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A biomechanical study of standard posterior pelvic ring fixation versus a posterior pedicle screw construct Jonathan M. Vigdorchik a, Xin Jin b, Anil Sethi a,*, Darren T. Herzog a, Bryant W. Oliphant a, King H. Yang b, Rahul Vaidya a a b
Department of Orthopedic Surgery, Detroit Receiving Hospital, Detroit Medical Center, 4201 St. Antoine Blvd., Suite 4G, Detroit, MI 48201, United States Department of Biomedical Engineering, Wayne State University, 818 West Hancock, Detroit, MI 48201, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Accepted 25 April 2015
Objectives: The purpose of this study was to biomechanically test a percutaneous pedicle screw construct for posterior pelvic stabilisation and compare it to standard fixation modalities. Methods: Utilizing a sacral fracture and sacroiliac (SI) joint disruption model, we tested 4 constructs in single-leg stance: an S1 sacroiliac screw, S1 and S2 screws, the pedicle screw construct, and the pedicle screw construct + S1 screw. We recorded displacement at the pubic symphysis and SI joint using highspeed video. Axial stiffness was also calculated. Values were compared using a 2-way ANOVA with Bonferroni adjustment (p < 0.05). Results: In the sacral fracture model, the stiffness was greatest for the pedicle screw + S1 construct (p < 0.001). There was no significant difference between the pedicle screw construct and S1 sacroiliac screw (p = 1). For the SI joint model, the S1 + S2 SI screws had the largest overall load and stiffness (p < 0.001). The S1 screw was significantly stronger than pedicle screw construct (p = 0.001). Conclusions: The pedicle screw construct biomechanically compares to currently accepted methods of fixation for sacral fractures when the fracture is uncompressible. It should not be used for SI joint disruptions as one SI or an S1 + S2 are significantly stiffer and cheaper. ß 2015 Elsevier Ltd. All rights reserved.
Keywords: Pelvis fracture Posterior stabilisation Pedicle screw
Introduction The treatment of unstable pelvic ring injuries continues to evolve. Posterior pelvic ring injuries commonly involve the ilium, sacroiliac (SI) joint, sacrum, or a combination [1]. The incidence of unstable sacral fractures in these injuries is between 17.4% and 30.4% as reported in several large series [2,3]. The mechanism is usually high-energy trauma, although in the setting of marked osteopenia and pathologic lesions, can be caused by low-energy. The stabilisation of these injuries can be difficult even in a patient with adequate bone stock and no concomitant medical comorbidities [4–8]. This treatment is further complicated when a patient presents with insufficient bone quality, comminution or a pathologic fracture [6–8]. These patients are usually not candidates for large open or lengthy procedures [6–8]. An anterior internal fixator has been employed that utilises the supraacetabular screws connected to a subcutaneous rod for structural
* Corresponding author. Tel.: +1 313 966 7852; fax: +1 313 966 8400. E-mail address: [email protected] (A. Sethi).
stability of the anterior ring. This techniques still incorporates posterior ring stabilisation using either percutaneous or open techniques in unstable pelvic ring injuries [9–13]. Many treatment methods have been studied for posterior pelvic fixation, including external fixation, open reduction internal fixation using plates, tension band constructs, or transiliac bars, and percutaneous Sacroiliac screws (SI) [4,5,14–27]. Currently, the standard of care for unstable sacral fractures involves closed reduction with percutaneous SI screw placement [17–20]. Recently, spinopelvic fixation (SPF) and triangular osteosynthesis (TOS) techniques have been shown to have improved properties over standard SI screw fixation in terms of vertical stability and sagittal plain rotation [28–33]. Not all patients, due to a variety of factors including congenital anomalies or poor bone stock, can be appropriately stabilised with percutaneous techniques [6,8,28,29]. We employed a percutaneous technique with, custom spinal implants in a manner similar to anterior subcutaneous pelvic fixation without lumbar spine instrumentation. The purpose of the study was to evaluate the biomechanical properties of established percutaneous methods of fixation for vertically and rotationally unstable pelvic ring injuries, and
http://dx.doi.org/10.1016/j.injury.2015.04.038 0020–1383/ß 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Vigdorchik JM, et al. A biomechanical study of standard posterior pelvic ring fixation versus a posterior pedicle screw construct. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.04.038
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compare them to a posterior pedicle screw construct used alone and in combination with an SI screw. By looking at the magnitude of load required to cause a specific deformation (stiffness of the constructs) and displacements at the pubic symphysis and at the fracture site, the following questions were posed: were two screws (one into S1 and one into S2) biomechanically superior to one S1 screw; is this pedicle screw construct alone superior to the SI screw constructs; is a combination of the pedicle screw construct and one SI screw superior to the SI screw constructs? Materials and methods Fracture model In a 3rd generation composite pelvis model (Sawbones, Vashon, WA, USA), we examined two different unilateral vertically and rotationally unstable pelvic ring injuries; one with a pure SI joint dislocation (OTA type 61-C1.2.a2.c5) and another with an ipsilateral transforaminal sacral fracture (OTA type 61C1.3.a2.c5). For the transforaminal fracture model, a saw was used to osteotomise an intact composite pelvis through the right sacral neural foramina creating a 1-cm