Vincent J Devlin MD and Darren L Bergey MD

OSTEOLOGY

1. Describe a typical lumbar vertebra.

The vertebral bodies are kidney-shaped with the transverse diameter exceeding the anteroposterior diameter (Fig. 3-1). The vertebral body may be divided by an imaginary line passing beneath the pedicles into an upper and lower half. Six posterior elements attach to each lumbar vertebral body. Three structures lie above this imaginary line (superior facet, transverse process, pedicle) and three structures lie below (lamina, inferior facet, spinous process). The pars interarticularis is located along this imaginary dividing line. The transverse processes are long and thin except at L5, where they are thick and broad and possess ligamentous attachments to the pelvis. The five lumbar vertebral bodies increase in size from L1 to L5.

Transverse process

Superior articular

process

Body \ .

Pedicle

\ Spinous process

Inferior articular

A

process

Superior articular process

•S) /--^v—Transverse process

/

rO^L\^^Body of vertebra

1 Lamina

O \ ^J Spinous process

B

\ Inferior articular process

_-—-Spinous process

Lamina

^.Mammillary process

i j \ /

^-—Superior articular process

V/

__^>j-Transverse process

---Pedicle

((

---

Vertebral canal

\ V

---Body

C ^

(. -

Figure 3-1. Typical lumbar vertebra. Lateral (A), posterior (B), and cranial (C) views. D, The six named posterior elements: SF, superior facet; IF, inferior facet; L, lamina; SP, spinous process; P, pedicle; TP, transverse process. (A, B, C, From Borenstein DG, Wiesel SW, Boden SD. Anatomy and biomechanics of the lumbosacral spine. In: Low Back Pain: Medical Diagnosis and Comprehensive Management. 2nd ed. Philadelphia: Saunders; 1995. D, From McCulloch JA, Young PH. Musculoskeletal and neuroanatomy of the lumbar spine. In: McCulloch JA, Young PH, editors. Essentials of Spinal Microsurgery. Philadelphia: Lippincott-Raven; 1998. p. 250.)

2. What region of the posterior elements of the spine is prone to failure when subjected to repetitive stress?

The pars interarticularis is an area of force concentration and is subject to failure with repetitive stress. A defect in the bony arch in this location is termed spondylolysis. The pars interarticularis is the concave lateral part of the lamina that connects the superior and inferior articular facets. The medial border of the pedicle is in line with the lateral border of the pars between L1 and L4. At L5 the lateral border of the pars marks the middle of the pedicle.

3. Describe the anatomy of the lumbar pedicles.

The pedicle connects the posterior spinal elements (lamina, transverse processes, facets) to the vertebral body. Lumbar pedicle widths are largest at L5 (18 mm) and smallest in the upper lumbar region (6 mm at L1). The pedicles in the lumbar spine possess a slight medial inclination, which decreases from distal to proximal levels. The pedicles angle medially 30° at L5 and 12° at L1.

4. What are the key anatomic features of the sacrum?

The sacrum is a triangular structure formed from five fused sacral vertebrae (Fig. 3-2). The S1 pedicle is the largest pedicle in the body. The sacral promontory is the upper anterior border of the first sacral body. The sacral ala (lateral sacral masses) are bilateral structures formed by the union of vestigial costal elements and the transverse processes of the first sacral vertebra. Four intervertebral foramina give rise to ventral and dorsal sacral foramina. The median sacral crest is formed by the fused spinous processes of the sacral vertebrae. The sacral cornu (horn) is formed by the S5 pedicles and is a landmark for locating the sacral hiatus. The sacral hiatus is an opening in the dorsal aspect of the sacrum due to absence of the fourth and fifth sacral lamina.

Figure 3-2. Anatomy of the sacrum and coccyx. Left, Anterior view. Right, Posterior view. (From Borenstein DG, Wiesel SW, Boden SD. Anatomy and biomechanics of the lumbosacral spine. In: Low Back Pain: Medical Diagnosis and Comprehensive Management. 2nd ed. Philadelphia: Saunders; 1995.)

5. What are the key anatomic features of the coccyx?

The coccyx is a triangular structure that consists of three, four, or five fused coccygeal vertebrae. The coccyx articulates with the inferior aspect of the sacrum.

ARTICULATIONS, LIGAMENTS, AND DISCS

6. Describe the anatomy of the facet joints of the lumbar spine.

The inferior articular process of the cephalad vertebra is located posterior and medial to the superior articular process of the caudad vertebrae. The upper and mid-lumbar facet joints are oriented in the sagittal plane. This orientation allows significant flexion-extension motion in this region but restricts rotation and lateral bending. The facets joints at L5-S1 oriented in the coronal plane, thereby permitting rotation and resisting anterior-posterior translation.

