Symphyseal internal rod fixation versus standard plate fixation for open book pelvic ring injuries: a biomechanical study G. Osterhoff, S. Tiziani, C. Hafner, S. J. Ferguson, H.-P. Simmen & C. M. L. Werner European Journal of Trauma and Emergency Surgery Official Publication of the European Society for Trauma and Emergency Surgery Incorporating the International Journal of Disaster Medicine ISSN 1863-9933 Eur J Trauma Emerg Surg DOI 10.1007/s00068-015-0529-5
1 23
Your article is protected by copyright and all rights are held exclusively by SpringerVerlag Berlin Heidelberg. This e-offprint is for personal use only and shall not be selfarchived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”.
1 23
Author's personal copy Eur J Trauma Emerg Surg DOI 10.1007/s00068-015-0529-5
ORIGINAL ARTICLE
Symphyseal internal rod fixation versus standard plate fixation for open book pelvic ring injuries: a biomechanical study G. Osterhoff1 · S. Tiziani1 · C. Hafner2 · S. J. Ferguson2 · H.‑P. Simmen1 · C. M. L. Werner1
Received: 2 January 2015 / Accepted: 30 March 2015 © Springer-Verlag Berlin Heidelberg 2015
Abstract Purpose This study investigates the biomechanical stability of a novel technique for symphyseal internal rod fixation (SYMFIX) using a multiaxial spinal screw-rod implant that allows for direct reduction and can be performed percutaneously and compares it to standard internal plate fixation of the symphysis. Methods Standard plate fixation (PLATE, n = 6) and the SYMFIX (n = 6) were tested on pelvic composite models with a simulated open book injury using a universal testing machine. On a previously described testing setup, 500 consecutive cyclic loadings were applied with sinusoidal resulting forces of 200 N. Displacement under loading was measured using an optoelectronic camera system and construct rigidity was calculated as a function of load and displacement. Results The rigidity of the PLATE construct was 122.8 N/ mm (95 % CI: 110.7–134.8), rigidity of the SYMFIX construct 119.3 N/mm (95 % CI: 105.8–132.7). Displacement in the symphyseal area was mean 0.007 mm (95 % CI: 0.003–0.012) in the PLATE group and 0.021 mm (95 % CI: 0.011–0.031) in the SYMFIX group. Displacement in the sacroiliac joint area was mean 0.156 mm (95 % CI: 0.051–0.261) in the PLATE group and 0.120 mm (95 % CI: 0.039–0.201) in the SYMFIX group.
* G. Osterhoff
[email protected] 1
Division of Trauma Surgery, University Hospital Zurich, University of Zurich, Raemistrasse 100, 8091 Zurich, Switzerland
2
Institute for Biomechanics, ETH Zurich, HCI‑E355.2 Wolfgang‑Pauli‑Strasse 10, 8093 Zurich, Switzerland
Conclusions In comparison to standard internal plate fixation for the stabilization of open book pelvic ring injuries, symphyseal internal rod fixation using a multiaxial spinal screw-rod implant in vitro shows a similar rigidity and comparable low degrees of displacement. Keywords Pelvic fracture · Pelvic ring injury · Internal fixation · External fixation · Symphyseal rod fixation
Introduction Operative treatment of unstable pelvic ring fractures accelerates recovery and improves functional outcome [1]. Antero-posterior compression or “open book” injuries of the pelvis with disruption of the symphysis (Young and Burgess APC 2/3 [2] or AO/Tile B1/3, C1 [3, 4]) are commonly treated by anterior fixation [3]. In the vast majority of cases this is addressed by standard anterior plate fixation through a Pfannenstiel incision or an extended pubic midline exposure with or without additional posterior fixation [5, 6]. In certain patients, the location of these approaches may be associated with increased risks for complications. Open wounds or scaring, obesity, a history of hernia repair with a mesh can be reasons for the surgeon to avoid open surgery. While partially stable lateral compression fractures can be stabilized only posteriorly [7], this is usually not an option for symphyseal disruption as seen in APC 2 or APC 3 injuries. One option is the use of external fixation until the injury has healed. However, external fixation is associated with pin tract infections in up to 50 % and limits the patient’s ability to mobilize [8]. With the advent of percutaneous implants for posterior spinal instrumentation, subcutaneous internal anterior screw-rod fixation with supraacetabular anchorage had been established [9–12]. Yet, it was
13
Author's personal copy G. Osterhoff et al.