bone loss, and through the pubic symphysis. The pure SI joint dislocation model was shipped from the company with an intact left SI joint, and disrupted right SI joint and pubic symphysis. The composite pelvis model has been shown to allow for more controlled and repeatable testing, as each specimen has the same properties [34–37]. The model simulates natural cortical bone using short e-glass fibres and epoxy resin pressure injected around a foam core to represent the cancellous bone of an average-sized adult male. Six composite pelvises were tested. Three composite pelvises were tested with the sacral fracture model with the 4 stabilisation methods applied in a random order. Three composite pelvises were tested with the SI joint dislocation model but only 3 stabilisation methods applied in random order were tested. This was because one construct (Pedicle screw construct by itself) was so inferior in this model that it was futile to continue testing it. Fixation construct Construct S1 was a standard 6.5-mm 100-mm partially threaded (32-mm) cannulated cancellous SI screw (Synthes, Paoli, PA, USA) placed under fluoroscopy and direct visualisation posteriorly from the ilium into the body of the first sacral vertebrae without anterior cortex penetration [19,20]. The screw was placed perpendicular [28] to the sacral fracture line or SI joint, and a 7 in.-pound torque screwdriver [27,38] was used to achieve equal compression in the SI dislocation model. No compression was applied in the sacral fracture model. The screw and washer were sunk to the level of the bone. Construct S1S2 utilised two SI screws, one into the S1 vertebral body, and the other into the S2 vertebrae (The S2 screw was 80 mm). Once again, a 7 in.-pound torque screwdriver was used in the SI dislocation model. No compression was applied in the sacral fracture model. The screw and washer were sunk to the level of the bone. The pedicle screw construct, utilised two 7.0-mm 80-mm titanium polyaxial pedicle screws and a 6.0-mm stiff titanium rod (Click’X Pedicle Screw System, Synthes, Paoli, PA, USA). Under direct visualisation and fluoroscopy using Judet oblique views, a 4.5-mm drill was placed with the starting point at the posteriorsuperior iliac spine (PSIS) directed towards the anterior–inferior iliac spine (AIIS). After drilling, the pedicle screw was inserted, with the same method repeated at the contralateral PSIS. The two screws were seated to the level of the bone by sinking them into the PSIS. A straight rod of appropriate length was placed in the screw heads, and locking caps applied. The construct was
Fig. 1. Posterior view of novel device + S1 screw in a composite pelvis.
compressed using a small c-clamp attached to the rod and compressor instruments from the spinal system; the locking caps were then tightened with a 7 in.-pound torque screwdriver in the SI dislocation model. No compression was applied in the sacral fracture model. Care was taken to cause no deformation at the SI joint or pubic symphysis during compression of the construct. The pedicle screws + S1 device (Fig. 1) is a combination of two constructs: one SI screw into the S1 vertebral body and the pedicle screw construct. Testing We created a single-leg stance model (Fig. 2) based on previous reports [15,27,39,40]. Each pelvis was securely mounted through the sacrum at the S1 level to a servo hydraulic testing machine (Instron Model 8500, Canton, MA, USA). The sacral mount was free to pivot in all planes to eliminate loading artifact. The anatomic standing vertical relationship of the anterior-superior iliac spine (ASIS) and pubic symphysis was maintained. For the femur, we used a 52-mm hemiarthroplasty, which freely articulated with the acetabulum. The femoral component was potted using Bondo putty (3M, St. Paul, MN, USA) in a position of 158 of adduction and 158 of anteversion to simulate single-legged stance, and the distal femur rigidly fixed to a laterally mobile base plate with confines on both sides. The abductor musculature and vectors were simulated using cables attached to the pelvis with two drill holes at the gluteus ridge, and a pulley system at the level of the greater trochanter on the prosthesis [27,41]. The chains were tightened manually and stabilised the hemi pelvis against rotation. Markers were placed at the fracture site and at the pubic symphysis. The markers were small white dots, uniform in size, placed at the superior and inferior aspects of the bilateral SI joints, and at the superior and inferior portion of the anterior pubic symphysis. A high-speed digital video camera (HG-2000, MotionXtra Inc., San Diego, CA, USA) aimed at the markers, recorded images at 125 frames per second. A vertical compressive displacement of 7 mm was applied through the sacral mount at the rate of 10 mm/s. This was chosen based on previous studies [25,26], and pilot data showing a more reproducible measurement of construct stiffness. For each pelvis, the constructs were applied in random order. The testing was repeated five times per construct to look for any degradation in the mechanical properties of the construct with repeated testing of the same pelvis. Each construct was re-tightened between tests. For each loading cycle, force and displacement data were recorded by a 32-bit Instron MAX V9.3 at 1 kHz, and connected to a
Please cite this article in press as: Vigdorchik JM, et al. A biomechanical study of standard posterior pelvic ring fixation versus a posterior pedicle screw construct. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.04.038
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adjustment for multiple comparisons using SPSS software (p < 0.05) (Fig. 3). Results
Fig. 2. Single-leg stance model. The white dots are the markers and the biplanar high speed cameras pick up the change in position of the parts.
