- Email: [email protected]
Locked versus standard unlocked plating of the symphysis pubis in a Type-C pelvic injury: A cadaver biomechanical study
Injury, Int. J. Care Injured 45 (2014) 748–751
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
Locked versus standard unlocked plating of the symphysis pubis in a Type-C pelvic injury: A cadaver biomechanical study Berton R. Moed a,b,*, Christopher P. O’Boynick a, J. Gary Bledsoe a,b a Department of Orthopaedic Surgery, Saint Louis University School of Medicine, 3635 Vista Avenue, 7th Floor Desloge Towers, St. Louis 63110, Missouri, United States b Department of Biomedical Engineering, Parks College of Engineering, Aviation and Technology, Saint Louis University, 3450 Lindell Boulevard, St. Louis 63103, Missouri, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 February 2013 Received in revised form 7 June 2013 Accepted 11 November 2013
Introduction: The benefits of locked plating for pubic symphyseal disruption have not been established. The purpose of this biomechanical study was to determine whether locked plating offers any advantage over conventional unlocked plating of the pubic symphysis in the vertically unstable, Type-C pelvic injury. Methods: In each of eight embalmed cadaver pelvis specimens, sectioning of the pubic symphysis in conjunction with a unilateral release of the sacroiliac, sacrospinous, and sacrotuberous ligaments and pelvic floor was performed to simulate a vertically unstable Type-C (Orthopaedic Trauma Association 61-C1.2) pelvic injury. The disrupted SI joint was then reduced and fixed using two 6.5 mm cannulated screws inserted into the S1 body. Using a six-hole 3.5 mm plate specifically designed for the symphysis pubis having both locked and unlocked capability, four pelvises were fixed with locked screws and four pelvises were fixed with standard unlocked bicortical screws. Both groups were similar based on a dualemission X-ray absorptiometry evaluation (P = 0.69). Each pelvis was then mounted on a servohydraulic materials-testing apparatus using a bilateral stance model to mainly stress the symphyseal fixation and was cycled up to 1 million cycles or failure, whichever occurred first. Results: Five specimens experienced failure at the jig mounting/S1 vertebral body interface, occurring between 360,000 and 715,000 cycles. Frank failure of the anterior or posterior instrumentation did not occur. However, end-trialing diastasis of the initial pubic symphysis reduction was found in all pelvises. There were no differences between the groups with respect to this loss of symphyseal reduction (P = 0.69) or average cycles to failure (P = 1.0). Conclusion: Pubic symphyseal locked plating does not appear to offer any advantage over standard unlocked plating for a Type-C (OTA 61-C1.2) pelvic ring injury. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: Symphyseal locked plating Locked plating Fixation failure
Introduction Plate fixation for disruption of the pubic symphysis as an addition to posterior fixation for completely unstable, Type-C (AO/ Orthopaedic Trauma Association [OTA] 61-C [1]) pelvic ring injuries has been shown to improve pelvic stiffness and stability [2–5]. Over the last decade new surgical plating systems have been developed with the capability of locking the screw heads to the plate and this locked plating technology has been shown to have advantages for diaphyseal and metaphyseal fracture fixation, especially involving osteoporotic bone [6–8]. Symphyseal plates with this locking capability are currently available. However, any
* Corresponding author. Tel.: +13145778850; fax: +13142685121. E-mail address: [email protected] (B.R. Moed). 0020–1383/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.injury.2013.11.017
benefit from the use of this technique for stabilization of an acute pubic symphyseal disruption has not been demonstrated. In fact, recent study indicates that locked plating of the pubic symphysis does not appear to offer any advantage over the standard unlocked technique for partially stable, open-book (OTA 61-B3.1 [1]) pelvic ring injuries [9]. The purpose of this biomechanical study was to determine whether locked plating offers any advantage over conventional unlocked plating of the pubic symphysis in the vertically unstable, Type-C pelvic injury. Materials and methods Eight embalmed pelvic specimens were obtained with the sacroiliac (SI), sacrospinous, sacrotuberous, symphyseal ligaments, and the pelvic floor intact. The mean age was 77 years (range,
B.R. Moed et al. / Injury, Int. J. Care Injured 45 (2014) 748–751
749
Table 1 Characteristics of the cadaver pelvic specimens. Specimen Unlocked group 1 2 3 4 Locked group 1 2 3 4 P value between groups a * **
DXA T-scorea
Age (years)
Sex
60 84 86 98
Female Female Female Female
1.1 3.2 2.1 3.0
59 68 77 84 0.20*
Male Female Female Female 1.0**
0.5 2.2 1.6 3.2 0.69*
DXA = dual-emission X-ray absorptiometry. Mann–Whitney U test. Fisher’s exact test.
59–98 years), and there were 1 males and 7 females. To ensure uniformity in bone density, each specimen had a dual-emission X-ray absorptiometry (DXA) scan performed prior to any soft tissue removal using a GE Lunar Scanner (GE Healthcare, UK). The DXA scan T-scores ranged from 0.5 to 3.2. Four specimens were classified as osteopenic (T-score between 1.0 and 2.5), three were classified as osteoporotic (T-score of 2.5 or less) and one was normal (greater than 1.0) [10]. The eight pelvises were then separated into two statistically similar groups of four specimens each (Table 1). The T-scores did not significantly differ between the two groups (P = 0.69, Mann–Whitney U test). Subsequently, the sacrospinous, sacrotuberous, and sacroiliac ligaments, and the pelvic floor were unilaterally released and the symphysis pubis was transected to simulate a vertically unstable Type-C (OTA 61-C1.2) injury. After this sectioning of the ligaments and symphysis pubis, the hemipelvis was rendered completely unstable (Fig. 1). The SI joint was then reduced and fixed using two Synthes (Synthes Inc., West Chester, PA, USA) 6.5 mm cannulated screws. Fluoroscopy was used to confirm appropriate reduction and screw placement (Fig. 2). Next, using a Synthes (Synthes Inc., West Chester, PA) six-hole 3.5 mm plate specifically designed for the symphysis pubis with the capability of fixation in locked or unlocked mode, four pelvises were fixed with locked screws and four pelvises were fixed with standard unlocked bicortical screws. A jig specially designed for this study was fabricated to ensure uniformity of screw placement. This jig was manually secured to
Fig. 1. Photograph after sectioning of the pubic symphysis in conjunction with a unilateral release of the left sacroiliac, sacrospinous and sacrotuberous ligaments and pelvic floor showing the completely unstable left hemipelvis.
