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Ann Thorac Surg 2002;74:739-744
© 2002 The Society of Thoracic Surgeons

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Original article: cardiovascular

Validation of a bone analog model for studies of sternal closure

^a Cardiac Surgery Research, Allegheny-Singer Research Institute, and Department of Surgery, Allegheny General Hospital, West Penn Allegheny Health System, Pittsburgh, Pennsylvania, USA

Accepted for publication April 11, 2002.

* Address reprint requests to Mr Trumble, Cardiac Surgery Research, 9th Floor, South Tower, Allegheny General Hospital, 320 East North Ave, Pittsburgh, PA 15212 USA
e-mail: trumble{at}wpahs.org

	Abstract

Top Footnotes Abstract Introduction Material and methods Results Comment References

 
Background. The incidence of serious sternal wound complications may be reduced with improvements in closure methods. Biomechanical testing of median sternotomy closures in cadavers has proven useful but is limited by availability, high cost, and wide variations in the material properties of the sterna. This study tests whether artificial sterna can be used to replace whole cadavers in sternal closure testing.

Methods. Two common wire closure techniques were tested using both whole cadavers and artificial sternal models formed from bone analogue material. Sternal models were molded from polyurethane foam (20 lbs/ft³) to simulate the mechanical properties observed in human cadaveric sterna. The force vector previously identified as the most detrimental to sternal cohesion (lateral traction) was used to stress the closures. Separation of the incision site was measured at the manubrium, midsternum, and xiphoid and data were compared between cadaver and bench test groups.

Results. Sternal separations recorded in cadavers were found to be similar to bench test results for both closure types. Data variability within test groups was found to be consistently lower using artificial sterna, where peak standard deviations for sternal motion averaged less than half that measured in cadavers.

Conclusions. Results suggest that anatomic sternal models formed from solid polyurethane foam can be used to approximate the biomechanical properties of cadaveric sterna and that reliable information regarding sternal closure stability can be secured through this means. Moreover, bench test data were shown to be less variable than cadaveric results, thus enhancing the power to detect small differences in sternal fixation stability.

	Introduction

Top Footnotes Abstract Introduction Material and methods Results Comment References

 
Despite the rising popularity of minimally invasive surgical techniques midline sternotomy incisions continue to be the preferred means by which many cardiac surgeons access the heart and surrounding vasculature. This approach affords maximum exposure of important cardiac structures and is generally well tolerated by most patients, roughly 98% of whom experience no serious difficulties with regard to postoperative sternal wound healing. Still, a small minority of sternotomy patients develop sternal instability, which can lead to devastating complications including mediastinitis and sternal dehiscence—conditions that carry alarmingly high rates of morbidity and mortality [1–3].

According to statistics currently published by the American Heart Association [4], more than 760,000 open heart surgeries are performed in the United States each year. Adopting a reasoned estimate of sternal wound complication rates based on recent published reports (2%) the sum total of patients suffering significant postoperative morbidity and mortality due to sternal nonunion figures to exceed 15,000 annually [5–7]. Given current mortality statistics, annual deaths from this condition can be expected to number in the thousands in this country alone [7]. It is therefore important that measures be taken to further reduce the incidence of sternal wound complications among this increasingly large patient population. We believe that this may best be accomplished by improving the mechanical stability of the closure itself thereby maintaining wound integrity and reducing the risk of bacterial infection.

Prior work using human cadavers has established that traditional sternal closure techniques may not always provide adequate fixation stability especially in the lower (xiphoid) region where the sternal halves are most easily separated by physiologic traction forces [8]. Given these preliminary test results the next logical step would be to examine numerous sternal approximation methods to determine which produce the most secure bond. Unfortunately, cadaveric models are of limited practical use in assessing a wide variety of sternal closure methods owing to high costs associated with testing and the variability of sternal mechanical properties among cadavers of different age, sex, body type, and medical history. Efforts to formulate less expensive and more reliable means to evaluate the relative merits of sternal closure techniques are therefore warranted.