Pentium Dual-Core E5300 personal computer. Force–displacement curves were generated for each test from the actuator of the test system. Total axial stiffness of fixation was calculated from the linear portion of the force–displacement curve. The displacement at the pubic symphysis and SI joint were analysed using TEMA (Photo-Sonics, Burbank, CA, USA) motion analysis software. Load, stiffness and displacement at the pubic symphysis and SI joint were analysed by 2-way ANOVA with a Bonferroni
The testing was repeated five times per construct to look for any degradation in the mechanical properties of each construct but no degradation was noted during the study. In the sacral fracture model, the average maximum load (Fig. 4A) for Construct pedicle screw + S1 was 414 N (SE, 8 N) which was significantly larger than constructs S1 (57 N; SE, 17 N; p < 0.001), S1S2 (244 N; SE, 63 N; p < 0.001), and pedicle screw alone (81 N; SE, 6 N; p < 0.001). Construct S1S2 was significantly stiffer than construct S1 and pedicle screw construct (p < 0.001). There was no significant difference between construct pedicle screws alone and construct S1 (p = 1). The stiffness data (Fig. 4B) correlated well with the force data. Construct pedicle screw + S1 had the largest stiffness of 54 N/mm (SE, 5 N/mm), and was significantly stiffer than constructs S1, S1S2, and pedicle screw alone (p < 0.001). Construct S1S2 had a stiffness of 29 N/mm (SE, 8 N/mm), and was significantly stiffer than constructs S1 and pedicle screw construct (p < 0.001). Once again, there was no significant difference (p = 0.342) between construct S1 (3 N/mm; SE, 1 N/mm) and construct pedicle screw alone (9 N/ mm; SE, 2 N/mm). We examined the displacement at the pubic symphysis (Fig. 5A) and SI joint (Fig. 5B). At the SI joint, construct pedicle screw + S1 (1.1-mm; SE 0.4-mm) displaced significantly less than construct S1 (6-mm; SE 3-mm; p = 0.001). There was no difference between construct S1, construct S1S2 (2.78-mm; SE 1.47-mm; p = 0.74) and construct pedicle screw alone (1.1-mm; SE 0.4-mm; p = 1). At the pubic symphysis, construct pedicle screw + S1 (15.4-mm; SE 0.7mm) displaced significantly less than construct S1 (23.5-mm; SE 2.8-mm; p = 0.009). There was no difference between construct S1, S1S2 (20.1-mm; SE 5.4-mm; p = 0.25), and pedicle screw alone (17.5-mm; SE 3-mm; p = 1). For the SI joint model, construct pedicle screws + S1 was not tested. This was because it was obvious that the S1 or S1S2 construct were so much stiffer in this model and the pedicle screw construct was very weak that it was not necessary. Construct S1S2 had the largest overall load (984 N; SE 159 N) (Fig. 5A). This was significantly higher than the loads for constructs S1 (625 N; SE 181 N; p < 0.001) and pedicle screw (71 N; SE 19 N; p < 0.001).
Fig. 3. (A) Maximum load (N) and (B) stiffness (N/mm) of the 4 constructs for the sacral fracture model. Construct novel + S1 is significantly stronger than all other constructs (p < 0.001). Construct S1S2 is significantly stronger than construct S1 and construct novel (p < 0.001).
Please cite this article in press as: Vigdorchik JM, et al. A biomechanical study of standard posterior pelvic ring fixation versus a posterior pedicle screw construct. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.04.038
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Fig. 4. Displacement at the pubic symphysis (A) and SI joint (B) for the sacral fracture model. Construct novel + S1 displaced significantly less than construct S1 at the pubic symphysis and SI joint (p = 0.009 and p = 0.001, respectively).
Construct S1 was also significantly stronger than pedicle screw alone (p = 0.001). The axial stiffness (Fig. 5B) results matched the load data, with construct S1S2 (233 N/mm; SE 49 N/mm) being significantly stiffer than constructs S1 (118 N/mm; SE 24 N/mm; p < 0.001) and pedicle screw alone (48 N/mm; SE 15 N/mm; p < 0.001). Construct S1 was also significantly stiffer than pedicle screw alone (p = 0.006). Discussion The purpose of this study was a biomechanical comparison of four different posterior fixation techniques for vertically and rotationally unstable pelvic ring injuries using a composite pelvis model. For the sacral fracture model and the SI joint disruption model, we found that two SI screws placed in the S1 and S2 vertebrae, (Construct S1S2) was significantly stiffer than one SI screw positioned in the S1 vertebrae (Construct S1). This result is contrary to previous findings that show no differences between 1, 2, or 3 SI screws [1,15] but in agreement with one previously published study [27]. The improved stability is likely due to improved sagittal plane rotational control of the unstable fracture, as the one screw construct rotated about the axis of the screw during testing. This
finding may be more significant in our study due to the fracture gap, as previous testing relied on the frictional and compressive forces on opposing bone edges that may have helped prevent rotation [13,20,28]. Also in the sacral fracture model, the pedicle screw construct alone was as strong as the S1 screw construct (p > 0.05) but significantly weaker than the S1S2 screw construct (p < 0.001). We believe this was due to the increased moment arm of our construct and increased bending strength of the two screws. A combination of the pedicle screw device and one SI screw was the stiffest construct compared to the other three (p < 0.05). The cumulative bending stiffness of the screw and the pedicle screw construct combined with the superior rotational control of the pedicle screw construct accounted for these findings. The results for the SI joint model were different. As shown in a previous study, 2 SI screws were significantly stronger than 1 SI screw [27]. The pedicle screw construct was significantly less stiff than both of the SI screw constructs. Therefore, we would not recommend using the pedicle screw construct on its own for a pelvic ring injury with SI joint disruption. The reason for the difference in the 2 models is the ability of SI screws to use friction when they can compress the injury in the SI Joint disruption model.
Fig. 5. (A) Maximum load (N) and (B) stiffness (N/mm) for the SI joint model. Construct S1S2 was significantly stronger than all other constructs (p < 0.001). Construct S1 was significantly stronger than construct novel (p = 0.001).