Fig. 2. Fluoroscopic view of the pelvis after the disrupted SI joint was reduced and fixed using two 6.5 mm cannulated screws inserted into the S1 body, the pubic symphysis was reduced and fixed using a six-hole 3.5 mm plate with unlocked screws, and an aluminum fixture was anchored at approximately 45 degrees onto the superior endplate of the S1 vertebral body with one sagittal and four axial 6.5 mm cancellous bone screws. A minor residual diastasis of the pubic symphysis reduction is evident.
the symphysis at four points of contact and allowed drill holes to be made in the pubic bones at uniform predetermined angles. The angles for screw insertion were provided by the plate manufacturer (Synthes Inc.). Therefore, the same angles of insertion were maintained in all eight specimens. Each pelvis was then mounted on a servohydraulic materialstesting machine, MTS 858 Mini Bionix (MTS Systems, Inc., Eden Prairie, MN) using the model developed by Hearn et al. and described by Varga et al., simulating bilateral stance with a vertical coronal plane aligning the anterior superior iliac spines and pubic tubercles [5,11]. This simulated two-legged stance specimen testing setup results in net tension through the pubic symphysis, providing a laboratory model for testing the biomechanical effects of symphyseal fixation [5,11]. Each pelvis was then loaded through a male ball bearing freely articulating with a female hemispherically machined aluminum plate. This plate was affixed to an aluminum fixture, which was anchored at approximately 45 degrees onto the superior endplate of the S1 vertebral body with one sagittal and four axial 6.5 mm cancellous bone screws [9]. Distally, the acetabuli articulated with appropriately sized bipolar hip prostheses with modular stems allowing free movement in all planes. These stems were potted to allow rotation without altering the center of rotation of the hip. The pots were recessed in ball bearing side plates in a track that allowed movement in the coronal plane while preventing anteroposterior motion (Fig. 3) [11]. Loading of the pelvis was based on data derived from previous study [9]. Each pelvis was stressed at 440 N, simulating a 100 lb normal compressive force in two-legged stance during gait, for a total of one million cycles (equivalent to 6.5 months of daily walking) at two cycles per second (2 Hz) or until fixation failure, whichever came first. Failure was defined as the point on the loaddisplacement curve when force measurement essentially declines rapidly toward zero with no further change in displacement or obvious visual failure of the fixation. Three measurements of the symphyseal gap were made on each specimen both before and after cyclic loading using an electronic Vernier LCD digital caliper accurate to 0.05 mm. These measurements were at the superior edge of the symphysis directly below the plate, 1 cm below the superior edge, and 2 cm below the superior edge. These three
750
B.R. Moed et al. / Injury, Int. J. Care Injured 45 (2014) 748–751
Fig. 4. Photograph of a pelvis specimen from an 86 year old woman shown in Fig. 2 after undergoing one million cycles on the materials-testing machine. A slight increase in the original diastasis was evident (white arrows).
Discussion
Fig. 3. Photograph of a pelvis specimen mounted on the materials-testing machine.
measurements were made to allow for the detection of any asymmetric symphyseal gapping. Appropriate for the number of specimens, the nonparametric Mann–Whitney U test was used to compare the two groups of cadaver pelvic specimens for all statistical evaluations with the exception gender, for which the Fisher’s exact test was used (Table 1). Level of statistical significance was defined as P < 0.05.
Results No frank failure of the symphyseal or the posterior fixation occurred during testing. However, five specimens experienced failure at the interface between the mounting jig and the S1 vertebral body. These five failures occurred through the S1 vertebral body, anterior to the 6.5 mm cannulated SI screws, between 360,000 cycles and 715,000 cycles. Of the three specimens completing the full one million cycles, one was in the locked group and two were in the unlocked group. Unlocked specimens averaged approximately 692,000 cycles before failure compared with 701,000 for the specimens with locked plates. This difference was not statistically significant (P = 1.0). A slight diastasis of the initial pubic symphysis reduction was found in all pelvises regardless of fixation method (Fig. 4) with an overall mean of 1.0 mm (range 0.2–1.7 mm). The symphyseal separation for the unlocked group before loading averaged 2.0, 2.7, and 3.3 mm, respectively for the three levels of measurement (i.e., superior edge of the symphysis directly below the plate, 1 cm below the superior edge, and 2 cm below the superior edge) and after loading this measurement averaged 3.0, 3.5, and 4.3 mm, respectively. The symphyseal separation for the locked group before loading averaged 1.4, 2.0, and 2.3 mm, respectively for the three levels of measurement and after loading this measurement averaged 2.5, 3.0, and 3.4 mm, respectively. At each of the three levels, no significant difference was found between the two groups when comparing symphyseal widening after cyclic loading (P = 0.69).
