012 −0.198 −0.006 −0.173 −0.582 −0.032 −0.130±0.178

B 0.379 0.248 0.064 0.466 0.185 −0.554 −0.166 −0.222 −0.664 −0.292 −0.056±0.385

C −0.352 0.034 −0.026 −0.214 0.072 −0.427 −0.208 −0.252 −0.505 −0.185 −0.206±0.191

Table 3 Variations in distance (mm) between markers of the upper left SIJ and lower left SIJ for each specimen and for each type of test at an applied load of 300 N Specimen 1 2 3 4 5 6 7 8 9 10 Mean±SD

Upper left SIJ A −0.186 −0.063 −0.087 −0.281 −0.235 −0.099 0.008 −0.201 −0.064 0.022 −0.119±0.102

B −0.272 −0.034 −0.033 −0.078 −0.220 −0.051 −0.014 −0.236 −0.208 0.046 −0.110±0.112

C −0.183 −0.056 −0.018 −0.229 −0.215 −0.052 −0.003 0.077 −0.042 0.052 −0.067±0.107

F UPPER RIGHT SIJ

UPPER LEFT SIJ

LOWER RIGHT SIJ LOWER LEFT SIJ

SYMPHYSIS SUPERIOR

R1

R2 SYMPHYSIS SUPERIOR

Fig. 7. Direction and location of the absolute displacements around the joints of the pelvic ring for an applied load of 300 N on the sacrum.

the increase in displacement among the markers was significantly greater in the injured pelvis, as expected (p = 0.002). In addition, it

Lower left SIJ A 0.043 −0.073 0.059 −0.430 −0.003 −0.177 −0.038 −0.251 −0.323 −0.019 −0.121±0.166

B 0.201 0.054 0.145 0.019 0.044 0.058 −0.032 −0.254 0.039 −0.040 0.023±0.121

C 0.045 −0.060 0.084 −0.433 −0.042 −0.148 −0.035 −0.256 −0.323 −0.042 −0.121±0.166

was found that in the injured pelvis, the two iliac bones tended to separate in the symphysis region. Significant differences between the intact pelvis and the injured pelvis were found only in the upper right sacroiliac joint (SIJ) (p = 0.02) and in the lower left SIJ (p = 0.009) (Tables 2 and 3). Overall, there were no differences between the intact pelvises and the pelvises fixed with crossed screws in any of the three joints; hence, both cases exhibited similar behaviours. When comparing distance variations and their signs, no significant differences were found in the upper symphysis (p = 0.82), lower symphysis (p = 0.74), or lower left SIJ (p = 0.94); however, there was a significant difference in the upper right SIJ (p = 0.04) and a non-significant difference in the lower right SIJ (p = 0.06). In the upper left SIJ, a significant difference was found (p = 0.01); however, in this case, the operated case was more rigid in comparison to the intact case. The relative rotations of each ilium were determined with respect to the sacrum using a pelvis with no load as a reference. When the load was applied to the intact pelvis, the “body weight” (F) fell over the upper face of the first sacral vertebrae, and the reactions at the hip joints (R1 and R2 ) produced a momentum

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Table 4 Results corresponding to flexion–extension rotation of the pelvis (in degrees). The rotations are contained in the sagittal plane (Rx ) Specimen 1 2 3 4 5 6 7 8 9 10 Mean±SD

Rx right ilium A −0.844 −0.244 −0.491 −1.718 −0.881 −1.097 −0.208 −0.551 −1.082 0.025 −0.709±0.520

B −1.667 −0.745 −0.774 −1.338 −1.590 −1.522 −1.491 −0.901 −1.667 −0.520 −1.221±0.438

C −0.928 −0.583 −0.469 −1.740 −0.893 −1.146 −0.744 −0.640 −0.803 −0.281 −0.823±0.405

Rx left ilium A −0.773 −0.209 −0.506 −1.730 −0.853 −1.112 −0.208 −0.552 −0.698 0.008 −0.663±0.504

B −0.695 −0.246 −0.437 −0.493 −0.736 −1.110 −0.389 −0.606 −0.902 −0.342 −0.595±0.269

C −0.904 −0.575 −0.380 −1.681 −0.876 −1.030 −0.632 −0.626 −0.650 −0.232 −0.759±0.402

Table 5 Results corresponding to internal–external rotation (in degrees). The rotations are contained in the transverse plane (Ry ) Specimen 1 2 3 4 5 6 7 8 9 10 Mean±SD

Ry right ilium A 0.182 −0.446 0.070 0.015 0.284 0.067 0.038 0.228 0.547 0.005 0.099±0.253

