Functional Anatomy Of The Lumbar Spine

Functional Anatomy of the Lumbar Spine Nabil A. Ebraheim, MD,* Ali Hassan, MD,† Ming Lee, MS,‡ and Rongming Xu, MD‡ The dysfunction of the lumbar spine has a pivotal role in etiology of low back pain. A thorough knowledge of the functional anatomy of the lumbar spine is needed to aid in understanding the mechanisms that lead to low back pain and to provide rationale of management. This article reviews functional anatomy of the lumbar spine involving the bony structures, articulation, ligaments, muscles, blood supply, and neural structures. Semin Pain Med 2:131-137 © 2004 Elsevier Inc. All rights reserved. KEYWORDS anatomy, lumbar spine, low back pain

T

he dysfunction of the lumbar spine has a pivotal role in etiology of low back pain. A thorough knowledge of the functional anatomy of the lumbar spine is needed to aid in understanding of the mechanisms that cause low back pain and to provide rationale of management. This article reviews the functional anatomy of the lumbar spine involving the bony structures, articulation, ligaments, muscles, blood supply, and neural structures.

Osseous Structures There are 5 lumbar vertebrae, followed by the sacrum. Each lumber vertebra has 2 parts, the vertebral body and neural arch. The vertebral body lies anteriorly, and its dimensions gradually increase from cephalad to caudal. The neural arch lies posterior to vertebral body and consists of a pair of pedicles emerging from the postero-lateral surface of the upper portion of vertebral body that joins with paired laminae, which are located further posteriorly (Fig. 1). When viewed from above, the superior surface of vertebral body is wider transversely and resembles to kidney’s shape. The spinal canal is triangular, which is most distinguishable at the L5 level. The angled lateral borders of the spinal canal are called the lateral recesses, which constitute the bony canal of the spinal nerve root. Pedicles are short and have a slight medial inclination. In general, the pedicle width increases gradually from L1 to L5 but the pedicle height varies between individuals (Table

*Department of Orthopaedic Surgery, Medical College of Ohio, Toledo, OH. †Department of Anesthesiology, Medical College of Ohio, Toledo, OH. ‡Department of Orthopaedic Surgery, Ningbo 6th Hospital, Ningbo, Zhejiang, P.R. China. Address correspondence to Nabil A. Ebraheim, MD, Department of Orthopaedic Surgery, Medical College of Ohio, 3000 Arlington Avenue, Toledo, OH 43614.

1537-5897/04/$-see front matter © 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.spmd.2004.08.004

1).1-3 The pedicle lengths measured between the dorsal and ventral cortex of the vertebra averages between 40 and 50 mm. The medial inclination of the lumbar pedicle increases consistently from L1 to L5. The projection point of the pedicle axis is located above the midline of the transverse process at the levels above L4. At L4, the projection point is close to the midline of the transverse process. At L5, this point is located inferior to the midline of the transverse process.1 The lamina is thicker and oriented in a more vertical direction in the sagittal plane as compared with the cervical and thoracic spines. The lamina may be divided into 2 portions: the cephalic and caudal.4 The cephalic portion is arched and has a smooth inner surface, whereas the caudal portion has a rough inner surface, which is the site for the attachment of the ligamentum flavum. The portion of the lamina between the superior and inferior articular processes and just below the level of the pedicle is the isthmus or pars interarticularis, which is the common site of stress fractures. The superior and inferior articular facets are quite different from the cervical and thoracic regions, which are orientated in the sagittal plane. In lumber region, the superior articular surface is concave and faces posteromedially, and the inferior articular surface is convex and faces anterolaterally. The facet angles relative to the sagittal plane ranges from 120° to 150°, with a trend of consistent decrease from L1 to L5.5 From the junction of 2 lamina, a spinous process arises posteriorly. It is almost horizontal, quadrangular, and thickened along its posterior and inferior borders.

Articulations and Ligaments The articulations include the intervertebral disc anteriorly and a pair of the facet or zygapophyseal joints posteriorly, reinforced by ligaments. The intervertebral discs, which are 131

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Figure 1 The superior, posterior, and lateral views of the lumbar vertebra.