Open reduction and internal fixation (ORIF) of the pubic symphysis has progressively evolved over the last half century from treatment with cerclage wiring to external fixation to modern techniques using plate and screw constructs [12]. Currently, the plate and screw construct is the preferred fixation method for the treatment of pubic symphyseal disruption in conjunction with an OTA 61-C pelvic injury [12,13]. With the arrival of locked plate technology, surgical implants designed specifically for internal fixation of the pubic symphysis have been made commercially available. Despite touted potential advantages of locked plating for fracture fixation, especially in osteoporotic bone, recent biomechanical study indicates that locked plating of the pubic symphysis does not appear to offer any advantage over the standard unlocked technique for partially stable, open-book (OTA 61-B3.12) pelvic ring injuries [9]. Furthermore, a recent case series of six patients further supports this contention [14]. In this admittedly small series, failure mechanisms of locked designspecific plate fixation of the pubic symphysis included those seen with conventional uniplanar fixation as well as those common to locked plate technology [14]. The authors concluded that the indications for the use of these implants remain to be determined. The pubic symphysis has inherent physiologic motion [2,11,15– 17]. Fixation with a locked screw construct provides angular stability, neutralizing bending forces through the screw/plate interface, creating a stiffer construct [6,18]. Therefore, its use to treat a pubic symphyseal disruption is counterintuitive. Consistent with this expectation, there was minor loss of symphyseal reduction in all of the unlocked pelvis specimens, which has been shown to be common with standard unlocked plating techniques [11,12,16,19,20]. Somewhat unexpected was this occurrence of this loosening at the screw–bone interface and gapping of the pubic symphyseal reduction with all the pelvises treated with locked fixation. However, this finding is consistent with the findings of previous studies [9,14]. This type of fixation failure has been shown to be clinically unimportant [21]. The testing apparatus used in this study is a proven model for biomechanical evaluation of pubic symphyseal fixation. It was initially developed by Hearn et al. at the biomechanics laboratory at Sunnybrook Health Sciences Centre in Toronto [22]. It has been used in numerous studies to evaluate the effectiveness and biomechanics of pelvic fixation [2,11,17,23]. Specifically, the twolegged stance apparatus is thought to be more appropriate than a single-leg stance model for the testing of symphyseal fixation because it produces distraction at the pubic symphysis [5,11].
B.R. Moed et al. / Injury, Int. J. Care Injured 45 (2014) 748–751
This study does have limitations. The cadaver specimens used for this study were embalmed and not fresh-frozen. However, Comstock et al. used embalmed cadaver specimens in a biomechanical evaluation of fixation of the posterior pelvic ring and found comparative results to studies performed with fresh frozen specimens [24]. More recently, van Zwienen et al. found embalmed pelvic specimens to be satisfactory for the biomechanical evaluation of unstable pelvic ring injuries [25]. Because of their relatively low bone density, as demonstrated by the DXA measurements, the specimens in this study should be considered appropriate to determine any advantage of locked plating over standard unlocked plating. Although the two groups were not statistically different by DXA measurement, if anything, the minor raw number differences in bone density would favor the locked over the unlocked group. We had hoped that the testing protocol would produce a few complete fixation failures. Another limitation is that five of the eight specimens failed at interface between the mounting jig and the S1 vertebral body without completing the planned one million cycles, occurring between 360,000 cycles and 715,000 cycles. We selected a load magnitude and a number of cycles that were thought to be adequate to produce fixation failures [26–28]. The fact that we did not produce any examples of progressive complete fixation failure is likely the result of the number of cycles rather than the magnitude of the load. These early failures, which occurred through the S1 body, most likely indicate that the S1 vertebral body was not of sufficient size and/or density to accommodate the two 6.5 mm SI fixation screws in combination with the mounting jig screws. While this issue did create minor differences in total cycles between the two groups it was not statistically significant (P = 1.0). In addition, using approximately 5000 steps per day for the average United States adult, each specimen experienced a minimum 2 to 3 months of simulated walking conditions before mounting failure [30]. A further limitation of this study is the small number of specimens tested, which is a universal issue with biomechanical cadaver studies. However, the appropriate use of nonparametric statistics should minimize this shortcoming, obviating the need for a power analysis. There are many described fixation methods for disruption of the pubic symphysis. These different techniques include variations in the types of plates used, the number of screws used and the types of screws used, including combinations of locked and standard unlocked screws [3,5,11–13,15,16,20,29]. It is possible techniques other than those examined in this study, such as longer plates with more screws or plating using locked and standard unlocked screws, could show improved dynamic biomechanical properties as compared to standard unlocked plating. However, these other techniques were not the subject of this report, and their evaluation must await further study. In conclusion, this study indicates that in the setting of an acute Type-C (OTA 61-C1.2) pelvic ring injury pubic symphyseal locked plating does not appear to offer any advantage over standard unlocked plating. Therefore, we recommend continued use of standard plating techniques for disruption of the pubic symphysis. References [1] Marsh JL, Slongo TF, Agel J, Broderick JS, Creevey W, DeCoster TA, et al. Fracture and dislocation classification compendium. J Orthop Trauma 2007;21(10 suppl.):S1–63.