7. What anatomic structures provide articulations between the lumbar vertebral bodies? Between the vertebral arches? Between L5 and the sacrum?

The structures that provide articulations between the lumbar vertebral bodies are the same as in the thoracic region: (1) anterior longitudinal ligament, (2) posterior longitudinal ligament, and (3) intervertebral disc.

The anatomic elements that provide articulations between the adjacent lumbar vertebral arches are the same as in the thoracic region: (1) articular capsules, (2) ligamentum flavum, (3) supraspinous ligaments, (4) interspinous ligaments, and (5) intertransverse ligaments.

Specialized ligaments connect L5 and the sacrum:

1. Iliolumbar ligament, which arises from the anteroinferior part of the transverse process of the fifth lumbar vertebra and passes inferiorly and laterally to blend with the anterior sacroiliac ligament at the base of the sacrum as well as the inner surface of the ilium.

2. Lumbosacral ligament, which spans from the transverse processes of L5 to the anterosuperior region of the sacral ala and body of S1.

8. Describe the alignment of the normal lumbar spine in reference to the sagittal plane.

The normal lumbar spine is lordotic (sagittal curve with its convexity located anteriorly). Normal lumbar lordosis (L1-S1) ranges from 30° to 80° with a mean lordosis of 50°. Normal lumbar lordosis generally begins at L1-L2 and gradually increases at each distal level toward the sacrum. The apex of lumbar lordosis is normally located at the L3-L4 disc space. Normally two-thirds of lumbar lordosis is located between L4 and S1 and one-third between L1 and L3 (Fig. 3-3).

Figure 3-3. Sagittal alignment of the lumbar spine. Average maximum lordosis as measured from superior L1 to superior S1. (Reproduced with permission from DeWald RL: Revision surgery for spinal deformity. In: Eilert RE, editor. Instructional Course Lectures, vol. 41. Rosemont, IL: American Academy of Orthopaedic Surgeons; 1992.)

9. Which contributes more significantly to the normal sagittal alignment of the lumbar region—the shape of the intervertebral discs or the shape of the vertebral bodies?

Eighty percent of lumbar lordosis occurs through wedging of the intervertebral discs, and 20% is due to the lordotic shape of the vertebral bodies. The wedge shape of the lowest three discs is responsible for one-half of total lumbar lordosis.

10. Describe the anatomy of the sacroiliac joint.

The sacroiliac joint is a small, auricular-shaped synovial articulation located between the sacrum and ilium (Fig. 3-4). The complex curvature and strong supporting ligaments of the sacroiliac joint minimizes motion. Ligamentous support is provided by anterior sacroiliac ligaments, interosseous ligaments, and posterior sacroiliac ligaments (most important). Other supporting ligaments in this region include the sacrospinous ligaments (ischial spine to sacrum) and sacrotuberous ligaments (ischial tuberosity to sacrum). Functionally, the sacrum and pelvis can be considered as one vertebra (pelvic vertebra), which functions as an intercalary bone between the trunk and lower extremities.

Figure 3-4. Lateral view showing the articular surface of the sacrum. (From Borenstein DG, Wiesel SW, Boden SD. Anatomy and biomechanics of the lumbosacral spine. In: Low Back Pain: Medical Diagnosis and Comprehensive Management. 2nd ed. Philadelphia: Saunders; 1995.)

NEURAL ANATOMY

11. Describe the contents of the spinal canal in the lumbar region.

The spinal cord terminates as the conus medullaris at the L1-L2 level in adults. Below this level, the cauda equina, composed of all lumbar, sacral, and coccygeal nerve roots, occupies the thecal sac. The lumbar nerves exit the intervertebral foramen under the pedicle of the same numbered vertebral body.

12. What structures comprise a lumbar anatomic segment?

The vertebral body, its associated posterior elements, and the disc below comprise an anatomic segment.

13. What is the difference between an exiting nerve root and a traversing nerve root?

Each lumbar anatomic segment can be considered to possess an exiting nerve root and a traversing nerve root. The exiting nerve root passes medial to the pedicle of the anatomic segment. The traversing nerve root passes through the anatomic segment to exit beneath the pedicle of the next caudal anatomic segment. For example, the exiting nerve root of the fifth anatomic segment is L5. This nerve passes beneath the L5 pedicle and exits the anatomic segment through the neural foramen of the L5 anatomic segment. The S1 nerve is the traversing nerve root and passes over the L5-S1 disc to exit beneath the pedicle of S1, which is located in the next caudad anatomic segment (Fig. 3-5).

