Clinical Comprehensive Guide: Sternal Dehiscence

1. Comprehensive Introduction & Overview

Sternal dehiscence, often clinically categorized under the broader umbrella of Deep Sternal Wound Infection (DSWI) or sternal instability, represents one of the most feared complications following a median sternotomy. A median sternotomy is the gold-standard surgical approach for cardiac procedures, including coronary artery bypass grafting (CABG) and valve replacements. Despite advancements in surgical technique, sternal dehiscence remains a significant source of morbidity, increased hospital length-of-stay, and mortality.

Clinically, sternal dehiscence is defined as the separation of the sternal edges following median sternotomy, which may occur with or without accompanying infection. When the sternum fails to achieve primary union—due to mechanical failure of the fixation, biological non-union, or infectious degradation—the result is a compromised chest wall, respiratory instability, and a high risk of mediastinitis.

2. Pathophysiology and Etiology

The pathophysiology of sternal dehiscence is multifactorial, involving an interplay between mechanical strain, patient-specific risk factors, and biological healing capacity.

The Mechanics of Sternal Healing

Under normal conditions, a sternotomy heals through primary intention. However, the sternum is subject to significant mechanical forces during respiration, coughing, and limb movement. If the fixation (typically stainless steel wires) fails to maintain strict apposition, micromotion occurs, preventing osteoblastic activity and promoting fibrous tissue formation instead of bone.

Etiological Factors

The etiology can be classified into three primary domains:

Pathophysiological Progression

1. Mechanical Instability: Initial failure of the fixation device leads to sternal "clicking" or "rocking."
2. Inflammatory Response: The lack of stability triggers a chronic inflammatory state, attracting inflammatory cells that may further degrade the local tissue.
3. Infection (Secondary): If the skin barrier is breached, the hematoma or dead space created by the dehiscence acts as a nidus for bacterial colonization (e.g., Staphylococcus aureus, Staphylococcus epidermidis).
4. Mediastinitis: The progression to deep infection of the retrosternal space, which can lead to osteomyelitis and systemic sepsis.

3. Clinical Staging and Grading

To standardize care, clinicians use the Oakley and Wright or similar classification systems to assess the severity of sternal wound issues.

Clinical Staging Table

4. Standard Presentation and Differential Diagnosis

Standard Presentation

• Audible/Palpable Clicking: Often the first complaint by the patient during movement or deep inspiration.
• Localized Pain: Sharp, non-pleuritic pain localized to the sternum.
• Wound Drainage: Presence of serosanguinous, purulent, or clear fluid from the incision line.
• Respiratory Distress: In severe cases, the paradoxical movement of the chest wall leads to reduced tidal volume and hypoxia.

Differential Diagnosis

It is critical to distinguish sternal dehiscence from other post-sternotomy conditions:
* Superficial Surgical Site Infection (SSSI): Infection limited to the subcutaneous tissue; the sternum remains stable.
* Costochondritis: Inflammation of the costochondral junctions, mimicking sternal pain but lacking mechanical instability.
* Sternal Non-Union: A biological failure of the bone to knit, which may occur without acute dehiscence.
* Pectoral Myositis: Often confused for deep infection, usually responds to conservative management.

5. Key Diagnostic Tests

Diagnostic accuracy is paramount to determining the necessity of surgical intervention versus conservative management.

1. Physical Examination: The "Rocking Test" or "Sternal Click" test is the gold standard. The clinician places hands on both sides of the sternum and applies gentle lateral pressure.
2. Computed Tomography (CT) Scan: The modality of choice. It provides high-resolution imaging of the sternal gap, the presence of retrosternal fluid collections (abscesses), and the integrity of the wires.
3. Microbiological Culture: Deep tissue biopsies (not superficial swabs) are required to identify causative pathogens in cases of suspected infection.
4. Blood Markers: Elevated CRP and WBC counts are non-specific but support the diagnosis of an infectious component.

6. Clinical Indications and Management Strategies

Management is contingent upon the presence or absence of infection (mediastinitis).

Non-Infected Sternal Dehiscence

If the sternum is stable but has minor separation, conservative management may include:
* Sternal Support Vests: Rigid bracing to minimize lateral motion.
* Activity Restriction: Avoidance of lifting >5 lbs and "no-reaching" protocols.