This report details development of a simple standardized method for comparing the strength and stability of various sternal approximation methods. The validity of using anatomic bone analogue models for the purpose of sternal closure testing is examined by comparing bench test data to results obtained from cadaveric testing of two common fixation methods. The goal is to establish a reliable means by which mechanical properties of sternal closure techniques may be quantified and compared in order to identify those methods most likely to yield positive clinical outcomes.

	Material and methods

Top Footnotes Abstract Introduction Material and methods Results Comment References

 
Cadaveric studies
Standard interrupted (SI) and figure-of-eight (F8) sternal closure techniques were tested in 5 human cadavers (2 male). Median sternotomies were performed with an oscillating saw and closed with no. 5 stainless steel wire. Mechanical testing of sternal closure strength was performed as previously described [8]. Briefly, the chest wall was instrumented to apply static traction forces across the sternum using a rigid metal cage, 1/16-inch steel cable, brass fixation plates, and a turnbuckle for tension adjustment (Fig 1). Traction forces were measured using a load cell stationed between the anchor cage and tensioning cable. Four pairs of piezoelectric crystal transducers (5 MHz, ± 0.04 mm resolution) were used to measure sternal separation across the incision site. Seven separate wires were used to complete the SI closure: 2 at the manubrium and 5 around the sternum in intercostal spaces 2 to 6. F8 closures were completed using 6 wires: 2 at the manubrium and 4 woven across the sterna at ribs 2 to 5. In all experiments SI closures were studied first, followed by the F8 configuration. Sternal wires were manually tightened before each experiment using the same technique employed clinically, that is, by twisting with a large needle-driver until the closure felt snug.

View larger version (57K): [in this window] [in a new window]   Fig 1. Experimental setup used to apply traction forces to the sterna of intact human cadavers. A rigid enclosure fashioned from steel pipe (0.85-inch outer diameter) was used to provide stable anchoring points for sternal traction in three mutually-orthogonal directions: lateral (shown here), rostral-caudal, and anterior-posterior. Force was applied through steel cables secured to the second, fourth, and sixth ribs at their insertion points and adjusted using a turnbuckle mechanism. Bone fracture was prevented by pressing each rib between two brass plates using a small metal hose clamp (to which the cable was attached). Sternal motion was monitored using four pairs of sonomicrometry crystals and tension measured by an S-beam load cell. Reproduced with permission from McGregor et al [8].

Bench studies
Bench-top analyses were performed using artificial sterna molded from rigid polyurethane foam—a material commonly used as an alternative test medium for cancellous bone (Fig 2). Sternal models were manufactured by 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 [9]. Artificial sterna were transected using a reciprocating saw and reapproximated using no. 5 sternal wire. Both test groups comprised five foam sterna closed in the same fashion as the cadaveric studies described above with the exception that a single wire was used to secure the manubrium (demonstrably the most stable portion of the sternal closure).

View larger version (72K): [in this window] [in a new window]   Fig 2. Synthetic sternal model (19 x 6.5 cm) separated bilaterally and reapproximated using six interrupted wire closures. All sterna used in this study were formed to replicate human anatomical structures and cast from solid polyurethane foam mixed to a density of 20 lbs/ft3.

View larger version (83K): [in this window] [in a new window]   Fig 3. Sternal closure test apparatus shown with rod/bushing stabilizing mount on the left and anchoring base plate on the right. The worm screw tensioning device and load cell (not shown) are stationed to the left of the stabilizing mount. Traction forces are applied through a series of cables and turnbuckles and uniform tension distribution maintained by a levered adjustment mechanism (middle right). Sternal motion is measured using three linear potentiometers mounted across the manubrium, midsternum, and xiphoid regions of the sternal model.

Data collection and statistical analysis
Sternal separation data were recorded at regular intervals of lateral traction force (20 N) in both cadaveric and bench-top experiments. Separation distances recorded in the upper-mid and lower-mid portions of the sterna in cadavers were averaged for comparison with in vitro studies where a single transducer was used to measure midsternal motion. In all cases, static traction forces were held steady for at least 10 seconds before data recording. Tests for significant differences between groups (p < 0.05) were made at each discrete tension level using unpaired t tests performed with StatView software (SAS Institute, Cary, NC). Summary data are presented as mean ± SD.

