Ann Thorac Surg 2008;85:287-293. doi:10.1016/j.athoracsur.2007.08.036
© 2008 The Society of Thoracic Surgeons
Original Articles: General Thoracic
Performance of a Novel Sternal Synthesis Device After Median and Faulty Sternotomy: Mechanical Test and Early Clinical Experience
Jacob Zeitani, MD, PhDa,*,
Alfonso Penta de Peppo, MDc,
Alessandra Bianco, PhDb,
Francesca Nanni, PhDb,
Antonio Scafuri, MDa,
Fabio Bertoldo, MDa,
Alessandro Salvati, MDa,
Saverio Nardella, MDa,
Luigi Chiariello, MDa
a Department of Cardiac Surgery, Science and Technology, Tor Vergata University, Italy, Rome
b Department of Science and Technology, Tor Vergata University, Italy, Rome
c Department of Cardiac Surgery, Second University of Naples, Naples, Italy
Accepted for publication August 14, 2007.
* Address correspondence to Dr Zeitani, Division of Cardiac Surgery, Tor-Vergata University, Viale Oxford 85, Rome, 00133, Italy (Email: zeitani{at}hotmail.com).
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Abstract
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Background: Reinforcement of chest closure may be required in patients with multiple risk factors of wound dehiscence. Performance of a light, size-adaptable closure reinforcement device (DSS: Sternal Synthesis Device; Mikai SpA, Vicenza, Italy) is presented.
Methods: A longitudinal median or paramedian incision was performed in artificial sternal models: closure was accomplished with simple interrupted steel wires or reinforced with the DSS. Forces required for separation of the rewired sternal halves during a monotonic tensile test were analyzed. A high velocity traction cycles test was also adopted to simulate the impact of coughing.
Results: After median incision, ultimate load values inducing break of the sternum models were 580 ± 35 N (Newton) in controls; failure of the test occurred at 1,200 ± 47 N in the reinforced group (p = 0.0002). More lateral displacement of sternal halves at increasing forces was observed in controls (p = 0.0001). After paramedian incision, ultimate load values inducing break of the constructs were lower in controls (220 ± 20 N vs 500 ± 25 N, p = 0.001), which also showed more lateral displacement of sternal halves than the reinforced group (p = 0.002). At the high velocity traction cycles test, the number of cycles required to break the models was lower in controls (2,250 ± 35 vs 3,855 ± 48 cycles, p = 0.0001). Preliminary clinical experience in 45 patients showed ease of implantation and low risk of complications.
Conclusions: The proposed sternal reinforcement device provides substantial sternal support at electromechanical testing after median and faulty sternotomy and may hopefully prevent sternal wires migration and bone fractures in high risk patients.
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Introduction
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Sternal wound dehiscence after cardiac surgery occurs in approximately 10% of patients [1–3], including superficial, deep wound dehiscence, and mediastinitis; the latter is characterized by high morbidity and mortality rates [4, 5]. The risk for such complications appears to increase seriously in patients with multiple known risk factors such as diabetes mellitus, obesity, and chronic obstructive pulmonary disease [6–10] or when a faulty sternotomy occurs [11, 12]. Standard closure might not be adequate in such patients and reinforcement is often required. Reinforcement of the chest closure with additional steel wires, as proposed with different techniques by Robicsek and colleagues [13] and other authors [14–18], may prevent postoperative dehiscence; also, titanium plates [19] and stainless steel coils or cables [20, 21] have been proposed in substitution of simple steel wires to improve postoperative sternum stability. To evaluate performance of sternal closure techniques, study models have been used adopting cadaveric sterna or artificial models [19–21]. The purpose of this study was to analyze the performance of a novel sternal reinforcement device in preventing wires cutting into the bone and thereby improving mechanical stability of the chest closure, by using a bone analogue model and a mechanical testing system.
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Material and Methods
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The sternal reinforcement device (DSS: Sternal Synthesis Device [Mikai SpA, Vicenza, Italy]) consists of separate clips made of 0.7 mm titanium sheet, sliding into each other to form two braces placed at either side of the sternum (up to five clips for each brace). In particular, the horizontal segments of the clips (5-mm wide) can slide into each other for a variable length, therefore adapting to the distance between the individual intercostal spaces and permitting to adjust the length of the full brace to fit the sternum size (as shown in Fig 1). The large vertical grooved arm of the clip (6.4-mm wide) is constructed to be placed into the intercostals space in a way that there is no direct contact between the stainless steel wire and side of the sternum, as described in the following Surgical Technique section. To better adapt to sternum sizes, the clips are available in two sizes: standard (40-mm horizontal and 32.2-mm vertical length, 1.6-g weight) and large (48-mm horizontal and 40.2-mm vertical length, 2.0-g weight).