disc degeneration, presented by the decrease in height of the disc. The annulus fibrosus consists mainly the collagenous fibers and looks like a laminated structure surrounding the nucleus pulposus. The posterior portion of annulus fibrosus is thinner as compared with its anterior portion. The fibers of the lamellae are arranged obliquely in concentric rings that overlap one another. The peripheral fibers of the annulus fibrosus insert into the cartilaginous end plates and anterior and posterior longitudinal ligaments. In the posterior portion of the disc, the fibers run in a more vertical direction.7 A thinner posterior annulus fibrosus and a more vertical arrangement of the fibers could account for increased incidence of posterior or posterolateral disc herniation as compared

avascular structures, are located in between adjacent vertebral bodies and allow flexion, extension, and lateral bending motions. They mainly consist of a centrally located nucleus pulposus, the annulus fibrosus encircling the nucleus pulposus, and cartilaginous end plates adjacent to the surfaces of the vertebral bodies (Fig. 2). The nucleus pulposus is composed mainly of mucoid material, which contains 70% to 90% water.6 On T2-weighted magnetic resonance imaging, the nucleus pulposus shows hyperintensity signal (white). The percentage of the water in the nucleus pulposus gradually decreases with aging. After the fifth decade, the nucleus pulposus becomes less distinguishable from the annulus fibrosis due to the loss of water. The loss of water could be the major factor responsible for

Table 1 Anatomic Parameters of Lumbar Pedicle (Mean ⴞ SD, mm) Zindrick3 L1 L2 L3 L4 L5

Panjabik2

Ebraheim15

PH

PW

PH

PW

PH

PW

15.4 ⴞ 2.8 15.0 ⴞ 1.5 14.9 ⴞ 2.4 14.8 ⴞ 2.1 14.0 ⴞ 2.3

8.7 ⴞ 2.3 8.9 ⴞ 2.2 10.3 ⴞ 2.6 12.9 ⴞ 2.1 18.0 ⴞ 4.1

15.9 ⴞ 0.8 14.9 ⴞ 0.5 14.4 ⴞ 0.6 15.5 ⴞ 0.5 19.6 ⴞ 0.9

8.6 ⴞ 0.9 8.3 ⴞ 0.7 10.2 ⴞ 0.6 14.1 ⴞ 0.3 18.6 ⴞ 1.0

14.1 ⴞ 1.3 14.0 ⴞ 1.2 13.9 ⴞ 1.4 12.8 ⴞ 1.7 11.4 ⴞ 1.4

7.5 ⴞ 1.5 8.2 ⴞ 1.3 9.8 ⴞ 1.1 12.7 ⴞ 1.9 18.0 ⴞ 2.4

PH, pedicle height; PW, pedicle width.

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Figure 2 The sagittal and transverse sections of the lumbar disc.

with anterior herniation. Disc herniations are commonly seen in the lumbar spines at the levels of L4 to 5 and L5 to S1. The cartilaginous end plate that is located between the vertebral body and disc, functions as a growth plate and transfuses nutrients from the vertebral body to the disc.

Facet Joints Facet joint (zygapophyseal or synovial joint) consists of the adjacent inferior and superior articular processes and the articular capsule. The articular surfaces are covered by hyaline cartilage, which allows sliding motion occurring in the posterior arch of the spinal column. The articular capsules are thin, and have an inner synovial and an outer fibrous membrane. They are attached peripheral to the articular surfaces of the facet joints. In the lumbar region narrowing of the joint space, thinning of articular cartilage, and hypertrophy of the subarticular cortical bone are the frequently observed changes due to aging process.8

Ligaments There are several ligaments that play an important role in stabilization of the spines as one unit. These include the anterior and posterior longitudinal ligaments, ligamentum flava or yellow ligaments, supraspinous and interspinous ligaments (Fig. 3). The anterior longitudinal ligament is a strong band that attaches to the whole anterior aspect of the vertebral bodies and intervertebral discs from the skull down to the upper

part of sacrum. It is thicker anteromedially and thinner laterally. Its most superficial fibers are the longest and extend over 3 to 4 vertebrae. Its deepest fibers extend over 2 vertebrae and are firmly attached to the inferior margin of the one vertebra and to the superior margin of the next. Limitation of extension of the spinal column is the main function of the anterior longitudinal ligament. Opposing the anterior longitudinal ligament, the posterior longitudinal ligament attaches to the posterior aspect of the vertebral bodies and discs, from the occipital bone to the sacrum. It is broad and uniform in the cervical region, but in the thoracic and lumber regions it is narrow over the middle of the vertebrae and broad over the discs. In the region of the intervertebral foramen, the posterior longitudinal ligament extends laterally and fuses with the lateral extensions of the anterior longitudinal ligament. Similar to the anterior longitudinal ligament, the superficial fibers of the posterior longitudinal ligament extend over 3 to 4 vertebrae, and the deeper fibers bridge merely the adjacent vertebrae. The role of the posterior longitudinal ligament is to stabilize the spinal column during flexion. The ligamentum flava are present in between the laminae of adjacent vertebrae and fuse with each other in the midline. They are mainly composed of the yellow elastic fibers, which run in vertical direction. The attachments of the ligamentum flavum extend from the lower portion of the anterior surface of the upper laminae to the upper portion of the posterior surface of the lower laminae, covering whole interlaminar