751
[2] Tile M, Hearn T, Vrahas M. Biomechanics of the pelvic ring. In: Tile M, Helfet DL, Kellam JF, editors. Fractures of the pelvis and acetabulum. 3rd ed., Philadelphia: Lippincott Williams & Wilkins; 2003. p. 32–45. [3] Sagi HC, Ordway NR, Dipasquale T. Biomechanical analysis of fixation for vertically unstable sacroiliac dislocations with iliosacral screws and symphyseal plating. J Orthop Trauma 2004;18:138–43. [4] Matta JM. Indications for anterior fixation of pelvic fractures. Clin Orthop Relat Res 1996;329:88–96. [5] Gorczyca J, Hearn T, Tile M. Biomechanics and methods of pelvic fixation. In: Tile M, Helfet DL, Kellam JF, editors. Fractures of the pelvis and acetabulum. 3rd ed., Philadelphia: Lippincott Williams & Wilkins; 2003. p. 116–29. [6] Kretteck C, Gosling T. Principles of internal fixation. In: Bucholz RW, Heckman JD, Court-Brown C, Tornetta P, Koval KJ, editors. Rockwood and Green’s fractures in adults. 6th ed., Philadelphia: Lippincott Williams & Wilkins; 2006 . p. 209–56. [7] Strauss EJ, Schwarzkopf R, Kummer F, Egol KA. The current status of locked plating: the good, the bad, and the ugly. J Orthop Trauma 2008;22:479–86. [8] Schutz M, Ruedi TP. Principles of internal fixation. In: Bucholz RW, CourtBrown C, Heckman JD, Tornetta PII, editors. Rockwood and Green’s fractures in adults. 7th ed., Philadelphia: Lippincott Williams & Wilkins; 2010. p. 162–90. [9] Grimshaw CS, Bledsoe JG, Moed BR. Locked versus standard unlocked plating of the symphysis pubis: a cadaver pelvic biomechanical study. J Orthop Trauma 2012;26:402–6. [10] Kanis JA. Assessment of fracture risk and its application to screening for postmenopausal osteoporosis: synopsis of a WHO report. Osteoporos Int 1994;4:368–81. WHO Study Group. [11] Varga E, Hearn T, Powell J, Tile M. Effects of method of internal fixation of symphyseal disruptions on stability of pelvic ring. Injury 1995;26:75–80. [12] Sagi HC, Papp S. Comparative radiographic and clinical outcome of two-hole and multi-hole symphyseal plating. J Orthop Trauma 2008;22:373–8. [13] Moed BR, Kellam JF, McLaren A, Tile M. Disruption of the pelvic ring: internal fixation for the injured pelvic ring. In: Tile M, Helfet DL, Kellam JF, editors. Fractures of the pelvis and acetabulum. 3rd ed., Philadelphia: Lippincott Williams & Wilkins; 2003. p. 217–93. [14] Moed BR, Grimshaw CS, Segina DN. Failure of locked design-specific plate fixation of the pubic symphysis: a report of six cases. J Orthop Trauma 2012;26:e71–5. [15] Webb L, Gristina A, Wilson J, et al. Two-hole plate fixation for traumatic symphysis pubis diastasis. J Trauma 1988;28:813–7. [16] Simonian PT, Schwappach JR, Routt Jr ML. Evaluation of new plate designs for symphysis pubis internal fixation. J Trauma 1996;41:498–502. [17] Vrahas M, Hearn TC, Diangelo D, Kellam J, Tile M. Ligamentous contributions to pelvic stability. Orthopedics 1995;18:271–4. [18] Lujan TJ, Henderson CE, Madey SM, Fitzpatrick DC, Marsh JL, Bottlang M. Locked plating of distal femur fractures leads to inconsistent and asymmetric callus formation. J Orthop Trauma 2010;24:156–62. [19] Collinge C, Archdeacon MT, Dulaney-Cripe E, Moed BR. Radiographic changes of implant failure after plating for pubic symphysis diastasis: an underappreciated reality? Clin Orthop Relat Res 2012;470:2148–53. [20] Simonian PT, Routt Jr ML, Harington RM, Tencer AF. Box plate fixation of the symphysis pubis: biomechanical evaluation of a new technique. J Orthop Trauma 1994;8:483–9. [21] Morris SAC, Loveridge J, Smart DKA, Ward AJ, Chesser TJS. Is fixation failure after plate fixation of the symphysis pubis clinically important? Clin Orthop Relat Res 2012;470:2154–60. [22] Tile M, Hearn T. Biomechanics. In: Tile M, editor. Fractures of the pelvis and acetabulum. 2nd ed., Baltimore, MD: Williams & Wilkins; 1995. p. 22–36. [23] Kim WY, Hearn TC, Seleem O, Mahalingam E, Stephen D, Tile M. Effect of pin location on stability of pelvic external fixation. Clin Orthop Relat Res 1999;361:237–44. [24] Comstock CP, van der Meulen MCH, Goodman SB. Biomechanical comparison of posterior internal fixation techniques for unstable pelvic fractures. J Orthop Trauma 1996;10:517–22. [25] van Zwienen CMA, van den Bosch EW, Snijders CJ, Kleinrensink GJ, van Vugt AB. Biomechanical comparison of sacroiliac screw techniques for unstable pelvic ring fractures. J Orthop Trauma 2004;18:589–95. [26] Walheim G, Olerud S, Ribbe T. Mobility of the pubic symphysis. Measurements by an electromechanical method. Acta Orthop Scand 1984;55:203–8. [27] Meissner A, Fell M, Wilk R, Boenick U, Rahmanzadeh R. Biomechanics of the pubic symphysis. Which forces lead to mobility of the symphysis in physiological conditions? Unfallchirurg 1996;99:415–21. [28] Cowin SC. Bone mechanics handbook. 2nd ed. Boca Raton, FL: CRC Press; 2001. [29] Pizanis A, Garcia P, Santelmann M, Culemann U, Pohlemann T. Reduction and fixation capabilities of different plate designs for pubic symphysis disruption: a biomechanical comparison. Injury 2013;44:183–8. [30] Bassett Jr DR, Wyatt HR, Thompson H, Peters JC, Hill JO. Pedometer-measured physical activity and health behaviors in United States adults. Med Sci Sports Exerc 2010;42:1819–25.