B −0.606 −0.243 0.118 −0.889 −0.079 0.092 0.131 0.056 0.254 0.090 −0.108±0.368

C 0.201 0.130 0.181 0.159 0.263 0.288 0.294 0.325 0.297 0.053 0.219±0.088

Ry left ilium A −0.124 0.106 −0.072 −0.190 0.048 −0.082 −0.058 −0.023 0.055 −0.009 −0.035±0.089

B 0.110 0.221 0.089 0.245 0.185 0.344 0.013 0.090 −0.025 0.001 0.127±0.119

C −0.147 0.066 0.057 −0.227 −0.031 0.182 0.211 0.007 0.063 0.000 0.018±0.133

B 0.435 0.219 0.136 0.535 0.436 0.553 0.112 0.286 0.495 0.187 0.339±0.170

C −0.119 −0.069 −0.042 0.180 0.067 −0.053 −0.138 0.062 0.030 0.005 −0.008±0.096

Table 6 Results corresponding to rotations contained in the coronal plane (Rz ) (in degrees) Specimen 1 2 3 4 5 6 7 8 9 10 Mean±SD

Rz right ilium A −0.191 0.204 −0.016 0.180 0.004 0.090 −0.004 −0.049 −0.156 0.014 0.008±0.127

B −1.336 −0.466 −0.274 −1.102 −0.364 −0.330 −0.561 −0.552 −0.751 −0.101 −0.584±0.382

C −0.256 −0.068 −0.094 −0.127 0.013 −0.068 −0.227 −0.172 −0.234 −0.038 −0.127±0.091

that caused the iliac bones to tilt towards the back with a rotation in flexion (negative Rx ) (Table 4). Table 5 shows that the Ry in the right iliac crest was generally positive, whereas in the left iliac crest, the opposite trend was observed. These trends of the bilateral internal rotation caused both iliac bones to move closer to the anterior part of the sacrum. Similarly, it is shown that the Rz rotation generally had a different sign for each ilium, that is, negative for the right and positive for the left (Table 6). Due to this nutation, the iliac crests moved closer to one another, whereas the ischial tuberosities separated. For the injured pelvis, the rotation around the x-axis is only influenced in terms of its magnitude and not in the sense of rotation (Table 4). The results indicate that the rotation in the flexion of the right ilium was greater in the injured pelvis than in the intact one (p = 0.006), whereas in the left ilium, no significant differences were observed between the intact and injured pelvises because the left SIJ was not injured (p = 0.45). With regard to the internal–external rotations of the iliac crests, there was no significant difference between the intact pelvises and injured pelvises in the right ilium (p = 0.25), whereas there was a significant difference in the left ilium in terms of rotation magnitude (p = 0.01) (Table 5); however, both iliac bones changed the sense of

Rz left ilium A −0.090 −0.034 0.058 0.314 0.030 0.097 0.008 0.145 0.244 0.015 0.079±0.124

their rotation once the pelvis was damaged due to the separation produced between the two bones upon sectioning the symphysis. From Table 6 it can be seen that, upon damage to the pelvis, the Rz rotation did not change sign, but did change in magnitude; however, in contrast to the Rx rotation, the rotation about the z-axis of both iliac bones was significantly greater in the injured pelvis (p = 0.003 in both cases). This rotation was slightly greater in magnitude for the right iliac crest (−0.584±0.382) than for the left one (0.339±0.170). A comparison of the flexion–extension rotation (Table 4) between the intact pelvis and the pelvis fixed with crossed screws showed no significant differences in the left ilium (p = 0.25). In the right SIJ, although there was a difference, it was not significant (p = 0.07). Although the sense of the rotation was maintained relative to the intact pelvis, upon dissecting the anterior ligaments of the right SIJ, the rotation of the right ilium in flexion relative to the sacrum was found to be greater than that of the left. The lateral rotation, Ry , of an intact pelvis was compared to that of a pelvis fixed with screws (Table 5), wherein a difference in the right ilium was observed, although it was not significant (p = 0.08)

