The Anatomy of the Low Back and the Limitations of Diagnostic Imaging of This Area

• Author: Samuel D. Hodge, Jr./Jack E. Hubbard
• Profession: Skilled litigator, is chair of the department of legal studies at Temple University/Professor of Neurology at the University of Minnesota
• Pages: 479-515

The Anatomy of

the Low Back and

the Limitations of

Diagnostic Imaging

of This Area

Routine X-rays or

MRIs for patients with

low-back pain does

not lead to improved

pain, function or

anxiety level, and

there were even some

trends toward worse

outcomes.

Roger Chou, M.D.

9

Scope of Low Back Problems1

The incidence of low back pain is a major economic problem globally. The medical litera-

ture reveals that at least 80 percent of the population experience back pain at least once

in their lifetime and 25 percent of these individuals will have recurrent discomfort within

one year.2 Simply put, low back pain is the price that humans have paid in going from a

four-legged creature to an upright one. And, it is quite an expensive price since the esti-

mated cost of low back pain is more than $90 billion annually.3 Nevertheless, most back

pain sufferers do quite well, with 60 percent recovering after only six weeks4 and 80 to 90

percent becoming asymptomatic by the twelfth week after onset of symptoms.5 Implicit

in these figures is the fact that a minority of people with chronic low back pain6 account

for the majority of the costs in treating this disorder.7 As the literature demonstrates,

70 percent of the cost associated with low back pain is generated by only 20 percent of

patients.8

Workers greatly influence these statistics.9 Approximately 2 percent of all employees

in the United States receive compensation for back injuries each year,10 and a mere 10

percent of those employees account for 70 percent to 80 percent of the costs associated

with this malady.11 Only 4.5 percent to 8 percent of disability claims involving low back

pain last longer than one year, and those claims that exceeded one year account for 64.9

percent to 84.7 percent of all of disability cases.12 In the vast number of instances, an

episode of low back pain limits a worker’s regular activity for at least 30 days.13 Less than

half of those individuals who miss 26 weeks of employment never return to their jobs and

almost none return to work if off more than 104 weeks.14 Demographically, low back pain

is the most common reason for disability in those under the age of 45 years.15

A number of diagnostic tests are available to help physicians discover the origins

of low back pain. These imaging modalities range from conventional x-rays to highly

sophisticated magnetic resonance imaging (MRI) scanning where few back abnormalities

escape detection. Unfortunately, such imaging is not infallible, for not every abnormality

is clinically significant and there can be legitimate pain complaints in the absence of an

480 ◆ CHAPTER 9

abnormal diagnostic test. This chapter discusses the anatomy of this region of the spine

and explores the limitations of low back diagnostic imaging.

Anatomy of the Low Back

The regions of the spine are adeptly explained in Kostel v. Schwartz as “three main groups

of vertebrae—the cervical vertebrae atop the spinal column, of which there are seven; the

thoracic vertebrae, situated below the cervical vertebrae, of which there are twelve; and

the lumbar vertebrae situated at the end. As discussed previously, the letters C, T, and L

are used, respectively, to designate cervical, thoracic and lumbar vertebrae. The sacrum is

located at the base of the spinal column, and below it, the coccyx or ‘tailbone.’ The five

sacral and four coccygeal vertebrae are fused and together are considered one bone.”16

The term low back typically refers to the lumbar and sacral (lumbosacral) regions of the

spine as well as the coccyx (tailbone).

To be more specific, the lumbar spine17 is the lower portion of the spine, where it curves

inward toward the abdomen, the region of the lumbar. The lumbar spine begins approxi-

mately five or six inches below the low tip of the shoulder blade, connecting with the tho-

racic spine at the top end and extending downward to connect with the sacrum at its lowest

end. The term “lumbar” is taken from the Latin lumbus, “lion,” describing its power and

flexibility (i.e., lifting and bending). The lumbosacral region is the lumbar plus the sacrum.18

Figure 9-1. The lumbosacral angle is formed at the juncture of the long axes of the lumbar

region of the spine and sacrum.19 The region’s major components are bones, joints, liga-

ments, disks, muscles, and nerve roots. Any of these structures can be a pain generator, a

fact that contributes to the difficulty in diagnosing the cause of a person’s low back pain.20

The lumbar spine has several important features:

• The lower the vertebral segment, the more weight it must support.