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
Locked versus standard unlocked plating of the symphysis pubis in a Type-C pelvic injury: A cadaver biomechanical study Berton R. Moed a,b,*, Christopher P. O’Boynick a, J. Gary Bledsoe a,b a Department of Orthopaedic Surgery, Saint Louis University School of Medicine, 3635 Vista Avenue, 7th Floor Desloge Towers, St. Louis 63110, Missouri, United States b Department of Biomedical Engineering, Parks College of Engineering, Aviation and Technology, Saint Louis University, 3450 Lindell Boulevard, St. Louis 63103, Missouri, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 February 2013 Received in revised form 7 June 2013 Accepted 11 November 2013
Introduction: The benefits of locked plating for pubic symphyseal disruption have not been established. The purpose of this biomechanical study was to determine whether locked plating offers any advantage over conventional unlocked plating of the pubic symphysis in the vertically unstable, Type-C pelvic injury. Methods: In each of eight embalmed cadaver pelvis specimens, sectioning of the pubic symphysis in conjunction with a unilateral release of the sacroiliac, sacrospinous, and sacrotuberous ligaments and pelvic floor was performed to simulate a vertically unstable Type-C (Orthopaedic Trauma Association 61-C1.2) pelvic injury. The disrupted SI joint was then reduced and fixed using two 6.5 mm cannulated screws inserted into the S1 body. Using a six-hole 3.5 mm plate specifically designed for the symphysis pubis having both locked and unlocked capability, four pelvises were fixed with locked screws and four pelvises were fixed with standard unlocked bicortical screws. Both groups were similar based on a dualemission X-ray absorptiometry evaluation (P = 0.69). Each pelvis was then mounted on a servohydraulic materials-testing apparatus using a bilateral stance model to mainly stress the symphyseal fixation and was cycled up to 1 million cycles or failure, whichever occurred first. Results: Five specimens experienced failure at the jig mounting/S1 vertebral body interface, occurring between 360,000 and 715,000 cycles. Frank failure of the anterior or posterior instrumentation did not occur. However, end-trialing diastasis of the initial pubic symphysis reduction was found in all pelvises. There were no differences between the groups with respect to this loss of symphyseal reduction (P = 0.69) or average cycles to failure (P = 1.0). Conclusion: Pubic symphyseal locked plating does not appear to offer any advantage over standard unlocked plating for a Type-C (OTA 61-C1.2) pelvic ring injury. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: Symphyseal locked plating Locked plating Fixation failure
Introduction Plate fixation for disruption of the pubic symphysis as an addition to posterior fixation for completely unstable, Type-C (AO/ Orthopaedic Trauma Association [OTA] 61-C [1]) pelvic ring injuries has been shown to improve pelvic stiffness and stability [2–5]. Over the last decade new surgical plating systems have been developed with the capability of locking the screw heads to the plate and this locked plating technology has been shown to have advantages for diaphyseal and metaphyseal fracture fixation, especially involving osteoporotic bone [6–8]. Symphyseal plates with this locking capability are currently available. However, any
* Corresponding author. Tel.: +13145778850; fax: +13142685121. E-mail address: [email protected] (B.R. Moed). 0020–1383/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.injury.2013.11.017
benefit from the use of this technique for stabilization of an acute pubic symphyseal disruption has not been demonstrated. In fact, recent study indicates that locked plating of the pubic symphysis does not appear to offer any advantage over the standard unlocked technique for partially stable, open-book (OTA 61-B3.1 [1]) pelvic ring injuries [9]. The purpose of this biomechanical study was to determine whether locked plating offers any advantage over conventional unlocked plating of the pubic symphysis in the vertically unstable, Type-C pelvic injury. Materials and methods Eight embalmed pelvic specimens were obtained with the sacroiliac (SI), sacrospinous, sacrotuberous, symphyseal ligaments, and the pelvic floor intact. The mean age was 77 years (range,
B.R. Moed et al. / Injury, Int. J. Care Injured 45 (2014) 748–751
749
Table 1 Characteristics of the cadaver pelvic specimens. Specimen Unlocked group 1 2 3 4 Locked group 1 2 3 4 P value between groups a * **
DXA T-scorea
Age (years)
Sex
60 84 86 98
Female Female Female Female
1.1 3.2 2.1 3.0
59 68 77 84 0.20*
Male Female Female Female 1.0**
0.5 2.2 1.6 3.2 0.69*
DXA = dual-emission X-ray absorptiometry. Mann–Whitney U test. Fisher’s exact test.
59–98 years), and there were 1 males and 7 females. To ensure uniformity in bone density, each specimen had a dual-emission X-ray absorptiometry (DXA) scan performed prior to any soft tissue removal using a GE Lunar Scanner (GE Healthcare, UK). The DXA scan T-scores ranged from 0.5 to 3.2. Four specimens were classified as osteopenic (T-score between 1.0 and 2.5), three were classified as osteoporotic (T-score of 2.5 or less) and one was normal (greater than 1.0) [10]. The eight pelvises were then separated into two statistically similar groups of four specimens each (Table 1). The T-scores did not significantly differ between the two groups (P = 0.69, Mann–Whitney U test). Subsequently, the sacrospinous, sacrotuberous, and sacroiliac ligaments, and the pelvic floor were unilaterally released and the symphysis pubis was transected to simulate a vertically unstable Type-C (OTA 61-C1.2) injury. After this sectioning of the ligaments and symphysis pubis, the hemipelvis was rendered completely unstable (Fig. 1). The SI joint was then reduced and fixed using two Synthes (Synthes Inc., West Chester, PA, USA) 6.5 mm cannulated screws. Fluoroscopy was used to confirm appropriate reduction and screw placement (Fig. 2). Next, using a Synthes (Synthes Inc., West Chester, PA) six-hole 3.5 mm plate specifically designed for the symphysis pubis with the capability of fixation in locked or unlocked mode, four pelvises were fixed with locked screws and four pelvises were fixed with standard unlocked bicortical screws. A jig specially designed for this study was fabricated to ensure uniformity of screw placement. This jig was manually secured to
Fig. 1. Photograph after sectioning of the pubic symphysis in conjunction with a unilateral release of the left sacroiliac, sacrospinous and sacrotuberous ligaments and pelvic floor showing the completely unstable left hemipelvis.
