Innervation of autologous flaps

Introduction

Post-mastectomy autologous breast reconstruction has evolved significantly since Tanzini described the first latissimus dorsi flap in 1906. The evolution has encompassed both limiting donor site morbidity as well as improvement in flap survival. While the focus of breast reconstruction up to this point has been on breast aesthetics, the next frontier aims to incorporate sensation.

The goal of neurotization of autologous breast reconstruction flaps is to provide protective and tactile sensation. Protective sensation is crucial to prevent thermal injury to the reconstructed breast, such as sunburns, and tactile sensation is essential for restoration of skin sensibility. A more utopian goal is to regain erogenous sensation, which is vital as it plays a role in intimacy and woman’s corporeal imagery. However, erogenous sensation is highly varied, and at the present time, we do not have a clinically useful way to measure it; thus, it may not be a realistic goal at this time. Successful reinnervation is dependent on thorough knowledge of breast neuroanatomy, nerve physiology, and nerve coaptation principles. Neurotization has recently become more popular due to our deeper understanding of peripheral nerve regeneration in breast reconstruction combined with patient demand, and the development of commercially available nerve grafts and conduits and is thus being offered routinely to women undergoing autologous reconstruction in our practice.

Breast sensation anatomy

The nerve supply to the breast comes from three sources: cervical plexus (C3/C4), anteromedial (T2–6), and anterolateral (T3–6) intercostal nerves ( Fig. 39.1 ). Supraclavicular nerves from lower fibers of C3/C4 provide innervation of the breast’s upper and lateral portions but do not contribute to nipple–areolar complex (NAC) sensation. The intercostal nerves originate from the ventral rami of the spinal cord. They travel along the chest wall in a plane between the internal intercostal and the transversus thoracis muscle groups, lying below the intercostal artery and vein along the lower border of the rib. The nerve gives off a lateral cutaneous branch that pierces the intercostal muscles and overlying serratus anterior along the midaxillary line, and divides into an anterior and posterior branch. The main intercostal nerve then continues along the lower border of the rib and terminates as the anterior cutaneous branch 1 cm lateral to the parasternal border. The anterior cutaneous branch splits into a medial and lateral branch. The medial branch supplies the skin over the sternum and does not contribute to breast sensation.

During a mastectomy, most if not all of the nerves supplying the NAC are transected. However, the lateral anterior cutaneous branch of the T4 intercostal nerve typically travels in the subcutaneous tissue and is more superficial. Due to this location, it can be preserved during a nipple-sparing mastectomy, maintaining sensation with an oncologically-sound mastectomy.

Laterally, the breast is innervated by the anterior branch of the lateral intercostal nerves of T2–T6, though the contribution of the anterior branch of T2 is small and limited to the axillary tail. The anterior branches enter the breast superficial to the fascia enveloping the glandular tissue and then divided into smaller branches that fan out close to the surface. This location allows the anterolateral branch of the T4 intercostal nerve to be preserved with careful dissection. This is particularly important as erogenous sensation to the NAC is derived from the lateral intercostal nerve.

The anterior lateral cutaneous branch of T4 splits into an anterior and “deep” branch. The anterior branch traverses the breast parenchyma and ascends directly to the NAC.

The deep branch arises lateral to the lateral margin of the pectoralis major. It passes through the retromammary space for 3–5 cm, lying in the loose areolar tissue at a level deeper than the vascular plane generally above the pectoralis major fascia. It then loops around through the inferolateral part of the breast and enters the subcutaneous tissue 2–3 cm from the surface. On its course toward the NAC, the nerve becomes more superficial, lying in the subdermal plane of the NAC with the other nerves, forming a plexus. There is evidence of a “deep” branch of T5, but this is only present in about 16% of patients. An additional contribution to NAC innervation is from T3 and T5 on the lateral side, and from T2, T3 and T5 on the medial side, with T3 branches to the NAC seen from both the anterior and lateral side in up to 50% of patients.

Anatomic studies by Schlenz et al . in 28 unilateral breast dissections in female cadavers found consistent innervation of NAC by the anterior and lateral cutaneous branches of the 3rd–5th intercostal nerves. The lateral cutaneous branch supplied posterior innervation of the nipple in 93% of cases. The 4th lateral cutaneous branch provided posterior innervation in 93% of cases and was the only source of nipple innervation in 79%. Anterior cutaneous branch had a superficial course to supply the medial aspect of NAC. The 3rd and 4th anterior cutaneous branches combined to provide innervation in 57% of cases. Given the more prominent role of the 4th lateral cutaneous nerve in nipple sensation, it is the preferred target for reinnervation during reconstruction.

The NAC has a rich nerve supply with plexus of nerves underneath it. This plexus receives a contribution from the lateral 4th nerve in all cases, from the 3rd in 83%, from the 5th in 33% and the 2nd in 25%. It also receives innervation from the anterior second through 5th cutaneous nerves. These nerves converge at the NAC from all directions and pass under the NAC in a subdermal plane to reach the nipple. However, the nerves supplying the NAC in two breasts in the same patient are not identical.