Applied Surgical Anatomy of the Posterior Approach to the Thoracic and Lumbar Spines
Overview
The posterior muscles of the thoracic and lumbar spines are arranged in three layers:
1. Superficial layer: The mooring muscles that attach the upper extremityto the spine
2. Intermediate layer: The muscles of accessory respiration
3. Deep layer: The paraspinal muscle system, the intrinsic muscles of theback
These distinct layers are not actually seen during surgical exposure of the spine, but the layering concept clarifies how the anatomy relates to the dissection.
The superficial layer of muscles can be subdivided into two layers: The most superficial layer consists of the trapezius and latissimus dorsi; the deeper layer is composed of the rhomboid major and minor.
The intermediate layer consists of the serratus posterior superior and
the serratus posterior inferior, which are small, laterally placed muscles that attach to the spine.
The deep layer includes the sacrospinalis muscles (erector spinalis) and a deep, obliquely running layer consisting of the semispinalis, multifidus, and rotator muscles.
The muscles of the superficial layer are supplied by the peripheral nerves: The trapezius by the spinal accessory nerve, the rhomboids by the nerve to them from C5, and the latissimus dorsi by the thoracodorsal nerve. They are not affected by a midline dissection.
The muscles of the intermediate layer are supplied by the anterior primary rami; they, too, are unaffected by the dissection.
The muscles of the deep layer are supplied segmentally at each level of the spine by the posterior rami of the thoracic and lumbar nerves. Their nerve supplies usually are safe, but they may be denervated partially by excessive lateral dissection.
Landmarks and Incision
The C7 and T1 spinous processes are the largest processes in the region, with T1 being slightly larger. They point directly posteriorly, with minimal caudal angulation, and are easily palpable. The large L5 spinous process, which also has minimal caudal angulation, can be palpated, but it cannot be differentiated from the other equally large lumbar spinous processes. The gluteal cleft, which runs between the protuberances of the gluteal (cluneal) muscles, is easy to see.
The skin on the posterior aspect of the spine is thicker than that on the anterior chest wall and abdomen. It usually heals with a fine line scar because there is so little tension across the sutured incision. The skin in the lumbar region (which is dissected subcutaneously to leave the iliac crest accessible for a bone graft) and the skin in the thoracic region (which is dissected subcutaneously to reach the ribs) heal well, despite the subcutaneous dissection. Dimpling of the skin over the iliac crest or ribs does not occur as long as the thick, subcutaneous, fatty tissue layer is taken with the skin to prevent it from adhering to the cut bony surfaces.
Superficial Surgical Dissection and Its Dangers
The tips of the spinous processes in the thoracic region are much narrower than are those in the lumbar area, and more muscles attach directly to their tips. As a result, dissection must approach the tips of the spinous processes exactly in the midline, without straying to either side. More bleeding occurs in the thoracic region, mainly because of the direct attachment of muscle fibers from the trapezius and rhomboid muscles; in the lumbar area, only the relatively avascular lumbodorsal fascia attaches to the tips of the lumbar and lower thoracic processes (Fig. 6-108). If the patient has scoliosis with extensive vertebral body rotation, the paraspinal muscles on the convex side of the curve may bunch up and roll over the tips of the spinous processes, causing further bleeding if the muscles are cut inadvertently during dissection.
Intermediate Surgical Dissection
The deep layer of the back consists of a superficial portion and a deep portion. The superficial portion is made up of the sacrospinalis muscle (the erector spinae), which runs longitudinally. In the lumbar area, the muscle is a single mass; in the thoracic region, it divides into three units, namely, from medial to lateral, the spinalis, the longissimus, and the iliocostalis (see Figs. 6-108 and 6-111).
The deep portion of the deep layer itself has three layers: Superficial, intermediate, and deep groups. The superficial group consists of three muscles. Laterally the iliocostalis muscle runs from the sacrum and inner side of the iliac crest to the angles of the lower six ribs. The muscle continues upward as the costalis muscle and the costocervicalis muscle. The intermediate muscle is the longissimus thoracis and the longissimus cervicis which arise from the sacrum to be inserted into the gutter between the transverse processes and the ribs. The medial muscle of the superficial layer is the weak spinalis muscle which runs alongside the spinous processes.
The intermediate layer also has three muscles in it. The multifidus fibers run from the laminae to the spinous processes; the semispinalis muscles, which span about five segments from origin to insertion run from the transverse processes to the spinous processes; and the levatores costarum muscles which run from the transverse processes to the upper border of the next rib.