Fig. 2. Fluoroscopic view of the pelvis after the disrupted SI joint was reduced and fixed using two 6.5 mm cannulated screws inserted into the S1 body, the pubic symphysis was reduced and fixed using a six-hole 3.5 mm plate with unlocked screws, and an aluminum fixture was anchored at approximately 45 degrees onto the superior endplate of the S1 vertebral body with one sagittal and four axial 6.5 mm cancellous bone screws. A minor residual diastasis of the pubic symphysis reduction is evident.
the symphysis at four points of contact and allowed drill holes to be made in the pubic bones at uniform predetermined angles. The angles for screw insertion were provided by the plate manufacturer (Synthes Inc.). Therefore, the same angles of insertion were maintained in all eight specimens. Each pelvis was then mounted on a servohydraulic materialstesting machine, MTS 858 Mini Bionix (MTS Systems, Inc., Eden Prairie, MN) using the model developed by Hearn et al. and described by Varga et al., simulating bilateral stance with a vertical coronal plane aligning the anterior superior iliac spines and pubic tubercles [5,11]. This simulated two-legged stance specimen testing setup results in net tension through the pubic symphysis, providing a laboratory model for testing the biomechanical effects of symphyseal fixation [5,11]. Each pelvis was then loaded through a male ball bearing freely articulating with a female hemispherically machined aluminum plate. This plate was affixed to an aluminum fixture, which was anchored at approximately 45 degrees onto the superior endplate of the S1 vertebral body with one sagittal and four axial 6.5 mm cancellous bone screws [9]. Distally, the acetabuli articulated with appropriately sized bipolar hip prostheses with modular stems allowing free movement in all planes. These stems were potted to allow rotation without altering the center of rotation of the hip. The pots were recessed in ball bearing side plates in a track that allowed movement in the coronal plane while preventing anteroposterior motion (Fig. 3) [11]. Loading of the pelvis was based on data derived from previous study [9]. Each pelvis was stressed at 440 N, simulating a 100 lb normal compressive force in two-legged stance during gait, for a total of one million cycles (equivalent to 6.5 months of daily walking) at two cycles per second (2 Hz) or until fixation failure, whichever came first. Failure was defined as the point on the loaddisplacement curve when force measurement essentially declines rapidly toward zero with no further change in displacement or obvious visual failure of the fixation. Three measurements of the symphyseal gap were made on each specimen both before and after cyclic loading using an electronic Vernier LCD digital caliper accurate to 0.05 mm. These measurements were at the superior edge of the symphysis directly below the plate, 1 cm below the superior edge, and 2 cm below the superior edge. These three
750
B.R. Moed et al. / Injury, Int. J. Care Injured 45 (2014) 748–751
Fig. 4. Photograph of a pelvis specimen from an 86 year old woman shown in Fig. 2 after undergoing one million cycles on the materials-testing machine. A slight increase in the original diastasis was evident (white arrows).
Discussion
Fig. 3. Photograph of a pelvis specimen mounted on the materials-testing machine.
measurements were made to allow for the detection of any asymmetric symphyseal gapping. Appropriate for the number of specimens, the nonparametric Mann–Whitney U test was used to compare the two groups of cadaver pelvic specimens for all statistical evaluations with the exception gender, for which the Fisher’s exact test was used (Table 1). Level of statistical significance was defined as P < 0.05.
Results No frank failure of the symphyseal or the posterior fixation occurred during testing. However, five specimens experienced failure at the interface between the mounting jig and the S1 vertebral body. These five failures occurred through the S1 vertebral body, anterior to the 6.5 mm cannulated SI screws, between 360,000 cycles and 715,000 cycles. Of the three specimens completing the full one million cycles, one was in the locked group and two were in the unlocked group. Unlocked specimens averaged approximately 692,000 cycles before failure compared with 701,000 for the specimens with locked plates. This difference was not statistically significant (P = 1.0). A slight diastasis of the initial pubic symphysis reduction was found in all pelvises regardless of fixation method (Fig. 4) with an overall mean of 1.0 mm (range 0.2–1.7 mm). The symphyseal separation for the unlocked group before loading averaged 2.0, 2.7, and 3.3 mm, respectively for the three levels of measurement (i.e., superior edge of the symphysis directly below the plate, 1 cm below the superior edge, and 2 cm below the superior edge) and after loading this measurement averaged 3.0, 3.5, and 4.3 mm, respectively. The symphyseal separation for the locked group before loading averaged 1.4, 2.0, and 2.3 mm, respectively for the three levels of measurement and after loading this measurement averaged 2.5, 3.0, and 3.4 mm, respectively. At each of the three levels, no significant difference was found between the two groups when comparing symphyseal widening after cyclic loading (P = 0.69).