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Figure 3 The sagittal section and anterior view of the lumbar ligaments.

space.9,10 Laterally, the ligamentum flavum fuses with the capsule of the facet joint. The ligamentum flavum is very thick in the lumbar spine. The lumbar ligamentum flavum has 2 layers, 1 superficial and another that is deeper.11 Hypertrophy or thickening of the ligamentum flavum in the lumbar spine is one of the common causes of spinal stenosis.12 The interspinous and supraspinous ligaments are the posterior ligaments of the spinal column, which connect the spinous processes with each other. The interspinous ligament is thin and extends from the lower border of one spinous process to the upper border of the next. The supraspinous ligament is stronger and extends over spinous processes from the occipital bone to the sacrum. The intertransverse ligaments are membranous structures located between the transverse processes, typically in the lumbar region. The lumbar nerves lateral to the intervertebral foramina lie directly underneath the intertransverse ligaments.13

Neural Structures The spinal cord ends between the disc levels of T12 to L1 and L2 to 3. The distal part of spinal cord forms a cone like structure called as conus medullaris, and is followed by a bundle of nerve roots termed the cauda equina (Fig. 4). The lumbar enlargement of the spinal cord is located between the L1 and S3 segments that gives rise to large nerves supplying the lower extremities.

The cauda equina is arranged in a consistent pattern when seen in cross-sectional plane.14 At L4 to 5 level, the L5 nerve is situated in the anterolateral corner of the dural sac, followed by the S1 and S2 to 5 nerves posteriorly. At L5-S1 level, the S1 nerve is situated in the anterolateral corner of the dural sac, followed by the S2 to 3 and S4 nerves posteriorly. The spinal cord is covered by three meninges, the dura mater, arachnoid, and pia mater. The dura mater is the outermost layer of the spinal meninges and is composed of dense, fibrous connective tissue. The space between the spinal canal and the dura matter is called the epidural space, and contains fat, loose connective tissue, and venous plexus, which function as padding around the spinal cord. The middle layer of the spinal meninges is the arachnoid, which consists of delicate connective tissue. There is a small space between the dural matter and arachnoid, termed the subdural space. The subdural space contains serous fluid. The inner thin transparent layer of the spinal meninges is the pia matter, which contains numerous nutrient vessels. The pia matter adheres closely to the surface of the spinal cord, and extends laterally to the dura matter between the ventral and dorsal nerve roots on either side. The lateral extensions of the pia matter are called the dentate ligaments, which serve to protect the spinal cord from injury during movement of the spinal column. The space between the arachnoid and pia matter is called the subarachnoid space, which contains the cerebrospinal fluid.

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Figure 5 Blood supply of the spinal cord.

the lower abdominal wall and part of the lower extremity. The femoral nerve is the largest branch from the lumbar plexus, which passes under the inguinal ligament to supply the flexor muscles of hip and extensor muscles of knee. Figure 4 The posterior view of spinal cord.

The Spinal Nerves The spinal nerves consist of ventral and dorsal roots that leave and enter the spinal cord, respectively. The ventral roots contain axons of motor neurons from the anterior gray horn of the spinal cord. The dorsal roots contain sensory axons that arise from the sensory cell bodies contained in ganglia, which are the enlargement of the dorsal roots. There are 11 pairs of spinal nerves in the lumbar region, including 5 lumbar, 5 sacral, and 1 coccygeal. The spinal nerves emerge below the corresponding vertebrae in the lumbosacral region. The spinal nerves exiting from the spinal canal are close to the medio-inferior border of the upper pedicle and lie in the upper portion of the intervertebral foramina in the lumbar spine.15,16 Most ganglia of the lumbar spinal nerves lie within the intervertebral foramen.17,18 After exiting from the intervertebral foramina, each spinal nerve divides into a small dorsal ramus and a large ventral ramus. The dorsal rami courses posteriorly to supply the spinal ligaments, muscles and skin of the back. The ventral rami are longer, and run in infero-lateral direction in the lumbar region to form the lumbar, and sacral plexuses. These plexuses give rise to nerves supplying the muscles, joints, and skin of the upper and lower extremities. The lumbar plexus consists of the ventral rami of L1, L2, L3, and L4 nerves. The lumbar plexus courses in the inferolateral direction, passes posterior to the psoas major muscle and anterior to the quadratus lumborum muscle. It supplies

Figure 6 The superficial layer of the posterior muscles of spine.

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Figure 7 The intermediate (right) and deep (left) layers of the posterior muscles of spine.

The sacral plexus consists of the ventral rami of L4, L5, and S1 through S4 nerves. Within the pelvis, the sacral plexus gives its peripheral nerves to supply the buttocks, perineum, and lower extremities. The largest branch from the sacral plexus is the sciatic nerve, which leaves the pelvis through the greater sciatic notch and supplies entire leg and foot. The main branches from the sciatic nerve are the common peroneal nerve and the tibial nerve. The common peroneal nerve further branches into the superficial and deep peroneal nerves.

