Serratus Posterior Inferior

The serratus posterior inferior is a thin, broad muscle of the lower back located at the junction between the thoracic and lumbar regions. It extends from the lower thoracic and upper lumbar vertebral column to the lowest four ribs, forming a series of muscular slips along the posterior aspect of the lower thoracic cage.

Together with serratus posterior superior, it belongs to the intermediate extrinsic muscles of the back. The two serratus posterior muscles occupy different regions and are separated by a considerable distance. Serratus posterior superior lies in the upper thoracic region, while serratus posterior inferior lies much lower, near the thoracolumbar junction.

Serratus posterior inferior has traditionally been described as a muscle that depresses the lower ribs and assists expiration. Its attachments are consistent with depression of ribs 9 to 12 during contraction, but evidence for a significant respiratory role is limited. A proprioceptive role related to movement and position of the thoracic wall has also been proposed.^[4][5]

Location

Serratus posterior inferior occupies the lower posterior thorax and upper lumbar region on each side of the vertebral column. It lies deep to latissimus dorsi and is closely associated with the posterior layer of the thoracolumbar fascia.

Medially, the muscle begins near the spinous processes at the thoracolumbar junction. Its fibers pass superiorly and laterally toward the lowest four ribs. This direction is opposite to that of serratus posterior superior, whose fibers pass inferiorly and laterally from the upper vertebral column toward the upper ribs.

The muscle overlies portions of the intrinsic back musculature. Laterally, its slips approach the posterior ends of ribs 9 to 12, where they attach near the rib angles.

Structure

Serratus posterior inferior is a thin, irregularly quadrilateral muscle. Its medial portion begins largely as an aponeurotic attachment that blends with the thoracolumbar fascia. As the muscle extends laterally, the fibers become fleshy and separate into distinct digitations that attach to the lower ribs.

The typical arrangement consists of four muscular slips, one for each of ribs 9, 10, 11, and 12. The slips are not completely identical in size or orientation, and variation in their development and exact attachment is common.

The lower two ribs are floating ribs and lack anterior attachment to the sternum. The relationship of serratus posterior inferior to these ribs places the muscle at the mobile lower margin of the thoracic cage.

Origin

Serratus posterior inferior arises by a broad aponeurosis from the lower thoracic and upper lumbar region. Its origin is commonly described from:

• The spinous processes of T11 and T12.
• The spinous processes of approximately L1 and L2, with some descriptions extending to L3.
• The associated supraspinous ligaments.
• The posterior layer of the thoracolumbar fascia, with which its aponeurotic origin blends.

The precise vertebral extent varies between individuals and among anatomical descriptions. The consistent pattern is a broad medial origin centered on the thoracolumbar junction.^[1][5]

Insertion

From its medial origin, the fibers pass superiorly and laterally and divide into muscular slips that insert onto the lower four ribs.

The usual insertion is onto the inferior borders of ribs 9 to 12 near their angles. The slips overlap the posterior portions of these ribs and establish a direct mechanical connection between the thoracolumbar region and the lower thoracic cage.^[1][2]

Innervation

Serratus posterior inferior is supplied by the anterior rami of the lower thoracic spinal nerves, generally T9 to T12. These reach the muscle through the ninth to eleventh intercostal nerves and the subcostal nerve associated with T12.^[2]

This innervation is an important distinction from the intrinsic muscles lying deeper in the back. Intrinsic back muscles such as erector spinae and the transversospinalis group are supplied by posterior rami, whereas serratus posterior inferior is an extrinsic back muscle supplied by anterior rami.

Blood Supply

The muscle receives its arterial supply primarily from branches of the lower posterior intercostal arteries and the subcostal artery. These vessels provide segmental branches to the muscle as they course through the lower thoracic region.^[2]

This segmental vascular pattern corresponds closely with the muscle's attachments to the lower ribs and its innervation by the lower thoracic spinal nerves.

Relations

The anatomical relationships of serratus posterior inferior help locate it within the layered musculature of the lower back.

• Superficially: Latissimus dorsi and the posterior layer of the thoracolumbar fascia.
• Deeply: Erector spinae and other intrinsic back muscles, with the lower ribs forming its lateral attachment sites.
• Medially: Spinous processes and supraspinous ligaments at the thoracolumbar junction.
• Laterally: Ribs 9 to 12 near their angles.

Its close association with the thoracolumbar fascia is particularly characteristic. The medial aponeurosis blends into this fascial system before the muscle becomes fleshy as it approaches the ribs.

Function

Contraction of serratus posterior inferior can draw ribs 9 to 12 inferiorly. Because of this mechanical action, the muscle has traditionally been described as assisting expiration by depressing the lower ribs.

The respiratory importance of this action, however, is uncertain. Electromyographic observations and anatomical studies have not demonstrated a substantial respiratory role for the serratus posterior muscles, and anatomical investigation has found no evidence of adaptations expected in a muscle with an important respiratory function.^[4][5]

It is therefore useful to distinguish the muscle's mechanical action from its proposed physiological role: serratus posterior inferior is capable of exerting an inferior pull on the lower ribs, but this does not necessarily mean that it makes a major contribution to normal or forced expiration.

Proprioceptive role

A proprioceptive function has been proposed for both serratus posterior muscles. In this interpretation, the muscles may contribute sensory information about movement and position of the thoracic cage rather than functioning primarily as respiratory muscles.^[4]

This proposed role is consistent with their broad attachments between the vertebral region and ribs, although the complete functional significance of the serratus posterior muscles remains incompletely established.

Anatomical Variations

Serratus posterior inferior varies in size, thickness, and the number and extent of its costal slips. The vertebral origin may extend over slightly different levels, particularly within the upper lumbar region.

The rib attachments can also vary. Individual slips may be reduced, partly tendinous, or blended with adjacent muscular or fascial structures. The muscle as a whole may be relatively thin, reflecting its modest muscular bulk compared with the large intrinsic back muscles lying beneath it.

Despite these variations, the usual arrangement remains a broad medial attachment at the thoracolumbar junction with fibers passing superolaterally to the lower four ribs.

Clinical Significance

Serratus posterior inferior is rarely examined as an isolated muscle because of its thin structure and deep position beneath latissimus dorsi. Its clinical anatomy is most relevant when interpreting the muscular and fascial layers of the thoracolumbar region.

Imaging anatomy

On cross-sectional imaging, serratus posterior inferior can be identified as a thin muscular layer extending from the thoracolumbar region toward the posterior aspects of the lower ribs. Its position deep to latissimus dorsi and superficial to the intrinsic back musculature helps distinguish it from the larger surrounding muscles.

Thoracolumbar fascia

The broad aponeurotic origin of serratus posterior inferior blends with the thoracolumbar fascia. This relationship places the muscle within an interconnected fascial and muscular system spanning the lower thorax and lumbar back rather than as an isolated muscular structure.

Posterior thoracic anatomy

The attachment of serratus posterior inferior to ribs 9 to 12 makes it a useful landmark when studying the transition between the posterior thoracic wall and lumbar region. Reflection of latissimus dorsi exposes the muscle and its serrated costal attachments, while deeper dissection reveals the intrinsic back muscles beneath it.

The serratus posterior inferior is therefore anatomically distinctive for its position at the thoracolumbar junction, its attachment to the lowest four ribs, and its innervation by anterior rather than posterior rami. Although traditionally classified as an accessory respiratory muscle, its precise physiological role remains less certain than its anatomy and mechanical effect on the lower ribs.^[4][5]