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and maintained the sense of the rotation; however, no significant differences were found in the left ilium (p = 0.26). The rotation around the z-axis (Table 6) presented significant differences in both the right (p = 0.004) and left ilium (p = 0.009). The Rz of the right ilium of the pelvis synthesised with screws had the same sense of rotation as that observed for the intact pelvis; however, the magnitude was much greater when the right anterior SIJ had its ligaments sectioned, and with fixation through the symphysis to the left ilium, it also rotated in the counter-clockwise direction, or rather, the set of the two iliac bones rotated as a block. Discussion Biomechanical experiments are used to study the behaviours of organs in response to loads, and they are often used to evaluate the capacity and effectiveness of implants and osteosynthesis systems before the clinical trial phase. However, there are few recorded biomechanical studies in the literature investigating pelvic injuries with rotational instability (Tile B34 or Young–Burgess type II35 ) and the different osteosynthesis techniques employed for their fixation.30,36–39 In the present biomechanical study, the displacements and rotations produced in the symphysis pubis and the sacroiliac joint were compared by applying an axial load of 300 N to pelvic bones in the standing position in the following situations: (A) intact pelvises, (B) pelvises simulating Tile type B1 rotational instability (Young– Burgess type II), and (C) injured pelvises that had been treated using two 6.5-mm symphyseal cannulated screws placed in a position parallel to the axis that runs perpendicular to the joint of the pubic symphysis. The goal of this study was to evaluate the effectiveness of a minimally invasive osteosynthesis of the pubic symphysis for the treatment of pelvic fractures with rotational instability (Tile B or Young–Burgess type II). This system may avoid the morbidity associated with the approach of Pfannenstiel.15–24 Moreover, this approach could decrease the amount of blood lost and the risk of infection of osteosynthesis materials, such as screw plates, in patients who have previously undergone abdominal surgery, as well as potentially shorten recovery times. The symphyseal osteosynthesis systems evaluated in other studies usually require an open approach for their application. The described systems range from plates to screws with cerclages.31 It is difficult to establish comparisons among the results associated with these systems due to the variability of the measurement systems used in each test and the variability in their precision or in the characteristics of the specimens in each study. In the current study, the intact pelvises served as a point of reference to determine the validity of the investigated system. In a qualitative analysis of the joint displacements and the rotations of the iliac bones with respect to the sacrum after the application of an axial load to intact pelvises in the standing position, the ilium–sacrum system exhibits the same dynamics as those described by Varga et al.31 (Fig. 8). As far as we know, there are no published biomechanical studies evaluating the osteosynthesis treatment described herein. There is a series of cases in the literature40 consisting of 8 patients who were treated with percutaneous osteosynthesis systems; however, the fixations employed in that study were prepared over heterogeneous injury patterns. The authors used 7.3 mm screws, using one screw for B1 type lesions and two screws for B2, B3, and C type lesions. It is unknown whether a single screw is sufficient to contain the symphyseal opening; however, having demonstrated the impact of an applied load on the rotations in the flexion of the ilium, it is worth considering the possibility of symphyseal rotation in an

Fig. 8. Position of the joint reference system of the sacrum. The directions that were considered to be positive are indicated for each rotation. Rx : rotation about the x-axis (extension–flexion); Ry : rotation about the y-axis (internal–external rotation); Rz : rotation about the z-axis.

isolated implant. According to Varga et al.,31 it is necessary to seek greater stability in the lower symphysis, which would translate into less sacroiliac mobility. In this vein, a system that stabilises the symphyseal plane at two points so as to establish a static anterior plane has been proposed. During the tests, there was no complete exteriorisation of the implants, although on two occasions, an imprint of the thread of the screws in the cortex was observed. Similar to what was explained above, it is impossible to determine the degree of severity of the Tile type B injuries in the specimens investigated in the aforementioned study. In the present study, Young–Burgess type II injuries were used, which are potentially more severe (with smaller displacements of the hemipelvis and the symphyseal opening32 ). Despite this fact, after being treated with the fixation proposed in this study, the anterior of the pelvic ring in the injured pelvises exhibited behaviour similar to the physiological behaviours observed in the tests (Fig. 9). No significant differences were found in the displacements of the anterior frame after performing osteosynthesis with two 6.5 mm cannulated screws, consistent with previous reports37,38 and inconsistent with the paper of MacAvoy et al.30 in that displacements similar to those found in intact pelvises in both the upper and lower symphyses were observed. Despite obtaining a stable anterior fixation via the screw-based osteosynthesis, statistically significant differences were observed in the sacroiliac joint of the specimens, both in the upper and lower parts. Various biomechanical studies have been published with the goal of demonstrating whether a combination of anterior osteosynthesis of the symphysis and posterior fixation of the sacroiliac joint in Tile type B1 fractures provides additional stability in comparison to anterior fixation of the symphysis alone. Simonian et al.37 concluded that the combination of anterior and posterior fixation is optimal for open-book injuries of the pelvis; however, they did not find any differences between posterior fixation with plates or screws. Dujardin et al.38 came to the same conclusion; with fixation using anterior and posterior plates, they obtained stabilities similar to those of intact pelvises. On the other hand, Van den Bosch et al.39 observed that the addition of a sacroiliac screw does not provide additional rotational rigidity, stability, or translation in open-book pelvic fractures, and in both cases, a similar stiffness to that of intact pelvises was obtained. Nevertheless, it cannot be confirmed whether additional posterior fixation is necessary for these types of injuries, given that it is unknown whether the displacements obtained with such a precise measuring system could have clinical repercussions. There are

