Summary of Key Points
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A solid understanding of anatomy is the foundation for any surgical procedure.
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This chapter provides an overview of the lumbar and sacral spine anatomy with emphasis on clinically relevant structures.
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Common surgical approaches to the anterior and posterior lumbar and sacral spine are described in detail.
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Potential complications associated with each surgical approach are discussed and strategies for avoidance are reviewed.
A solid understanding of anatomy is the foundation for any surgical procedure. This principle holds true for surgical approaches to the lumbar and sacral spine as well. A thorough knowledge of the pertinent bony, neural, vascular, muscular, and visceral structures helps to reduce surgical complications while ensuring an adequate surgical corridor to access the pathology. This is especially important in the setting of complex spinal reconstruction where normal anatomy is distorted, or during minimally invasive procedures where the visualization of local anatomy is limited.
This chapter provides an overview of the lumbar and sacral spine anatomy with emphasis and highlights on clinically relevant structures.
Anatomy
Bony Anatomy
The lumbar vertebra consists of the vertebral body, pedicles, laminae, and superior and inferior articular processes, along with the transverse and spinous processes. The spinal canal is formed by the vertebral body ventrally, the pedicles laterally, and the lamina dorsally. The bone bridging the superior and inferior articular processes is the “pars interarticularis.”
The vertebral body is a cylindrically shaped structure. The outer cortical bone layer, which consists of the end plates and side walls, provides load-bearing strength for the vertebral body; the inner part of the vertebral body consists of cancellous bone and has functions in hematopoiesis. The pedicles are broad and thick bony structures widely spaced on the rostral dorsolateral aspect of the body. Their angles in the transverse and sagittal planes increase and decrease, respectively, as the lumbar spine is descended ( Fig. 48-1 ). The transverse processes are flat and long from L1 to L4; they usually become small stubs at L5. The mamillary processes are large in the lumbar region, providing attachment sites for multifidus and intertransversarii muscles. The superior and inferior articular processes have complementary relationships. The superior articular process from the caudal vertebra is concave and faces dorsomedially to meet the inferior articular process from above, which faces ventrolaterally. They form a synovial joint known as the “zygapophyseal joint,” or commonly referred to as the “facet joint.” The junction of the two facets forms the roof of the neural foramina ( Fig. 48-2 ).
The sacrum has a triangular shape and consists of five fused vertebrae. It joins with the fifth lumbar vertebra and forms the dorsal aspect of the pelvis. The five fused vertebrae have homologous structures referable to the lumbar spine. The transverse processes form the laterally projecting alae that articulate with the pelvis. A prominent midline dorsal ridge represents the fused spinous processes. More laterally, another ridge forms the sacral articular crest with a functional superior articular process at S1. This facet faces rostrally and dorsally. Because of the sacrovertebral angle created by the tilt of the sacrum as it joins the fifth lumbar vertebra, this joint prevents ventral displacement of the lumbar spine on the sacrum. Ventrally, transverse ridges represent fused vertebrae and enclose remnants of intervertebral discs. Foramina dorsally and ventrally provide sites of exit for the ventral and dorsal divisions of the sacral nerves. The sacrum is the most variable portion of the spine. Lengthening or shortening of the lumbar spine by deletion or addition of segments to and from the sacrum is not uncommon ( Fig. 48-3 ).
Although it is not the main focus of this chapter, thoracic spine anatomy is pertinent to surgery involving the thoracolumbar junction and thus is briefly summarized here. Thoracic vertebrae T2 to T9 have points of articulations for each rib: one on the vertebral body and the other on the corresponding transverse process. T11 and T12 have a single costal facet on their pedicles ( Fig. 48-4 ). The typical rib has a head, a neck, a tubercle, and a shaft or body. The crest of the head is joined to the intervertebral disc by an intra-articular ligament with two surface articulations, one on the numerically corresponding vertebra and one on the vertebra above it. The neck is the nonarticulating portion of rib between the head and tubercle. The tubercle is on the dorsal portion of the rib at the junction of the neck and shaft. The tubercle of most ribs has a smooth convex facet that articulates with the transverse process of the corresponding vertebra and a rough nonarticular surface to which the lateral costotransverse ligament attaches. The body of the rib is thin and flat, with its greatest diameter in the rostral to caudal orientation. The point of greatest curvature is called the angle of the rib. The costal groove and the flange formed by the caudal border of the rib accommodate and protect the intercostal vessels and nerves that accompany the rib. The 11th and 12th ribs are short and capped with cartilage. They have a single facet on their heads and no neck or tubercle. The 11th rib has a slight angle and a shallow costal groove. The 12th rib has neither of these features. Minet classifies the 12th rib as long, medium, or short. The long type is parallel to 11th rib, and the short is horizontal and less oblique than the long type. For thoracolumbar surgery, it is important to understand the relationship of the pleural sac to the 12th rib. The pleural sac passes caudally over the inferior border to the 12th rib and continues in this direction for 1 to 2 cm. From there it passes horizontally, crossing caudally and 3 to 4 cm lateral to the 12th rib head, and it continues to pass along the 12th rib for another 7 to 8 cm ( Fig. 48-5 ).