associated with complications like irritation of the lateral femoral cutaneous nerve or heterotopic ossifications [9, 13] and, more important, direct reduction of the symphysis is not possible. This study describes a novel technique for symphyseal internal rod fixation (SYMFIX) of a pelvic open book injury using a multiaxial spinal screw-rod implant that allows for direct reduction and can be performed percutaneously. The purpose was to investigate the biomechanical stability of this technique and compare it to standard internal plate fixation of the symphysis.
Materials and methods Testing samples The experimental setup was designed to assess the rigidity and to progressively measure the damage accumulation of the implant–bone construct [14]. Two fixation techniques were tested on 12 composite analog whole pelvis models (Pelvis; Synbone, Malans, Switzerland). These bone surrogates consist of solid foam (polyurethane foam), which represents dense cancellous bone, and a rigid outer layer replicating cortical bone. An open book pelvic ring (Young and Burgess APC 3 or AO/OTA C1.2) injury with a pure sacroiliac joint dislocation was simulated by transecting the left sacroiliac joint and the symphysis. In the group PLATE (n = 6), a four-hole pelvic reconstruction steel plate (3.5 mm, Synthes, Zuchwil, SO, Switzerland) was fixed to the right and left aspect of the symphysis of the pelvic model using two 3.5 mm cortical screws (length 50 mm) on each side (Fig. 1a, c). In the group SYMFIX (n = 6), two multiaxial pedicle screws (5.5 mm × 50 mm) of a percutaneous spinal internal rod-screw fixation system (CD Horizon Longitude, Medtronic, Minneapolis, MN, USA) were placed in the right and left aspect of the symphysis (Fig. 1b, d). The screws were connected by a straight titanium rod (4.5 mm × 40 mm). Couplings in the multiaxial screw heads were tightened with a torque screwdriver according to the manufacturer’s instructions. The posterior pelvic ring was stabilized with a sacroiliac screw (90 mm) in S1 in all testing samples. Biomechanical testing A likely loading scenario in patients who underwent pelvic ring stabilization after an APC 2 or APC 3 injury is postoperative partial weight-bearing on crutches. In their work on physiologic loads of on the pubic symphysis in patients with a symphyseal rupture who ambulate with
13
partial weight-bearing, Meissner et al. [15] stated that neutralization of 169 N in the vertical direction and 68 N in the sagittal direction must be achieved by an adequate fixation device. To simulate ambulation as realistic as possible, a cyclic loading protocol with peak resulting forces of 200 N was chosen as described previously [16–18]. Each pelvis was mounted through the sacrum to a universal testing machine (ElectroPulse E10000, Instron, Norwood, MA, USA) in a way to allow for free to pivoting in all planes. It was tilted 45° anteriorly to simulate the anatomic pelvic tilt. The pelvises left hip socket articulated with a 54-mm femoral arthroplasty head that was fixed to the testing machines table being embedded with PMMA (SCS-Beraycry-Powder D-28, Suter Kunststoffe AG, Fraubrunnen, Switzerland) in a Plexiglas tube in 15° of anteversion. The hip abductor muscles were simulated by two cable pulleys attached to the iliac crest [17, 18] (Fig. 2). After preloading the construct with 20 N, each specimen was subjected to total of 500 consecutive cyclic sinusoidal axial loadings through the sacrum at 1 Hz. During testing, loading forces and displacement were registered by a ±1000 N (compression/distraction forces) and a 25-Nm (torque forces) load cell (Dynacell, Instron, Norwood, MA, USA). The applied loads were plotted by this load cell as a function of the recorded displacement and the rigidity of the construct (N/mm) was then defined as the slope of this loading curve. To measure the relative displacement of the symphysis and the sacroiliac joint under simulated physiological loading, a non-contact, optoelectronic camera system (CamBar B2 Type 1.2, Axios 3D Services GmbH, Oldenburg, Germany) with a precision of 40 μm was used. Markers of 6 mm diameter were placed on the pelvic bone and their displacements were measured. Three markers in a static order form a locator and deliver three-dimensional information (Fig. 