Ann Thorac Surg 2007;84:1288-1293
© 2007 The Society of Thoracic Surgeons
Original Articles: Cardiovascular
Single Wire Versus Double Wire Loops for Median Sternotomy Closure: Experimental Biomechanical Study Using a Human Cadaveric Model
Julian E. Losanoff, MDa,b,*,
Marc D. Basson, MD, PhDa,b,
Scott A. Gruber, MD, PhDb,
Harold Huff, MSc,
Fu-hung Hsieh, PhDc
a Department of Surgery, John D. Dingell VA Medical Center, Detroit, Michigan
b Department of Surgery, Wayne State University, Detroit, Michigan
c Department of Biological Engineering, University of Missouri, Columbia, Missouri
Accepted for publication May 9, 2007.
* Address correspondence to Dr Losanoff, Department of Surgery (11S), John D. Dingell VA Medical Center, 4646 John R, Detroit, MI 48201 (Email: jelosanoff{at}yahoo.comtel).
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Abstract
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Background: Healing of median sternotomy requires reliable sternal fixation. Although both single and double wire kits are commercially available, no experimental study has compared the two closures in a human cadaveric model. We used a recently described human experimental cadaveric model to compare the stability of the closures.
Methods: Sixteen fresh adult human cadaveric sternal plates with adjacent ribs were fixed with custom designed spiked stainless steel clamps and attached to a biomechanical testing device. Single No. 5 peristernal and double peristernal closures were tested. The stability of the unions was tested using perpendicular, repetitive force loads increasing from 0 to 800 Newtons at a rate of 0.5 mm/second.
Results: The two study groups did not differ in age or sex. No clamp failures or damage to the specimens occurred. The double peristernal closure exhibited a significantly lower permanent displacement than the single wire group, suggesting a superior strength and stability of that closure.
Conclusions: To the extent to which this human cadaveric model resembles in vivo median sternotomy, these data suggest that the biomechanical stability of the peristernal double wire closure may exceed that of single wires.
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Introduction
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Midline sternotomy incisions continue to be the preferred means by which cardiac surgeons access the heart and great vessels. According to the American Heart Association, 666,000 open heart operations were performed in the United States in 2003 (http://www.americanheart.org/presenter.jhtml?identifier=4674).
Thus, estimates of annual sternal wound disruption rates between 0.3% and 8% [1–4] suggest that between 3,300 and 53,000 patients may potentially suffer from this significant postoperative morbidity each year in the US. Indeed, healing complications after median sternotomy are associated with a 14% to 47% mortality rate if mediastinitis supervenes [2, 3]. The treatment cost of a deep sternal wound infection was estimated to exceed $80,000 in 2001 [5]. The complications of median sternotomy are best avoided by achieving durable stability at the union [2, 3].
However, postoperative sternal disruption remains a serious, persistent, but also potentially preventable problem [4]. Stainless steel wires are among the most frequently used closure materials. Although several previously published clinical studies have used various wire configurations to optimize the closure, their retrospective nature and lack of detailed statistical comparisons have not established the best method. Our recently introduced human cadaveric model tested single peristernal, alternating single peristernal and transsternal, single transsternal, figure-eight peristernal, figure-eight pericostal, and Robicsek closures and demonstrated that the first two methods provided significantly greater mechanical stability [6]. Some cardiothoracic surgeons claim that using commercially available looped double wire (DoubleWire; A&E Medical Corp, Farmingdale, NJ) is not only faster but also more reliable than the traditionally used single wire closures. A recent prospective, randomized study from Europe using looped double wires (Fumedica GmbH, Herne, Germany) showed significantly improved results [7]. We used our previously described human cadaveric model to compare the patterns of sternal dehiscence with peristernal single wires versus commercial looped double wires.
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Material and Methods
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No Institutional Review Board review was required because federal regulations do not require approval of research on deceased patients by the Board. The study was conducted in compliance with the protected health information of decedents.
Sixteen fresh autopsy sternal plates with 4 cm adjacent ribs on both sides of the sternum were used. They were stored at 2°C for 12 to 48 hours prior to the experiment. The tests were conducted at room temperature. Median sternotomies were performed with an oscillating saw. The specimens were gently but firmly grasped through the ribs with spiked stainless steel clamps. The sternal halves were then approximated manually.