Please cite this article in press as: Vigdorchik JM, et al. A biomechanical study of standard posterior pelvic ring fixation versus a posterior pedicle screw construct. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.04.038
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This compression reconstructs the keystone structure of the 2 ilia and sacrum to combat vertical force. The inability to reconstruct this structure in the fracture gap model is why all constructs are poor in resisting vertical force in a sacral fracture model where compression is not possible and the pedicle screw construct becomes useful in this pathology. Many previous studies have been performed testing internal fixation techniques for posterior pelvic ring injuries. It is difficult to extrapolate data across studies as many different fixation methods and loading techniques were examined: transiliac plates, 1, 2, or 3 sacroiliac screws, transiliac rods, and tension band plates have all been examined in single [15,16,22,23,40,49] and double leg stance [1,3,21,42,46,47] models with varying loads, from 250 to 2000 N. Different injury patterns were also tested, with or without stabilisation of the anterior pelvis [1,23,43,46]. Between studies and between constructs, no significant differences in stability were shown. However, all methods of fixation are inferior to the intact pelvis [1,23,42]. Our data for the SI screw techniques is in agreement with previous studies showing a difference between one and two screws [27]. Also, our force data for construct pedicle screw + S1 compares to recent literature for triangular osteosynthesis with average loads of 344–360 N [32]. Transforaminal sacral fractures are unstable in the vertical, horizontal, and rotational planes. Multiplanar stability is of utmost importance to prevent sacral nerve root injuries [28]. Techniques such as triangular osteosynthesis have been developed to try to address this issue. The lumbopelvic fixation adds vertical stability, while the iliosacral fixation adds the horizontal component [28]. The fracture may extend to the L5 pedicle, requiring the lumbopelvic fixation to be extended to the L4 pedicle [28]. Most trauma surgeons are not comfortable instrumenting the lumbar spine. This pedicle screw construct builds on these principles, is a percutaneous technique, and combines the benefit of avoiding the lumbar spine. The iliosacral screw provides horizontal stability and some bending strength. The bilateral iliac pedicle screws are directed obliquely, creating a large moment opposing both vertical displacement and sagittal plane rotation, and when placed between the inner and outer iliac tables directed towards the AIIS, have excellent fixation over a large surface [31]. Of clinical importance, this technique is familiar to most trauma surgeons, and is analogous to supra-acetabular screws used in external fixation [50] This study has several limitations. We attempted to simulate an in-vivo loading and testing environment although we did not attempt to simulate in-vivo forces. We chose a single-leg stance model, as this was the most unstable scenario, with greater shear, bending and rotational forces than a double-leg stance [15]. This has been postulated to be more relevant to clinical application [15,22]. Further, six composite pelvises were tested. Of these 3 pelvises were tested with the sacral fracture model and 3 with the SI joint dislocation model. This number may be considered small, however we were able to obtain results that were statistically significant with these numbers. Another limitation to this study could be that by creating a 1-cm fracture gap, we did not simulate the friction of the bone edges as might be seen with sacral fractures but we wanted to test the worst case scenario where no compression was possible. Also, the anterior pelvis was osteotomised through the pubic symphysis and left free of fixation to eliminate any confounding of anterior fixation [42]. We used a 3rd generation composite pelvis model, providing consistent material properties as compared to cadaver pelvises, and thus minimizing variations between specimens and allowing us to achieve relatively small standard deviations. We were not concerned about the bone screw interface which would confound testing in cadaver models. We used high-speed video cameras to measure movement. Other studies have used strain gauges, inclinometers, linear voltage transducers, or electromagnetic motion sensors to
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measure displacements, rotations, and 3-dimensional motions [1,15,26,27,42–48]. We chose high-speed video because it allowed us to evaluate in real-time the complex motions across the pelvis, and gave us an accurate (up to 0.8 mm) detection of displacement to measure the stiffness of each construct. Although we were unable to quantify rotational and 3-dimentional displacements, we were able to quantify displacement in the x and y directions and visually observe any rotation of the pelvis. We feel that indications for this pedicle screw construct would include comminuted sacral fractures where no compression is possible with SI screws, Pelvic fractures with sacral tumors usually from Metastasis where an SI screw may have no purchase, and a fracture in the Ilium or SI Joint area which interferes with the insertion point of percutaneous SI screws. In situations where one would want 2 SI screws but it is difficult or unfeasible the construct would be useful to supplement a single SI screw. We do not think it is indicated in SI joint disruptions or fractures where you are able to compress the posterior injury as SI screws are cheaper ($150 vs. $1350) and make a stiffer construct. The pedicle screw construct would be beneficial in treating osteoporotic fractures both anteriorly and posteriorly as its strength relies more on the cortical wall of the ilium and not the cancellous osteoporotic bone of the sacrum or the superior pubic ramii and can be placed with four percutaneous incisions (2 AIIS and 2 PSIS). In the future, this construct should be compared to lumbopelvic fixation techniques. Conclusion This biomechanical study has shown that two iliosacral screws (one in S1 and one in S2) are stiffer than one iliosacral screw and appeared to have less movement in rotational planes for the SI Joint disruption model and the Sacral fracture model. One SI screw combined with the pedicle screw construct was stiffest in the sacral fracture model and one SI screw was equal to the pedicle screw construct in this model as well. The pedicle screw construct alone was weakest in the SI joint disruption model as compared to 1 SI screw or 2 SI screws and we do not recommend its use in compressible posterior injuries where the SI screws are much cheaper and more effective. Conflict of interest statement The authors declare that they have no Conflict of Interest in the preparation of this publication: We also declare that: Dr. Vaidya has received payment for lectures from AO Trauma and Synthes Trauma for teaching at Courses. Dr. Vaidya has received payments for consulting with Stryker Corporation for implant development. Dr. Oliphant has received payments for lectures from Synthes Trauma for teaching at courses. The institution of the authors has received funding from Synthes for the testing materials used in this study through a research grant. The implants used in this study are not FDA approved for use in the technique described. References [1] Simonian PT, Routt Jr C, Harrington RM, Tencer AF. Internal fixation for the transforaminal sacral fracture. Clin Orthop Relat Res 1996;323:202–9. [2] Denis F, Davis S, Comfort T. Sacral fractures: an important problem. Retrospective analysis of 236 cases. Clin Orthop Relat Res 1988;227:67–81. [3] Gibbons KJ, Soloniuk DS, Razack N. Neurological injury and patterns of sacral fractures. J Neurosurg 1990;72:889–93. [4] Matta JM, Saucedo T. Internal fixation of pelvic ring fractures. Clin Orthop Relat Res 1989;242:83–97. [5] Matta JM, Tornetta 3rd P. Internal fixation of unstable pelvic ring injuries. Clin Orthop Relat Res 1996;329:129–40.
