Post-Cardiac Surgery Sternal Instability

Comprehensive Clinical Guide: Post-Cardiac Surgery Sternal Instability (PCSSI)

1. Introduction and Clinical Overview

Post-Cardiac Surgery Sternal Instability (PCSSI), frequently manifesting as Sternal Dehiscence, represents one of the most feared and morbid complications following median sternotomy. While median sternotomy remains the gold standard approach for cardiac surgical procedures—including Coronary Artery Bypass Grafting (CABG) and valve replacements—the surgical disruption of the sternum introduces significant biomechanical vulnerability.

PCSSI occurs when the rigid fixation of the sternal halves is compromised, leading to abnormal motion at the osteotomy site. This instability ranges from minor, asymptomatic clicking to life-threatening deep sternal wound infection (DSWI) and mediastinitis. The incidence of sternal complications is reported between 0.5% and 5% in the general population, though this rate increases exponentially in high-risk cohorts, such as diabetic, obese, or elderly patients.

This guide provides a clinical framework for the diagnosis, classification, and management of PCSSI, intended for orthopedic surgeons, cardiothoracic specialists, and critical care clinicians.

2. Pathophysiology and Biomechanical Mechanisms

The sternum is a flat bone that functions as a structural anchor for the thoracic cage. During a median sternotomy, the structural integrity is severed, and reliance is placed on mechanical fixation devices (typically stainless steel wires) to maintain apposition until primary bone healing occurs.

The Mechanics of Failure

Failure of the sternal closure mechanism occurs when the forces exerted on the sternal edges exceed the mechanical strength of the fixation or the bone itself.

• Tension Cycles: Respiratory excursions and coughing create cyclical loading on the sternum. If the fixation is insufficient, these forces cause "micromotion."
• Bone Density Factors: Osteopenia or osteoporosis reduces the "pull-out strength" of the wires, allowing them to saw through the cancellous bone (the "cheese-cutter" effect).
• Vascular Compromise: The internal thoracic arteries (ITAs) are critical for the sternum's blood supply. Bilateral ITA harvesting significantly reduces perfusion to the sternal edges, leading to impaired osteoblastic activity and delayed union.

Key Pathophysiological Phases

3. Clinical Staging and Grading

To standardize clinical communication, the following grading system is utilized to categorize the severity of sternal instability.

The Modified Sternal Instability Scale (MSIS)

• Grade 0 (Stable): No palpable movement; no pain.
• Grade 1 (Minor Instability): Minimal, localized clicking; mild discomfort on palpation.
• Grade 2 (Moderate Instability): Palpable "rocking" or "sliding" motion; moderate pain; absence of erythema.
• Grade 3 (Sternal Dehiscence): Gross instability; visible separation of the wound edges; clinical signs of infection (purulence, fever).

4. Clinical Presentation and Diagnostic Evaluation

Standard Presentation

Patients typically present in the early postoperative period (days 5–21) complaining of:
1. Clicking sensations during deep breathing, coughing, or postural changes.
2. Localized pain that is disproportionate to the typical surgical recovery.
3. Wound drainage (serosanguinous or purulent).
4. "Springing" sensation when the patient transitions from supine to sitting.

Diagnostic Modalities

5. Differential Diagnosis

Distinguishing between simple sternal instability and mediastinitis is the most critical clinical decision.

• Sternal Non-Union (Aseptic): Mechanical failure without infection. Usually presents with mechanical clicking but minimal systemic symptoms.
• Mediastinitis: Sternal instability accompanied by systemic inflammatory response (fever, leukocytosis) and deep space infection. This is a surgical emergency.
• Costochondritis: Inflammation of the costal cartilages; typically more lateral than the sternum and non-mechanical in origin.
• Incisional Hernia: Rare, but can involve protrusion of mediastinal fat through a sternal defect.

6. Risks, Side Effects, and Contraindications

Risk Factors for Development

• Metabolic: Diabetes Mellitus (HbA1c > 7.0%), Obesity (BMI > 30), and advanced age.
• Surgical: Bilateral ITA harvest, prolonged cardiopulmonary bypass time, and excessive use of electrocautery.
• Post-operative: Early aggressive physiotherapy without sternal support, chronic obstructive pulmonary disease (COPD) causing excessive coughing.

