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Ann Thorac Surg 2004;78:912-917
© 2004 The Society of Thoracic Surgeons

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Original article: cardiovascular

Postoperative sternal dehiscence in obese patients: Incidence and prevention

^a Division of Cardiovascular and Thoracic Surgery, Department of Surgery, University of Minnesota Medical School, Minneapolis, Minnesota, USA

Accepted for publication March 15, 2004.

^* Address reprint requests to Dr Molina, Cardiovascular and Thoracic Surgery, University of Minnesota Medical School, 420 Delaware St SE, MMC 182, Minneapolis, MN 55455; e-mail: molin001@umn.edu; USA.

Presented at the Fortieth Annual Meeting of The Society of Thoracic Surgeons, San Antonio, TX, Jan 26–28, 2004.

	Abstract

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BACKGROUND: Obesity has been identified as the single most important risk factor for postoperative sternal infection in coronary bypass surgery patients. It is also a major risk factor for sternal dehiscence, with or without infection, for any type of cardiac operation. We assessed whether prophylactic measures could prevent this complication.

METHODS: Two studies were conducted. In study A, 3,158 heart surgery patients were analyzed at 3 cardiac units. Obesity was defined as body mass index (BMI) more than 30. Group I (1,253 obese [39.7%]) was compared with group II (1,905 nonobese [60.3%]). Sternal closure was done at the surgeon's preference: (a) plain wires through and through the bone; (b) peristernal figure-of-eight wires; or (c) peristernal method, using stainless-steel cables. In study B, 123 obese patients were prospectively divided into 2 subgroups. Group B-1 (54 patients) underwent lateral prophylactic sternal reinforcement before placement of peristernal wires. Group B-2 (69 patients) had standard sternal closure, as in study A.

RESULTS: In study A, group I had 81 dehiscences (6.46%); 78 also suffered deep sternal infection and mediastinitis (96%). Despite treatment, dehiscence recurred in 13, and mortality was 38.4%. In group II nonobese patients, 31 dehisced (1.6%, p = 0.000), with no mortality. In study B, group B-1 (54) had 0% dehiscence versus group B-2 (69) with 6 dehiscences (8.7%).

CONCLUSIONS: In our study, the rate of obesity is high ( 40%). Sternal dehiscence is real when the BMI is more than 30 (6.46%), and has high morbidity and mortality. Prophylactic sternal reinforcement seems to prevent this complication.

	Introduction

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Obesity has reached epidemic proportions in the United States [1]. It is often associated with diabetes [2], and has been known to pose a high risk for any type of abdominal or thoracic surgical intervention. Literature reports indicate a high incidence of sternal wound infections in obese patients undergoing coronary bypass surgery (CAB) [3–6]. Obesity has been found to be the single most important risk factor for sternal infection— even though that finding has come from countries like France, where a great proportion of patients undergoing heart surgery are not obese [7]. In the United States, obesity has also been found to significantly affect morbidity and mortality because of concomitant complications after surgery [8, 9], particularly heart surgery [10].

In our present report, we conducted two studies to investigate the problems of obesity among adults undergoing all types of cardiac procedures. Study A is a retrospective assessment and analysis of the incidence of obesity, the occurrence of sternal dehiscence, and the results of its treatment in patients operated during the last 3 1/2 years. In study B, we prospectively examined the effects of prophylactic sternal reinforcement on sternal dehiscence in another group of obese patients.

	Material and methods

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Three cardiac units are represented in study A and study B: the Fairview-University Medical Center, the Veterans Affairs Medical Center (VA) in Minneapolis, and Regions Hospital Saint Paul. The same team of surgeons operate in the three hospitals using the same prophylactic antibiotic protocol (Cephazolin). After study A was completed, the second study was launched, with no overlap of patients. Study B was not a randomized sample, but it was conducted simultaneously in the 3 cardiac units with the same team of surgeons. The surgeons decided how they wanted to close the sternum. The pattern of surgery performed and the gender distribution of the patients differed between the cardiac units. At the Veterans Affairs Medical Center, the population of female patients in study A was very small; 3 women versus 997 men. At Fairview-University Medical Center and Regions Hospital Saint Paul units, the proportion of female patients is about 33.7% to 36% (Table 1). Therefore, in a non-VA institution 1 out of every 3 patients is female. In the obese group, most of the procedures involving midline sternotomy were coronary artery bypass operations (Table 2), comprising about 74% of the entire group. Table 2 also shows the percentage of patients undergoing valve replacement or a combination of procedures. Among the 3 cardiac units, there is an obvious difference in the number of heart transplants and left ventricular assist devices, which are implanted exclusively at Fairview-University Medical Center.