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Fig 1. Illustration of the "DSS" device, showing how clips are assembled, sliding into each other for a variable length to adapt to sternal size.
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Surgical Technique
To facilitate insertion of the device, the muscle fascia is dissected from the sternal border by electrocautery for the length requiring reinforcement (Fig 2A). Once the fascia is mobilized, the hemisternum is elevated by two hand retractors to optimize view of the surgical field, clips are assembled to form a brace of the required length, and the vertical segments of the clips are inserted into the intercostal spaces. The two pliable vertical "fingers" of each segment are then bent outward to wrap around the ribs so that the clips are held firmly in place (Fig 2B). Reapproximation of the sternum is then achieved by means of single interrupted stainless steel wires (2–3) through the manubrium, and the others placed around the grooved arms of the clips at the intercostal spaces (Figs 2C; 2D). The fascia and the subcutaneous layers are then routinely closed above the device with two continuous absorbable 0 polyglycolic acid suture lines and the skin with an intradermic absorbable 4-0 polyglycolic acid suture line.
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Fig 2. Technique of implantation of the DSS device: (A) dissection of the muscle fascia; (B) insertion and fixation of the clips in the intercostal spaces; (C) placement of single steel wires; (D) the reinforced sternal closure after wires are tightened.
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Study Procedure
A longitudinal median or paramedian incision was performed with an oscillating saw in 22 artificial sternal models formed from 20 lbs/ft3 density polyurethane foam (Pacific Research Labs Inc, Vashon Island, WA). All models were rewired by a cardiothoracic surgeon with five peristernal number 5 steel wires (AE Medical Corporation, Farmingdale, NJ) placed in simple interrupted fashion at intercostal spaces, passing around five DSS clips and the sternum (reinforced groups) or only around the sternum (control groups). The endpoint of the study was to evaluate the forces required for separation of the rewired sternal halves after lateral traction in a mechanical testing system and load values at time of break of the constructs.
Electromechanical Test Description
The electromechanical test was conducted on the 22 rewired artificial sternal models mounted onto an electromechanical testing system (model 8055; Instron Corp, Canton, MA) as previously described [12]. In particular, six lateral anchoring holes were made in each side of the sterna, five at the costal junctions and one in the manubrium, and lateral distraction forces were applied through pairs of steel cables secured across the holes through small metal hoses. Three strain gauges (Vishay Intertechnology Inc, Malvern, PA) were applied over the closure line on the posterior surface of the manubrium, midsternum (at the third intercostals space), and xiphoid, and separation of the two sternal halves was measured with a Spider 8 apparatus in half-bridge configuration (HBM, Marlborough, MA).
Monotonic Tensile Test
On 16 sternal models (8 reinforced, 5 median and 3 paramedian sternotomy; 8 controls, 5 median and 3 paramedian sternotomy) a continuously increasing tensile test at 2 mm/minute was conducted to generate a load-displacement curve until the construct would break. Force (Newton) and displacement (mm) data were recorded every 0.25 seconds using a data capture card (Amplicon PC 20G, Amplicon, UK).
Repetitive Dynamic Cycles Test
To simulate respiratory chest motion and high peak velocities occurring during coughing, a repetitive cycling loads electromechanical test was performed on six other sternal models cut by median sternotomy (three reinforced and three controls). The system was programmed at 200 mm/minute to develop forces from 0 to 500 N rapidly, and the 500N traction force value was obtained in approximately 2.5 seconds. Displacement and number of cycles required to break the models were recorded.
Statistical Analysis
Separation data of the electromechanical monotonic test were compared by two-way repeated measurements analysis of variance to detect influence of the reinforcement system and type of sternotomy, and increasing loads on lateral displacement of sternal halves. Ultimate load values at time of fracture of the sternal models were compared by the unpaired Student t test. Displacement and number of cycles required to break the construct at the repetitive cycles test were also compared by the unpaired Student t test. Variables are presented as mean ± 1 standard deviation. A p value less than 0.05 was considered statistically significant. Statistical analysis was done by SPSS statistical software package (SPSS Inc, Chicago, IL).
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Results
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Monotonic Tensile Test
After median sternotomy, tests were interrupted because of failure of the closure due to transverse fracture of sternal modelgroup or failure of the test for fracture at the anchoring holes sites at high traction force values in the reinforced group. In particular, fractures in the control group models were noted at the sterno-xiphoid junction in three and the lower third segment of the sternum in two and occurred after steel wires cutting through the casts for a variable extent. Ultimate load value inducing break of the sternum models was 580 ± 35 N. In the reinforced group, tests were interrupted because of fractures at the anchoring holes sites at 1,200 ± 47 N (p = 0.0002). Lateral displacements at increasing tensile forces in the two groups are reported in Table 1. Comparison between mean lateral displacement of sternal halves during traction at increasing loads showed significantly more separation in the control group than in the reinforced group (p = 0.0001) (Fig 3).