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JINJ-6186; No. of Pages 6 Injury, Int. J. Care Injured xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
A biomechanical study of standard posterior pelvic ring fixation versus a posterior pedicle screw construct Jonathan M. Vigdorchik a, Xin Jin b, Anil Sethi a,*, Darren T. Herzog a, Bryant W. Oliphant a, King H. Yang b, Rahul Vaidya a a b
Department of Orthopedic Surgery, Detroit Receiving Hospital, Detroit Medical Center, 4201 St. Antoine Blvd., Suite 4G, Detroit, MI 48201, United States Department of Biomedical Engineering, Wayne State University, 818 West Hancock, Detroit, MI 48201, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Accepted 25 April 2015
Objectives: The purpose of this study was to biomechanically test a percutaneous pedicle screw construct for posterior pelvic stabilisation and compare it to standard fixation modalities. Methods: Utilizing a sacral fracture and sacroiliac (SI) joint disruption model, we tested 4 constructs in single-leg stance: an S1 sacroiliac screw, S1 and S2 screws, the pedicle screw construct, and the pedicle screw construct + S1 screw. We recorded displacement at the pubic symphysis and SI joint using highspeed video. Axial stiffness was also calculated. Values were compared using a 2-way ANOVA with Bonferroni adjustment (p < 0.05). Results: In the sacral fracture model, the stiffness was greatest for the pedicle screw + S1 construct (p < 0.001). There was no significant difference between the pedicle screw construct and S1 sacroiliac screw (p = 1). For the SI joint model, the S1 + S2 SI screws had the largest overall load and stiffness (p < 0.001). The S1 screw was significantly stronger than pedicle screw construct (p = 0.001). Conclusions: The pedicle screw construct biomechanically compares to currently accepted methods of fixation for sacral fractures when the fracture is uncompressible. It should not be used for SI joint disruptions as one SI or an S1 + S2 are significantly stiffer and cheaper. ß 2015 Elsevier Ltd. All rights reserved.
Keywords: Pelvis fracture Posterior stabilisation Pedicle screw
Introduction The treatment of unstable pelvic ring injuries continues to evolve. Posterior pelvic ring injuries commonly involve the ilium, sacroiliac (SI) joint, sacrum, or a combination [1]. The incidence of unstable sacral fractures in these injuries is between 17.4% and 30.4% as reported in several large series [2,3]. The mechanism is usually high-energy trauma, although in the setting of marked osteopenia and pathologic lesions, can be caused by low-energy. The stabilisation of these injuries can be difficult even in a patient with adequate bone stock and no concomitant medical comorbidities [4–8]. This treatment is further complicated when a patient presents with insufficient bone quality, comminution or a pathologic fracture [6–8]. These patients are usually not candidates for large open or lengthy procedures [6–8]. An anterior internal fixator has been employed that utilises the supraacetabular screws connected to a subcutaneous rod for structural
* Corresponding author. Tel.: +1 313 966 7852; fax: +1 313 966 8400. E-mail address: [email protected] (A. Sethi).
stability of the anterior ring. This techniques still incorporates posterior ring stabilisation using either percutaneous or open techniques in unstable pelvic ring injuries [9–13]. Many treatment methods have been studied for posterior pelvic fixation, including external fixation, open reduction internal fixation using plates, tension band constructs, or transiliac bars, and percutaneous Sacroiliac screws (SI) [4,5,14–27]. Currently, the standard of care for unstable sacral fractures involves closed reduction with percutaneous SI screw placement [17–20]. Recently, spinopelvic fixation (SPF) and triangular osteosynthesis (TOS) techniques have been shown to have improved properties over standard SI screw fixation in terms of vertical stability and sagittal plain rotation [28–33]. Not all patients, due to a variety of factors including congenital anomalies or poor bone stock, can be appropriately stabilised with percutaneous techniques [6,8,28,29]. We employed a percutaneous technique with, custom spinal implants in a manner similar to anterior subcutaneous pelvic fixation without lumbar spine instrumentation. The purpose of the study was to evaluate the biomechanical properties of established percutaneous methods of fixation for vertically and rotationally unstable pelvic ring injuries, and
http://dx.doi.org/10.1016/j.injury.2015.04.038 0020–1383/ß 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Vigdorchik JM, et al. A biomechanical study of standard posterior pelvic ring fixation versus a posterior pedicle screw construct. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.04.038
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compare them to a posterior pedicle screw construct used alone and in combination with an SI screw. By looking at the magnitude of load required to cause a specific deformation (stiffness of the constructs) and displacements at the pubic symphysis and at the fracture site, the following questions were posed: were two screws (one into S1 and one into S2) biomechanically superior to one S1 screw; is this pedicle screw construct alone superior to the SI screw constructs; is a combination of the pedicle screw construct and one SI screw superior to the SI screw constructs? Materials and methods Fracture model In a 3rd generation composite pelvis model (Sawbones, Vashon, WA, USA), we examined two different unilateral vertically and rotationally unstable pelvic ring injuries; one with a pure SI joint dislocation (OTA type 61-C1.2.a2.c5) and another with an ipsilateral transforaminal sacral fracture (OTA type 