Open reduction and internal fixation (ORIF) of the pubic symphysis has progressively evolved over the last half century from treatment with cerclage wiring to external fixation to modern techniques using plate and screw constructs [12]. Currently, the plate and screw construct is the preferred fixation method for the treatment of pubic symphyseal disruption in conjunction with an OTA 61-C pelvic injury [12,13]. With the arrival of locked plate technology, surgical implants designed specifically for internal fixation of the pubic symphysis have been made commercially available. Despite touted potential advantages of locked plating for fracture fixation, especially in osteoporotic bone, recent biomechanical study indicates that locked plating of the pubic symphysis does not appear to offer any advantage over the standard unlocked technique for partially stable, open-book (OTA 61-B3.12) pelvic ring injuries [9]. Furthermore, a recent case series of six patients further supports this contention [14]. In this admittedly small series, failure mechanisms of locked designspecific plate fixation of the pubic symphysis included those seen with conventional uniplanar fixation as well as those common to locked plate technology [14]. The authors concluded that the indications for the use of these implants remain to be determined. The pubic symphysis has inherent physiologic motion [2,11,15– 17]. Fixation with a locked screw construct provides angular stability, neutralizing bending forces through the screw/plate interface, creating a stiffer construct [6,18]. Therefore, its use to treat a pubic symphyseal disruption is counterintuitive. Consistent with this expectation, there was minor loss of symphyseal reduction in all of the unlocked pelvis specimens, which has been shown to be common with standard unlocked plating techniques [11,12,16,19,20]. Somewhat unexpected was this occurrence of this loosening at the screw–bone interface and gapping of the pubic symphyseal reduction with all the pelvises treated with locked fixation. However, this finding is consistent with the findings of previous studies [9,14]. This type of fixation failure has been shown to be clinically unimportant [21]. The testing apparatus used in this study is a proven model for biomechanical evaluation of pubic symphyseal fixation. It was initially developed by Hearn et al. at the biomechanics laboratory at Sunnybrook Health Sciences Centre in Toronto [22]. It has been used in numerous studies to evaluate the effectiveness and biomechanics of pelvic fixation [2,11,17,23]. Specifically, the twolegged stance apparatus is thought to be more appropriate than a single-leg stance model for the testing of symphyseal fixation because it produces distraction at the pubic symphysis [5,11].
B.R. Moed et al. / Injury, Int. J. Care Injured 45 (2014) 748–751
This study does have limitations. The cadaver specimens used for this study were embalmed and not fresh-frozen. However, Comstock et al. used embalmed cadaver specimens in a biomechanical evaluation of fixation of the posterior pelvic ring and found comparative results to studies performed with fresh frozen specimens [24]. More recently, van Zwienen et al. found embalmed pelvic specimens to be satisfactory for the biomechanical evaluation of unstable pelvic ring injuries [25]. Because of their relatively low bone density, as demonstrated by the DXA measurements, the specimens in this study should be considered appropriate to determine any advantage of locked plating over standard unlocked plating. Although the two groups were not statistically different by DXA measurement, if anything, the minor raw number differences in bone density would favor the locked over the unlocked group. We had hoped that the testing protocol would produce a few complete fixation failures. Another limitation is that five of the eight specimens failed at interface between the mounting jig and the S1 vertebral body without completing the planned one million cycles, occurring between 360,000 cycles and 715,000 cycles. We selected a load magnitude and a number of cycles that were thought to be adequate to produce fixation failures [26–28]. The fact that we did not produce any examples of progressive complete fixation failure is likely the result of the number of cycles rather than the magnitude of the load. These early failures, which occurred through the S1 body, most likely indicate that the S1 vertebral body was not of sufficient size and/or density to accommodate the two 6.5 mm SI fixation screws in combination with the mounting jig screws. While this issue did create minor differences in total cycles between the two groups it was not statistically significant (P = 1.0). In addition, using approximately 5000 steps per day for the average United States adult, each specimen experienced a minimum 2 to 3 months of simulated walking conditions before mounting failure [30]. A further limitation of this study is the small number of specimens tested, which is a universal issue with biomechanical cadaver studies. However, the appropriate use of nonparametric statistics should minimize this shortcoming, obviating the need for a power analysis. There are many described fixation methods for disruption of the pubic symphysis. These different techniques include variations in the types of plates used, the number of screws used and the types of screws used, including combinations of locked and standard unlocked screws [3,5,11–13,15,16,20,29]. It is possible techniques other than those examined in this study, such as longer plates with more screws or plating using locked and standard unlocked screws, could show improved dynamic biomechanical properties as compared to standard unlocked plating. However, these other techniques were not the subject of this report, and their evaluation must await further study. In conclusion, this study indicates that in the setting of an acute Type-C (OTA 61-C1.2) pelvic ring injury pubic symphyseal locked plating does not appear to offer any advantage over standard unlocked plating. Therefore, we recommend continued use of standard plating techniques for disruption of the pubic symphysis. References [1] Marsh JL, Slongo TF, Agel J, Broderick JS, Creevey W, DeCoster TA, et al. Fracture and dislocation classification compendium. J Orthop Trauma 2007;21(10 suppl.):S1–63.