Contraindications for Conservative Management

If any of the following are present, conservative management (sternal vests) is contraindicated, and surgical debridement is required:
1. Evidence of purulent mediastinal discharge.
2. Positive blood cultures for Staphylococcus aureus or Pseudomonas.
3. Radiographic evidence of sternal sequestration or osteomyelitis.
4. Hemodynamic instability.

7. Management Strategies

Conservative Management

For Grade 1 or 2 instability without infection, management consists of:
* Sternal Support Vests: External bracing to minimize shearing forces.
* Activity Restriction: Avoidance of lifting >5 lbs or overhead reaching for 6–12 weeks.
* Pain Management: Multimodal analgesia to facilitate deep breathing and prevent atelectasis.

Surgical Management

For Grade 3 instability or infected cases:
* Surgical Debridement: Aggressive excision of necrotic bone and infected tissue.
* Rewiring/Plating: Utilizing titanium sternal plating systems to achieve rigid fixation.
* Muscle Flaps: Transposition of pectoralis major or omental flaps to fill dead space and improve vascularity.

8. Frequently Asked Questions (FAQ)

1. How long does it take for the sternum to fully heal?

Bone union typically takes 8 to 12 weeks, though complete remodeling can take up to 6 months.

2. Is "clicking" always a sign of infection?

No. Clicking is a mechanical symptom of instability. It only indicates infection if accompanied by fever, redness, or purulent drainage.

3. What is the role of titanium plating in sternal closure?

Titanium plates provide rigid fixation that is superior to wire cerclage, especially in patients with poor bone quality or those undergoing repeat sternotomy.

4. Can I exercise if I have sternal instability?

No. Physical activity must be strictly limited until the sternum is clinically stable to prevent further dehiscence.

5. What are the signs of mediastinitis I should look for at home?

Look for fever, increased redness around the incision, foul-smelling discharge, or the sensation of the sternum "shifting" when you move.

6. Does smoking affect sternal healing?

Yes. Nicotine is a potent vasoconstrictor that significantly impairs bone healing and increases the risk of wound complications by 3x.

7. What is the "cheese-cutter" effect?

This occurs when the sternal wires cut through the bone, usually due to osteoporosis, leading to progressive sternal instability.

8. How is the diagnosis of sternal instability confirmed?

Diagnosis is primarily clinical via physical examination (palpation) and is often supported by CT imaging to assess the gap between sternal edges.

9. What is the prognosis for someone with PCSSI?

With prompt intervention, most patients achieve complete healing. However, untreated mediastinitis carries a high mortality rate, necessitating aggressive, early management.

10. Can I prevent sternal instability?

Prevention includes strict glucose control, smoking cessation, and the use of "sternal precautions" (no lifting, pushing, or pulling) during the first 6 weeks post-surgery.

9. Long-Term Prognosis and Rehabilitation

The long-term outlook for patients with PCSSI is generally favorable provided the infection is cleared and mechanical stability is restored. Patients who undergo plate fixation often report higher satisfaction scores and faster return to function compared to those managed with wire alone.

Rehabilitation Protocol:
1. Weeks 0–6: Focus on wound care and strict sternal precautions.
2. Weeks 6–12: Gradual introduction of range-of-motion exercises for the shoulders.
3. Months 3+: Progressive resistance training, provided there is radiographic confirmation of bone union.

Conclusion

Post-Cardiac Surgery Sternal Instability is a multifaceted clinical challenge that requires a high index of suspicion. By understanding the biomechanical failures and utilizing objective staging systems, clinicians can effectively differentiate between benign mechanical clicking and life-threatening infection, thereby optimizing patient outcomes and reducing the burden of morbidity in cardiothoracic surgery.

Disclaimer: This guide is intended for educational purposes for healthcare professionals. It does not replace institutional protocols or individual clinical judgment. Always consult with a cardiothoracic surgeon regarding specific patient cases.