Soft-Tissue Anatomy
Many important soft-tissue structures surround the bony cylindrical canal of the lumbar spine and triangular sacrum. These include exiting nerve roots and plexus, vasculatures, muscles, ligaments, and viscera, as well as synovial and nonsynovial tissues. Given a solid understanding of these intimate anatomic relationships is crucial for successful surgery in the lumbar and sacral spine, they are described in detail in the following sections.
Lumbar Spine
Muscles and Ligaments.
The ligaments important to the lumbar spine include anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL), ligamentum flavum, interspinous ligaments, supraspinous ligament, and intertransverse ligaments. The ALL runs along the anterior aspect of the spine and helps to resist extension of the spine. The PLL runs along the posterior part of the vertebral body; it marks the anterior boarder of the spinal canal and helps to resist flexion of the spine. The ligamentum flavum bridges the space between adjacent laminae, attaching to the ventral surface of the upper lamina and rostral lip of the lower one. It usually thins out as it ascends to the upper lamina, and this feature can be useful in assessing the extent of laminectomy required during spinal decompression. The interspinous and supraspinous ligaments help to provide the posterior tension band and help to resist flexion of the spine; however, this function is much less important in the lumbar as compared to the cervical region of the spine.
The intrinsic and extrinsic muscles adjacent to the spine are commonly dissected in approaches to the spine and provide important landmarks in specific strategies. Intrinsic muscles consist of the erector spinae, multifidus, quadratus lumborum, and deep muscles.
The large erector spinae muscle is divided into three columns: the iliocostalis, longissimus, and spinalis muscles. The lateral column or iliocostalis muscle is the most lateral of the group and arises from the iliac crest and inserts into the ribs. The intermediate column or longissimus muscle runs between the transverse processes of the vertebrae and is the largest, longest, and thickest of the erector spinae. The medial column or the spinalis muscle, small and often incomplete, inserts and attaches to spinous processes in the lumbar and thoracic region. All three columns of the erector spinae muscle extend the vertebral column and bend the vertebral column laterally.
Central to the erector spinae muscles are several short muscles that interconnect adjacent and nearby vertebral bodies. This group of small muscles, called the multifidus muscles, originates on the mamillary processes of the superior articular process and runs rostrally and medially to insert on the spinous processes of vertebrae two to four segments above.
The quadratus lumborum muscle is located ventral and lateral to the erector spinae muscles. This muscle originates on the iliac crest and iliolumbar ligament and runs obliquely to insert ventrally on the lowest rib and transverse processes of the upper four lumbar vertebrae. Ventral and medial to this muscle are the small intertransversarii muscles that span the transverse processes. Ventrolateral and adjacent to the lumbar vertebral bodies are the psoas muscles, which originate from the lateral aspects of the vertebral bodies and transverse processes of L1 to L5 and pass through the pelvis and into the thigh dorsal to the posterior inguinal ligament ( Figs. 48-6 and 48-7 ).
The extrinsic musculature consists of the rectus abdominis, external oblique, internal oblique, transversalis, latissimus dorsi, and serratus dorsalis caudalis muscles (see Fig. 48-7 ). The rectus abdominis muscles run bilaterally on the ventral abdominal wall from the pubis to the middle ribs. The internal and external oblique muscles and the transversalis muscles are layered superficial to deep as described. They arise from the ribs and thoracodorsal fascia dorsally and insert on the iliac crest caudally and the linea alba medially. The latissimus muscle is a large and diffuse muscle that originates on the sacrum, dorsal iliac crest, and 10th, 11th, and 12th ribs. The fibers in the costoiliac interval run rostrally and laterally. The serratus dorsalis caudalis muscle originates from the lower four ribs, runs caudally and medially, and inserts on the thoracolumbar fascia ventral to the latissimus dorsi muscle.
Exiting Nerve Roots.