751
[2] Tile M, Hearn T, Vrahas M. Biomechanics of the pelvic ring. In: Tile M, Helfet DL, Kellam JF, editors. Fractures of the pelvis and acetabulum. 3rd ed., Philadelphia: Lippincott Williams & Wilkins; 2003. p. 32–45. [3] Sagi HC, Ordway NR, Dipasquale T. Biomechanical analysis of fixation for vertically unstable sacroiliac dislocations with iliosacral screws and symphyseal plating. J Orthop Trauma 2004;18:138–43. [4] Matta JM. Indications for anterior fixation of pelvic fractures. Clin Orthop Relat Res 1996;329:88–96. [5] Gorczyca J, Hearn T, Tile M. Biomechanics and methods of pelvic fixation. In: Tile M, Helfet DL, Kellam JF, editors. Fractures of the pelvis and acetabulum. 3rd ed., Philadelphia: Lippincott Williams & Wilkins; 2003. p. 116–29. [6] Kretteck C, Gosling T. Principles of internal fixation. In: Bucholz RW, Heckman JD, Court-Brown C, Tornetta P, Koval KJ, editors. Rockwood and Green’s fractures in adults. 6th ed., Philadelphia: Lippincott Williams & Wilkins; 2006 . p. 209–56. [7] Strauss EJ, Schwarzkopf R, Kummer F, Egol KA. The current status of locked plating: the good, the bad, and the ugly. J Orthop Trauma 2008;22:479–86. [8] Schutz M, Ruedi TP. Principles of internal fixation. In: Bucholz RW, CourtBrown C, Heckman JD, Tornetta PII, editors. Rockwood and Green’s fractures in adults. 7th ed., Philadelphia: Lippincott Williams & Wilkins; 2010. p. 162–90. [9] Grimshaw CS, Bledsoe JG, Moed BR. Locked versus standard unlocked plating of the symphysis pubis: a cadaver pelvic biomechanical study. J Orthop Trauma 2012;26:402–6. [10] Kanis JA. Assessment of fracture risk and its application to screening for postmenopausal osteoporosis: synopsis of a WHO report. Osteoporos Int 1994;4:368–81. WHO Study Group. [11] Varga E, Hearn T, Powell J, Tile M. Effects of method of internal fixation of symphyseal disruptions on stability of pelvic ring. Injury 1995;26:75–80. [12] Sagi HC, Papp S. Comparative radiographic and clinical outcome of two-hole and multi-hole symphyseal plating. J Orthop Trauma 2008;22:373–8. [13] Moed BR, Kellam JF, McLaren A, Tile M. Disruption of the pelvic ring: internal fixation for the injured pelvic ring. In: Tile M, Helfet DL, Kellam JF, editors. Fractures of the pelvis and acetabulum. 3rd ed., Philadelphia: Lippincott Williams & Wilkins; 2003. p. 217–93. [14] Moed BR, Grimshaw CS, Segina DN. Failure of locked design-specific plate fixation of the pubic symphysis: a report of six cases. J Orthop Trauma 2012;26:e71–5. [15] Webb L, Gristina A, Wilson J, et al. Two-hole plate fixation for traumatic symphysis pubis diastasis. J Trauma 1988;28:813–7. [16] Simonian PT, Schwappach JR, Routt Jr ML. Evaluation of new plate designs for symphysis pubis internal fixation. J Trauma 1996;41:498–502. [17] Vrahas M, Hearn TC, Diangelo D, Kellam J, Tile M. Ligamentous contributions to pelvic stability. Orthopedics 1995;18:271–4. [18] Lujan TJ, Henderson CE, Madey SM, Fitzpatrick DC, Marsh JL, Bottlang M. Locked plating of distal femur fractures leads to inconsistent and asymmetric callus formation. J Orthop Trauma 2010;24:156–62. [19] Collinge C, Archdeacon MT, Dulaney-Cripe E, Moed BR. Radiographic changes of implant failure after plating for pubic symphysis diastasis: an underappreciated reality? Clin Orthop Relat Res 2012;470:2148–53. [20] Simonian PT, Routt Jr ML, Harington RM, Tencer AF. Box plate fixation of the symphysis pubis: biomechanical evaluation of a new technique. J Orthop Trauma 1994;8:483–9. [21] Morris SAC, Loveridge J, Smart DKA, Ward AJ, Chesser TJS. Is fixation failure after plate fixation of the symphysis pubis clinically important? Clin Orthop Relat Res 2012;470:2154–60. [22] Tile M, Hearn T. Biomechanics. In: Tile M, editor. Fractures of the pelvis and acetabulum. 2nd ed., Baltimore, MD: Williams & Wilkins; 1995. p. 22–36. [23] Kim WY, Hearn TC, Seleem O, Mahalingam E, Stephen D, Tile M. Effect of pin location on stability of pelvic external fixation. Clin Orthop Relat Res 1999;361:237–44. [24] Comstock CP, van der Meulen MCH, Goodman SB. Biomechanical comparison of posterior internal fixation techniques for unstable pelvic fractures. J Orthop Trauma 1996;10:517–22. [25] van Zwienen CMA, van den Bosch EW, Snijders CJ, Kleinrensink GJ, van Vugt AB. Biomechanical comparison of sacroiliac screw techniques for unstable pelvic ring fractures. J Orthop Trauma 2004;18:589–95. [26] Walheim G, Olerud S, Ribbe T. Mobility of the pubic symphysis. Measurements by an electromechanical method. Acta Orthop Scand 1984;55:203–8. [27] Meissner A, Fell M, Wilk R, Boenick U, Rahmanzadeh R. Biomechanics of the pubic symphysis. Which forces lead to mobility of the symphysis in physiological conditions? Unfallchirurg 1996;99:415–21. [28] Cowin SC. Bone mechanics handbook. 2nd ed. Boca Raton, FL: CRC Press; 2001. [29] Pizanis A, Garcia P, Santelmann M, Culemann U, Pohlemann T. Reduction and fixation capabilities of different plate designs for pubic symphysis disruption: a biomechanical comparison. Injury 2013;44:183–8. [30] Bassett Jr DR, Wyatt HR, Thompson H, Peters JC, Hill JO. Pedometer-measured physical activity and health behaviors in United States adults. Med Sci Sports Exerc 2010;42:1819–25.