The Conundrums of Defect Management in Deep Sternal Wound Infection: Review

Deep Sternal Wound Infection (DWSI) remains an important cause of morbidity and mortality following Cardiac surgery. Equally many methods of preventing it from occurring have been stated however when it does occur, the management by debridement and provision of cover for the defect can be challenging and thus different treatment options are instituted with various degrees of success. The use of Vacuum Assisted Closure therapy was stated as been mere effective in treatment of DSWI with mechanism not known but better used as an adjunct to flaps and with a potential for profuse bleeding. The major advantage in the use of Pectoralis Major Flap was in its bulky size that allows for the cover of defect following sternectomy. However, it had a serious draw back in neonates and infants as the compressive effect on the heart and the distortion of the mammary line and its preclusion in patients with previous radionecrosis and the divergent opinions for further resternontomy. The omental flap has the advantage of provision of cover as may be occasioned by deep defect; in extensive sepsis because of the immunogenic functions and does not lead to deformity but serious complications as the herniation of the transverse colon as an important drawback to its use. This present review gives the omental flap a slight superior edge over the other two when considering the cover for defect; taking care of local sepsis and resternotomy. However, what is needed now is a multicenter randomised study to determine the option that best suit this condition.


Introduction
Deep Sternal Wound Infection (DSWI) is a rare but one of the challenging problems that can occur after open heart surgery ,especially with the management of defect; causing prolonged hospitalization, increased hospital costs, and increased morbidity and mortality [1][2][3][4][5][6]. There have been many novel surgical approaches that are geared toward the provision of adequate covers for the defects, as may occur in many cases; especially after extensive debridement [5][6][7][8][9][10][11]. The choice of flaps ranges from the use of muscles to the use of omentum as well as the use of Vacuum assisted wound closure (VAC) as a therapeutic strategy [2,12].

Method
We reviewed the relevant English literature relating to this topic via a MEDLINE and Google scholar search using the following terms: Omental flap and Deep Sternal Wound Infection; Vacuum Assisted Closure and DSWI; Pectoralis major flap and DSWI with no limitations to years of study. Expert opinions were sought in both conceptualizing and the write-up of this work.

The Epidemiology
The incidence of DSWI as reported in the literatures is between 0.2-3.6% [1,13,14] and this can be higher especially when the following risk factors like age, diabetes mellitus, obesity, Hypertension, postoperative mechanical ventilation, and early surgical re-exploration, and some surgical techniques such as double harvest of Internal thoracic artery(ITA) in pedicled fashion ;combined valve and coronary artery bypass graft are adopted [15]. In the preoperative period, parameters like diabetes mellitus, COPD and preoperative renal insufficiency were independent risk factors for postoperative sternal complications [16,17]. The prolonged hospital stay and attended escalated cost as notable morbidities. Mortality rates may vary between 0 to 20% [1,18]. The most causative microbiological agents of SWI are staphylococci species [19]. The incidence of infection caused by Gram-negative bacteria and Mycobacterial species are been seen [1] and fungi is less common [20].

Prevention
The rate of decreases with the adoption of d methicillin-resistant Staphylococcus aureus (MRSA) screening, bacterial decolonisation measures, good prophylactic use of antibiotic [1,14]. The other important step in preventing the DSWI is to identify the patients at risk in developing DSWI; Patients with co-morbid conditions like diabetes be properly optimized even before surgery. Diabetes mellitus has been an important risk factor from previous reports. Thus efforts at prevention of DSWI in such patients are of utmost importance. The work done by Furnary et al. [21] in patients with diabetes in which a continuous insulin infusion was used in such patients, the conclusion was that perioperative adoption of that method significantly reduced DSWI [20]. Also, the harvesting of ITA in diabetes, it was observed that the risk of DSWI was minimized in diabetic patients undergoing coronary artery bypass graft surgery by harvesting the ITA in a skeletonized fashion instead of pedicled harvest [21].
Another important mark is the technique of closure of the sternum after sternotomy. This is desirable in other to ensure optimal sternal stability. A retrospective-prospective descriptive and comparative study of two sternal closure techniques, divided into: group A, steel band closure and group B, conventional technique closure , a statistically significant difference was found in the frequency of sternal dehiscence between both groups (p=0.022) in favor of group A [22].
In another method of achieving sternal stability, 53 fresh adult human cadaveric sternal plates with adjacent ribs were fixed with specially designed spiked stainless steel clamps and attached to a texture analyzer. Single peristernal and transsternal, alternating single peristernal and transsternal, figure-eight peristernal, figureeight pericostal, and Robicsek closures using no. 5 stainless steel wires were tested. The bone density, stiffness, and displacement using perpendicular, repetitive variable force loads of 800 the single peristernal and alternating peristernal and transsternal closures proved superior in strength and stability (p<0.001). The figure-eight peristernal, then the single transsternal, then the Robicsek were next stablest groups in decreasing order. The figure-eight pericostal closure had the highest failure rate (p<0.001) [23]. Equally in another study, it was demonstrated that the number of wires had no influence on sternal complications in entire patient cohort [24]. Generally, in orthopaedic surgeries, rigid bone fixation is the standard of care for all bone reconstructions except that of sternotomy, thus sternal reconstruction after median sternotomy using rigid fixation with plates when compared with sternal wiring with respect to bone healing and reduction in pain and concluded that Sternal reconstruction using rigid fixation with plates improved bone healing and reduced early postoperative pain compared with sternal wiring [25].