Each manubrium was approximated with three No. 5 single transsternal wires (Ethicon, Somerville, NJ). The peristernal double wiring used three looped wires and a single standard No. 5 wire, which was placed to further reinforce the xiphoid end of the sternotomy (Fig 1). Placement of the double wires proceeded according to the instructions on the manufacturer’s web site (http://www.aemedical.com/Doublewire.htm). Three DoubleWire sutures (A&E Medical Corp) (Fig 2) were placed between the ribs by passing its leader needle around each side of the sternum. The needle was then removed and the ends of each loop were made equal. The hook of a manual wire twister (A&E Medical catalog number 040-400) was placed in each loop with slight upward pressure (Fig 3). While continuous upward pressure was applied, the twister was rotated causing the loops to braid. The rotation was continued as the loops formed a single cable. Optimum tension was reached until the double cable began to bend. The braided cable was then cut, leaving a tip up to 1 cm long. The tip was then flattened into the sternum. The single distal wire was twisted with a wire holder.
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Fig 1. Schematic drawing illustrating the two wiring methods. (Left) single peristernal with four single wire loops; (right) double peristernal with three double loops (arrows). Note a fourth, single wire loop at the bottom of the closure. Each manubrium is closed with three single wires.
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The stainless steel clamps were placed on either side of the closure, approximately 2.5 cm from the incision, and secured with end bolts. The clamps were attached to the jigs of a computerized biomechanical tester (TAHDi texture analyzer with texture expert exceed software; Stable Micro System, Texture Technologies Corp, Scarsdale, NY). The testing was performed with the sternum oriented horizontally (Fig 4). The closures were stressed with perpendicular, repetitive variable force ranging from 0 to 800 Newtons and cycling at 0.5 mm/second for 40 minutes. At the conclusion of each test, the sternum was removed and a 3 to 4 g portion of its manubrium cut away, cleaned of muscle, and submitted to a density test. The test used an air pycnometer (VM-100; Horiba Instruments, Irvine, CA) which determines volume and density using displacement of helium gas at constant pressure. All data were digitally stored.
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Fig 4. Experimental setup showing a sternal specimen in place during a test. Note that the sternotomy is oriented horizontally and parallel to the clamps.
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The variables analyzed included age, sex, and bone density, as well as stiffness and displacement (DSPL) at cycles 1, 10, and 25. Permanent displacement (PDSPL) after the first, fifth, tenth, and twenty-fifth cycles, and the maximum displacement were evaluated. The DSPL and PDSPL were analyzed in the same way as in our previously published study [6] in order not to deviate from the original experimental protocol. Maximum displacement was measured at the end of each test (160 cycles), therefore max DSPL was greater than DSPL 25. Stiffness was defined as the ratio of force divided by displacement (high stiffness means that a large force produces a small displacement and vice versa). Stiffness was measured in Newtons per millimeter. The displacement was defined as the change in distance between the sternal halves as traction is applied; it was recorded in millimeters. Permanent displacement was defined as part of the force-displacement curve after the first cycle; it showed the change in displacement without a change in force. Bone density was measured to determine correlation between the durability of the closure and bone quality.
The "sex ratios" were tested using a row-by-column 
2 test. Other parameters were tested using one-way analysis of variance to determine if there were differences between the two closure groups. The strength of association between bone density and permanent displacement was calculated using the Pearson r (product-moment) correlation. Statistical software (SigmaStat for Windows version 2.03; SPSS, Chicago, IL) was used for the analyses, with the significance level set at p less than 0.05.
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Results
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Sixteen sternal samples were available for the test including 13 males (81%).
We detected no significant differences in age and sex (Table 1). None of the patients had received long-term corticosteroid treatment or had clinical osteoporosis. Bone density differed between the groups and was found to be statistically significantly higher in the PSD group by retrospective analysis (Table 1). The above described experimental protocol was not changed throughout the study.
All specimens were able to complete the full cycle of the experiment. Clamp slippage with resulting mechanical damage to the specimens was not observed. A macroscopically observed gap was initially observed in each of the specimens’ xiphoid end at the time of maximum pull (Fig 5); this invariably progressively increased in size throughout the tests. Permanent gap in the incision at the conclusion of the test was evident in all specimens and was considered macroscopic evidence of failure. Close inspection of the sterna after they were removed from the machine revealed that the permanent gap resulted from wires cutting through the bone. The suture lines in the manubria macroscopically displayed no or very little gaping at the conclusion of the test. No sternal fractures or disruption of the ribs, muscles, or any of the wires were observed.