Surgical techniques
The sternum was closed in a standard manner, depending on the particular surgeon's preference: (A) plain 6-gauge stainless-steel wire going through-and-through the bone; (B) figure-of-eight wire stitches around the sternum; (C) a peristernal method using stainless-steel cables. Regardless of which technique was used, the manubrium was always closed with 3 simple through-and-through stitches of 6-gauge stainless-steel wire. These closures did not involve any type of lateral reinforcement, such as staples [11], sternal plates [12, 13], or "sleeve-rope closure" [14]. In all three methods, the wire or cable was in direct contact with the sternal bone.

In study A, all patients who suffered sternal dehiscence with mediastinitis (deep sternal infection) were treated using the method described in 1993 by one of the authors [15]. Treatment of dehiscence involves two components: implementation of a lateral reinforcement of the sternum by lacing a gauge-6 wire around each rib parallel to the sternum from the costal margin up to the manubrium, and returning in the opposite direction. The suture is tightened at the bottom end and cut. The maneuver is repeated on the opposite side. The double-wire peristernal sutures are then placed to approximate the sternum.

The second component entails the implantation of an irrigation-suction tube system [15] in a staggered manner behind and in front of the sternum carried out with a large volume of 100 mL/h per tube for 48 hours, and then diminishing to 50 mL/h by the end of one week. During the following week, the chest tubes are gradually pulled back 2 inches per day until all tubes are removed. Throughout the treatment period, patients were maintained on intravenous antibiotics, and then switched to oral antibiotics for 12 days. If the treatment failed, other procedures were implemented to place pectoral or rectus abdominal muscle flaps and skin flaps to heal the infected area.

In study B, when the sternum was prophylactically reinforced laterally, we used the technique described by Robicsek and colleagues [16], as described above, but without the irrigation-suction system (Fig 1). We use an A&E Medical Corporation (A&E Medical Corporation, Durham, NC) suture kit, which provides individual double-wire sutures swaged to a blunt needle to prevent bleeding. We did not distinguish between first-time operations and reoperations. If postoperative bleeding occurred and the patient was returned to the operating room for reexploration, only the original operation is listed. However, the type of closure listed is the one used for the patient's last intervention.

View larger version (123K): [in this window] [in a new window]   Fig 1. The results and correct placement of sutures for the reinforced closure of the sternum. The image on the left shows how the repair should appear on a radiograph, and the illustration on the right shows the correct placement of the sutures.

	Results

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Study A: We analyzed all cases of sternal dehiscence, with and without infection. Most patients who suffered dehiscence (96%), however, developed mediastinitis and sepsis. In study A, of the 1,253 obese patients in group I, 81 (6.46%) suffered sternal dehiscence versus 31 (1.63%) of the 1,905 nonobese patients in group II. The 95% confidence interval for the difference was (0.034, 0.063) and the difference was statistically significant (p = 0.000). Obesity has a significant effect (p = 0.000), and the obesity of the patient increases the odds of the patient suffering dehiscence by 3.93 times (coefficient = 1.37). Table 4 shows the incidence of dehiscence by standard sternal closure technique. We found practically no difference between the cases closed with single stitches through the bone versus the cases using figure-of-eight stitches or cable closure. However, the greater the degree of obesity, the higher the incidence of dehiscence (Fig 2). Of the 81 obese patients who suffered sternal dehiscence, the incidence was 4.59% when the BMI was 30 to 34, but the rate rose to 21.43% when the BMI was greater than 49 (Fig 2).

View larger version (21K): [in this window] [in a new window]   Fig 2. Incidence of sternal dehiscence at the 5 levels of obesity. = percentage of dehiscence. (BMI = body mass index.)