The use of Vacuum Assisted device Closure (VAC)
The use of the VAC therapy as a definitive closure strategy in the management of DSWI and its defect has been recently reported. However, the approach seemed to be mostly beneficial in reducing dependence on regional flaps such as the muscle or musculocutaneous flaps, or at best, may be used as adjunct for definitive wound closure with flap [4,32]. It was stated as been effective in taking care of DSWI but the mechanism on how it could be achieved was not well elucidated [33]. The therapy was employed to aid wound healing in different areas of the body and equally in sternotomy wounds. The treatment of the sternum in postoperative DSWI enhances sternal preservation and increases the rate of possible rewiring. In using this modality, two separate layers of polyurethane foam are placed to fit between the sternal edges and subcutaneous area with a continuous negative pressure of about 125 mm Hg applied [34]. Prolonged use of the VAC system as a replacement for surgical closure of sternal defect wound appeared to be, viciously, associated with recurrent problems of the sternal wound infection and thus strategic use of it for a short duration followed by early surgical closure is advocated. Furthermore, VAC therapy required specialist supervision by Clinicians with experience and requisite training on the use of the technique [4,[27][28][29][30].
In a recent report, 34 patients who had VAC therapy for sternal wound infection following cardiac surgery showed that the overall length of hospitalization was 34.6 days (range 9 to 62) which was prolonged and the therapy was solely used as a bridge to definite wound closure [35]. A group of Experts developed clinical guidelines on the practical application of VAC therapy in DSWI; the consensus reached at was that VAC therapy should be instituted early with specialist surgical supervision by clinicians with adequate experience and training in the use of the technique [29]. Beyond the preclusions on the use of VAC, there are fearful complications of profuse bleeding which may occur [31]. Equally, a meta-analysis conducted showed that VAC therapy resulted in a decrease of 7.18 days in hospital length of stay (confidence interval (CI) 95%: 10.82, 3.54), with no significant impact on mortality with the conclusion that there was a robust evidence of the effectiveness of VAC therapy in the management of DSWI [32]. From the aforementioned, the use of VAC therapy was stated as been mere effective in treatment of DSWI with mechanism not known but better used as an adjunct to flaps and with a potential for profuse bleeding.

The use of pectoralis major muscle flap
The review on the use of pectoralis muscle flap (PMF) showed that it was an easy technique, promoted wound healing and covered all of the sternal wound defects without tension and without requiring additional flaps; it produced minimal growth and developmental problems, and may not preclude resternotomy [36,37]. Although the use of muscle flaps was not free of complications, it was an effective and reliable method for the management of patients with complicated cardiothoracic problems [38,39]. That was similarly experiencen in the use of PMF in the treatment of neonates with sternal wound infection, and stated that though the wounds healed successfully but there was the fear on the potential for the disturbance of future breast development especially in female patients. Furthermore the delay in the use of PMF because of myocardiac compression consequent upon postoperative myocardial oedema and small chest cavity in the neonate were all stated as notably drawbacks [6,35]. And in small infants the pectoralis muscle can be thin and inadequate for the cover of large sternal defects [32]. There have also been flap dehiscence, persistent infection and even mortality with the use of PMF, and the unalloyed fear of reoperation for repeated cardiac surgeries in the patients [30] in contrary to an previously stated as non preclusion for resternotomy [36]. However its use with subsequent quality of life assessments of those patients showed no significant limitations but a disturbed sleep pattern with mild restriction of executing hobbies and social activities [31]. In another review, the experience with the treated 54 patients with complicated cardiothoracic problems by reconstruction with pedicled myocutaneous or muscle flaps showed that chest wall with multiple scars from previous procedures, limited the use of the pectoralis muscle; the use of PMF does not require skin grafting and the combination of immunocompetent bulky muscle tissue used to obliterate the sternal cavity [40].
The major advantage in the use of PMF is in its bulky size that allows for the cover of defect following sternectomy. However, it also had serious draw back in neonates and infants as the compressive effect on the heart and the distortion of the mammary line and its preclusion in patients with previous radionecrosis and the divergent opinions for further resternontomy.