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Fig 5. A dynamic observation at the beginning (A) and during a maximal pull showing a gap at the closure’s xiphoid end (B).
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A summary of the variables of the test is presented in Table 2. There were no significant differences in stiffness, DSPL 1, DSPL 10, DSPL 25, and max DSPL even though the PSS group had a lower bone density than the PSD group. Indeed, the PSD group also had a much lower permanent displacement than the PSS group. The permanent displacement was significantly higher in the PSS group.
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Comment
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A review of the published experimental studies of median sternotomy closure reveals a multitude of experimental models, methods, and aims, and only a few using human sterna [1, 6, 8, 9]. Based on the assumption that human cadaveric sterna are uniquely suitable for sternotomy closure research, we used a previously established model [6] for the present study. There were no significant differences between our previous [6] and present studies with regard to the preparation of the specimens or experimental setup.
The jig used for the study did not recreate the complete range of torsional and distracting forces that human movement can place on the sternum. However, exploring the predominant distracting forces directed perpendicular to the closure simplifies the model and facilitates the reproduction of the data. The cadaveric tissue model cannot evaluate clinical characteristics including pain caused by the closure, discomfort associated with the wire knot under the skin, and the propensity for infection associated with an implanted foreign body. The effect of such factors on varying sternal closures can only be explored through large scale living animal or human studies that may be motivated by smaller ex vivo controlled studies such as this one.
The repetitive variable force of 0 to 800 Newtons used was chosen based on a previously published porcine sternotomy study, which used a similar experimental setup [10]. The study observed no tissue disruption or clamp failure at pulling forces averaging 730 to 916 Newtons [10]. A mathematical estimate of the lateral forces across the sternotomy during coughing was calculated to be between 400 and 1,200 Newtons, describing the average to maximum possible physiologic force produced by coughing [11]. The force used in our study falls within the above range. It must be acknowledged, however, that using a uniaxial testing method has potential shortcomings. It has been shown that the anterior-posterior force needed to achieve 2.0 mm distraction was 263 ± 74 Newtons, in comparison with 220 ± 40 Newtons for a lateral force [11]. The directionality of the forces applied to the sternum may therefore also be important. Including other force vectors in this experiment could have improved correspondence to the in vivo situation if the relative contributions of such different force vectors in vivo could be accurately defined, but only at the cost of substantial increased analytic complexity.
The published literature contains a small number of studies including commercially available double wires, contrary to the statement by many cardiothoracic surgeons that they prefer the double wires because of the easy application and increased stability. In the United States the double wires were developed by A&E Medical (http://www.aemedical.com/Doublewire.htm) with the help of the cardiothoracic surgeon Garth R. McDonald, MD, who joked that his idea for the wires is based on the maxima: "Two is better than one" (personal communication). The following is based on data from the manufacturer and the authors’ experience with the double wire closure. The wires are manufactured from very flexible and pliable steel. Thus, they do not resist tightening and twisting. The installation method using an eccentric hook prevents over-tightening and over-twisting. The double wires conform more closely to the sternal contour, achieving an even distribution of the forces, and an extremely strong and tight sternal approximation. In Europe, a similar product is provided by Fumedica GmbH (Herne, Germany) (http://www.fumedica.de/nav/fs_3_f.htm). A recently published trial from Germany used double wires from Fumedica GmbH to prevent sternal dehiscence in high-risk patients and showed improved results [7]. To the best of our knowledge, the present study is the only experimental biomechanical trial that compares the double wire with the more established single-loop peristernal closure. Using a human cadaveric model, the results suggest that the double peristernal closure provides an increased stability.
Our study shows that the wires typically cut through the bone rather than breaking or becoming unraveled; this suggests that bone strength plays an important role in the closure. The higher bone density found in the PSD group and the small number of specimens precludes definitive conclusions about the superiority of the DoubleWire (A&E Medical) closure. Using a bone analogue or another material such as wooden blocks could have avoided the differences. Our experimental setup, however, approximates the clinical situation more closely, represents a continuation of our previous trial, and permits detailed comparisons with other closures using the same biological model [6]. The superiority of double over single wires showed by the present study suggests that the force is perhaps distributed more widely on the bone; thus, the added stability at the wire-cortical bone interface may be better exploited in the PSD closure.