61C1.3.a2.c5). For the transforaminal fracture model, a saw was used to osteotomise an intact composite pelvis through the right sacral neural foramina creating a 1-cm bone loss, and through the pubic symphysis. The pure SI joint dislocation model was shipped from the company with an intact left SI joint, and disrupted right SI joint and pubic symphysis. The composite pelvis model has been shown to allow for more controlled and repeatable testing, as each specimen has the same properties [34–37]. The model simulates natural cortical bone using short e-glass fibres and epoxy resin pressure injected around a foam core to represent the cancellous bone of an average-sized adult male. Six composite pelvises were tested. Three composite pelvises were tested with the sacral fracture model with the 4 stabilisation methods applied in a random order. Three composite pelvises were tested with the SI joint dislocation model but only 3 stabilisation methods applied in random order were tested. This was because one construct (Pedicle screw construct by itself) was so inferior in this model that it was futile to continue testing it. Fixation construct Construct S1 was a standard 6.5-mm 100-mm partially threaded (32-mm) cannulated cancellous SI screw (Synthes, Paoli, PA, USA) placed under fluoroscopy and direct visualisation posteriorly from the ilium into the body of the first sacral vertebrae without anterior cortex penetration [19,20]. The screw was placed perpendicular [28] to the sacral fracture line or SI joint, and a 7 in.-pound torque screwdriver [27,38] was used to achieve equal compression in the SI dislocation model. No compression was applied in the sacral fracture model. The screw and washer were sunk to the level of the bone. Construct S1S2 utilised two SI screws, one into the S1 vertebral body, and the other into the S2 vertebrae (The S2 screw was 80 mm). Once again, a 7 in.-pound torque screwdriver was used in the SI dislocation model. No compression was applied in the sacral fracture model. The screw and washer were sunk to the level of the bone. The pedicle screw construct, utilised two 7.0-mm 80-mm titanium polyaxial pedicle screws and a 6.0-mm stiff titanium rod (Click’X Pedicle Screw System, Synthes, Paoli, PA, USA). Under direct visualisation and fluoroscopy using Judet oblique views, a 4.5-mm drill was placed with the starting point at the posteriorsuperior iliac spine (PSIS) directed towards the anterior–inferior iliac spine (AIIS). After drilling, the pedicle screw was inserted, with the same method repeated at the contralateral PSIS. The two screws were seated to the level of the bone by sinking them into the PSIS. A straight rod of appropriate length was placed in the screw heads, and locking caps applied. The construct was
Fig. 1. Posterior view of novel device + S1 screw in a composite pelvis.
compressed using a small c-clamp attached to the rod and compressor instruments from the spinal system; the locking caps were then tightened with a 7 in.-pound torque screwdriver in the SI dislocation model. No compression was applied in the sacral fracture model. Care was taken to cause no deformation at the SI joint or pubic symphysis during compression of the construct. The pedicle screws + S1 device (Fig. 1) is a combination of two constructs: one SI screw into the S1 vertebral body and the pedicle screw construct. Testing We created a single-leg stance model (Fig. 2) based on previous reports [15,27,39,40]. Each pelvis was securely mounted through the sacrum at the S1 level to a servo hydraulic testing machine (Instron Model 8500, Canton, MA, USA). The sacral mount was free to pivot in all planes to eliminate loading artifact. The anatomic standing vertical relationship of the anterior-superior iliac spine (ASIS) and pubic symphysis was maintained. For the femur, we used a 52-mm hemiarthroplasty, which freely articulated with the acetabulum. The femoral component was potted using Bondo putty (3M, St. Paul, MN, USA) in a position of 158 of adduction and 158 of anteversion to simulate single-legged stance, and the distal femur rigidly fixed to a laterally mobile base plate with confines on both sides. The abductor musculature and vectors were simulated using cables attached to the pelvis with two drill holes at the gluteus ridge, and a pulley system at the level of the greater trochanter on the prosthesis [27,41]. The chains were tightened manually and stabilised the hemi pelvis against rotation. Markers were placed at the fracture site and at the pubic symphysis. The markers were small white dots, uniform in size, placed at the superior and inferior aspects of the bilateral SI joints, and at the superior and inferior portion of the anterior pubic symphysis. A high-speed digital video camera (HG-2000, MotionXtra Inc., San Diego, CA, USA) aimed at the markers, recorded images at 125 frames per second. A vertical compressive displacement of 7 mm was applied through the sacral mount at the rate of 10 mm/s. This was chosen based on previous studies [25,26], and pilot data showing a more reproducible measurement of construct stiffness. For each pelvis, the constructs were applied in random order. The testing was repeated five times per construct to look for any degradation in the mechanical properties of the construct with repeated testing of the same pelvis. Each construct was re-tightened between tests. For each loading cycle, force and displacement data were recorded by a 32-bit Instron MAX V9.3 at 1 kHz, and connected to a
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adjustment for multiple comparisons using SPSS software (p < 0.05) (Fig. 3). Results
Fig. 2. Single-leg stance model. The white dots are the markers and the biplanar high speed cameras pick up the change in position of the parts.