The use of omental flap
In a heterotopic location, the omentum still could continue its peritoneal role and thus will be useful in the treatment of cavitationally infected as well as broken-down irradiated wounds [7]. The various uses of omental flap were for suppurative and radionecrotic wound; to complement muscle flap, or to cover chest wall prosthesis, and when used to treat post-sternotomy mediastinitis after cardiac surgery as, in this situation, the omental flap improves control of infection and prognosis of the patient [7,8,35]. The use of the omental flap was noted as having a superior impact in situations that requires tackling local sepsis especially in diabetes who are prone to such. One study noted a higher failure rate especially in diabetes when muscle flap was use, which subsequently necessitated the use of omental flap in such patients and another; reported a high mortality rate of 14% in patients in whom pectoralis major muscle flap was used in treatment and also stated that raising of pectoralis muscle flap in radionecrosis can be difficult if not impossible [7]. In a 3-year review of 25 patients who underwent transposition of the greater omentum, either alone or in combination with muscle flaps, for treatment of recalcitrant DSWI with most of the patients undergoing radical sternectomy for extensive sternal wounds; the outcome was good when omental flap was used [7]. The advantage of using omentum comes to fore again even when the need for reoperation in a patient who had previous DSWI and had omental flap done. Even in complex cardiac surgery e.g. cardiac retransplantation, absence of significant thick scar and calcification within the retrosternal space may be a good indicator that the procedure can be safely performed without injury to underlying structure [32]. Also, in a different account, it was observed that omental flap can be used in the management of sternal wound infection even after heart transplant and there was no fear of it contraindicating subsequent reoperations in these patients [41,42]. There was the fear of infection spreading to the abdomen with resulting peritonitis with the use of omental flap in sternal wound infection, but this is quite rare [43,44]. There may also be gastric ileus after surgery but it resolves with time also the complication of epigastric hernia [32] is not a too much of a threat than the possibility of tube in VAC eroding into the heart or the necrosis of the medial portion pectoralis major flap or the overlying skin failure [33,45]. A systemic review using 333 citations which focused on publications from single institutions with experience with both types of flap in the treatment of DSWI covering 1046 patients, the conclusion was that the weight of the evidence was insufficient to prove the superiority of reconstruction with Muscle flap to a laparotomy-harvested, over omental flap in the treatment of DSWI and also suggested that use of the omentum may be associated with lower mortality and fewer complications [46]. The limitation to the use of omental flap in patients severe low cardiac output or malnutrition as it may compressing the heart and have attenuated immunogenic properties [46]. The use of omental flap was precluded in patients who may be having peritoneal dialysis and the use of pectoralis major was a difficult technique in neonates and the haemodynamic conditions were poor in the cases they reviewed [30]. An important but rare complication is the transverse colon herniation into the anterior mediastinum that required emergency exploration and colon resection as reported by Halldorsson et al. [45] in a 59-year-old man who developed a DSWI after an emergency cardiac surgery. Omental flap transposition was used to cover the sternal defect and several days later, the patient developed a transverse colon herniation into the anterior mediastinum that required emergency exploration and colon resection. In recent reviews; findings in the treatment of High risk patients such as diabetic or Hypertensive; Stump et al. [9] observed that diabetic patients who were treated with omental flap were 5.4 times less likely to require reoperation for sternal wound management than were patients in whom PMF were used for treatment [8]. The authors more recent findings furthermore showed that the omental flap offered good surgical outcome when used in a diabetic and hypertensive patient with suppurating wound and thereby concluded that the use of omental flap was an effective surgical option in dealing with this condition.
The omental flap has the advantage of provision of cover as may be occasioned by deep defect; in extensive sepsis because of the immunogenic functions and does not lead to deformity but serious complications as the herniation of the transverse colon will severe as an important drawback to its use.

Conclusion
This present review gives the omental flap a slight superior edge over the other two when considering the cover for defect; taking care of local sepsis and resternotomy. However, what is needed now is a multicenter randomized study to determine the option that best suit this condition.