	Results

Top Footnotes Abstract Introduction Material and methods Results Comment References

 
Summary data from cadaveric and bench testing of SI wire closures are shown in Figure 4 where sternal motion is plotted against traction forces at the manubrium, midsternum, and xiphoid process. Sternal separation at the manubrium was extremely small in both test groups, peak traction forces producing 0.17 ± 0.09 mm distraction in the human cadaver model and 0.09 ± 0.07 mm motion using artificial sterna. Peak distraction distances at the midsternum for cadavers and artificial sterna were 0.88 ± 0.55 mm and 0.50 ± 0.20 mm respectively. Motion at the xiphoid process was nearly identical in both groups, where peak separation in artificial sterna exceeded cadavers by less than 0.1 mm (0.89 ± 0.28 mm versus 0.83 ± 0.50 mm). In every instance differences between groups failed to reach statistical significance.

View larger version (21K): [in this window] [in a new window]   Fig 4. Plots of sternal displacement versus traction force for standard interrupted closures in human cadavers and sternal models. Top, middle, and lower graphs show motion at the manubrium, midsternum, and xiphoid regions respectively. Data are mean ± SD.

View larger version (20K): [in this window] [in a new window]   Fig 5. Plots of sternal displacement versus traction force for figure-of-eight closures in human cadavers and sternal models. Top, middle, and lower graphs show motion at the manubrium, midsternum, and xiphoid regions respectively. Data are mean ± SD.

	Comment

Top Footnotes Abstract Introduction Material and methods Results Comment References

Data from these whole-body cadaveric experiments produced several findings of fundamental importance to this current work. First, traction forces were observed to cause more sternal motion in the lateral direction than along other angles of distraction. Such anisotropy is believed to occur because only lateral separation can be achieved without the need to overcome bone-on-bone frictional forces. Biomechanical testing of sternal closure stability was thus determined to be most sensitive when using distraction forces applied in the lateral direction, orthogonal to the sagittal plane. Second, it was found that different regions of the sternum were at various risk for disruption—the lower, xiphoid region exhibiting much less stability than the manubrial portion of the closure. Anatomic factors proposed to explain this finding included the fact that the manubrium is significantly thicker than the lower sternum and may be stabilized further by the clavicle and other structures of the upper thorax. The relative importance of these factors could not be established however.

In addition, experience gained through sternal testing in cadavers exposed numerous practical concerns regarding the application of this technique to more large-scale testing of multiple closure methods. Use of a single specimen in multiple experiments for the sake of economy raises questions concerning the validity of testing closure strength on previously stressed sterna. On the other hand, the costs of restricting sternal testing to one trial per cadaver can quickly become prohibitive. This problem is exacerbated by the natural variability of mechanical properties among cadaveric specimens creating the need to collect more samples in order to secure meaningful results. Together these elements provide a strong impetus to seek an alternate in vitro approach to sternal closure testing.

The study described in this report was conducted to determine whether experimental results similar to those acquired using human cadavers could be reproduced through simpler means. The overriding goal was to reproduce the important mechanical aspects of cadaveric testing on the bench while eliminating the expense and specimen variability inherent to work involving human tissues. Validation of such alternative methods would allow a wide variety of sternal closure mechanisms to be tested more quickly and efficiently thereby promoting accelerated development of improved sternal fixation techniques.

Two common wire closure techniques were tested using both cadavers and artificial sternal models formed from bone analogue material. The force vector previously identified as the most detrimental to sternal cohesion (ie, lateral traction) was used to stress the closures. Separation of the incision site was measured at the manubrium, midsternum, and xiphoid in all experiments and results compared between cadaver and bench test groups.

