Authors: Caroline Lilja, MD., Emma Tubæk Nielsen, stud.med., Magnus Balslev Avnstorp, MD., and Stephan Thunich, MD.
Illustrator: Caroline Lilja, MD.

The latissimus dorsi muscle is a rather large and flat muscle located at the back, belonging to the superficial layer of the back muscles. It has its origin from four places:

1) Vertebral origin: originate from the spinous process of the 7^th-12^th vertebras.

2) Iliac origin: originate from the posterior third of the iliac crest.

3) Costal origin: adhering to the 9^th-12^th ribs.

4) Scapular origin: emanates from the inferior angle of the scapula.

The muscle extends superiorly and laterally, before inserting at the proximal humerus, in the intertubercular sulcus between the pectoralis major and teres major muscles, partially forming the posterior axillary fold with its anterior rim. ^{1, 2} 

The muscle’s external surface is adherent to the serratus anterior muscle and four lower ribs. ²

Functionally, the muscle causes movement of the scapula, producing motion of the shoulder joint such as internal rotation, adduction and extension of the arm.¹

The arterial supply is ensured by two vascular systems:

1) the thoracodorsal artery .

2) perforators of the posterior intercostal arteries from the 9^th-11^th ribs and the 1^st-3^rd lumbar arteries.

From the third part of the axillary artery, the subscapular artery originates, travelling along the inferior border of the subscapular muscle to the inferior angle of the scapula. Here it anastomoses with the lateral thoracic artery and the intercostal arteries. 

Shortly after its origin, the artery gives off two branches:

1) the scapular circumflex artery

2) the thoracodorsal artery. ^{3, 4}

The thoracodorsal artery is the dominant perforator of the latissimus dorsi muscle, and travels in continuation of the subscapular artery along with its venae commitantes. It enters the muscle at its profound surface, near the lateral border of the muscle, about 8.5-9 cm from its origin. ^{1, 2} 

There is often a larger branch from the thoracodorsal artery supplying the serratus anterior muscle, which must be ligated during the harvest of the latissimus, unless the serratus anterior muscle also is harvested, in order to make a separate muscular paddle.¹

Most literature would agree that once the artery is located intramuscularly, it divides into two branches, a vertical descending branch and a transverse branch. However, there is often also a third branch travelling radially between these two.

The posterior intercostal arteries and lumbar arteries are responsible for the blood supply of the inferior and medial parts of the muscle. Because these often are ligated during the surgical procedure, the chances of viability are slim for this part of the flap. 

Motor innervation of the muscle is ensured by the thoracodorsal nerve, arising from the posterior branch of the brachial plexus, originating from C6-C8. It travels alongside the thoracodorsal artery.

The nerve is usually sacrificed during the surgical procedure. ¹

Biomechanical properties

As a pedicled flap, it is possible to cover defects on the shoulder, upper arm, armpit, anterior chest wall and above the clavicle. Secondary breast reconstruction can also be performed. For reconstruction of the breast, the LD flap may either be used alone or with an implant for an immediate reconstruction, or together with as a tissue expander for at staged reconstruction. ⁵ Due to the size of the flap, the implant can be covered over a large area and the thick vascular pedicle allows safe ventral rotation. 

As a free flap large defects, for example in the extremities, can be reconstructed.

References

1. Little S. C. Latissimus Myocutaneous Flap. Medscape, Clinical Procedures. 17.06.2019. https://emedicine.medscape.com/article/880878-overview#showall 
2. Smith M. Latissimus Dorsi Muscle. Kenhub, Back. 03.12.2020. https://www.kenhub.com/en/library/anatomy/latissimus-dorsi-muscle
3. Grujičić R. Subscapular artery. Kenhub, Nerves and vessels. 29.10.2020. https://www.kenhub.com/en/library/anatomy/subscapular-artery
4. Crumbie L. Axillary artery. Kenhub, Nerves and vessels. 13.10.2020. https://www.kenhub.com/en/library/anatomy/the-axillary-artery
5. Sood R. et Al. Latissimus Dorsi Flap in Breast Reconstruction: Recent Innovations in the Workhorse Flap. Cancer Control, 2018.