It must be stressed that the single wires are made of harder, less compliant stainless steel that conforms to the surface of the bone less; the double wires, on the other hand, are made of softer steel, which is more pliable and follows the contour of the bone more smoothly. A significantly greater permanent displacement was observed in the PSS group. This was most likely a result of the less compliant closure material compared with that group. The hardness of a metal is typically measured by either the Rockwell or the Brinell hardness tests. The former test is based on the net increase in depth of impression as a load is applied. The latter forces a hard steel or carbide sphere of a specified diameter under a specified load into the surface of a material and measuring the diameter of the indentation left after the test (http://www.calce.umd.edu/general/Facilities/Hardness_ad_.htm). The impression that A&E wires are more pliable than Ethicon is based on the personal experience of the authors and many other surgeons rather than objective data. We regret that numerical data with regard to the hardness of the material used by A&E or Ethicon are not available from the literature or the manufacturers. The hardness of the steel used by A&E Medical was never compared with the standard stainless steel used by Ethicon and other companies (personal communication, A&E Medical).
The difference in the compliance of the wires might explain the dissimilarity in permanent displacement. In other words, because the double wires are more pliable and follow the contour of the bone more smoothly than the harder, less compliant stainless steel of the single wires, specimens with PSD had significantly less permanent displacement than those with PSS, despite the fact that no differences were observed for displacements after 1, 10, and 25 cycles between PSD and PSS. Our study leaves a number of questions unanswered. As we explained above, numerical data about the hardness of the wires are not available. Certainly, mechanical properties such as pliability, hardness, or compliance could have affected the results. In addition, the single versus the double wire nature of the closure itself might have altered the manner in which equivalent forces were applied to individual parts of the sterna where they were contacted by wires. A more detailed analysis of the contribution of such individual mechanical factors to sternal dehiscence may help to design better sternal closures in the future, but exceeded the scope of the present study. However, since the closures studied here are the only available choices for single versus double wire closure, our data still stand as highly relevant for sternal closure in the current clinical situation.
There are potential limitations to the sternotomy model described and our use of the wires. The low negative correlation between stiffness and bone density is most likely due to the uniaxial testing method used in our study. The testing does not explore how a lateral distracting force displaces various sternal parts under different stress. In addition, we did not measure the cortical thickness of the specimens, which would have allowed a correlation with the support it provides. It is not clear how our findings would apply to people with extremely fragile or infected sterna.
We reanalyzed the data presented in our previous work [6]. Indeed, the correlation between stiffness and bone density was also very low (r = 0.026). However, our choice of bone density as a key parameter was influenced by its availability to clinical measurement and interpretation. The use of a single distal wire loop in the second group somewhat deviates from the principle of double loops. In our experience, however, the predominant cartilaginous structure of the region is more susceptible to mechanical deformation by the double wires. Moreover, a distal single wire loop is suggested on the web site of the manufacturer, which makes it the most likely choice among the cardiothoracic surgeons who adopt the method.
Nonbiologic sternal analogues such as the recently described solid polyurethane foam [12] can be used to approximate the biomechanical properties of cadaveric sterna. Because they are uniformly manufactured, they are likely to provide less variable stability data. Such analogues are more expensive than the biological sterna. More importantly, they lack the infrastructure of the cadaveric material. In addition, the biological variability of cadaveric sterna seems likely to reflect the variability of human sterna in vivo. Study of a uniform artificial construct might therefore miss variations in effect dependent upon variations in mechanical characteristics of the sternum. It therefore remains not entirely clear how closely the nonbiologic sterna resemble the clinical situation. So far there is no published experimental comparison between the two models. Several published median sternotomy studies have discussed alternative closures, including stainless steel plates, bands, cables, and various other devices; the majority were summarized in a recent review article [2]. None of these closures has so far achieved wide popularity. A large enough randomized clinical trial is expected to recommend the most durable closure.
Conclusions
This second human cadaveric study using the same biological model confirms that the human specimens are uniquely valuable in comparing clinical closures of median sternotomy. Our model approximates a clinical scenario and provides a cost-effective and easily reproducible means of biomechanical testing. Our study suggests that the double wire loops provide a more stable closure compared with single wires. A clinical randomized trial comparing standard closure with the DoubleWire (A&E Medical) should be encouraged as a next step in exploring a better median sternotomy closure.
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References
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Abstract
Introduction