The lumbar plexus is formed within the psoas major muscle. The largest and most important branches of the lumbar plexus are the obturator and femoral nerves (L2, L3, and L4). The ilioinguinal and iliohypogastric nerves are derived from L1, enter the abdomen dorsal to the medial arcuate ligament, and pass inferolaterally, ventral to the quadratus lumborum muscle, piercing it near the anterior superior iliac spine. The genitofemoral nerve (L1 and L2) pierces the fascia iliaca and the ventral surface of the psoas major muscle and divides lateral to the common and external iliac arteries into two femoral and genital branches. The lumbosacral trunk (L4 and L5) is a large flat nerve, from which the L4 component descends through the psoas major muscle on the medial part of the transverse process of the L5 vertebra and passes closely over the ala of the sacrum to join the first sacral nerve ( Fig. 48-8 ).
The sympathetic and parasympathetic nerves are distributed to the abdominal viscera via a tangle of plexuses and ganglia located on the ventral surface of the aorta. The principal components of this system are the celiac plexus ganglia, which are located on each side of the celiac trunk at the level of the rostral aspect of the first lumbar vertebra. The greater, lesser, and least splanchnic nerves are branches of thoracic sympathetic ganglia 5 to 12. The hypogastric plexus runs on the ventral surface of the aorta. It receives contributions from the lateral rami of the right and left lumbar sympathetic trunks and from median rami of the celiac plexus and the superior and inferior mesenteric plexuses. It spans the distance from the fourth lumbar to the first sacral vertebra. Its shape and bifurcations can be variable. In males, the plexus innervates the bladder, vas deferens, and seminal vesicles and is important in the neurophysiology of ejaculation ( Fig. 48-9 ). This is clinically relevant during anterior lumbar interbody fusion (ALIF) procedures, because a small percentage of patients may experience retrograde ejaculation if the hypogastric plexus is injured.
Soft-Tissue Structures.
The thoracolumbar junction is one of the more complicated areas of the vertebral column and has important soft-tissue anatomic structures that require special attention. The thoracolumbar fascia is made up of dorsal, intermediate, and ventral layers. The dorsal layer surrounds the erector spinae muscles dorsally. It arises with the tendon of the latissimus dorsi on the sacrum and iliac crest and attaches on the spinous processes of the lumbar vertebrae. The intermediate layer of the thoracolumbar fascia attaches to all of the transverse processes of the lumbar vertebrae and to the caudal border of the 12th rib. The lumbocostal ligament of Henle arises from the transverse process of L1 and runs rostrolaterally, inserting to the caudal border of the 12th rib close to its medial end. The ventral layer of the thoracolumbar fascia is attached to the lateral arcuate ligament rostrally, to the iliac crest caudally, to the transversalis fascias laterally, and to the psoas fascia medially. This layer covers the quadratus lumborum muscle and is in contact with the retroperitoneal contents ( Fig. 48-10 ; see also Fig. 48-6 to 48-9 ).
The lateral arcuate ligament arises from the L1 transverse process and crosses the proximal portion of the quadratus lumborum muscle to attach to the lower border of the 12th rib lateral to the insertion of the quadratus lumborum muscle.
The diaphragm consists of a peripheral muscular portion and a central aponeurosis called the central tendon, upon which the muscular portion converges ( Fig. 48-11 ). The muscular portion is divided into three parts based on the origins of its fibers: sternal, costal, and lumbar. The sternal part of the diaphragm arises from the xiphoid process. The costal part of the diaphragm arises from the internal surface of the caudal six ribs at the costal margin. The lumbar part of the diaphragm arises from the lumbar vertebrae by two crura and three arcuate ligaments. The musculotendinous crura envelop the aorta and attach ventrolaterally to the rostral two lumbar vertebral bodies on the left and the upper three on the right. The crura blend with the anterior longitudinal ligament of the lumbar spine. Three arcuate ligaments give rise to fibers of the diaphragm. The median arcuate ligament unites the medial sides of the two crura. The medial arcuate ligament on each side is a thickening of the ventral thoracolumbar fascia over the rostral part of the psoas muscle. From the crus it runs over the psoas and has an attachment to the transverse process of the first lumbar vertebra. The lateral arcuate ligament is a thickening of the anterior thoracolumbar fascia running over the rostral aspect of the quadratus lumborum muscle forming attachments to the 12th rib and transverse process of the first lumbar vertebra.
The abdominal aorta begins at the aortic hiatus in the diaphragm at the level of the T12-L1 intervertebral disc and ends at about the level of L4 by dividing into the two common iliac arteries. The inferior vena cava begins ventral to the fifth lumbar vertebra by the union of the common iliac veins and ascends to the right of the median plane and pierces the central tendon of the diaphragm at the level of the eighth thoracic vertebra. Five anatomic variants of the aortocaval axis are outlined according to the level of bifurcation and origin of the aorta and vena cava, respectively ( Fig. 48-12 ). The aorta is ventral to the vena cava and lumbar vertebral bodies and sits slightly to the left, whereas the vena cava is located slightly to the right.
