Serratus Posterior Superior

The serratus posterior superior is a thin, flat muscle of the upper back that extends from the lower cervical and upper thoracic vertebral region to the upper ribs. It lies deep to the rhomboid muscles and forms four characteristic slips that attach to ribs 2 to 5.

Together with serratus posterior inferior, it is classified among the intermediate extrinsic muscles of the back. Despite its location on the posterior trunk, its motor innervation comes from anterior rami of spinal nerves rather than the posterior rami that supply the intrinsic back muscles.

Serratus posterior superior has traditionally been described as an accessory muscle of inspiration because contraction can elevate its attached ribs. Its importance in respiration, however, has been questioned, and a proprioceptive role related to movement of the thoracic wall has also been proposed.^[4][5]

Location

Serratus posterior superior lies in the upper thoracic region on either side of the vertebral column. Its medial attachment begins near the lower cervical and upper thoracic spinous processes, and its fibers pass inferiorly and laterally toward the upper ribs.

The muscle is positioned deep to trapezius and the rhomboids. Rhomboid major and rhomboid minor are particularly important superficial relationships because they cover much of serratus posterior superior. Deeper to the muscle lie the upper ribs and portions of the intrinsic back musculature.

Serratus posterior superior and serratus posterior inferior are widely separated rather than forming a continuous muscular sheet. The superior muscle occupies the upper thoracic region, while serratus posterior inferior lies near the thoracolumbar junction.

Structure

Serratus posterior superior is a thin, roughly quadrilateral muscle. Its medial attachment is partly aponeurotic, while the lateral portion becomes fleshy and divides into a series of digitations that reach the ribs.

The typical muscle has four costal slips, corresponding to ribs 2, 3, 4, and 5. These slips create the serrated appearance reflected in the muscle's name.

The fibers run obliquely from a superomedial origin toward inferolateral rib attachments. This orientation differs from serratus posterior inferior, whose fibers pass superiorly and laterally from the thoracolumbar region toward the lower ribs.

Origin

Serratus posterior superior arises from a thin aponeurosis attached to structures at the cervicothoracic junction. Its origin is commonly described from:

• The lower part of the nuchal ligament.
• The spinous process of C7.
• The spinous processes of approximately T1 to T3.
• The associated supraspinous ligaments.

The exact vertebral extent can vary slightly, but the origin is centered on the lower cervical and upper thoracic midline.^[1][5]

Insertion

From its medial origin, the fibers pass inferiorly and laterally before separating into four muscular slips. These insert onto the superior borders of ribs 2 to 5, near and just lateral to their angles.

The rib attachments place serratus posterior superior directly across the posterior portion of the upper thoracic cage and provide the anatomical basis for its ability to exert an upward pull on these ribs.^[1][2]

Innervation

Serratus posterior superior is supplied by the second through fifth intercostal nerves, derived from the anterior rami of approximately T2 to T5.^[1][2]

This innervation helps distinguish the serratus posterior muscles from the intrinsic muscles of the back. Although serratus posterior superior lies on the posterior trunk, it is an extrinsic back muscle and is therefore supplied by anterior rather than posterior rami.

Blood Supply

The arterial supply of serratus posterior superior is primarily segmental, with branches from the upper posterior intercostal arteries supplying the muscle in the upper thoracic region.

These vessels reach the muscle locally near its rib attachments and neighboring intercostal spaces, matching the segmental organization of the upper posterior thoracic wall.

Relations

Serratus posterior superior occupies a relatively deep plane in the upper back, and its relationships help distinguish it from the larger muscles surrounding the scapula.

• Superficially: Trapezius and the rhomboid muscles, particularly rhomboid major and rhomboid minor.
• Deeply: Upper ribs, intercostal spaces, and portions of the intrinsic back musculature.
• Medially: Nuchal and supraspinous ligaments and the spinous processes at the cervicothoracic junction.
• Laterally: Ribs 2 to 5 near their angles.

The muscle does not attach to the scapula. This is an important distinction from the nearby rhomboids and from serratus anterior, despite the shared use of the term "serratus" in their names.

Function

Contraction of serratus posterior superior can elevate ribs 2 to 5. This mechanical action led to its traditional description as an accessory inspiratory muscle, since elevation of the upper ribs can contribute to expansion of the thoracic cage.

The extent to which serratus posterior superior actually contributes to respiration in vivo is less certain. Anatomical and electromyographic observations have questioned whether the serratus posterior muscles have a substantial respiratory role.^[4][5]

It is therefore useful to separate the muscle's clear mechanical capability from its traditionally assigned respiratory function. Its attachments allow it to exert an elevating force on the upper ribs, but the physiological importance of that force during breathing has not been firmly established.

Proprioception

A proprioceptive role has been proposed for the serratus posterior muscles. Proprioception provides sensory information about the position and movement of body structures, and the serratus posterior muscles may contribute information related to movement of the ribs and thoracic wall.^[4]

This interpretation remains a proposed functional explanation rather than a reason to discard the traditional description of rib elevation. The anatomical action on the ribs is well established, while the broader physiological role of the muscle remains less certain.

Anatomical Variations

Serratus posterior superior varies in the number and development of its muscular slips. Although four slips attaching to ribs 2 to 5 are typical, individual slips may be reduced, duplicated, or extend to a neighboring rib.

The medial origin may also vary in its vertebral extent, and the relative amount of muscular and aponeurotic tissue differs between individuals. In some specimens the muscle is particularly thin and may blend with surrounding fascial structures.

These variations do not alter the characteristic arrangement of the muscle: a medial attachment at the cervicothoracic junction with oblique fibers passing inferolaterally toward the upper ribs.

Clinical Significance

Serratus posterior superior is not usually assessed as an isolated muscle during routine examination because it is thin and covered by larger superficial muscles. Its clinical anatomy is more useful for understanding the layered structure of the upper posterior thorax and for distinguishing neighboring muscles on imaging and during dissection.

Imaging anatomy

On CT or MRI, serratus posterior superior appears as a thin muscular layer deep to the rhomboids and superficial to structures of the posterior thoracic wall. Following the muscle medially toward the upper thoracic spinous processes and laterally toward ribs 2 to 5 helps distinguish it from surrounding muscles.

Posterior thoracic anatomy

The muscle may be encountered during anatomical dissection and posterior approaches involving the upper thoracic region. Reflection of trapezius and the rhomboids exposes serratus posterior superior and demonstrates its characteristic slips extending toward the upper ribs.

Distinction from serratus anterior

Serratus posterior superior should not be confused with serratus anterior. Serratus anterior is a much larger muscle of the lateral thoracic wall that attaches the ribs to the medial border of the scapula and plays a major role in scapular movement and stabilization. Serratus posterior superior does not attach to the scapula and does not function as a scapular protractor.

The anatomy of serratus posterior superior is therefore defined most clearly by its cervicothoracic origin, four slips to ribs 2 to 5, and anterior ramus innervation. Its ability to elevate the upper ribs is anatomically clear, while its traditional classification as an accessory inspiratory muscle should be interpreted in light of the continuing uncertainty about the broader function of the serratus posterior muscles.^[4][5]