 The latissimus dorsi muscle is the largest muscle
in the body, up to 20 by 40 centimeters, allowing coverage of extremely large
wounds. In spite of its size, no significant donor functional deficit
results from removal of the muscle. It is the largest flap that can be harvested
on a single predicle, and can even be combined with the serratus, scapular or
parascapular flaps, to create a flap complex that can cover massive wounds. In
the normal population the muscle is quite thin (less than 1 centimeter thick),
allowing it to be draped over irregular surfaces with ease. With the
rectus muscle and radial forearm flap, it represents one of the workhorse flaps
in reconstructive microsurgery. When re-innervated using the thoracodorsal
nerve, the latissimus can be used as a functional muscle.Anatomy
The muscle takes origin on the iliac crest
inferiorly and the thoracolumbar fascia posteriorly near the midline. It
inserts into the humerus where it acts as a humeral adductor and internal
rotator. The posterior axillary fold is made up of the most superior
aspect of the muscle that begins to narrow before it forms the tendon of
insertion. The nerve supply is via the thoracodorsal nerve, a branch of
the posterior cord of the brachial plexus. Lesions of C-7 will affect
latissimus function. The nerve closely accompanies the thoracodorsal
artery.
Arterial Anatomy
The latissimus muscle blood supply is via the
subscapular artery, a branch of the axillary artery.
The subscapular sends off a
circumflex scapular branch posteriorly and then distributes a serratus branch
before it enters the substance of the muscle on its undersurface.
On
the subscapular system a 5 to 10 centimeter pedicle can be obtained. There
is typically a single venae comitans accompanying the artery. The pedicle
can be approached directly by dissecting the latissimus from the axilla, or it
can be found by following the undersurface of the muscle in a distal to proximal
approach. Because the artery divides in the substance of the muscle, the
muscle can be split longitudinally to to form a bi-lobed or two tongued flap.
The subscapular artery can be from 2 to 5 millimeters in size, while the
thoracodorsal artery ranges from 1 to 3 millimeters. The venae comitans is
usually slightly larger.
The muscle is also supplied by perforators from the
thoracic intercostal and lumbar arteries that allow it to be used as a pedicled
flap that can resurface posterior defects. These vessels are quite small
with short leashes and not used for microsurgical reconstruction.
Operative Procedure
The patient is placed in the lateral decubitus
position on a beanbag, with an axillary roll placed in the dependent axilla.
The ipsilateral arm is prepped completely and left in the operative field,
allowing it to be freely moved about the field. For most of the procedure
it is kept abducted and resting on a Mayo stand placed anterosuperiorly to the
patient. The anterior border of the latissimus is marked with a marking
pen. The incision is then marked extending from the axilla, anterior or on
the posterior axillary fold, inferiorly and posteriorly over the latissimus
muscle.
The length of muscle needed will dictate the
incision length. Anterior and posterior flaps are raised superficial to
the muscle to expose the latissimus.
The figure below reveals the superior, posterior,
inferior and anterior edges of the latissimus exposed. The serratus is
visible anteriorly.
The pedicle can be approached directly in the
axilla. We prefer to elevate the muscle first, starting at the superior
aspect, near the midline and posterior (green arrow). A plane is developed
developed easily the latissimus.
The posterior edge of the muscle is then transected
with electrocautery as the plane under the muscle is developed inferiorly.
The posterior edge is then exposed. The
inferior plane near the iliac crest is slightly more difficult to develop.
Once the inferior plane is developed the muscle is
raised superiorly, looking for the appearance of the pedicle in the musculature.
Usually it becomes visible near the axilla after the latissimus is separated
from the serratus. Caution must be observed not to dissect under the
serratus muscle, but between the serratus and latissimus.
When the pedicle becomes visible it is dissected
free, noting the serratus and circumflex scapular branches. The nerve is
transected depending on length required, if any.
The wound is closed with a deep and superficial
layer. Two suction drains are place through the anterior flap.
Postoperative Care
This donor site often forms a seroma. Drains
are often left in for 2 weeks or longer until the output is diminished.
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