Pentium Dual-Core E5300 personal computer. Force–displacement curves were generated for each test from the actuator of the test system. Total axial stiffness of fixation was calculated from the linear portion of the force–displacement curve. The displacement at the pubic symphysis and SI joint were analysed using TEMA (Photo-Sonics, Burbank, CA, USA) motion analysis software. Load, stiffness and displacement at the pubic symphysis and SI joint were analysed by 2-way ANOVA with a Bonferroni
The testing was repeated five times per construct to look for any degradation in the mechanical properties of each construct but no degradation was noted during the study. In the sacral fracture model, the average maximum load (Fig. 4A) for Construct pedicle screw + S1 was 414 N (SE, 8 N) which was significantly larger than constructs S1 (57 N; SE, 17 N; p < 0.001), S1S2 (244 N; SE, 63 N; p < 0.001), and pedicle screw alone (81 N; SE, 6 N; p < 0.001). Construct S1S2 was significantly stiffer than construct S1 and pedicle screw construct (p < 0.001). There was no significant difference between construct pedicle screws alone and construct S1 (p = 1). The stiffness data (Fig. 4B) correlated well with the force data. Construct pedicle screw + S1 had the largest stiffness of 54 N/mm (SE, 5 N/mm), and was significantly stiffer than constructs S1, S1S2, and pedicle screw alone (p < 0.001). Construct S1S2 had a stiffness of 29 N/mm (SE, 8 N/mm), and was significantly stiffer than constructs S1 and pedicle screw construct (p < 0.001). Once again, there was no significant difference (p = 0.342) between construct S1 (3 N/mm; SE, 1 N/mm) and construct pedicle screw alone (9 N/ mm; SE, 2 N/mm). We examined the displacement at the pubic symphysis (Fig. 5A) and SI joint (Fig. 5B). At the SI joint, construct pedicle screw + S1 (1.1-mm; SE 0.4-mm) displaced significantly less than construct S1 (6-mm; SE 3-mm; p = 0.001). There was no difference between construct S1, construct S1S2 (2.78-mm; SE 1.47-mm; p = 0.74) and construct pedicle screw alone (1.1-mm; SE 0.4-mm; p = 1). At the pubic symphysis, construct pedicle screw + S1 (15.4-mm; SE 0.7mm) displaced significantly less than construct S1 (23.5-mm; SE 2.8-mm; p = 0.009). There was no difference between construct S1, S1S2 (20.1-mm; SE 5.4-mm; p = 0.25), and pedicle screw alone (17.5-mm; SE 3-mm; p = 1). For the SI joint model, construct pedicle screws + S1 was not tested. This was because it was obvious that the S1 or S1S2 construct were so much stiffer in this model and the pedicle screw construct was very weak that it was not necessary. Construct S1S2 had the largest overall load (984 N; SE 159 N) (Fig. 5A). This was significantly higher than the loads for constructs S1 (625 N; SE 181 N; p < 0.001) and pedicle screw (71 N; SE 19 N; p < 0.001).
Fig. 3. (A) Maximum load (N) and (B) stiffness (N/mm) of the 4 constructs for the sacral fracture model. Construct novel + S1 is significantly stronger than all other constructs (p < 0.001). Construct S1S2 is significantly stronger than construct S1 and construct novel (p < 0.001).
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Fig. 4. Displacement at the pubic symphysis (A) and SI joint (B) for the sacral fracture model. Construct novel + S1 displaced significantly less than construct S1 at the pubic symphysis and SI joint (p = 0.009 and p = 0.001, respectively).
Construct S1 was also significantly stronger than pedicle screw alone (p = 0.001). The axial stiffness (Fig. 5B) results matched the load data, with construct S1S2 (233 N/mm; SE 49 N/mm) being significantly stiffer than constructs S1 (118 N/mm; SE 24 N/mm; p < 0.001) and pedicle screw alone (48 N/mm; SE 15 N/mm; p < 0.001). Construct S1 was also significantly stiffer than pedicle screw alone (p = 0.006). Discussion The purpose of this study was a biomechanical comparison of four different posterior fixation techniques for vertically and rotationally unstable pelvic ring injuries using a composite pelvis model. For the sacral fracture model and the SI joint disruption model, we found that two SI screws placed in the S1 and S2 vertebrae, (Construct S1S2) was significantly stiffer than one SI screw positioned in the S1 vertebrae (Construct S1). This result is contrary to previous findings that show no differences between 1, 2, or 3 SI screws [1,15] but in agreement with one previously published study [27]. The improved stability is likely due to improved sagittal plane rotational control of the unstable fracture, as the one screw construct rotated about the axis of the screw during testing. This
finding may be more significant in our study due to the fracture gap, as previous testing relied on the frictional and compressive forces on opposing bone edges that may have helped prevent rotation [13,20,28]. Also in the sacral fracture model, the pedicle screw construct alone was as strong as the S1 screw construct (p > 0.05) but significantly weaker than the S1S2 screw construct (p < 0.001). We believe this was due to the increased moment arm of our construct and increased bending strength of the two screws. A combination of the pedicle screw device and one SI screw was the stiffest construct compared to the other three (p < 0.05). The cumulative bending stiffness of the screw and the pedicle screw construct combined with the superior rotational control of the pedicle screw construct accounted for these findings. The results for the SI joint model were different. As shown in a previous study, 2 SI screws were significantly stronger than 1 SI screw [27]. The pedicle screw construct was significantly less stiff than both of the SI screw constructs. Therefore, we would not recommend using the pedicle screw construct on its own for a pelvic ring injury with SI joint disruption. The reason for the difference in the 2 models is the ability of SI screws to use friction when they can compress the injury in the SI Joint disruption model.
Fig. 5. (A) Maximum load (N) and (B) stiffness (N/mm) for the SI joint model. Construct S1S2 was significantly stronger than all other constructs (p < 0.001). Construct S1 was significantly stronger than construct novel (p = 0.001).