Figure 3-5. The exiting nerve root and traversing root(s) of an unnumbered spinal segment. At the open arrow, the traversing nerve root becomes the exiting root of the anatomic segment below. (From McCulloch JA, Young PH. Musculoskeletal and neuroanatomy of the lumbar spine. In: McCulloch JA, Young PH, editors. Essentials of Spinal Microsurgery. Philadelphia: Lippincott-Raven; 1998. p. 249-327, http://www.lww.com.)

14. What analogy is commonly used to localize spinal pathology from caudad to cephalad within a lumbar anatomic segment?

The analogy of a house with three floors is most commonly used to localize spinal pathology (Fig. 3-6A). The first story of the anatomical house is the level of the disc space. The second story is the level of the neural foramen and lower vertebral body. The third story is the level of the pedicle and includes the upper vertebral body and transverse process.

15. How is the spinal canal subdivided into zones from medial to lateral to precisely locate compressive spinal pathology within a lumbar anatomic segment?

Neural compression may affect the thecal sac, nerve roots, or both structures. Central spinal stenosis refers to neural compression in the region of the spinal canal occupied by the thecal sac. Lateral stenosis involves the nerve root and its location is described in terms of three zones (see Fig. 3-6B and C) using the pedicle as a reference point. Zone 1 (also called the subarticular zone, entrance zone, or lateral recess) includes the area of the spinal canal medial to the pedicle and under the superior articular process. Zone 2 (also called the foraminal or midzone) includes the portion of the nerve root canal located below the pedicle. Zone 3 (also called the extraforaminal or exit zone) refers to the nerve root in the area lateral to the pedicle.

16. An L4-L5 posterolateral disc protrusion located entirely within zone 1 results in compression of which nerve root?

The most common location for a disc protrusion is posterolateral. This type of disc herniation impinges on the traversing nerve root of the L4 anatomic segment. This nerve is the L5 nerve root.

17. An L4-L5 lateral disc protrusion located entirely within zone 3 results in compression of which nerve root?

This describes the so-called far lateral disc protrusion. This type of disc protrusion impinges on the exiting nerve of the L4 anatomic segment. This nerve is the L4 nerve root.

Figure 3-6. A, Conceptualization of the lumbar anatomic segment as a house. B and C, Zone concept of the lumbar spinal canal. (From McCullough JA. Microdiscectomy: The gold standard for minimally invasive disc surgery. Spine State Art Rev 1997;11(2):382.)

Figure 3-6. A, Conceptualization of the lumbar anatomic segment as a house. B and C, Zone concept of the lumbar spinal canal. (From McCullough JA. Microdiscectomy: The gold standard for minimally invasive disc surgery. Spine State Art Rev 1997;11(2):382.)

Extraforaminal

Extraforaminal

18. Describe the location and significance of the superior hypogastric plexus. What can happen if it is injured during exposure of the anterior aspect of the spine?

The superior hypogastric plexus is the sympathetic plexus located along the anterior prevertebral tissues in the region of the L5 vertebral body and anterior L5-S1 disc. This sympathetic plexus is at risk during anterior exposure of the L5-S1 disc space. Disruption of this plexus in men may cause retrograde ejaculation and sterility. Erection would not be affected because it is a parasympathetically mediated function (Fig. 3-7).

VASCULAR STRUCTURES

19. Describe the blood supply to the lumbar vertebral bodies.

Each lumbar vertebra is supplied by paired lumbar segmental arteries. The segmental arteries for L1 to L4 arise from the aorta. The origin of the segmental arteries for L5 is variable and may arise from the iliolumbar artery, fourth lumbar segmental artery, middle sacral artery or aorta. As the segmental artery courses toward the intervertebral foramen, it divides into three branches:

1. The anterior branch (supplies the abdominal wall)

2. The posterior branch (supplies paraspinous muscles and facets)

3. The foraminal branch (supplies the spinal canal and its contents)

The venous supply of the lumbar region parallels the arterial supply. It consists of an anterior and posterior ladderlike configuration of valveless veins that communicate with the inferior vena cava.

20. What is Batson's plexus?

Batson's plexus is a system of valveless veins located within the spinal canal and around the vertebral body. It is an alternate route for venous drainage to the inferior vena cava system. Because it is a valveless system, any increase in abdominal pressure (e.g. secondary to positioning during spine surgery) can cause blood to flow preferentially toward the spinal canal and surrounding bony structures. Batson's plexus also serves as a preferential pathway for metastatic tumor and infection spread to the lumbar spine.