The branches of the abdominal aorta may be grouped into four types: (1) those arising rostrally: the celiac (T12), superior mesenteric (L1), and inferior mesenteric (L3) arteries; (2) those arising laterally: the renal (L1), the middle suprarenal (L1), and the testicular or ovarian (L2) arteries; (3) those arising dorsolaterally: the parietal branches of the inferior phrenic arteries, which give rise to the superior suprarenal arteries, and the four pairs of lumbar arteries; and (4) an unpaired parietal artery, the sacral artery, which arises from the dorsal surface of the aorta just proximal to its bifurcation ( Fig. 48-13 ). The lumbar arteries pass dorsomedially. On the right they run dorsal to the inferior vena cava, dividing between the transverse processes into the ventral and dorsal branches. The ventral branch passes deep to the quadratus lumborum muscle to anastomose with the inferior epigastric arteries. Each dorsal branch passes dorsally lateral to the articular processes and supplies the spinal cord, cauda equina, meninges, erector spinae muscles, and overlying skin. The radicular arteries, which supply blood to the posterior and anterior spinal arteries, arise from these dorsal branches. The largest of these, the arteria radicularis magna (also known as the spinal artery of Adamkiewicz), supplies most of the blood to the caudal spinal cord, including the lumbosacral enlargement ( Figs. 48-14 and 48-15 ).
Tributaries of the inferior vena cava include the common iliac veins (L5), the lumbar veins, the right testicular or ovarian vein (the left drains into the left renal vein), the renal veins, the azygos vein, the right suprarenal vein (the left also drains into the renal vein), the inferior phrenic veins, and the hepatic veins. The lumbar veins consist of four or five segmental pairs. They may drain separately into the inferior vena cava or the common iliac vein, but they are usually united on each side by a vertical connecting vein, the ascending lumbar vein that lies dorsal to the psoas major muscle. Each ascending lumbar vein passes dorsal to the medial arcuated ligament of the diaphragm to enter the thorax. The right ascending lumbar vein joins the right subcostal vein to form the azygos vein, whereas the left subcostal vein forms the hemiazygos vein.
The cisterna chili, also known as receptaculum chili, is a saclike structure located between the origin of the abdominal aorta and the azygos vein. It lies on the right sides of the bodies of the first two lumbar vertebrae and is located dorsal to the right crus of the diaphragm. The thoracic duct begins at the cisterna chyli.
Each kidney lies dorsal to the peritoneum on the dorsal abdominal wall. The kidneys lie along the vertebral column against the psoas muscle. The ureter is retroperitoneal throughout its length. It adheres to the peritoneum and is usually retracted with it during retroperitoneal approaches to the spine. The ureter descends nearly vertically along the psoas major muscle. On the right, it is next to the vena cava, and on both sides, it crosses the brim of the pelvis and the external iliac artery, just beyond the bifurcation of the common iliac artery.
Sacral Spine
Synovial and Nonsynovial Tissue.
The joints of the sacrum consist of the bilateral facet joints articulating with the fifth lumbar vertebrae superiorly, the sacrococcygeal joint connecting the coccyx via the cornua caudally, and the sacroiliac joints connecting with the pelvic bone laterally. The anterior longitudinal ligament passes over the sacral promontory. The posterior longitudinal ligament runs across the dorsal surface of the lumbosacral disc forming the ventral margin of the sacral canal. The sacrococcygeal joint contains a disc and is secured by four ligaments (ventral, dorsal, and two lateral ligaments). The sacroiliac joint is strengthened ventrally by ventral and lumbosacral ligaments. Other accessory ligaments are the sacrospinous, sacrotuberous, and iliolumbar ligaments.
Nerve Roots.
The sacral canal contains sacral and coccygeal nerve roots. The filum terminale consists of two parts: the interna and the externa. The interna extends from the tip of the conus to the level of the second sacral neuroforamen. The externa begins at the level of the second sacral neuroforamen and attaches to the first coccygeal vertebra. The dorsal root ganglia lie in the sacral canal, central and rostral to the foramina from which their respective rami emerge. The sacral plexus is complex, formed by the lumbosacral trunk and the sacral nerves ( Fig. 48-16 ). The predominant nerves of the plexus are the sciatic nerve, superior and inferior gluteal nerves, and pudendal nerve. The sympathetic trunk passes deep to the common iliac artery to run on the ventral surface of the sacrum. Each trunk continues caudally to the coccyx to form a single ganglion, the ganglion impar. The pelvic plexus lies embedded in the subperitoneal serosa lateral to the sacrum on the rostral surface of the obturator internus ( Fig. 48-17 ).
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