In 1957, the introduction of the median sternotomy to allow access to intrathoracic organs by Julian et al revolutionized the field of thoracic surgery. Since this landmark introduction, sternal wound infection and dehiscence have been reported to occur in approximately 0.5-8.4% of cases. Sternal dehiscence is the process of separation of the bony sternum, which often is accompanied by mediastinitis (infection of the deep soft tissues). In thoracic and trunk reconstruction, plastic surgeons play a crucial role in addressing wound healing issues and reconstructive techniques of the chest wall. The management of chest wall reconstruction from sternal dehiscence has evolved considerably over the past half century.
History of the Procedure
After introduction of the midline sternotomy into clinical practice, sternal infection rates began to rise; this was directly associated with high complication rates. Initially, sternal dehiscence was treated conservatively with open drainage and debridement with packing. Complications included graft exposure, desiccation of wound margins, osteomyelitis, and, ultimately, death. These complications often led to significant morbidity, with reported mortality rates exceeding 50%. Shumaker and Mandelbaum introduced the concept of closed management with catheter-antibiotic irrigation in 1963.  This reduced mortality rates from 50% to 20% overall. Despite this dramatic reduction, catheter-induced erosion of major vessels and resultant fatal hemorrhage was a major risk. Thus, incentive to improve methods of wound care continued.
One of the most common reasons for mediastinal infections, and ultimate sternal dehiscence, is sternal instability. Following a sternotomy, the structural integrity of the sternum is significantly compromised. This explains the importance for the surgeon to take time and use meticulous technique when performing a sternotomy. If a midline sternotomy is not placed properly, sternal instability is almost certain to follow, and the patient is at a much higher risk for mediastinitis.
The management of sternal wounds
Managing infected sternal wounds changed with the introduction of the principles of wide debridement and muscle and myocutaneous flap transposition. This management strategy of sternal wound infections was instituted in 1976 when Lee et al introduced the concept of the flaps to reduce dead space in the anterior mediastinum by using the greater omental flap. [2, 3] In 1980, Jurkiewicz et al introduced the concept of muscle and myocutaneous flaps, which dramatically improved effectiveness of management of sternal dehiscence and infection.  Mathes also contributed with the concept of using muscle flaps in wounds with osteomyelitis. 
The use of vascularized regional tissue allowed for greater blood flow, obliteration of dead space, and faster healing time directly from quicker infection resolution. Since the introduction of the omental flap, several other flaps have been introduced to repair chest wall defects. The institution of flaps have led to a decrease in mortality rate of 10%.
Despite these advances, sternal infection and mediastinitis continue to pose clinical management issues. Currently, management of sternal wounds involves a multidisciplinary approach. Time-sensitive, nonsurgical management techniques include early debridement, microbiological analysis, and broad-spectrum antibiotics. Soft tissue flaps do not address repair of the bony sternum, which can lead to chronic pain, paradoxical motion, impaired pulmonary function testing, and cosmetic disapproval from the patient. A relevant development to sternal wound issues has been the development of vacuum-assisted closure devices (VAC) by Argenta and Morykwas, which serve as a bridge between debridement and reconstruction. 
A broad range of surgical strategies are currently being used including myo/myocutaneous flaps. In order to address the issue of bony sternal repair, several institutions are now turning toward sternal rewiring and reconstruction with rigid sternal fixation using transverse and longitudinal plates. [7, 8, 9, 10, 11, 12, 13, 14] Additionally, a sternal clamp device, to reduce sternal instability, has now been introduced into the market with promising results, although long-term clinical data are not yet available.
Problems associated with sternal dehiscence include issues both before and after debridement. Predebridement issues include frank infection and bony destruction of the sternum. Postdebridement issues include possibly sternal instability, paradoxical motion, and pain.
Current literature reflects that anywhere from 0.5-8.4% of median sternotomy incisions are complicated with wound infections, leading to sternal dehiscence. [8, 11, 12] With the introduction of flaps to cover sternal defects, mortality rate from sternal wound dehiscence is approximately 10%. [8, 11, 12] Long-term mortality studies following rigid fixation have yet to be reported.
Factors associated with sternal wound dehiscence
Many mechanisms have been proposed to explain the development of sternal wound infection and dehiscence. These theories include inadequate sternal fixation leading to instability and dehiscence of the overlying skin incision and inadequate surgical drainage. Further theories suggest a localized ischemic osteomyelitis. This theory suggests that sternal wires become loose, leading to sternal instability, which ultimately leads to skin dehiscence and osteomyelitic infection. The most commonly cultured organism is Staphylococcus Aureus.