The rate of diabetes was 11% in the nonobese group versus 18% in obese patients (p = 0.05). Chronic obstructive pulmonary disease was 9% in nonobese versus 5% in obese (not sufficient [NS]). Postoperative bleeding occurred in 2.3% of nonobese versus 2% in the obese (NS) Left internal thoracic artery (ITA) was used in 89% of patients undergoing CAB surgery (92% of nonobese vs 81% of obese). None of the obese patients had bilateral ITA implants.

In study B of 123 obese patients, group B-1 (54 patients, BMI of 31 to 54), had no dehiscences. In group B-2 (69 patients, BMI of 30 to 50), closed with standard closure technique, 6 cases of dehiscence occurred (8.7%) (p = 0.001).

Morbidity and mortality of sternal dehiscence
In 67 of the 81 obese patients in study A group I who suffered dehiscence, the irrigation-suction system was 100% effective in clearing the infection, but reinforcement of the sternal closure failed to provide complete sternal healing and stability in about 19% of the cases. Reinforcement was successful in 55 patients (80.6%), which is significantly less than the success rate obtained in nonobese patients (100%). Thirty (96.7%) of the 31 patients in the nonobese group II of study A were completely healed using the same therapy. Only 1 partial failure required reintervention, after which the patient healed properly.

Among the 13 patients (19.4%) in group I who failed treatment, 5 deaths occurred (38.46%). Fourteen patients in group I did not undergo treatment with the irrigation-suction system, as elected by their treating physician; they instead underwent one or more procedures, including creation of subcutaneous flaps or muscular flaps, and sternectomy. Several of them underwent from 2 to 6 operations. Nine of these patients are surviving (64.29%), which is significantly less than the survival rate obtained using the irrigation-suction system. Five of the patients died, resulting in a mortality rate of 35.7%.

	Comment

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Sternal dehiscence is a very serious complication after cardiac surgery. It leads to high mortality and morbidity, due to the instability of the chest wall and the frequently concurrent presence of infection. Our studies focused on patients suffering from obesity—a major risk factor for any type of complication after surgery. Since the incidence of obesity in the United States is very unlikely to significantly decrease or disappear within the next few years, measures need to be taken to prevent sternal dehiscence, particularly in obese patients. Diabetes may also play a significant role in the lack of healing of sternotomy incisions. However, the incidence of diabetes in the obese patients in our study was found to be 18% versus 11% in the nonobese group. The difference did not prove to be statistically significant.

A previous report based on the Society of Thoracic Surgeons database by Prabhakar and colleagues [5] showed an association between extreme obesity and deep sternal infection within 1 month after surgery, but did not address sternal dehiscence in the absence of infection, nor did it account for the frequent infections which occur beyond 4 weeks. Their study was also limited only to patients undergoing CAB operations. Many different sternal closure techniques have been implemented and described during the past few decades, in an attempt to find a foolproof method of preventing sternal dehiscence [17–19]. The studies published by Trumble and colleagues [19] and McGregor and colleagues [20], using cadavers and sternal models, showed that dehiscence occurs from the stainless-steel wire cutting through the bone. Depending on the amount of movement and tension placed on the point of contact, the bone will often yield before the period of healing is complete.

Sternal dehiscence usually occurs when the wire or cable cuts through the bone and causes multiple fractures of the sternum. Rarely does it occur because of breakage of the suture material. Therefore, as long as we continue to use only standard sternal closure techniques in obese patients, the incidence of sternal dehiscence will continue to occur in obese patients at the high rates we have found. If the rate of obesity continues to increase—as seems to be the pattern in the United States—we will face an ever higher incidence of sternal dehiscence and a resultant increase in mortality and morbidity, as well as a rise in hospital costs.

The need to provide lateral reinforcement of the sternum has led to the development of several techniques, such as the use of lateral staples and lateral plates. These techniques change the point of contact from metal-to-bone to metal-to-metal. In principle, the technique published by Zurbrügg and colleagues [11] and the techniques using lateral sternal plates [12] and sleeve-rope closure [14] seem to help. However, these methods were tried only in a very small number of patients, and we have found that no further publications have appeared reporting the use of these systems. In our study B, we applied our reinforcement method prophylactically in a prospective manner. This technique has several advantages over the others: it changes the site of pressure and provides wider support due to the metal sternal reinforcement, which is evenly distributed along the entire length of the bone from the manubrium to the xiphoid. This even distribution proved to be an advantage over the technique that places isolated pieces of metal lateral to the sternum; such pieces are somewhat cumbersome to apply, and fail to distribute the pressure to a broad area. Moreover, there is always the risk of losing pieces of metal in the operative field.