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This compression reconstructs the keystone structure of the 2 ilia and sacrum to combat vertical force. The inability to reconstruct this structure in the fracture gap model is why all constructs are poor in resisting vertical force in a sacral fracture model where compression is not possible and the pedicle screw construct becomes useful in this pathology. Many previous studies have been performed testing internal fixation techniques for posterior pelvic ring injuries. It is difficult to extrapolate data across studies as many different fixation methods and loading techniques were examined: transiliac plates, 1, 2, or 3 sacroiliac screws, transiliac rods, and tension band plates have all been examined in single [15,16,22,23,40,49] and double leg stance [1,3,21,42,46,47] models with varying loads, from 250 to 2000 N. Different injury patterns were also tested, with or without stabilisation of the anterior pelvis [1,23,43,46]. Between studies and between constructs, no significant differences in stability were shown. However, all methods of fixation are inferior to the intact pelvis [1,23,42]. Our data for the SI screw techniques is in agreement with previous studies showing a difference between one and two screws [27]. Also, our force data for construct pedicle screw + S1 compares to recent literature for triangular osteosynthesis with average loads of 344–360 N [32]. Transforaminal sacral fractures are unstable in the vertical, horizontal, and rotational planes. Multiplanar stability is of utmost importance to prevent sacral nerve root injuries [28]. Techniques such as triangular osteosynthesis have been developed to try to address this issue. The lumbopelvic fixation adds vertical stability, while the iliosacral fixation adds the horizontal component [28]. The fracture may extend to the L5 pedicle, requiring the lumbopelvic fixation to be extended to the L4 pedicle [28]. Most trauma surgeons are not comfortable instrumenting the lumbar spine. This pedicle screw construct builds on these principles, is a percutaneous technique, and combines the benefit of avoiding the lumbar spine. The iliosacral screw provides horizontal stability and some bending strength. The bilateral iliac pedicle screws are directed obliquely, creating a large moment opposing both vertical displacement and sagittal plane rotation, and when placed between the inner and outer iliac tables directed towards the AIIS, have excellent fixation over a large surface [31]. Of clinical importance, this technique is familiar to most trauma surgeons, and is analogous to supra-acetabular screws used in external fixation [50] This study has several limitations. We attempted to simulate an in-vivo loading and testing environment although we did not attempt to simulate in-vivo forces. We chose a single-leg stance model, as this was the most unstable scenario, with greater shear, bending and rotational forces than a double-leg stance [15]. This has been postulated to be more relevant to clinical application [15,22]. Further, six composite pelvises were tested. Of these 3 pelvises were tested with the sacral fracture model and 3 with the SI joint dislocation model. This number may be considered small, however we were able to obtain results that were statistically significant with these numbers. Another limitation to this study could be that by creating a 1-cm fracture gap, we did not simulate the friction of the bone edges as might be seen with sacral fractures but we wanted to test the worst case scenario where no compression was possible. Also, the anterior pelvis was osteotomised through the pubic symphysis and left free of fixation to eliminate any confounding of anterior fixation [42]. We used a 3rd generation composite pelvis model, providing consistent material properties as compared to cadaver pelvises, and thus minimizing variations between specimens and allowing us to achieve relatively small standard deviations. We were not concerned about the bone screw interface which would confound testing in cadaver models. We used high-speed video cameras to measure movement. Other studies have used strain gauges, inclinometers, linear voltage transducers, or electromagnetic motion sensors to
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measure displacements, rotations, and 3-dimensional motions [1,15,26,27,42–48]. We chose high-speed video because it allowed us to evaluate in real-time the complex motions across the pelvis, and gave us an accurate (up to 0.8 mm) detection of displacement to measure the stiffness of each construct. Although we were unable to quantify rotational and 3-dimentional displacements, we were able to quantify displacement in the x and y directions and visually observe any rotation of the pelvis. We feel that indications for this pedicle screw construct would include comminuted sacral fractures where no compression is possible with SI screws, Pelvic fractures with sacral tumors usually from Metastasis where an SI screw may have no purchase, and a fracture in the Ilium or SI Joint area which interferes with the insertion point of percutaneous SI screws. In situations where one would want 2 SI screws but it is difficult or unfeasible the construct would be useful to supplement a single SI screw. We do not think it is indicated in SI joint disruptions or fractures where you are able to compress the posterior injury as SI screws are cheaper ($150 vs. $1350) and make a stiffer construct. The pedicle screw construct would be beneficial in treating osteoporotic fractures both anteriorly and posteriorly as its strength relies more on the cortical wall of the ilium and not the cancellous osteoporotic bone of the sacrum or the superior pubic ramii and can be placed with four percutaneous incisions (2 AIIS and 2 PSIS). In the future, this construct should be compared to lumbopelvic fixation techniques. Conclusion This biomechanical study has shown that two iliosacral screws (one in S1 and one in S2) are stiffer than one iliosacral screw and appeared to have less movement in rotational planes for the SI Joint disruption model and the Sacral fracture model. One SI screw combined with the pedicle screw construct was stiffest in the sacral fracture model and one SI screw was equal to the pedicle screw construct in this model as well. The pedicle screw construct alone was weakest in the SI joint disruption model as compared to 1 SI screw or 2 SI screws and we do not recommend its use in compressible posterior injuries where the SI screws are much cheaper and more effective. Conflict of interest statement The authors declare that they have no Conflict of Interest in the preparation of this publication: We also declare that: Dr. Vaidya has received payment for lectures from AO Trauma and Synthes Trauma for teaching at Courses. Dr. Vaidya has received payments for consulting with Stryker Corporation for implant development. Dr. Oliphant has received payments for lectures from Synthes Trauma for teaching at courses. The institution of the authors has received funding from Synthes for the testing materials used in this study through a research grant. The implants used in this study are not FDA approved for use in the technique described. References [1] Simonian PT, Routt Jr C, Harrington RM, Tencer AF. Internal fixation for the transforaminal sacral fracture. Clin Orthop Relat Res 1996;323:202–9. [2] Denis F, Davis S, Comfort T. Sacral fractures: an important problem. Retrospective analysis of 236 cases. Clin Orthop Relat Res 1988;227:67–81. [3] Gibbons KJ, Soloniuk DS, Razack N. Neurological injury and patterns of sacral fractures. J Neurosurg 1990;72:889–93. [4] Matta JM, Saucedo T. Internal fixation of pelvic ring fractures. Clin Orthop Relat Res 1989;242:83–97. [5] Matta JM, Tornetta 3rd P. Internal fixation of unstable pelvic ring injuries. Clin Orthop Relat Res 1996;329:129–40.
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