Rhomboid Minor

The rhomboid minor is a small, quadrilateral muscle of the upper back that connects the lower cervical vertebral column to the medial border of the scapula. It lies immediately superior to rhomboid major and works closely with it to retract and stabilize the scapula.

Rhomboid minor belongs to the superficial extrinsic muscles of the back. Although it is located on the posterior trunk, its primary actions are on the pectoral girdle rather than the vertebral column. It is supplied by the dorsal scapular nerve, which arises from the anterior ramus of C5.

The muscle is largely covered by trapezius, so it is not normally visible at the surface. Its short, oblique fibers extend from the lower part of the nuchal ligament and the C7-T1 spinous processes toward the medial end of the scapular spine.

Location

Rhomboid minor lies in the upper back between the vertebral column and the medial border of the scapula. It is positioned superior to rhomboid major and inferior to levator scapulae, forming part of a muscular sequence along the medial border of the scapula.

The muscle lies deep to trapezius. When trapezius is reflected, rhomboid minor can be seen extending laterally and slightly inferiorly from the lower cervical and uppermost thoracic midline toward the scapula.

At its scapular attachment, rhomboid minor is associated with the medial end, or root, of the spine of the scapula. This attachment provides a useful landmark for distinguishing it from rhomboid major, which attaches farther inferiorly along the medial border.

Structure

Rhomboid minor is a relatively thin, flat muscle with fibers that run approximately parallel to those of rhomboid major. From their medial origin, the fibers pass laterally and slightly inferiorly toward the scapula.

The muscle is considerably smaller than rhomboid major. Despite their separate names, the two rhomboids are closely related structurally and functionally and may have little or no clear separation between portions of their adjacent borders.

Rhomboid minor forms part of the muscular connection between the axial skeleton and the scapula. Because the scapula has no direct bony articulation with the axial skeleton, muscles such as the rhomboids play an important role in controlling its position against the thoracic wall.

Origin

Rhomboid minor arises medially from:

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

These attachments place its origin across the cervicothoracic junction, immediately superior to the vertebral origin of rhomboid major.^[1][2]

Insertion

Rhomboid minor inserts onto the medial border of the scapula at the level of the root of the scapular spine. Descriptions commonly place the attachment along the smooth triangular area at the medial end of the spine and the immediately adjacent medial border.

This insertion lies between the attachment of levator scapulae superiorly and rhomboid major inferiorly. The arrangement allows these muscles to exert different but coordinated forces along the medial border of the scapula.

Innervation

Rhomboid minor is innervated by the dorsal scapular nerve, primarily from the C5 spinal nerve. The dorsal scapular nerve usually arises from the anterior ramus of C5, often close to the origin of the upper trunk of the brachial plexus.

After arising in the neck, the nerve commonly passes through the middle scalene muscle and descends along the medial border of the scapula. It supplies both rhomboid minor and rhomboid major and commonly also supplies levator scapulae, although levator scapulae receives additional cervical contributions.^[1][2]

The origin of the dorsal scapular nerve from an anterior ramus is consistent with the classification of the rhomboids as extrinsic rather than intrinsic back muscles.

Blood Supply

The principal arterial supply to rhomboid minor comes from the dorsal scapular artery. This vessel descends along the medial border of the scapula in close association with the dorsal scapular nerve and supplies the rhomboid muscles and neighboring structures.

The dorsal scapular artery may arise directly from the subclavian artery or represent the deep branch of the transverse cervical artery. This variation changes the vessel's origin but not its relationship with the rhomboid region.^[1]

Relations

Rhomboid minor occupies an important plane between trapezius and the posterior thoracic wall. Its relationships also help define the arrangement of muscles attached along the medial border of the scapula.

• Superficially: Trapezius.
• Inferiorly: Rhomboid major.
• Superiorly: Levator scapulae.
• Laterally: Medial border of the scapula at the root of the scapular spine.
• Deeply: Serratus posterior superior and structures of the upper posterior thoracic wall.

The dorsal scapular nerve and artery descend in the region of the medial scapular border in close association with the rhomboid muscles. Their course is important when examining the neurovascular anatomy of the posterior shoulder girdle.

Function

Rhomboid minor acts with rhomboid major to control the position of the scapula. Its principal actions are retraction, downward rotation, and stabilization of the scapula.

Scapular retraction

Contraction draws the medial border of the scapula toward the vertebral column, producing retraction. The rhomboids work with the middle fibers of trapezius during movements that bring the scapulae toward the midline.

Downward rotation

Rhomboid minor contributes to downward rotation of the scapula. In this movement, the glenoid cavity is directed more inferiorly as the inferior angle moves medially. The rhomboids act with levator scapulae in producing this rotational effect.

Scapular stabilization

The rhomboids also help hold the medial border of the scapula against the thoracic wall. This stabilization provides a controlled base from which muscles acting on the humerus can produce movements of the upper limb.

Rhomboid minor usually acts together with rhomboid major rather than functioning as an isolated scapular mover. Their similar fiber direction and shared innervation reflect this close functional relationship.^[2]

Anatomical Variations

The degree of separation between rhomboid minor and rhomboid major varies. In some individuals, the two muscles are clearly separated by a small interval, while in others their adjacent fibers may blend.

The vertebral origin and width of the scapular insertion may also vary. Accessory muscular slips have occasionally been described around the rhomboid region, reflecting variation in the muscular connections between the vertebral column and scapula.

The dorsal scapular artery also has a well-recognized variation in origin. It may arise independently from the subclavian artery or continue as the deep branch of the transverse cervical artery.

Clinical Significance

The clinical anatomy of rhomboid minor is closely related to the dorsal scapular nerve and to the muscle's role in maintaining the position of the medial border of the scapula.

Dorsal scapular nerve injury

Injury to the dorsal scapular nerve can weaken both rhomboid minor and rhomboid major. Weakness reduces the ability to retract and stabilize the scapula and may alter its resting position and movement during upper limb activity.

Because the dorsal scapular nerve also has a close relationship with levator scapulae, the functional effects of nerve injury are not necessarily limited to the rhomboids. Isolated dorsal scapular neuropathy is uncommon compared with injuries involving larger nerves of the shoulder girdle.

Scapular winging

Weakness of the rhomboids can contribute to abnormal prominence or displacement of the medial border of the scapula. This should be distinguished from the more characteristic winging associated with serratus anterior paralysis following long thoracic nerve injury.

The direction and circumstances of scapular displacement differ because serratus anterior and the rhomboids exert different forces on the scapula. Examination of scapular movement can therefore provide information about which muscular or neural structures are affected.

Surgical anatomy

The dorsal scapular nerve and artery travel near the medial border of the scapula deep to the rhomboid muscles. Their position is relevant during posterior approaches involving the medial scapular region and when the rhomboids are reflected or mobilized.

Imaging anatomy

On CT and MRI, rhomboid minor can be identified as a thin muscle extending from the lower cervical and upper thoracic midline to the medial end of the scapular spine. Its position superior to rhomboid major and deep to trapezius provides the clearest landmarks for identifying it.

The combination of its C7-T1 origin, attachment at the root of the scapular spine, and dorsal scapular nerve supply distinguishes rhomboid minor from neighboring muscles. Although small, it forms an important part of the muscular system that positions and stabilizes the scapula during movements of the upper limb.