Using the prophylactic reinforcement method for all obese patients seems to prevent sternal dehiscence, even for those obese patients who were noncompliant regarding arm movement restrictions routinely implemented during the postoperative period. Building on the fact that sternal dehiscence usually begins in the lower part of the sternum due to the patient's movements, Dasika and colleagues [13] showed that reinforcing the inferior portion of the sternum improves its stability and helps prevent dehiscence, shown also by Casha and colleagues [21].

We recommend the reinforcement kit we used in our study B. It features a blunt needle attached to a 30-inch-long, malleable, 6-gauge or 8-gauge stainless-steel wire. This kit makes it easy to go around each rib lateral to the sternum without having to dissect and expose the intercostal space, and eliminates bleeding. The double wires—either 6-gauge or 8-gauge—used for the peristernal closure are sufficiently strong to hold the sternum together.

A few technical points need to be mentioned regarding the use of this technique. First, careful attention must be paid to the placement of the peristernal wires lateral to the parallel reinforcing wires. Second, figure-of-eight wire sutures should not be used in conjunction with lateral reinforcements, because the wire is stiff and will not slide through the two halves of the sternum. Instead, single or double wires—as we currently use—should be placed in each intercostal space in a single through-and-through fashion. Third, it is also effective to use cables for the peristernal reinforcement, as they slide easily and are more pliable than the rigid 6-gauge wires.

In conclusion, we highly recommend the sternal closure reinforcement technique described in our study B for all patients with any degree of obesity (BMI > 30). Our results most definitely also show that the higher the degree of obesity, the stronger the need to reinforce the sternal closure in order to prevent the devastating results of dehiscence.

	Discussion

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DR KEITH ALLEN (Indianapolis, IN): I enjoyed your talk. We have instituted in our group very aggressive diabetic management along with nasal Bactroban and our internal data would suggest that we have neutralized the risk of both diabetes and obesity regarding sternal wound complications. I applaud your technique, but it is somewhat onerous to do that on every patient, and I wonder if you could comment on how you manage your obese diabetic patients, i.e., keeping their blood sugars at a very reasonable level. We try to maintain them at less than 150.

DR MOLINA: Thank you for the question. As you may know, the University of Minnesota is a very large transplant center, the largest pancreas center in the States for diabetics. We just completed a series of almost 100 patients who needed to undergo coronary bypass prior to kidney and pancreas transplant for diabetes type I; they are severely diabetic. Of course, the management of diabetes is of the utmost importance there. But it doesn't seem to be as important as the degree of obesity, since in this group of almost 100 patients that we have, there was no dehiscence. There was only one patient with superficial infection, and they were handled the same as the rest. So I think diabetes is an important factor for healing, but the dehiscence is probably mostly the primary factor and the obesity of the person

DR A. NORMAN LEWIN (Buffalo, NY): There was no reference made to the use or nonuse of internal mammary arteries (IMAs) in these patients. Could you make a comment about that?

DR MOLINA: No, none of these patients received that treatment

DR MICHAEL ACKER (Philadelphia, PA): No, no. What was the incidence of single IMA versus bilateral IMAs for revascularization?

DR MOLINA: 76–78% of the patients received an IMA implant.

DR ACKER: Would you use a bilateral IMA in a patient with a BMI over 35?

DR MOLINA: No, I would not.

DR JOHN D. PUSKAS (Atlanta, GA): How about if the patient is nondiabetic but has a BMI of 35?

DR MOLINA: It depends. Some of these persons are so huge that they don't clear the retractor to expose the mammaries. So it becomes a very difficult problem, particularly due to the clinical situation. If some of these patients have unstable angina or a recent infarction, we would not try to harvest the IMA. But in general, in most of them, we do.

DR PUSKAS: I think most of us would shy away from double mammaries in the really gargantuan patient and I would echo Dr. Allen's comments about the importance of perioperative insulin management. We use intravenous insulin infusions and maintain really vigorous diabetic control for the first 24–36 hours. I think that makes a significant impact on rates of sternal wound infections.

	References

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