Ann Thorac Surg 2003;75:1618-1621
© 2003 The Society of Thoracic Surgeons
Original article: general thoracic
Lower sternal reinforcement improves the stability of sternal closure
Uday K. Dasika, MDa,
Dennis R. Trumble, MSa,
James A. Magovern, MDa*
a Department of Cardiothoracic Surgery, Allegheny General Hospital, Pittsburgh, Pennsylvania, USA
Accepted for publication December 12, 2002.
* Address reprint requests to Dr Magovern, Department of Cardiothoracic Surgery, Allegheny General Hospital, 320 East North Ave, 14th St, Pittsburgh, PA15212, USA
e-mail: jmagover{at}wpahs.org
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Abstract
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BACKGROUND: This study uses a mechanical testing system to evaluate three methods of sternal closure.
METHODS: Twelve sternal replicas composed of a polyurethane foam bone analogue were divided in the midline and reapproximated using three stainless steel wire techniques: six simple wires (6S), six figure-of-eight wires (6F8), or seven simple wires (7S), which included an extra wire at the lower sternum. The closures were subjected to increasing lateral distraction from 0 to 400 Newtons (N) (1 N = 0.224 lbs), and motion was measured using transducers stationed across the manubrium, midsternum, and lower sternum.
RESULTS: With each method of closure, the manubrium was the most stable, the lower sternum the least stable, and the midsternum intermediate between the other two. There were also differences between sternal closure methods, but only at the lower sternum. Less sternal distraction was measured with the 7S than the 6S and 6F8 methods, starting at 100 N (0.20 ± 0.06 mm vs 0.48 ± 0.19 and 0.39 ± 0.10, p = 0.003), and progressively increasing until the study was stopped at 400 N (1.64 ± 0.39 mm vs 4.92 ± 1.73 and 5.1 ± 1.43 mm, p = 0.003).
CONCLUSIONS: These data show that the lower sternum is the site of greatest instability and that reinforcement of this area with an additional wire effectively stabilizes the closure. Figure-of-eight wires are not superior to simple wires.
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Introduction
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Median sternotomy for access to the heart and great vessels is a familiar and time-tested approach, but major morbidity occurs when dehiscence or mediastinitis develops. This occurs with an incidence of 0.5% to 2.5%, and results in considerable excess morbidity and mortality [1]. Various methods for sternal closure have been published, but most are based on practical experience rather than biomechanical testing.
This laboratory has published a biomechanical analysis of sternal closure, which demonstrated that physiologic levels of force were sufficient to cause minor sternal instability and that the lower aspect of the sternum was more prone to disruption than the manubrium [2]. We subsequently developed a sternal-closure testing model using a bone analogue, and validated this model against a human cadaver model [3]. This project was undertaken using the bone analogue model to compare two commonly used sternal closure techniques: interrupted simple wires versus interrupted figure-of-eight wires. In addition, we have tested the effect of additional reinforcement at the lower end of the sternum.
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Material and methods
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Bench analyses were performed using 15 artificial sternal models molded from rigid polyurethane foam, a material commonly used as an alternative test medium for cancellous bone [4]. The sternal models were checked to ensure uniform size and weight before being divided equally among the three test groups. The models were manufactured by Sawbones Corporation, Pacific Research Labs, Inc (Vashon Island, WA) using foam densities of 20 lbs/ft3 to reproduce the pull-through properties observed in human cadaveric sterna [2]. The artificial sterna were transected using a reciprocating saw and reapproximated using No. 5 stainless steel sternal wire (US Surgical Corporation, Norwalk, CT ). Three methods of closure were evaluated: (1) six simple wires (6S); (2) six figure-of-eight wires (6F8); and (3) seven simple wires (7S) (Fig 1).
In each method, the first wire was placed through the manubrium. In the 6S group, an additional five wires were placed around the sternum. In the 6F8 group, four additional figure-of-eight wires were placed in a parasternal location. The 7S group was the same as the 6S group, except that one additional wire was placed through the sternum at the sterni-xiphoid junction.
The rewired sternal models were then mounted onto a traction device comprising an anchoring base plate, 1/16-inch steel cables, miniature turnbuckles, sternal fixation plates (four pairs), a rod/bushing stabilizing mount, a heavy-duty load cell, and a worm screw tensioning device. Sternal fixation plates were bent slightly to approximate the contour of the sternal model and secured across the "ribs" using eight 1/4-inch bolts. Connection to the base plate was made by looping a pair of cables around the bolt stems (between the fixation plates) and securing them to a pair of turnbuckles, which in turn, were fastened to a third turnbuckle anchored to the base plate. A similar arrangement was used on the opposite side to connect the sternum to the rod/bushing mount. Uniform tension distribution to both top and bottom portions of the sterna was maintained by placing a simple levered adjustment mechanism at the junction of the three turnbuckles that connect the sternal model to the base plate. A manually operated worm screw tensioning device, fitted with a model LCDB-200 load cell (Omega Engineering, Inc, Stamford, CT), was used to apply static traction forces across the sternal closure. Sternal distraction was carried out to a maximum force of 400 Newtons (N) (89.8 lbs).
A set of three model LP804-01 linear potentiometers were used to measure sternal separation at the manubrium, midsternum, and xiphoid regions. Each potentiometer was securely mounted across the fissure using eyelet screws to fix both a transducer body and measurement arm. Signals were calibrated against a high-resolution linear positioner (Velmex, Inc, Bloomfield, NY) and displayed on a laptop computer using Windaq data acquisition software (Dataq Instruments, Akron, OH).