Infected Sternal Dehiscence (Mediastinitis)

This is a surgical emergency requiring:
* Surgical Debridement: Aggressive removal of necrotic bone, infected wires, and soft tissue.
* Vacuum-Assisted Closure (VAC) Therapy: The application of negative pressure wound therapy to promote granulation and clear infection.
* Flap Reconstruction: Pectoralis major muscle flaps or omental flaps are often utilized to fill the dead space once the infection is controlled.

7. Risks, Side Effects, and Contraindications

Risks of Intervention

• Surgical Revision: High risk of further bone trauma and recurrent dehiscence.
• Anesthesia Risks: Given the patient population (often cardiac patients), there is a significant risk of hemodynamic instability during revision surgery.
• Failure of Flap: Potential for necrosis of the transposed muscle tissue.

Contraindications to Primary Re-wiring

• Severe Osteomyelitis: If the sternal bone is too friable or necrotic, primary re-wiring is contraindicated as the wires will simply cut through the bone (the "cheese-cutter" effect).
• Active Sepsis: Patient must be hemodynamically stabilized before undergoing major chest wall reconstruction.

8. Long-Term Prognosis

The prognosis for patients with sternal dehiscence is variable.
* Uncomplicated Dehiscence: Patients generally achieve full stability with secondary fixation or strict immobilization within 6–12 weeks.
* Infected Dehiscence (Mediastinitis): Associated with a mortality rate ranging from 10% to 30%. Long-term survivors may suffer from chronic chest wall pain, restrictive pulmonary disease, and psychological distress related to the chronic wound.

9. Frequently Asked Questions (FAQ)

1. What is the difference between Sternal Dehiscence and Mediastinitis?

Sternal dehiscence is the mechanical separation of the sternum. Mediastinitis is a deep infection of the mediastinal space. They often co-occur, but dehiscence can exist without infection.

2. Can sternal dehiscence heal on its own?

Minor, non-infected dehiscence can sometimes heal with strict immobilization, but if the gap is significant, surgical intervention is usually required to prevent long-term non-union.

3. Why is diabetes a major risk factor?

Diabetes causes microvascular disease, which impairs the delivery of oxygen and nutrients to the sternal edges, significantly slowing down osteoblastic bone formation.

4. What is the "Sternal Click"?

It is the audible or palpable sound of the two sternal halves shifting against each other when the patient breathes, coughs, or moves. It is a pathognomonic sign of dehiscence.

5. How long does the sternum take to heal?

Full osseous union typically takes 6 to 12 weeks, though the initial "soft" stability occurs much sooner.

6. Is VAC therapy always necessary?

VAC therapy has revolutionized the treatment of infected sternal wounds by cleaning the wound bed and reducing the size of the defect before definitive closure. It is not always necessary for clean, non-infected dehiscence.

7. Can I exercise with a sternal dehiscence?

No. Exercise is strictly contraindicated. Any activity that puts tension on the sternum can worsen the separation.

8. What are the signs of a deep infection?

Fever, persistent redness, warmth, foul-smelling drainage, and systemic malaise are classic signs of a deep infection (mediastinitis).

9. Will I need plastic surgery?

If the sternum is significantly infected or the soft tissue is deficient, a plastic surgeon is often consulted to perform a muscle flap to provide healthy, vascularized tissue to the area.

10. How can I prevent dehiscence after surgery?

Adherence to the "sternal precautions" (no lifting, no pushing, no pulling), maintaining blood sugar control, and smoking cessation are the three most effective prevention strategies.

10. Conclusion for Clinical Practitioners

Sternal dehiscence is a high-stakes clinical scenario that requires a multidisciplinary approach. Surgeons, infectious disease specialists, and wound care nurses must work in concert to manage both the mechanical instability and any underlying infectious processes. Early identification through vigilant post-operative monitoring—specifically looking for the "click"—can mean the difference between a simple revision and a protracted course of mediastinitis. By adhering to standardized protocols for wound management and patient education, the incidence of this complication can be minimized, and the quality of life for the post-cardiac surgery patient can be vastly improved.