2b). One locater was set on each side of the pubic symphysis (markers 1–6) and one locator on each side of the left sacroiliac joint (markers 7–12) as relative motion of the fragments could be expected in these areas. A measurement of the relative motion was performed with the camera system after every 100 cycles over ten cycles. Symphyseal displacement beyond 10 mm was determined as construct failure resulting in an immediate stop of testing. In case of implant loosening or cut-out, the testing was continued until definitive failure occurred (as determined above) or 500 cycles were accomplished. Statistical analysis Before testing, a sample size calculation was performed using PS power and sample size calculations 3.0 (alpha error: 0.05) [19]. Based on previous data on anterior pelvic
Author's personal copy Symphyseal internal rod fixation…
Fig. 1 Testing samples. In group PLATE, a four-hole reconstruction steel plate was fixed to the right and left aspect of the symphysis using cortical screws (a, c). In group SYMFIX, two multiaxial pedi-
cle screws of a percutaneous spinal rod-screw fixation system were placed in the right and left aspect of the symphysis and connected by a rod (b, d)
ring fixation with a comparable testing setup [17, 18], we expected a displacement of 5 mm was considered clinically relevant, resulting in an expected difference in means of 4 mm and a standard deviation of 3 mm. The calculated number of samples to be able to reject the null hypothesis that the population means of displacement in the SYMFIX and the PLATE groups are equal with a probability (power) of 0.85 was five per group. To anticipate problems with the testing setup, we planned a sample size of six per group. To compare means of displacement and construct rigidity, 95 % confidence intervals (95 % CI) were calculated for all values using SPSS for Windows V21.0 (SPSS, Chicago, IL, USA).
Results The average overall rigidity of the PLATE construct was 122.8 N/mm (95 % CI: 110.7–134.8). The average overall rigidity of the SYMFIX construct was 119.3 N/mm (95 % CI: 105.8–132.7; Fig. 3). Displacement in the symphyseal area was mean 0.007 mm (95 % CI: 0.003–0.012 mm) in the PLATE group and 0.021 mm (95 % CI: 0.011–0.031 mm) in the SYMFIX group (Fig. 4). This difference in means of symphyseal displacement of 0.014 mm was close to statistical significance but considered not clinically relevant—especially as the precision of the optical tracking system as stated by the manufacturer is only 40 μm.
13
Author's personal copy G. Osterhoff et al.
Fig. 2 Testing setup. a On a universal testing machine, 500 consecutive cyclic loadings were applied with sinusoidal resulting forces of 200 N. Loads were applied through the sacrum that was allowed to pivot in all planes while the left hip socket articulated with a fixed femoral arthroplasty head. Hip abductor muscles were simulated by
cable pulleys attached to the iliac crest. b Displacement under loading was measured using an optoelectronic camera system. Markers of 6 mm diameter were placed on the pelvic model near the pubic symphysis and the left sacroiliac joint
Fig. 3 Construct rigidity. PLATE standard plate fixation, SYMFIX symphyseal internal rod fixation. Box plots represent the median with 1 and 3 quartile, bars represent minimum and maximum. Outliers are shown as filled circles
Fig. 4 Displacement under loading as measured at the pelvic model’s symphysis and the sacroiliac joint. PLATE standard plate fixation, SYMFIX symphyseal internal rod fixation. Box plots represent the median with 1 and 3 quartile, bars represent minimum and maximum. Outliers are shown as filled circles
Displacements were slightly higher in the sacroiliac joint area with mean 0.156 mm (95 % CI: 0.051– 0.261 mm) in the PLATE group and 0.120 mm (95 % CI: 0.039–0.201 mm) in the SYMFIX group (Fig. 4). Again,
this difference in means of sacroiliac displacement of 0.036 mm was not considered to be of clinical relevance. Temporary deformation was solely caused by elastic bending of the rod or plate and the screws. Persistent
13
Author's personal copy Symphyseal internal rod fixation…
plastic deformation was not seen. Failure or loosening of the screws in either constructs or the rod-screw interfaces was not observed with the applied loads.