Several retrospective and prospective studies have identified factors relating to increasing risk of sternal dehiscence. Patient risk factors include obesity, diabetes mellitus, chronic obstructive lung disease (COPD), chronic cough from tobacco abuse, steroid therapy, hypertension, immunosuppression, and advanced age. Other risk factors, as reported in a study by Fu et al, include congestive heart failure and respiratory failure. 
Operative risk factors include single or bilateral internal mammary artery (IMA) harvesting (significantly decreases blood supply to ipsilateral hemithorax), prolonged operation, excessive hemorrhage, reoperation, break in sterile technique, and the use of an intra-aortic balloon pump. The Fu study also identified the presence of two or more arterial conduits as a risk factor. 
Wound dehiscence is the partial or complete separation of the layers of the incision. A compromise in factors responsible for wound healing can lead to wound dehiscence. These factors include inadequate nutrition, compromised circulation, and surgical factors. Surgical factors include placing sutures under excessive tension or insufficient tension. Circulatory factors include the presence of diabetes, coagulopathies, or other vascular-related issues. Adequate nutritional intake is also required for proper wound healing.
Factors associated with mortality include septicemia, perioperative myocardial infarction, and use of the intra-aortic balloon pump. Strict aseptic technique, attention to hemostasis, and precise motionless sternal approximation are advocated to prevent mediastinitis.
In the clinical evaluation of suspected mediastinitis or sternal dehiscence, careful repeated examination of the patient is warranted. If the patient has multiple risk factors for dehiscence or impairment to wound healing, he or she must be examined at closer intervals. Findings of erythema, fever, tachycardia, increased leukocyte count, purulent discharge, and sternal instability are clinical indicators of sternal dehiscence.
If clinical deterioration of the patient or further signs of breakdown are observed (ie, increased erythema, drainage, separation of incision), immediately obtain wound cultures, administer broad-spectrum antibiotics, and perform swift aggressive debridement. Follow with either the vacuum-assisted closure (VAC) device (to serve as a bridge to reconstruction) or reconstruction with flap coverage or rigid sternal plating. This combination can reduce the incidence of mortality, decrease hospital stay, rapidly propel the patient's recovery from thoracic surgery, and avert the severe complications of mediastinitis.
Pairolero and Arnold have based their classification of sternal wounds on timing of presentation of infection; this classification divides wounds into 3 categories.  This classification system does not indicate the type of reconstruction necessary for management of each type of sternal wound. Type II and III wounds are typically referred to plastic surgeons for reconstruction.
Type I: Type I wounds occur in the first few days postoperatively, contain early wound separation with or without sternal instability, and are characterized by serosanguineous drainage without cellulitis, osteomyelitis, or costochondritis.
Type II: Type II wounds occur within the first few weeks and are characterized by drainage, cellulitis, mediastinal suppuration, and positive cultures. Type II wounds are characterized by fulminant mediastinitis.
Type III: Type III wounds occur months to years after surgery and are characterized by the presence of chronic draining sinus tracts, localized cellulites, osteomyelitis, or retained foreign bodies. Mediastinitis is a rare complication of type III wounds.
The main indications for sternal reconstruction are sternal instability with dehiscence, early or subacute infections, and repair after tumor resection.
The chest wall skeleton consists of the sternum in the anterior midline, bilateral clavicles, 12 pairs of ribs, with the eleventh and twelfth not associated with costal cartilage. The first seven ribs are "true" ribs, and the last 5 are "false" ribs. The eighth, ninth, and tenth ribs attach to the sternum via costal cartilage. The eleventh and twelfth ribs articulate posteriorly with the vertebrae. Anterior chest wall muscles allow for movement of the extremities. The elasticity of the chest wall supports the mechanics of ventilation.
The paired internal thoracic arteries and the deep epigastric arteries provide the main blood supply to the ventral aspect of the chest. This system connects the major vessels of the neck to those in the groin. Many flaps are based on understanding this vascular supply. Collateral blood supply from the acromiothoracic axis is also important to recognize.
The relevant muscles and structures utilized for sternum reconstruction are the pectoralis major, rectus abdominus, latissimus dorsi, and greater omentum. All except the latissimus dorsi can be harvested in the supine position; the latissimus dorsi should be harvested in the lateral decubitus position. The blood supplies for each flap are well-established and described under each individual section.
Understanding the advantages and disadvantages of all reconstructive options is important. For example, the pedicled latissimus flap is a great flap for lateral chest wall defects but often is inadequate to cover large midline sternal defects.
The blood supplies for each flap are well-established, as shown below.
Wounds with active purulence require extensive debridement prior to flap coverage and/or rigid fixation.
Additional contraindications for sternal reconstruction are found in patients who are unstable for surgery, including those with poor pulmonary function, poor cardiac reserve, or terminal illness. Hemodynamic stability is required for surgical intervention in patients with sternal dehiscence.
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