Applied Surgical Anatomy of the Posterior Approach to the Lumbar Spine‌

Overview

The muscles of the lumbar spine are made up of superficial and deep layers. The superficial layer consists of the latissimus dorsi, a powerful muscle of the posterior axillary wall that originates from the spinous processes and supraspinous ligaments from the spine of the seventh thoracic vertebra down to the sacrum and inserts into the intertubercular groove of the humerus. The surgically important deep layer consists of the paraspinal muscles and itself is divided into two layers: The superficial portion, which contains the sacrospinalis muscles (erector spinae), and the deep portion, which consists of the multifidus and rotator muscles (Fig. 6-10).

This arrangement is not apparent during surgery, because the approach involves detaching all these muscles in a single mass.

Landmarks and Incision

Landmarks

Spinous Processes. The spinous processes in the lumbar area are thick. The distal end of the tip of the spinous process is bulbous and extends slightly caudally. Each process separates the paraspinal muscles on each side. In a growing patient, the processes are capped by cartilaginous

apophyses, which, when split, make it easier to remove the paraspinal muscles subperiosteally.

Figure 6-10 An overview of the musculature of the lumbosacral spine. In the lumbar spine, the sacrospinalis system is composed of the multifidi, longissimus, and iliocostalis muscles. Note the intertransversarii muscles located deeper. Note the dorsal sacroiliac ligaments.

Posterior Superior Iliac Spine and Crest of the Ilium. The broad iliac crests run posteriorly at a 45-degree angle toward the midline. Because muscles either take origin from or insert into the crest (none cross it), it has a palpable subcutaneous border. The palpable, visible dimples over the buttocks lie directly over the posterior superior iliac spines. A line drawn between the two posterior superior iliac spines crosses the second part of the sacrum; a line drawn between the highest points of the iliac crest crosses between the spinous processes of L4 and L5 (Fig. 6-11).

Incision

The midline incision follows the course of the spinous processes. It tends to heal with a fine, thin scar, because it is not under tension after suturing and is attached firmly to underlying fascia. No major cutaneous nerves cross the midline.

Superficial Surgical Dissection and Its Dangers

The dorsal lumbar fascia and the supraspinous (supraspinal) ligaments lie between the skin and the spinous processes. The fascia is a broad, relatively thick, white sheet of tissue that forms a sheath for the sacrospinalis muscles and attaches to the spinous processes (see Fig. 6-10). It extends to the cervical spine, where it becomes continuous with the nuchal fascia of the neck. Medially, it is attached to the spinous processes of the vertebrae, the supraspinous ligaments, and the medial crest of the sacrum. Inferiorly, it is attached to the iliac crests. Laterally, it is continuous with the origin of the aponeurosis of the transversus abdominis and latissimus dorsi muscles.

The supraspinous ligaments extend from vertebra to vertebra, connecting the spinous processes. They blend intimately with the attachment of the dorsal lumbar fascia to the spinous processes (Fig. 6-10). Further dissection consists of detaching the two layers of muscle from bone. Because these muscles are detached in a single mass, their critical feature, in regard to their surgical anatomy, lies in their blood supply and not in their structure. The segmental lumbar vessels arise directly from the aorta. They wrap around the waist of each vertebral body and then ascend close to the pedicle, where they divide into two branches. One supplies the spinal cord; the other, larger branch then comes directly posteriorly to supply the paraspinal musculature. During the approach, these vessels appear between the transverse processes, close to the facet joints (see Fig. 6-12). They often bleed as dissection is carried out. In addition, the arteries branch within the muscle bodies, frequently creating a very vascular field. For this reason, the dissection should be kept as close to the midline as possible; no major vessels cross the midline, and the plane is safe for use

(Fig. 6-12).

Figure 6-11 The bony anatomy of the lumbosacral spine and the posterosuperior aspect of the pelvis. The facet joint capsules, ligamentum flavum, and interspinous ligaments are shown. A line drawn across the crest of the ilium intersects the L4-5 interspinous space. A line crossing the posterior superior iliac spine intersects the second part of the sacrum.

Deep Surgical Dissection and Its Dangers

The ligamentum flavum is the most important structure in the deep layer. Consisting of yellow elastic tissue, the ligament takes origin from the leading edge of the lower lamina and inserts into the anterior surface of the lamina above, about halfway up onto a small ridge (Fig. 6-13). The two ligamenta flava, one from each side, meet in the midline, but generally do not fuse; the plane between the ligamentum flavum and the underlying dura fat can be entered most easily at that point. Because of its attachments, the ligamentum flavum is removed best from the leading edge of the lower lamina through sharp dissection or curettage (see Fig. 6-5A).

The major danger in the deep dissection involves damage to the dura. Once the ligamentum flavum is entered, a thin spatula should be placed beneath it to protect the underlying dura from being torn (see Fig. 6-6A). The cord itself and the nerve roots often are difficult to see as a result of

bleeding from epidural veins. The veins, which are thin-walled and easy to rupture, even with blunt dissection, can be controlled by direct pressure using a pattie or by bipolar cautery.

Figure 6-12 Cross section at the L3-4 disc space, looking distally. The segmental lumbar vessels branch directly from the aorta. They wrap around the waist of each individual vertebral body and then ascend close to the pedicle, where they divide into two branches. One branch supplies the cord; the other, larger branch proceeds directly posterior to supply the paraspinal musculature. During the surgical approach, these vessels appear between the transverse processes, close to the facet joints. Note that the posterior primary rami and the posterior branches of the lumbar vessels appear between the transverse processes close to the pedicle and descending facet.

Figure 6-13 A sagittal section through the lamina of a lumbar vertebra. Note the origin and insertion of the ligamentum flavum as well as the supraspinous and interspinous ligaments. The nerves exit at the inferior aspect of the pedicle.