Discussion This study tested the in vitro biomechanical stability of a novel symphyseal internal rod fixation (SYMFIX) in comparison to standard internal plate fixation when used for the stabilization of an open book pelvic ring injury. No clinical relevant differences neither with regard to construct rigidity nor to symphyseal and sacroiliac displacement were found between the two techniques. The experimental setup used in this study was designed to simulate forces resulting from partial weight-bearing on crutches. This setup and the loads applied in this study were chosen as described previously in the literature for comparable investigations [16–18]. However, the forces acting on the pelvic ring under these loading conditions have not been fully understood. It is known, that loads acting on the hip can approximate body weight even with non-weightbearing [20]. In contrast to the hip, however, where muscular pulley is responsible for a large amount of the forces acting on the joint socket, the forces acting on the symphyseal area are supposed not to exceed the weight of the lower extremity in this setting. The relative weight of the lower extremity ranges around 15 % of the body weight. The 200 N of loading used in this study, therefore, would correspond to a body weight of 130 kg and higher loads would not be realistic for non-weightbearing—even though the displacements in the symphyseal area were minimal. These loads resemble the forces measured by Meissner et al. [15] for mobilization of patients with a symphyseal rupture with partial weight-bearing. This study used a single sacroiliac screw for posterior fixation. The use of either one or two posterior screws in unstable open book injuries (Young and Burgess APC III) of the pelvis is controversial. It is known that, if the anterior ring is stabilized, the presence of a second screw does not offer any significant additional benefit in APC injuries [21, 22]. As we tested a novel anterior pelvic ring fixation method, we did not want to apply a posterior construct to rigid to diminish all loads going through the anterior construct. Our results are consistent with previously published biomechanical data on internal plate fixation showing less than 1 mm of symphyseal displacement under similar loading conditions [17]. In the clinical patient, postoperative secondary fragment displacement usually is not a result of a single trauma or loading. It rather occurs due to repetitive motion (i.e.,
walking on crutches) as is suggested by the typical appearance of symmetric loosening of the pins in pelvic external fixation [8]. Therefore, a cyclic loading protocol was used in this study as this seems to better approximate clinical loading scenarios. To avoid confounding factors as bone quality and pelvic size and shape, we used composite bone models for the biomechanical testing. Composite bone models have been shown to allow for controlled and repeatable testing, as each specimen has the same properties [23, 24]. They might, however, not completely resemble the conditions at the bone-implant interface as they can be found in vivo. Our study showed only minimal displacement and no loosening of the screws of either of both implant configurations. On one hand, this might be a consequence of the rather small number of cycles (500) used in our protocol, on the other hand it is consistent with our experience and expectations for the initial postoperative period after internal anterior pelvic ring fixation [1, 5, 25, 26]. The pubic symphysis is a nonsynovial amphiarthrodial joint with anatomical characteristics that are very similar to those found in the intervertebral disc space. Good fixation strength and low rates of loosening have been shown for minimally invasive percutaneous pedicle screw fixation using multiaxial screw-rod implants in the stabilization of spinal injuries [27, 28]. Together with the in vitro data of the present study, this suggests a safe use of symphyseal internal rod fixation with multiaxial spinal screwrod implants for the stabilization of open book pelvic ring injuries. The implants and instruments used for percutaneous fixation of spinal injuries would also allow for percutaneous implant insertion and reduction. Especially in the treatment of pelvis ring injuries patients with open wounds or scaring, obesity, or a history of hernia repair, the SYMFIX would be an interesting option. It is important to mention, in this context, that the urinary bladder is in close vicinity to the posterior aspect of the symphysis. Pure percutaneous placement of the SYMFIX screws therefore should be performed with careful adherence to the surgical anatomy and under fluoroscopic guidance only. As could be shown for the supraacetabular anterior subcutaneous internal fixateur [11, 13], the rod can be safely placed percutaneously when running through the subcutaneous layers and superficial to the rectus abdominis muscle.