The deep group, also consists of three muscles which span adjacent segments (Figs. 6-109 and 6-110). The interspinales join adjacent borders of the spinous processes. The intertransversales join adjacent transverse processes. The most significant muscles in this layer are the rotator muscles which pass in a lateral to medial direction, with the distal end of the muscle being more lateral. The resulting angle between the muscle and its insertion makes stripping the muscles in a caudad to cephalad direction in the thoracic region easier (see Figs. 6-105 and 6-110). In addition, because the spinous processes are angled more caudally in the thoracic area than in the lumbar area (where they stand erect, almost directly over the vertebral bodies), it is easier to dissect the paraspinal muscles free from the thoracic spinous processes in a distal to proximal direction. Finally, the short rotators take origin from the caudal end of the spinous processes and are detached easily and dissected out laterally onto the transverse processes (see Figs. 6-105 and 6-110).
The transverse processes themselves should be stripped of musculature in a distal to proximal direction. The transverse processes become larger from T12 to T1.
Intermediate surgical dissection avoids the middle layer of back muscles, the muscles of respiration; these are placed more laterally.
The posterior primary rami of the paired thoracic and lumbar nerves may be injured during dissection of the muscles, particularly laterally between the transverse processes where the rami are located. Although the loss of one or two posterior primary rami may denervate the paraspinal muscles partially, the significant overlap of the segmental nerve supply to these muscles prevents total denervation. Excessive lateral retraction and cauterization at each level, however, can cause muscle denervation.
Segmental vessels come directly off the aorta in the lumbar and thoracic areas; they are located between the transverse processes, close to the posterior primary rami. The vessels constitute the main blood supply to the paraspinal muscles. Cauterizing them does not appear to cause significant loss of blood supply to the muscles. If they are cut, they must be cauterized or tied off; they branch directly from the aorta and may cause postoperative bleeding under pressure (Fig. 6-111; see Figs. 6-105 and 6-106B).
Deep Surgical Dissection
The lumbar facet joints and their capsules are much larger than their thoracic counterparts and protrude further posteriorly. Their size is mainly the result of their large articulating processes and large mamillary processes that sit on the posterior aspect of the ascending processes, extending the bone even further posteriorly. The lumbar facet joints lie in the sagittal plane (see Fig. 6-111B). The joint capsules themselves are shiny, usually quite white, and are continuous with the ligamentum flavum, which is yellow-white. In the thoracic region, the joints are smaller, do not protrude as far posteriorly, are flatter, and are placed in the frontal plane (see Fig. 6-111A). The facet joints are vulnerable during removal of the joint capsules.
The ligamentum flavum, which originates from the leading edge of the inferior vertebra and extends upward to a ridge under the lamina of the next vertebra, covers the blue-white dura and its layer of epidural fat. The dura must be protected; any epidural tear must be closed off (see Figs. 611 and 6-13).
Figure 6-108 The musculature of the back. The most superficial layer is seen, including the trapezius, the latissimus dorsi, and the lumbodorsal fascia (left). The trapezius and latissimus dorsi have been resected to reveal the deep layer, the sacrospinalis muscles, including the spinalis, longissimus, and iliocostalis muscles (right). A portion of the rhomboid major muscle of the superficial layer is seen inserting into the medial border of the scapula.
Trapezius. Origin. From all spinous processes of the cervical spine except C1; from all spinous processes of thoracic vertebrae (T1-T12); and from superior nuchal line. Attachment to cervical spine is indirect, via ligamentum nuchae. Insertion. Upper fibers from upper third of muscle, passing laterally and inferiorly to flattened posterior border of lateral third of clavicle and its upper surface. Intermediate muscle fibers pass laterally in a horizontal direction to adjacent part of upper surface of acromion and to associated upper lip of crest of spine of scapula. Lower fibers ascend, passing superiorly and laterally, inserting into tubercle on lower lip of spine of scapula. Action. Stabilizing muscle of shoulder girdle. Nerve supply. Spinal accessory nerve; cranial nerve XI.
Figure 6-109 The sacrospinalis system has been resected to reveal the deep portion of the deep layer, which consists of the semispinalis and multifidi. Note the intertransversarii muscles and the insertion of the iliocostalis muscles into the borders of the ribs.
Figure 6-110 The muscles are resected further to reveal the deep muscles of the deep layer (i.e., the rotators as well as the intertransversarii muscles and the interspinous muscles) and the facet joint capsules.
The cup-shaped ascending articulating process is closest to the nerve root. Arthritis of the medial end of the ascending facet can cause compression of the nerve in the foramen. The nerve root is safe during the foraminotomy if the anatomic arrangement of the facet joints to the nerve root is appreciated. When the medial portion of the ascending process is being removed take care to protect the nerve root because this is the portion of bone that is closest to it (see Fig. 6-111B).
Figure 6-111 A: Cross section through the level of a thoracic vertebra. Superficial and deep layers of the thoracic spine are visualized, as well as their nerve and blood supply. B: Cross section through the level of a lumbar vertebra. Note that the individual muscles of the sacrospinalis musculature are one paravertebral mass at this level. Note that the medial end of the cup-shaped ascending articulating process is closest to the lumbar nerve root.