The magnitude of sternal separation in cadavers was found to be similar to bench test results at all points along the incision site for both types of sternal closure methods tested. Near-perfect matches in sternal motion were seen at the xiphoid using the SI closure and near the midsternum using the F8 style approximation technique. SI closures appeared to be somewhat less stable at the manubrium and midsection in cadavers but differences were too small to be confirmed statistically. Likewise, cadaveric sterna tended to be slightly more compliant near the xiphoid while secured with the F8 closure but appeared less compliant than artificial sterna at the manubrium. Moreover, data variability within test groups was found to be consistently lower using artificial sterna in which peak standard deviations for sternal motion averaged less than half that measured in cadavers.

The fact that the manubrium in both groups remained much more stable than the middle and lower portions of the sterna suggest that closure stability in this region has less to do with surrounding support structures than was previously hypothesized and that sternal thickness may be the predominant factor in maintaining cephalad sternal union. It is important to note, however, that the transmission of forces to the sternum through the rib cage is likely to be less uniform in live patients than in our test fixture owing to the anatomy of the human thoracic cavity. Sharp increases in intrathoracic pressure due to sneezing, coughing, or valsalva maneuvers may indeed place more stress on the lower sternum owing to the common articulation of ribs 6 to 10 at the xiphoid process. The degree to which the outward, lateral forces produced by air pressure within the thoracic cavity is counterbalanced by the simultaneous contraction of muscles surrounding the rib cage remains largely unknown. It is reasonable to speculate, however, that the relatively shallow depth of the lower sternum combined with multiple rib articulation at the xiphoid may contribute to sternal dehiscence in patients.

Regarding study limitations, it should be emphasized that our use of synthetic models for sternal closure testing is not intended to reproduce the biomechanical complexities found in living patients where the incidence of sternal wound complications can often be traced to one or more disparate risk factors including obesity, diabetes, chronic obstructive pulmonary disease, advanced age, prior sternotomy, and smoking (among others). Indeed, it is precisely because of these diverse risk factors and the anatomic variability among patients that a standard means to test sternal closure stability is needed. The mechanical testing of sternal models provide an opportunity to assess each closure method in isolation under uniform, reproducible conditions apart from confounding variables endemic to clinical studies. In this way the relative merits of numerous closure methods can be quantified and the most appropriate fixation technique chosen based on the individual needs of the patient.

Results from just this kind of biomechanical bench testing have already been reported by Cohen and Griffin [10] who recently evaluated several sternotomy closure techniques (wire, cable, and plate) using sternal models similar to those tested here. In their study, 54 foam sterna were stressed to failure in order to compare three fixation schemes under conditions of lateral distraction, longitudinal shear, and transverse shear—an experiment that would have been difficult to perform using more expensive, less uniform cadaver sterna.

Our results suggest that studies of this sort not only provide valuable information regarding the relative efficacy of closure techniques but also show how actual human sterna of like size might react under similar test conditions. However, because sterna of disparate dimensions (as in the case of pediatric applications) may respond differently to mechanical stressors, it is important that studies of this type be repeated using sternal models of similar size and shape in order to gauge the efficacy of sternotomy closures in these special patient populations.

Conclusions
Results from this study suggest that anatomic sternal models formed from solid polyurethane foam can be used to approximate the biomechanical properties of cadaveric sterna and that reliable information regarding sternal closure stability can be secured through this means. These experiments further indicate that data collected using sternal models tend to be less variable than data gathered using human tissues, thereby enhancing the power to detect small differences in sternal fixation stability between and among closure types. The most important benefits of testing sternal closures with bone analogue materials are that such studies (a) are considerably less costly than cadaveric testing, (b) can be performed more quickly, (c) yield less data variability, and (d) eliminate the need to perform multiple tests on a single sternum. In combination these factors serve to substantially enhance the diagnostic power of sternal closure testing by both improving data quality and making the examination of large test groups more practical.

	Footnotes

Top Footnotes Abstract Introduction Material and methods Results Comment References

 
This work was supported by the Allegheny Heart Institute, Pittsburgh, Pennsylvania.

	References

Top Footnotes Abstract Introduction Material and methods Results Comment References

 

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Walter E. McGregor James A. Magovern Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Trumble, D. R. Articles by Magovern, J. A. Search for Related Content PubMed PubMed Citation Articles by Trumble, D. R. Articles by Magovern, J. A. Related Collections Chest wall