Conclusions In comparison to standard internal plate fixation for the stabilization of open book pelvic ring injuries, symphyseal internal rod fixation using a multiaxial spinal screw-rod implant in vitro shows a similar rigidity and comparable
13
Author's personal copy G. Osterhoff et al.
low degrees of displacement. This technique might provide a useful tool for the orthopedic and trauma surgeon in the treatment of anterior pelvic ring injuries, especially in patients where extensive open surgery needs to be avoided. Acknowledgments We would like to thank Synthes (Zuchwil, SO, Switzerland; group PLATE) and Medtronic (Minneapolis, MN, USA; group SYMFIX) for donating the implants that were used for this study. Conflict of interest Georg Osterhoff, Simon Tiziani, Carina Hafner, Stephen J. Ferguson, Hans-Peter Simmen, and Clément M. L. Werner declare that they have no conflicts of interests other than the funding of implants as stated in the Acknowledgments. Compliance with ethical standards Thus study was conducted in accordance with the ethical guidelines of the European Journal of Trauma and Emergency Surgery. The research did not involve human participants or animals.
References 1. Papakostidis C, Kanakaris NK, Kontakis G, Giannoudis PV. Pelvic ring disruptions: treatment modalities and analysis of outcomes. Int Orthop. 2009;33(2):329–38. 2. Young JW, Burgess AR, Brumback RJ, Poka A. Pelvic fractures: value of plain radiography in early assessment and management. Radiology. 1986;160(2):445–51. 3. Rüedi T, Buckley R, Moran C. AO principles of fracture management, vol. 2. 2nd ed. NewYork: Thieme; 2007. 4. Tile M. Pelvic ring fractures: should they be fixed? J Bone Joint Surg. 1988;70B:1–12. 5. Putnis SE, Pearce R, Wali UJ, Bircher MD, Rickman MS. Open reduction and internal fixation of a traumatic diastasis of the pubic symphysis: one-year radiological and functional outcomes. J Bone Joint Surg Br. 2011;93(1):78–84. 6. Adams MR, Scolaro JA, Routt ML Jr. The pubic midline exposure for symphyseal open reduction and plate fixation. J Orthop Traumatol Off J Italian Soc Orthop Traumatol. 2014;15:195–9. 7. Osterhoff G, Ossendorf C, Wanner GA, Simmen HP, Werner CM. Posterior screw fixation in rotationally unstable pelvic ring injuries. Injury. 2011;42(10):992–6. 8. Mason WT, Khan SN, James CL, Chesser TJ, Ward AJ. Complications of temporary and definitive external fixation of pelvic ring injuries. Injury. 2005;36(5):599–604. 9. Kuttner M, Klaiber A, Lorenz T, Fuchtmeier B, Neugebauer R. The pelvic subcutaneous cross-over internal fixator. Unfallchirurg. 2009;112(7):661–9. 10. Osterhoff G, Tiziani S, Ferguson SJ, Spreiter G, Scheyerer MJ, Spinas GL, Wanner GA, Simmen HP, Werner CM. Mechanical testing of a device for subcutaneous internal anterior pelvic ring fixation versus external pelvic ring fixation. BMC Musculoskelet Disord. 2014;15:111. 11. Scheyerer MJ, Zimmermann SM, Osterhoff G, Tiziani S, Simmen HP, Wanner GA, Werner CM. Anterior subcutaneous internal fixation for treatment of unstable pelvic fractures. BMC Res Notes. 2014;7:133. 12. Vaidya R, Colen R, Vigdorchik J, Tonnos F, Sethi A. Treatment of unstable pelvic ring injuries with an internal anterior fixator