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Data collection and statistical analysis
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Sternal separation data were recorded at 20-N intervals of lateral traction force held steady for at least 10 seconds before data recording. Tests for significant differences among the three closure techniques (p < 0.05) were made at each sternal location using two-way analysis of variance. Correction for repeated measures was computed using Tukey’s HSD. All summary data are presented as mean ± SD.
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Results
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The amount of sternal distraction at the manubrium, midsternum, and lower sternum was analyzed for each closure method. This showed significant differences at each location, with stability decreasing from the manubrium to the lower sternum. These findings are shown graphically in Figure 2.
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Fig 2. Sternal separation as a function of distracting force for (A) six simple wires, (B) figure-of-eight wires, and (C) seven simple wires. Data are presented at means ± SD.
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There were also
differences among the three closure methods. The data collected
at 100, 200, 300 and 400 N distracting forces are listed in Tables 1, 2, and 3.
The 7S technique reduced sternal distraction at the lower sternum at all levels of force from 100 to 400 N, in comparison with the 6S and 6F8 technique. At 400 N distracting force, the lower sternum opened 4.92 ± 1.73 mm with the 6S and 5.10 ± 1.43 mm with 6F8 closure, but only 1.64 ± 0.39 mm with the 7S closure (p = 0.003). Figure 3
shows the relative separation of the lower sternum measured using these three closure techniques.
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Fig 3. Comparison of sternal separation at the lower sternum with the three methods of wire closure. Data are presented as means ± SD.
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Comment
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Previous studies have used mechanical testing systems to compare sternal closure methods in human cadaveric sternal bone. Casha and associates also compared methods of sternal closure, but used a steel plate as a sternal substitute [1]. Several years ago, we described a mechanical apparatus to study the process of sternal disruption in cadavers [2]. Recently, we have developed a testing system that uses a bone analogue rather than cadaver sternum and compared this with the cadaver model [3]. These data show a remarkable convergence with both models, and validates the bone analogue model. Multiple publications in the orthopedic literature also show that polyurethane foam models provide a legitimate approximation of human bone [4].
There is no consensus among surgeons on the optimal method for sternal closure. Many techniques have been described, including various wiring configurations, mersiline tapes, and nylon bands. For the most part, surgeon preferences are based on personal experience rather than scientific analysis [5–11]. This study shows no difference between simple wires and figure-of-eight wires with regard to sternal stability. This has also been reported by several other authors who have done mechanical analysis of closure techniques [12]. Many surgeons believe that figure-of-eight wires provide better stability, but there is no scientific demonstration of this in the literature. Recently, rigid internal fixation with specifically designed hardware has been described and recommended. This approach has been universally adopted in orthopedics and cranio-facial surgery, but has not been popular in cardiac surgery, probably because of the increased time required for internal fixation, and concern that these methods will hinder rapid reentry [7].
Fundamental to this controversy is a lack of information on the mechanical forces that can disrupt or retard sternal union. This study shows that distracting forces are not equally born by all regions of the sternum and that the lower sternum is especially susceptible to dehiscence, a situation that can be readily addressed by reinforcement of the closure in this region. It follows that the materials and configurations are probably less important than recognition of anatomic and mechanical considerations. The key issue is to fully stabilize the sternum, especially the lower end, to insure reliable healing. This can be done with plates, but also with the use of additional wires.
The relative instability of the lower sternum can be explained by several anatomic factors. During normal respiration, the abdomen and lower thorax move a greater distance than the upper thorax. In addition, ribs 7 through 10 all attach to the seventh costal cartilage at the lower sternum, a situation that tends to concentrate forces at this region. Other factors that may contribute include the greater transverse and anterior-posterior dimensions of the lower versus upper thorax and the reduced thickness of the lower sternum in comparison with the manubrium.
Optimal sternal wound healing is the result of many factors, but the most important is a secure closure. Additional factors such as acuity of surgical procedure, cigarette smoking, obesity, hypertension, interior mammary artery utilization, diabetes, and prior sternotomy increase the incidence of wound complications, but may be less important than mechanical instability [13]. In most instances, sternal wound infection is precipitated by sternal instability rather than the other way around. Increased attention to the lower end of the sternum by means of additional wires or rigid internal fixation should reduce the morbidity of the midline sternotomy incision.
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Conclusions
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These data show that the lower sternum is a site of potential instability and that reinforcement of this area significantly stabilizes the closure. The clear implication is that more wires, especially at the lower aspect of the sternum, will reduce the complication rate of sternal closure. Figure-of-eight wires provide no improvement in sternal stability in comparison to simple wires.
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References
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- McGregor W.E., Trumble D.R., Magovern J.A. Mechanical analysis of midline sternotomy wound closure. J Thorac Cardiovasc Surg 1999;117:1144-1150.[Abstract/Free Full Text]
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- Szivek J.A., Thomas M., Benjamin J.B. Characterization of a synthetic foam as a model for human cancellous bone. J Appl Biomater 1993;4:269-272.[Medline]
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- Goodman G., Palatianos G.M., Bolooki H. Technique of closure of median sternotomy with trans-sternal figure-of-eight wires. J Cardiovasc Surg (Torino) 1986;27:512-513.[Medline]
- Taber R.E., Madaras J. Prevention of sternotomy wound disruptions by use of figure-of-eight pericostal sutures. Ann Thorac Surg 1969;8:367-369.[Medline]
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- Casha A.R., Ashraf S.S., Kay P.H., Cooper G.J. Routine sternal closure using interlocking multitwisted wires. Eur J Cardiothoracic Surg 1999;16:353-355.[Abstract/Free Full Text]
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Abstract
Introduction
