Dissection Of The Sternocleidomastoid Muscle

Usually, the first step of the operation is the dissection of the fascia that covers the sternocleido-mastoid muscle. The goal of this maneuver is to completely unwrap the muscle from its surrounding fascia.

Prior to approaching the fascia of the sternocleidomastoid muscle, the external jugular vein must be ligated and divided. Usually, three sections of the external jugular vein are required in functional and selective neck dissection (Fig. 4-9): (1) at the tail of the parotid gland, where the

Figure 4-8 Boundaries of a complete functional neck dissection on the right side of the neck. ML, midline; BM, inferior border of the mandible; C, clavicle; TM, trapezius muscle; ga, great auricular nerve; SC, sternocleidomastoid muscle; sm, strap muscles; pm, platysma muscle; ej, external jugular vein; aj, anterior jugular vein; SG, submandibular gland.

Figure 4-8 Boundaries of a complete functional neck dissection on the right side of the neck. ML, midline; BM, inferior border of the mandible; C, clavicle; TM, trapezius muscle; ga, great auricular nerve; SC, sternocleidomastoid muscle; sm, strap muscles; pm, platysma muscle; ej, external jugular vein; aj, anterior jugular vein; SG, submandibular gland.

Figure 4-9 Points of division of the external jugular vein on a right functional neck dissection. 1, tail of the parotid gland; 2, posterior border of the sternocleidomastoid muscle; 3, supraclavicular fossa; SC, sternocleidomastoid muscle; ga, great auricular nerve.

external jugular vein begins by the union of the retromandibular and posterior auricular veins; (2) at the external surface of the sternocleidomastoid muscle; and (3) at a later step of the operation, within the posterior triangle of the neck when this nodal region is included in the dissection.

The dissection of the sternocleidomastoid muscle begins with a longitudinal incision over the fascia, along the entire length of the muscle. This cut is made with a number 10 knife blade and must be placed near the posterior border of the muscle (Fig. 4-10). This facilitates the dissection of the sternocleidomastoid muscle because the cleavage plane between the fascia and the muscle is much easier to identify in a forward direction. The external jugular vein should be thus transected as close to the posterior border of the sternocleidomastoid muscle as possible. The vein is then included in the specimen and dissected forward with the fascia of the sternocleidomastoid muscle (Fig. 4-11).

Using several hemostats, one of the assistants retracts the fascia medially while the surgeon carries the dissection toward the anterior margin of the muscle (Fig. 4-11). Fascial retraction should be done with extreme care because the thin superficial layer of the cervical fascia is the only tissue now included in the specimen.

We strongly recommend performing this, as well as most other parts of the operation, using knife dissection. The fascial planes of the neck are mainly avascular and can be easily followed with the scalpel. For knife dissection to be most effective the tissue must be under traction. An important task of the assistants throughout the operation is to apply adequate pressure to the dissected tissue.

When the dissection reaches the anterior border of the sternocleidomastoid muscle the hemo-stats that have been used to retract the fascia may be left lying on the medial part of the surgical field hanging toward the opposite side. This will maintain the required amount of traction while freeing the assistants' hands. Then the muscle is retracted posteriorly to continue the dissection over its medial face. Retraction is performed initially by one of the assistants, who holds the muscle posteriorly by means of a retractor, while the surgeon continues the dissection over the sterno-cleidomastoid muscle (Fig. 4-12). When the dissection reaches the deep medial face of the muscle

Figure 4-10 Incision of the fascia over the sternocleidomastoid muscle on the right side. Note the posterior placement of the incision with respect to the muscle. SC, sternocleidomastoid muscle; SG, submandibular gland; ej, external jugular vein; ga, great auricular nerve.
Figure 4-11 The fascia of the sternocleidomastoid muscle is dissected medially. The external jugular vein is included in the fascia (right side). ej, external jugular vein; F, fascia; SC, sternocleidomastoid muscle.

Figure 4-12 Lateral retraction of the sternocleidomastoid muscle allows the dissection of the medial surface of the muscle. The dissected fascia is carefully pulled medially (right side). IJ, internal jugular vein shinning through the fascia; SC, sternocleidomastoid muscle; F, dissected fascia; tm, trapezius muscle; SG, submandibular gland.

Figure 4-12 Lateral retraction of the sternocleidomastoid muscle allows the dissection of the medial surface of the muscle. The dissected fascia is carefully pulled medially (right side). IJ, internal jugular vein shinning through the fascia; SC, sternocleidomastoid muscle; F, dissected fascia; tm, trapezius muscle; SG, submandibular gland.

(close to the carotid sheath) the retractor is removed and further separation of the sternocleidomastoid muscle is performed by the surgeon using a hand with a gauze pad.

Until this point, the cleavage plane between the muscle and the fascia is avascular. However, when the deep medial face of the muscle is approached, small perforating vessels are found entering the muscle through the fascia (Fig. 4-13). The assistant must now cauterize the vessels while the surgeon continues the dissection over the entire medial surface of the sternocleidomas-toid muscle. The surgeon must be extremely careful at the upper half of this region, where the spinal accessory nerve enters the muscle. One or more small vessels usually accompany the spinal accessory nerve, which often divides before entering the muscle. The vessels should be cauterized without injuring the nerve, and all branches of the nerve must be preserved to obtain the best shoulder function. More details concerning the dissection of the spinal accessory nerve are given in a later stage of the operation.

After all the small vessels entering the sternocleidomastoid muscle have been cauterized, a new avascular fascial plane is entered and the dissection continues posteriorly along the entire length of the muscle. The internal jugular vein can now be seen through the fascia of the carotid sheath (Fig. 4-14).

The muscle is now almost completely separated from its covering fascia except for a small portion at the posterior border. This part of the muscle will be dissected in a later stage of the procedure. Wet surgical sponges are now introduced in the lower half of the sternocleidomastoid muscle, between the muscle and its dissected fascia. They will serve two purposes: (1) maintain the desired moisture of the dissected tissues while the attention shifts to the upper part of the surgical field, and (2) serve as a reference for the dissection of the fascia that still covers the posterior border of the sternocleidomastoid muscle, in a later stage of the operation.

The surgeon now moves to the upper part of the surgical field to complete the identification of the spinal accessory nerve. For a better understanding of the following steps of the operation, at this point it may help the reader to take a short pause in the technical details to realize how the surgical approach is made with respect to the sternocleidomastoid muscle when the posterior triangle is included in the resection.

Figure 4-13 Small vessels enter the sternocleidomastoid muscle through its medial face (right side). SC, Sternocleidomastoid muscle retracted laterally; IJ, Internal jugular vein shining through the fascia; V, Vascular pedicle entering the sternocleidomastoid muscle.
Figure 4-14 The dissection of the medial face of the sternocleidomastoid muscle has been completed (right side). IJ, internal jugular vein; ej, external jugular vein; aj, anterior jugular vein; SC, sternocleidomastoid muscle; F, dissected fascia.
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