Pectoralis major

The pectoralis major is a large, fan-shaped muscle located in the anterior chest wall. It plays a crucial role in movements of the shoulder and upper limb, contributing to adduction, medial rotation, and flexion of the humerus. Its clinical importance extends to reconstructive surgery and sports medicine due to its functional and anatomical significance.

Anatomy

Origin

The pectoralis major has multiple heads that originate from different regions of the thoracic skeleton and associated structures.

• Clavicular Head: Originates from the anterior surface of the medial half of the clavicle.
• Sternocostal Head: Arises from the anterior surface of the sternum and the costal cartilages of the first six ribs.
• Abdominal Portion: Originates from the aponeurosis of the external oblique muscle.

Insertion

The fibers converge to insert on the lateral lip of the bicipital groove of the humerus, forming a tendon that is closely associated with the deltoid and other shoulder muscles.

• Provides leverage for upper limb movements including adduction, medial rotation, and flexion.
• Interacts with adjacent muscles such as the deltoid and pectoralis minor for coordinated shoulder function.

Structure and Fiber Orientation

The muscle has a distinct arrangement of fibers that influences its biomechanical function.

• Superficial and deep fiber layers contribute to different movements of the humerus.
• Pennate arrangement allows for strong adduction and medial rotation.
• Orientation of fibers from clavicular and sternocostal heads provides versatility in movement and strength distribution.

Blood Supply

• Pectoral branches of the thoracoacromial artery supply the majority of the muscle.
• Lateral thoracic artery contributes to the inferior and lateral regions.
• Minor contributions from intercostal perforators enhance vascularization.

Innervation

• Medial pectoral nerve innervates the sternocostal portion and abdominal fibers.
• Lateral pectoral nerve primarily innervates the clavicular head.
• Coordination between these nerves allows precise control of shoulder and upper limb movements.

Functions

The pectoralis major is a powerful muscle responsible for a range of movements at the shoulder joint. Its actions depend on the specific head and fiber orientation involved.

• Adduction of the Humerus: Brings the arm toward the body midline.
• Medial Rotation of the Humerus: Rotates the arm inward, contributing to activities such as throwing or pushing.
• Flexion of the Humerus (Clavicular Head): Raises the arm forward from the anatomical position.
• Assisting in Respiration: Acts as an accessory muscle during forced inspiration when the shoulder girdle is fixed.

Biomechanics

The pectoralis major works in concert with other muscles of the shoulder and upper limb to facilitate complex movements. Its biomechanical role is critical for functional strength and coordination.

• Role in Shoulder Joint Movements: Provides powerful adduction, flexion, and medial rotation necessary for pushing, lifting, and throwing motions.
• Interaction with Other Muscles: Works synergistically with the deltoid, latissimus dorsi, and rotator cuff muscles for dynamic stability and movement control.
• Contribution to Upper Limb Strength: Essential for activities requiring forceful upper limb actions, such as weightlifting and climbing.

Clinical Significance

Injuries

The pectoralis major is prone to various injuries, especially in athletes and individuals engaging in high-intensity upper body activities.

• Muscle Strain and Tears: Partial or complete tears often occur during weightlifting or contact sports.
• Avulsion Injuries: Detachment of the tendon from the humerus, commonly associated with bench press or overhead activities.
• Mechanisms of Injury: Sudden eccentric loading or overstretching of the muscle fibers is the primary cause.

Deformities and Variations

Congenital or acquired variations can impact muscle function and appearance.

• Congenital Absence or Hypoplasia: Rare conditions affecting muscle size or function, sometimes associated with syndromes.
• Poland Syndrome: Characterized by unilateral absence or underdevelopment of the pectoralis major and minor.
• Anatomical Variants: Variations in origin, insertion, or muscle fibers can influence surgical planning and injury risk.

Surgical Considerations

The pectoralis major is frequently utilized in reconstructive and orthopedic procedures due to its size and accessibility.

• Muscle Flaps in Reconstructive Surgery: Used for chest wall, breast, or head and neck reconstruction.
• Tendon Repair Techniques: Surgical repair of tears or avulsions restores function and strength.
• Implications for Orthopedic and Plastic Surgery: Knowledge of vascular supply and innervation is critical for successful surgical outcomes.

Imaging and Diagnosis

Accurate imaging is essential for diagnosing pectoralis major injuries and planning appropriate management.

• MRI: Gold standard for assessing muscle tears, tendon injuries, and extent of fiber disruption.
• Ultrasound Evaluation: Useful for dynamic assessment and guiding interventions in superficial injuries.
• X-ray and CT: Limited role but may identify associated bony injuries or avulsion fractures.

Rehabilitation and Physical Therapy

Rehabilitation of the pectoralis major is essential after injury or surgery to restore strength, flexibility, and functional capacity. A structured physiotherapy program facilitates recovery and prevents re-injury.

• Post-Injury Rehabilitation Protocols: Gradual progression from passive range of motion to active exercises to promote healing.
• Strengthening Exercises: Focus on isotonic and resistance training to rebuild muscle mass and power.
• Stretching Exercises: Maintain flexibility of the muscle and surrounding structures to prevent contractures.
• Prevention of Recurrence: Emphasis on proper warm-up, technique during sports, and ergonomic modifications.

References

1. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Netter FH. Atlas of Human Anatomy. 7th ed. Philadelphia: Elsevier; 2019.
3. Peterson P, et al. Pectoralis Major Muscle Injuries: Diagnosis and Management. J Shoulder Elbow Surg. 2012;21(3):412-420.
4. Bak K, Cameron EA. Rupture of the Pectoralis Major Muscle: Surgical Repair and Outcomes. Am J Sports Med. 2000;28(1):53-62.
5. Galano GJ, et al. Pectoralis Major Ruptures: A Review of the Literature. Orthop Clin North Am. 2007;38(4):507-518.
6. Kretzler B, et al. Anatomy and Variations of the Pectoralis Major Muscle: Implications for Surgery. Clin Anat. 2010;23(8):907-913.
7. Beason DP, et al. Rehabilitation After Pectoralis Major Repair. J Orthop Sports Phys Ther. 2013;43(3):173-180.
8. Moorman CT, et al. Pectoralis Major Tendon Injuries in Athletes. Sports Med Arthrosc. 2005;13(3):165-172.