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(a)

(b)

(c)

Fig. 9. Field of view of the displacement of the left camera: (a) intact pelvis; (b) injured pelvis; (c) injured pelvis reconstructed with symphyseal screws.

authors who have observed cases of residual pain and arthritis of the sacroiliac joint41,42 among patients who have had pelvic injuries due to anterior–posterior compression with a partial injury of the sacroiliac joint and were surgically treated with only anterior fixation, as this clinical presentation could be due to the micromovement of the sacroiliac joint. More studies are needed on this topic to specify the indications of additional posterior fixation in Tile type B1 injuries. With respect to rotation, in the specimens treated with symphyseal screws, the injured hemipelvis exhibited external rotations and flexions not significantly different from those of the intact pelvises, and significant differences were found with respect to the Rz rotation. These observations indicate that absolute pelvic ring stabilisation was not achieved, although as indicated above, it would be necessary to determine whether lower-degree rotations generate sufficient instability to create clinical pain or defects in pelvic reconstruction. Open reduction and fixation with plates has been established as the gold standard for treatment of these injuries;43–47 there are many factors that suggest the need to reconsider the use of Pfannenstiel’s approach for the fixation of these fractures. The vascular anatomy of the anterior pelvis has been identified as a potential cause of significant bleeding17–21 and has occasionally been related to injuries of the corona mortis (lower epigastric system) during dissection. Injuries of the superficial iliohypogastric and ilioinguinal nerves have been described in the process of synthesis with a symphyseal plate. The damage to these nerves may cause pain at the incision site, deep in the abdomen, in the region of the labia or scrotum, and in the upper part of the thigh.15,16 Several authors have hypothesised that the abdominal wall plays an indirect role in the stability of the pelvis, and it has been demonstrated in studies on cadavers that a midline laparotomy may significantly increase displacement of the pelvis with unstable trauma.48 In addition to the above, patients with pelvic trauma may have undergone previous surgeries or may have previous abdominal complications that could impede the use of fixation. The current study is based on an experimental biomechanical study and, therefore, possesses methodological limitations inherent to this type of study. The selection of the subjects for the tests was not random, which means there is a selection bias in the sample. A certain degree of sample homogeneity was pursued in this study, resulting in 1 male and 9 female specimens. Despite this, the sample was relatively small with only 10 individuals, and the specimens may have had subject-dependent characteristic-induced biases that could modify the biomechanical behaviour of the bone, such as age and weight. The ages of the specimens were heterogeneous, which may

not appropriately represent the real population and may negatively affect the internal validity of the study. Despite using fresh samples, the fact that bone injury patterns are being analysed impedes the use of cadavers with all of their organ structures intact, including muscles and fascias or skin, as there are various tissues whose stiffness was not taken into account in the experiments. Although this study has sought to reproduce the worst possible conditions of both injury and load, the variables studied were obtained at a single moment in time (crosssectional study) in a situation with a static pelvic load, which does not simulate the real conditions of such patients, who often change from a static supine position to the sitting and then to the standing position. Another factor that was not considered in this study is the biomechanical repercussions of the healing process. The fixation system proposed herein does not diminish the stabilising role conferred by the abdominal wall but does decrease the risks associated with Pfannenstiel’s approach. Thus, this system offers a lower cost and lower morbid-mortality rate in comparison to other techniques, and moreover, the applicability of this system is almost obligatory in cases where there are injuries of the abdominal wall or active fistulas in the symphysis region, which would disqualify the patient for any type of open surgery. The system described herein presents clear results regarding its effectiveness relative to anterior fixation of the pelvis, and moreover, it is capable to produce relative displacements of the pelvic bones that are similar to the physiological conditions of the pelvis. Despite these positive results, sacroiliac mobility was observed, which could contribute to the debate on posterior fixation of the pelvis in openbook injuries. It is unknown whether the micromovements depicted herein could lead to subsequent clinical repercussions. More biomechanical studies are needed to facilitate a better comparison of the osteosynthesis system described in this study to traditional plate systems. The development of well-designed prospective clinical studies could reduce the biases inherent to this study and would allow researchers to determine whether the obtained data can be extrapolated to biological reality. Disclosure statement There are no conflicts of interest. Acknowledgments The authors are indebted to Villanueva J and Prada F, Ph.D., Anatomy Department, Universidad de Sevilla, and to CATEC for their

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