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Table 1 Mean Displacement Values (mm) and Corresponding Forces (N) in the Monotonic Tensile Test Groups, After Midline Sternotomy
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Fig 3. Sternal models separation data in reinforced and control monotonic tensile test groups after midline incision. (* = displacement differs significantly from other group [p = 0.0001]; – – – = unreinforced model; — = reinforced model.)
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After paramedian sternotomy, ultimate load values inducing breaks were also lower in the control group (220 ± 20 N vs 500 ± 25 N, p = 0.001). In the control group, fractures were noted at the lower third segment of the sternum; in the reinforced group, fractures were noted at the unreinforced xiphoid segment. Lateral displacement at increasing tensile forces in the two groups is reported in Table 2. Again, comparison between mean lateral displacement of sternal halves during traction at increasing loads showed significantly more separation in the control group than in the reinforced group (p = 0.002) (Fig 4).
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Table 2 Mean Displacement Values (mm) and Corresponding Forces (N) in the Monotonic Tensile Test Groups, After Paramedian Sternotomy
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Fig 4. Sternal models separation data in reinforced and control monotonic tensile test groups after paramedian incision. (* = displacement differs significantly from other group [p = 0.001]; – – – = unreinforced model; — = reinforced model.)
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Repetitive Dynamic Cycles Test
At the high velocity traction cycles tests fractures occurred at the lower third of the sternum in controls (n = 3), after steel wires cutting through the casts; in the reinforced group (n = 3) break of the constructs occurred at the anchoring holes sites. Breaks occurred after 2,250 ± 35 cycles in controls versus 3,855 ± 48 cycles in the reinforced group (p = 0.0001). Comparison between mean lateral displacement of the sternal halves at the mid segments shortly before breaking showed significantly more separation in the control group than in the reinforced group (2.8 ± 0.8 vs 0.3 ± 0.5 mm, respectively, p = 0.0002).
Preliminary Clinical Experience
From December 2004 to December 2006, 45 nonconsecutive patients, submitted to median sternotomy for coronary artery bypass grafting (n = 41) or valve replacement (n = 4), and presenting a faulty paramedian sternotomy or at least three preoperative risk factors of sternal wound complications (obesity, diabetes mellitus, chronic obstructive pulmonary disease, peripheral vascular disease, depressed left ventricular function), underwent primary sternal reinforcement with the DSS. Decision of device implantation was left to the surgeon. Ethical Review Board approval (November 2004) and individual patient informed consent for surgical and additional radiologic procedures were obtained. Preoperative and intraoperative characteristics of patients are shown in Table 3. Endpoints of the clinical observations were to test surgical feasibility of the technique and to detect occurrence of potential complications such as bleeding, infection, and failure of the chest closure. Failure of the chest closure was defined as instability of the sternum without evidence of tissue infection or sternal wound infection, according to the Guidelines for Prevention of Surgical Site Infection of the Hospital Infection Control Practices Committee [22]. Patients were followed during hospitalization and at the out-patient clinic during the first three postoperative months and follow-up was 100% complete.
Time required for implantation of the reinforcement device was approximately 4 to 6 minutes for each sternal half. There were no intraoperative complications apparently related to implantation of the device. Three patients experienced postoperative respiratory failure and required prolonged mechanical ventilation and, of those, one patient developed superficial wound dehiscence with a positive culture for Staphylococcus Epidermidis. He was treated by antibiogram-guided antibiotic treatment, surgical debridement, and primary closure of the superficial wound dehiscence, as previously described [2]; chest closure remained stable and the patient recovered completely.
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Comment
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Both artificial and human cadaveric sterna have been used in biomechanical studies to test a variety of surgical techniques and materials, and to reduce the incidence of sternal wound complications. Trumble and colleagues [23] compared the mechanical properties of cadaveric and artificial sternal models formed from polyurethane foam (20 lbs/ft3) in mechanical closure testing, and found that data of both sternal models were comparable. These findings underline the theoretical advantage of using artificial models in electromechanical tests, including availability, low cost, and material homogeneity, and prompted us to use artificial sterna in our investigation. We also chose application of lateral distraction forces to the rewired models, as forces from that direction seem crucial to test stability of the sternal closure. Indeed, McGregor and colleagues [21] tested distracting forces from different directions applied on cadaveric models and found that less force was required in the lateral direction for separation of the rewired sternal halves.
