Sternal Debridement and Application of Muscle Flaps

Sean M. Fisher

Jeff J. Kim

David H. Song

DEFINITION

Deep sternal wound infections (DSWIs) are a feared postoperative complication of open heart surgery, with a reported global incidence of 0.25% to 2.8%.¹
- Sternal infections may be classified based on the system popularized by Pairolero and Arnold¹^,²^,³:
  - Type I: Demonstrate serosanguineous drainage without evidence of deep soft tissue, bone, or cartilage involvement.
  - Type II: Demonstrate purulent drainage, cellulitis, and mediastinal suppuration.
  - Type III: Osteomyelitis, costochondritis, and/or foreign bodies are commonly present.
- Alternative classification systems are based on the extent of damage to underlying tissue or relative anatomic location of the wound.²
- Sternal necrosis can result in critical-size defects in the thoracic wall with exposure of underlying thoracic structures, necessitating the involvement of reconstructive surgeons.
Sternal nonunion is defined as pain or clicking with objective measures of instability lasting greater than 3 months, albeit in the absence of signs of infection.⁴

ANATOMY

The thoracic cage has mechanical and protective properties, serving as an attachment point for a number of chest wall muscles and acting as a key protective barrier for thoracic organs.
The sternum acts as a cornerstone of the thoracic cage and is composed of the manubrium, the body, and the xiphoid process.
- Manubrium
  - Superior most aspect of the sternum, helping anchor the pectoral girdle via the sternoclavicular joints, and as the anterior aspect of the superior thoracic aperture (FIG 1A)
  - Sternal angle serves as a critical surface landmark, composed of the manubrium angled posteriorly to the body at the manubriosternal joint.
  - Contains articular surfaces at the superolateral aspect, allowing for articulation with the left and right clavicle
  - Facets inferior to the sternoclavicular joints represent the site of attachment for the first costal cartilage, whereas demifacets at the inferolateral aspect allow for articulation with the superior aspect of the second costal cartilage (FIG 1B).
  - Serves as the site of attachment for the sternocleidomastoid, pectoralis major, sternohyoid, and sternothyroid muscles
- Body
  - Narrow, flat bone that is oriented longitudinally with palpable transverse ridges
  - Contains facets along the lateral aspect, allowing for articulation with the 3rd to 6th costal cartilages
  - Demifacets at the superior and inferior aspect allow for shared articulation between the 2nd and 7th costal cartilages with the adjacent sternal components (see FIG 1B).
  - Serves as the major attachment for the pectoralis major
- Xiphoid process
  - Inferior aspect of the sternum and demonstrates variable morphology
  - Along with the body of the sternum, the xiphoid process articulates with the 7th costal cartilage via superolateral demifacets.
  - Serves as an attachment point for the diaphragm and rectus abdominis

PATHOGENESIS

The primary insult in DSWIs may begin as a site of focal sternal osteomyelitis without external evidence of infection when sternal fixation is inadequate or may be due to skin breakdown with subsequent penetration of bacteria to deeper layers.
Presentation often demonstrates a highly variable time course, ranging from days to years following cardiac surgery.²
In cases requiring surgical debridement, Gram-positive organisms, namely Staphylococcus, have been found to be overwhelmingly responsible for infection.⁵
In addition to male sex, conditions associated with patients’ habitual state have been shown to be risk factors in the development of DSWIs and associated morbidity and mortality.
- Diabetes mellitus (DM), COPD, BMI over 30, chronic steroid use, and smoking are known predictors of DSWI.
- Bilateral internal mammary artery (IMA) grafting for CABG also increases the risk of postoperative DSWI, as it may reduce sternal vascularity, and should be avoided in those patients with DM and other high-risk comorbidities.¹^,²
Sternal nonunion represents a similar, yet separate, diagnosis that may necessitate sternal debridement as a treatment strategy.
Both intrinsic and extrinsic factors may contribute to an increased propensity in developing sternal nonunion following median sternotomy or traumatic injury.
- Factors that are dependent on patients’ habitual state closely mimic those risk factors associated with DSWI.
  - These include DM, COPD, obesity, chronic steroid use, malnutrition, and osteoporosis.⁴
- Intraoperative technical errors, as well as postsurgical factors, may also play a role in the development of sternal nonunion.⁴

FIG 1 • A. The sternum represents an anterior anchor point for the pectoral girdle, attaching via the sternoclavicular joint bilaterally. The superior thoracic aperture, noted in red, houses the trachea, esophagus, and thoracic duct, as well as a variety of nerves and vessels. B. Lateral view of the sternum demonstrates the manubrium, the body, and the xiphoid process with corresponding facets and demifacets for articulation with corresponding costal cartilage.

NATURAL HISTORY

Julian et al. first described the median sternotomy in 1957, which initially met resistance due to reported rates of infection exceeding 5%.¹
As the technique gained traction in open cardiac surgery, and advances were made allowing for surgical intervention in patients with more extensive comorbidities, DSWIs began being described with incidence ranging from 3% to 5%¹^,⁶^,⁷ and associated mortality as high as 50%.¹^,²
Classification of the wound often typifies the time course to presentation¹^,²:
- Type I: Often presents within the first few days following surgery
- Type II: Typically occurs a few weeks following surgery
- Type III: Can occur months to years following surgery
A second classification system proposed by Jones et al. attempts to further qualify the severity of the wound by addressing the extent of involvement in underlying tissues⁸:

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