Blood Supply The blood supply of the lumbar spine and spinal cord comes mainly from the segmental arteries that originate from the intercostal and lumbar arteries. Each of the segmental arteries gives off a spinal branch supplying the vertebrae, spinal cord and cauda equina. The spinal branch enters the spinal canal through the intervertebral foramen and anastomoses with the

spinal arteries above and below. The fifth lumbar vertebra and sacrum receive their blood supply from the fourth lumbar artery, the iliolumbar arteries, and the middle and lateral sacral arteries. The main blood supply of the spinal cord is by a single anterior spinal artery, paired posterior spinal arteries and several radicular (medullary) arteries (Fig. 5). The number of the medullary arteries varies from the cervical to lumbar region because only a few segmental arteries branch off the medullary arteries to join with the anterior spinal artery.19 There are usually one to three medullary arteries in lower thoracic and lumbosacral cord regions.20 The most caudal medullary artery is the largest with a mean diameter of 0.9 mm and also called Adamkiewicz’s artery. It usually originates from the lower intercostal or upper lumbar artery.21 The medullary arteries provide vital contributions to blood supply of the anterior spinal artery. The risk of ischemic injury to spinal cord greatly increases if the anterior spinal artery is compromised by osteophytes, disc herniation or fracture, or if

Functional anatomy of the lumbar spine medullary artery gets injured. The radicular arteries may also provide blood supply to the cauda equina.22,23 Veins surrounding the spinal cord include an anterior internal vertebral venous plexus and a posterior internal vertebral venous plexus, which are valveless venous structures in the epidural space. The anterior internal venous plexus consists mainly of two longitudinal veins located between the pedicles and the posterior longitudinal ligament. These veins anastomose with each other and also with the basivertebral sinus that drains blood from the vertebral body. The posterior internal venous plexus is lesser dense, and anastomoses with the anterior internal venous plexus. Blood from the internal venous plexus is drained through the intervertebral foramen into the segmental veins.

Muscle Distribution The muscles surrounding the lumbar spine are divided into three groups: posterior, lateral, and anterior, depending on their location. The posterior muscles of the lumbar spine generally consist of superficial, intermediate and deep layers. The superficial layer is the thoracolumbar fascia, which is thick and strong, and may play an important role in rotation of the trunk and stabilization of the lower back (Fig. 6).24 The intermediate layer of the muscles in the lumbar region is made by serratus posterior inferior, which origins from the spinous processes of the cervicothoracic and thoracolumbar spine and inserts into the ribs. The deep layer of the posterior muscles consists of the erector spinae muscles, which are vertically oriented muscle bundles from the iliosacrolumbar region up to the cervical region (Fig. 7). There are 3 distinct columns of the erector spinae muscles under the thoracolumbar fascia in the lumbar region: iliocostalis laterally, longissimus centrally, and spinalis medially. The iliocostalis is divided into the iliocostalis lumborum, iliocostalis thoracis, and iliocostalis cervicis based on its distribution. It originates from the posterior iliac crest and gives muscle slips to the ribs and the transverse processes of the lower cervical spine. The longissimus is the largest muscle of the erector spinae, and is also divided into 3 portions: longissimus thoracis, longissimus cervicis, and longissimus capitis. It originates from the posterior surface of the sacrum and gives muscle slips to the transverse processes of the thoracic and cervical spine and the mastoid process. The spinalis is the smallest of all the erector spinae, and has three portions: spinalis thoracic, spinalis cervicis, and spinalis capitis. The spinalis originates from the spinous processes of the upper lumbar spine and gives muscle slips to the spinous processes above. Under the erector spinae muscle, there are several short muscles called the semispinalis, multifidi, and rotators. These muscles course in an oblique direction between the spinous processes and the transverse processes of the spine. Most of the posterior muscles of the spine are supplied by the dorsal rami of the spinal nerves and the dorsal branches of segmental arteries. Their function includes extension, lateral bending, and rotation of the spine. The lateral or anterolateral muscles in the lumbar region in-

137 clude, the iliopsoas major and quadratus lumborum. The psoas major originates from the anterolateral aspect of the vertebral bodies and discs, from the anterior aspect of the transverse processes of entire lumbar spine, and inserts onto the lesser trochanter of the femur. The quadratus lumborum is a rectangular muscle that originates from the L5 transverse process and iliac crest, and inserts onto the transverse processes above and the twelfth rib. The anterior and lateral muscles of lumber spine are innervated by the ventral rami of the spinal nerves. Most of them contribute to flexion and rotation of the lumbar spine.

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