21. Where is the bifurcation of the aorta and vena cava located?

Most commonly, the bifurcation is over the L4-L5 disc or L5 vertebral body (Fig. 3-7).

Truncus sympatheticus

Aorta ,

Vena cava .

Superior hypogastric plexus s

Truncus sympatheticus

Aorta ,

Vena cava .

Superior hypogastric plexus s

Figure 3-7. Bifurcation of aorta and vena cava in relation to the spine. The superior hypogastric plexus. (From Hanley EN, Delmarter RB, McCulloch JA. Surgical indications and techniques. In: Wiesel SW, Weinstein JN, Herkowitz H. The Lumbar Spine. 2nd ed. Philadelphia: Saunders; 1996. p. 492-524.)

22. What is the significance of the iliolumbar vein?

The iliolumbar vein is a branch of the iliac vein that limits mobilization of the iliac vessels off of the anterior aspect of the spine (Fig. 3-8). This vein should be carefully isolated and securely ligated before attempting to expose the anterior aspect of the spine at the L4-L5 disc level.

Figure 3-8. Anatomy of the iliolumbar vein and environs. (From Canale ST, Beaty J. Campbell's Operative Orthopedics. 11th ed. Philadelphia: Mosby; 2007.)

FASCIA, MUSCULATURE, AND RELATED STRUCTURES

23. Why should a spine specialist be knowledgeable about the anatomy of the abdominal and pelvic cavities?

There are many important reasons why a spine specialist must possess a working knowledge of anatomy and pathology relating to the abdominal and pelvic cavities. Extraspinal pathologic processes within the abdominal and pelvic cavities (e.g. aneurysm, infection, tumor) may mimic the symptoms of lumbosacral spinal disorders. Surgical treatment of many spinal problems involves exposure of the anterior lumbar spine and/or sacrum through a variety of surgical approaches. Evaluation of complications after spinal procedures requires assessment not only of the vertebral and neural structures but also of vascular and visceral structures (e.g. bladder, intestines, spleen, kidney, ureter).

24. What muscles of the posterior abdominal wall cover the anterolateral aspect of the lumbar spine?

Psoas major and minor. These muscles originate from the lumbar transverse processes, intervertebral discs, and vertebral bodies and insert distally at the lesser trochanter and iliopectineal region, respectively. They must be mobilized during exposure of the anterior lumbar spine, taking care to avoid nerves that cross the psoas muscles (genitofemoral nerve, sympathetic trunk) as well as the lumbar plexus, which passes within the substance of these muscles.

Key Points

1. Lumbar lordosis begins at L1-L2 and gradually increases at each distal level toward the sacrum.

2. Six named posterior osseous elements attach to each lumbar vertebral body.

3. The spinal cord normally terminates as the conus medullaris at the L1- L2 level in adults.

4. The cauda equina occupies the thecal sac distal to the L1-L2 level in adults.

Websites

1. See lumbar spine anatomy: http://www.orthogate.org/patient-education/lumbar-spine/lumbar-spine-anatomy.html

2. See spine anatomy section, lumbar spine: http://www.spineuniverse.com/displayarticle.php/article1286.html

BiBLiOGRAPHY

1. Borenstein DG, Wiesel SW, Boden SD: Anatomy and biomechanics of the lumbosacral spine. In: Low Back Pain: Medical Diagnosis and Comprehensive Management, 2nd ed. Philadelphia: Saunders; 1995, pp 1-16.

2. Daubs, MD: Anterior lumbar interbody fusion. In Vaccaro AR, Baron EM, editors. Spine Surgery. Philadelphia: Saunders; 2008, pp 391-400.

3. Herkowitz HN, Dvorak J, Bell G, et al., editors. The Lumbar Spine. 3rd ed. Philadelphia: Lippincott; 2004.

4. Herkowitz HN, Garfin SR, Eismont FJ, editors. The Spine. 5 ed., Philadelphia: Saunders; 2006.

5. McCulloch JA, Young PH Musculoskeletal and Neuroanatomy of the Lumbar Spine. In: McCulloch JA, Young PH, editors. Essentials of Spinal Microsurgery. Philadelphia: Lippincott-Raven; 1998, p. 249-327.

6. Wong DA: Open lumbar microscopic discectomy. In: Vaccaro AR, Albert TJ, editors. Spine Surgery, Tricks of the Trade. 2nd ed. New York: Thieme; 2009, p. 119-121.

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