Primary reinforcement of the chest closure site is usually considered advantageous in patients with multiple risk factors of sternal wound dehiscence, such as obesity, diabetes, and chronic obstructive pulmonary disease [3, 5, 6]. In addition, a faulty paramedian incision appears to affect, strongly, postoperative chest wound stability [11], as underlined by recent observations from our group [12]. Several closure techniques and devices have been proposed in high risk patients, trying to distribute closure forces over a large bone surface; indeed, the common mechanism leading to sternum instability and chest pain is wires cutting into the bone, causing fractures of sternal halves. For example, McGregor and colleagues [20] tested stability of the chest closure on a cadaveric sternal model after simple interrupted wiring with 3.0-mm diameter stainless steel coils, and found significant improvement in sternal stability compared with the use of simple stainless steel wires. Similarly, use of cables [24, 25] or steel bands [21] has been proposed by other authors. Titanium plates have also been adopted to reinforce sternal closure, anchoring the devices to the bone by means of screws [26]. Other authors also suggested the combined use of titanium [27] or stainless steel plates [28] with steel wires. Zurbrügg and colleagues [29] presented a simplified method of stabilizing the chest closure, which includes fixing of short staples on the anterior surface of the sternal halves adjacent to each outlet of the transsternal wire sutures by using a surgical staple gun. Although all the proposed techniques appear to improve stability of the chest closure, it should be considered that usefulness of techniques adopting large plates combined with screws and(or) wires might be jeopardized in patients with particularly narrow or severely osteoporotic bone. On the other end, light devices as coils, cables, bands, or staples appear easy to use but might not offer adequate support in large patients when elevated lateral distraction forces are applied to the sternal closure.
We propose a novel device, the DSS, which presents some theoretic advantages. First, it consists of small light modules, which can be assembled so that part or the full length of sternum halves can be reinforced; moreover, the design allows assembling of the clips to form braces of various lengths to adapt to the sternum size. Also, the wide grooved vertical arm, which is positioned in the intercostal space, avoids direct contact of the wires with the lateral side of the bone and distributes the lateral traction forces on a wide surface. It appears to offer adequate support against elevated lateral distraction forces, as shown by the high force values reached at electromechanical testing, without any distortion of shape of the device. Stability of the rewired sternal models was tested after both midline and paramedian incisions; displacement of sternal halves after insertion of the DSS was significantly less compared with controls, at all periodic measurements (Figs 3; 4). In the midline sternotomy test, values over 1,200 N were reached in the reinforced group before failure of the test occurred, due to breaking of the construct at the attachments of the mechanical grip system to the model. Those high values might approximate the high peak values of traction forces created during coughing, which is very common in the early postoperative period. Simulation of high velocity forces during coughing, not produced in the monotonic tensile test, was attempted by applying cycles of high velocity traction (0 to 500 N in approximately 2.5 seconds) in six sternal models (three reinforced and three controls); findings confirmed that significantly less displacement of the sternal halves was observed after reinforcement. Also, failure of the test at the anchoring site occurred in the latter group after many more cycles than number of cycles inducing fractures of the models in controls.
It has to be considered, however, that in laboratory studies forces reached during electromechanical tests are usually lower than peak forces that may be produced in life. Casha and colleagues [30] calculated that peak traction forces to the sternum during coughing may reach 1,500 N and McGregor and colleagues [21] obtained similar values using a cadaveric chest model, showing how easily stability of the sternal closure can be jeopardized by coughing during the postoperative course. In our investigation, failure of the test in reinforced models was due to breaking of the anchoring system and occurred at approximately 1,200 N. Therefore, despite the satisfactory experimental and preliminary clinical findings, performance of the reinforced sternal closure needs more extensive investigations in the clinical setting. Also, implantation of the proposed device presents some potential complications: for example, bleeding due to inadvertent damage to the internal thoracic vessels during implantation, tissue damage during mobilization of the superficial muscle fascia, and infection at implantation sites. Moreover, the use of the DSS increases somewhat the surgical costs; in our series, we calculated an additional cost of Euro 300 to 350 per patient. Nevertheless, the preliminary clinical experience in a selected group of patients at high risk of sternal wound complications shows that the proposed technique consumes little time, presents ease of implantation of the devices, and has a low risk of complications.
In conclusion, use of the DSS, a novel light and size-adaptable device, increases resistance of the rewired sternum to distracting lateral forces at electromechanical tests and seems a promising technique to prevent chest wound instability in patients at high risk of wound dehiscence.
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Abstract
Introduction