13
and posterior fixation: initial clinical series. J Orthop Trauma. 2012;26(1):1–8. 13. Vaidya R, Kubiak EN, Bergin PF, Dombroski DG, Critchlow RJ, Sethi A, Starr AJ. Complications of anterior subcutaneous internal fixation for unstable pelvis fractures: a multicenter study. Clin Orthop Relat Res. 2012;470(8):2124–31. 14. Ferguson SJ, Winkler F, Nolte LP. Anterior fixation in the osteoporotic spine: cut-out and pullout characteristics of implants. Eur Spine J. 2002;11(6):527–34. 15. 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(6):415–21. 16. Clements JP, Moriaty N, Chesser TJ, Ward AJ, Cunning ham JL. Determination of pelvic ring stability: a new technique using a composite hemi-pelvis. Proc Inst Mech Eng H. 2008;222(5):611–6. 17. Vigdorchik JM, Esquivel AO, Jin X, Yang KH, Onwudiwe NA, Vaidya R. Biomechanical stability of a supra-acetabular pedicle screw Internal Fixation device (INFIX) vs External Fixation and plates for vertically unstable pelvic fractures. J Orthop Surg Res. 2012;7(1):31. 18. Vigdorchik JM, Esquivel AO, Jin X, Yang KH, Vaidya R. Anterior internal fixator versus a femoral distractor and external fixation for sacroiliac joint compression and single stance gait testing: a mechanical study in synthetic bone. Int Orthop. 2013;37(7):1341–6. 19. Dupont WD, Plummer WD. Power and sample size calcula tions for studies involving linear regression. Control Clin Trials. 1998;19:589–601. 20. Schwachmeyer V, Damm P, Bender A, Dymke J, Graichen F, Bergmann G. In vivo hip joint loading during post-operative physiotherapeutic exercises. PLoS One. 2013;8(10):e77807. 21. Giraldez-Sanchez MA, Lazaro-Gonzalvez A, Martinez-Reina J, Serrano-Toledano D, Navarro-Robles A, Cano-Luis P, Fragkakis EM, Giannoudis PV. Percutaneous iliosacral fixation in external rotational pelvic fractures. A biomechanical analysis. Injury. 2014;46:327–32. 22. van Zwienen CM, van den Bosch EW, van Hoek Dijke GA, Snijders CJ, van Vugt AB. Cyclic loading of sacroiliac screws in Tile C pelvic fractures. J Trauma. 2005;58(5):1029–34. 23. Chong AC, Miller F, Buxton M, Friis EA. Fracture toughness and fatigue crack propagation rate of short fiber reinforced epoxy composites for analogue cortical bone. J Biomech Eng. 2007;129(4):487–93. 24. Osterhoff G, Baumgartner D, Favre P, Wanner GA, Gerber H, Simmen HP, Werner CM. Medial support by fibula bone graft in angular stable plate fixation of proximal humeral fractures: an in vitro study with synthetic bone. J Shoulder Elbow Surg. 2011;20(5):740–6. 25. Matta JM. Indications for anterior fixation of pelvic fractures. Clin Orthop Relat Res. 1996;329:88–96. 26. Sagi HC, Papp S. Comparative radiographic and clinical outcome of two-hole and multi-hole symphyseal plating. J Orthop Trauma. 2008;22(6):373–8. 27. Cappuccio M, Amendola L, Paderni S, Bosco G, Scimeca G, Mirabile L, Gasbarrini A, De Iure F. Complications in minimally invasive percutaneous fixation of thoracic and lumbar spine fractures. Orthopedics. 2013;36(6):e729–34. 28. Ringel F, Stoffel M, Stuer C, Meyer B. Minimally invasive transmuscular pedicle screw fixation of the thoracic and lumbar spine. Neurosurgery. 2006;59(4 Suppl 2):ONS361–6 (discussion ONS366–367).
