The Internal Thoracic Vessels and their use in Free Flap Reconstruction

Anatomy of the Internal Thoracic Vessels

Internal Thoracic Artery

The internal thoracic artery is the second major branch of the subclavian artery. It arises distal to the origin of vertebral artery and proximal to the thyrocervical trunk.

After its origin, the internal thoracic artery descends inferiorly within the anterior mediastinum of the thorax, approximately 1-2cm lateral to the sternal border. It travels in a plane deep to the innermost intercostal muscle, but superficial to the transverse thoracis and pleura of the lungs.

As it descends, several branches arise from the internal thoracic artery:

• Anterior and posterior intercostal arteries
• Perforating branches
• Pericardiophrenic artery

These contribute to the arterial supply of structures of the anterior chest – breast, sternum, thymus gland, and well as the chest wall itself.

The internal thoracic artery terminates by bifurcating into terminal branches at the sixth intercostal space – the superior epigastric artery and the musculophrenic artery.

Internal Thoracic Vein(s)

The internal thoracic vein accompanies the internal thoracic artery along its course. It receives tributaries from intercostal and perforating veins.

At its distal course, most individuals have two pairs of internal thoracic veins. These run either side of the internal thoracic artery as vena comitantes. These unite to form a single vein (which usually lies medial to the artery) before emptying into the brachiocephalic vein. The two veins unite higher on the left (commonly 2^nd costal cartilage) than the right (3^rd costal cartilage).

Surgical Access to the Internal Thoracic Vessels in Breast Reconstruction

The internal thoracic vessels are commonly used as recipient vessels in free flap reconstruction of the breast. They offer several advantages:

• Reliable anatomy between individuals
• Good size match for free flap vessels
• Easily accessible with minimal morbidity
• Close to the recipient bed for the flap

To be used as recipient vessels, the artery and vein(s) must be accessed and dissected clear of the surrounding tissues. To do this, a window is made in the anterior chest wall, typically at the level of the 2^nd intercostal space.

It is crucial to have a strong understanding of the layers of the chest wall to avoid damaging both the vessels themselves and the underlying pleura. The layers from superficial to deep are:

• Skin, subcutaneous fat and breast tissue
• Pectoralis major fascia and muscle (pectoralis minor originates from ribs 3-5 more laterally so is not present at this level)
• Intercostal fascia and intercostal muscles:
  • External intercostal – fibres orientated in a superolateral to inferomedial direction anteriorly.
  • Internal intercostal – fibres orientated in an inferolateral to superomedial direction anteriorly.
  • Innermost intercostal – fibres orientated in the same direction as the internal intercostals in an inferolateral to superomedial direction anteriorly.
  • Of note the external intercostal can be very thin and tendonous anteriorly as can the innermost intercostal
• Perivascular fat containing the internal thoracic vessels – encased by the thin layer of innermost intercostal anteriorly and a thin layer of medial attachment of transversus thoracis posteriorly
• Transversus thoracis
• Endothoracic fascia and parietal pleura

Rib-Sacrificing and Rib-Sparing Approaches

There are two different surgical approaches for accessing the internal thoracic vessels: rib-sacrificing or rib sparing:

In the rib-sacrificing approach, the pectoralis major fibres are separated. Part of the third costal cartilage is resected and the intercostal muscles above and below the resected costal cartilage are removed. This has the advantage of providing excellent exposure to the vessels, but at the cost of increased post-operative pain and higher risk of pneumothorax.

In contrast, in the rib-sparing approach, the costal cartilage is left intact, and the intercostal muscles are resected from the 2^nd/3^rd intercostal space. This provides less space to perform the anastomosis but is associated with less post operative pain.

Anterograde and Retrograde Flow

The terms anterograde and retrograde refer to the direction of blood flow into and out of the free flap.

In anterograde flow, the arterial blood flows through the subclavian artery, into the internal thoracic artery and into the free flap via an anastomosis with the flap vessels. Venous return is into the brachiocephalic vein via the internal thoracic vein.

In contrast, the path of arterial and venous supply is much longer in retrograde flow. Arterial blood flows through the superior epigastric artery and into the distal portion of the internal thoracic artery. Venous drainage is into the superior epigastric vein via the internal thoracic vein.

Surgical Pitfalls

When considering the anatomy of the internal thoracic vessels, there are several pitfalls that can occur when accessing and preparing the vessels:

Incising the Pectoralis Major Too High:

• The pectoralis major should be divided at the level of the 3rd costal cartilage.
• If the incision is placed too high, the pedicle may become kinked by the upper free edge of the pectoralis muscle, potentially compromising blood flow.

Incorrect Placement of the Intercostal “Window”:

• The internal thoracic vessels run 1–2 cm lateral to the sternal border.
• When creating the 2–3 cm window in the intercostal muscles, ensure it is not placed too laterally.
• If the window is too far from the sternum, the perivascular fat layer beneath the innermost intercostal muscle is absent, increasing the risk of pleural injury.

Injuring Small Blood Vessels:

• Small branches, such as sternal vessels and intercostal arteries, arise from the internal thoracic vessels at each intercostal space.
• Damage to these vessels can result in bleeding that is challenging to control, especially as they pass beneath the sternum or ribs.