Paraspinal muscles

Erector spinae

The intermediate layer of the intrinsic back muscles consists of the erector spinae (ES) muscle group, which is primarily responsible for the extension and lateral flexion of the trunk and neck (see Chapter 1 ). The ES consists of three thin columns of muscle: iliocostalis, longissimus, and spinalis, each covered in thoracolumbar fascia and distinguished by their points of attachment ( Table 15.1 ). The muscle groups extend caudocranially from the common origin (sacrum, pelvis, or spinous and transverse processes of T1 to L5) to the ribs, thoracic transverse processes, cervical spinous or transverse processes, or base of the skull. Specifically, they are found within grooves between the spinous processes and the angle of the ribs on each side of the vertebral column, coursing deep to the serratus posterior, rhomboids, splenius cervicis, and splenius capitis muscles [ ]. Only those muscles related to the lumbar function are discussed in detail in this section.

The iliocostalis group is the most lateral of the ES ( Fig. 15.1 ) and consists of three muscles: iliocostalis lumborum, iliocostalis thoracis, and iliocostalis cervicis. Iliocostalis lumborum is the most inferior, originating from the ES aponeurosis and transverse processes of L1-L4 and traversing superiorly to the posteroinferior surfaces of the angles of ribs 3–6. Bilateral activation of the iliocostalis lumborum extends the thoracolumbar trunk, while unilateral activation mediates lateral flexion. Superior to the iliocostalis lumborum is the iliocostalis thoracis, which spans from the angle of ribs 7–12 to the superior borders of the angle of ribs 3–6 and the transverse process of C7. This muscle is also activated to extend and laterally flex the spine, instead solely acting within the thoracic region. The third and most superior muscle is the iliocostalis cervicis that mainly extends and laterally flexes the cervical spine. Each muscle of the iliocostalis group is innervated by the dorsal ramus of its respective underlying spinal nerves [ ].

Medial to the iliocostalis is the longissimus muscle group composed of the longissimus thoracis, longissimus cervicis, and longissimus capitis. Longissimus thoracis, the largest of all erector spinae muscles, primarily originates from the ES aponeurosis, though many fibers additionally stem from the transverse processes of the lumbar vertebrae. The superior attachment of the longissimus thoracis demonstrates a unique anatomic pattern. The upper fascicles insert into the tips of the transverse processes of T1-T4, while the lower fascicles demonstrate a bifid insertion to both the transverse processes and adjacent ribs of T5-T12 [ ]. Overall, this muscle serves in extension and lateral flexion of the thoracolumbar spine. Innervation is supplied by the lateral branches of the thoracic and lumbar posterior primary divisions of spinal nerves [ ]. The longissimus cervicis is located cranial to the longissimus thoracis, while longissimus capitis is the most superior of the longissimus group. They act together to control neck and head movement.

The third and most medial portion of the ES muscles is the spinalis group. Spinalis thoracis acts to extend or laterally flex the thoracic spine, while spinalis cervicis and spinalis capitis work together to extend, ipsilaterally flex, and/or rotate the neck [ ].

Of note the ES aponeurosis, often referred to as the common tendon, is a point of attachment for all inferior ES muscles. Attached to the lumbar spinous processes, supraspinous ligament, median sacral crest and dorsal iliac crest, the ES aponeurosis is found just superficial to the sacrum and blends with the thoracolumbar fascia covering affiliated muscles [ ].