• The lumbar spine joins the sacrum at the lumbosacral joint (L5-S1). This area

allows the pelvis and hips to swing when a person walks and runs.

• The lowest two spinal segments of the lumbar spine, L4-L5 and L5-S1, support

the most weight, so they are the most prone to degradation and injury.

• The spinal cord ends around T12-L1. At this juncture, nerve roots continue down

the vertebral column, forming the cauda equina.

• Because the spinal cord ends at the lowest portion of the thoracic spine, a lum-

bar problem such as a herniated disk or lumbar stenosis does not cause injury to

the spinal cord.21 However, all of the nerves going to the legs, bladder, and rectal

sphincter go through the lumbar spine as the cauda equina, which can be injured,

resulting in paralysis of the legs and bowel/bladder incontinence.

Bones

The bones of the low back consist of five lumbar vertebrae and two sets of fused bones, the

sacrum and coccyx. The five sacral vertebrae fuse together in adults and four coccygeal ver-

tebrae join together to create the coccyx.22 Bones themselves are not pain sensitive; it is the

membrane covering these structures, the richly innervated periosteum,23 that causes dis-

comforts from bone trauma, such as a fracture,24 or a nontraumatic cause, such as cancer.

Like the rest of the spine, the five lumbar vertebrae are stacked on top of each other like

napkin rings. They are labeled L1 to L5 consecutively; the L1 vertebra is located around

the level of the belly button. Figure 9-2. These vertebrae have massive bodies and rugged

lamina and lack costal facets for rib attachment. The lumbar vertebrae are large because

the body weight they support increases at the end of the spine25 such that the L5 vertebra

is the largest of all movable vertebrae because it supports the weight of the entire upper

body.26 The lumbar vertebrae are most frequently involved in back pain because of their

weight-bearing responsibility and stresses along the spine.27 The lumbar region has a

lordotic curve, meaning that it is has the shape of a backwards “C.” By way of analogy, if

one views the entire spine from the right side it appears to have an “S”-like shape, with

the lumbar region at the bottom of the “S.”28 Figure 9-2.

THE ANATOMY OF THE LOW BACK AND T HE LIMITATIONS OF DIAGNOSTIC IMAGING ◆ 481

FIGURE 9-1.

9-1a: Posterior view of the lumbo-

sacral region of the spine superim-

posed upon the human form. This

termination region of the spine

extends from the L1 vertebra to

the sacrum. The sacroiliac (SI) joints

are the points of attachment of

the sacrum to the pelvis. In a thin

person, the SI joints are visible as

two dimples on either side of the

low back.

9-10b: Side and front view of

the lumbosacral region of the spine

showing its attachment to the

pelvis. The lumbosacral region

consists of five lumbar vertebrae

and the single upside-down tri-

angular-shaped sacrum formed by

fusing of the sacral vertebrae. The

sacrum is attached to the ilium of

the pelvis at the sacroiliac (SI) joint.

The entire weight of the upper

body is carried by and exerted

onto the lumbosacral region. The

coccyx (“tailbone”) is attached at

the end of the sacrum and has no

known function.

a

b

In addition to their role in supporting the weight of the upper portion of the body, the

lumbar vertebrae allow movement in the low back, provide for muscle attachment to the

spine, and protect the nerve roots that extend from the spinal cord through openings in

the spine on their journey down into the legs. These functions are reflected in the design

and shape of these bones. For example, the massive, roughly kidney-shaped body of each

vertebra is built to support the trunk as it bears the vertical weight. Figure 9-1. In the

spinal vertebrae, a delicate vertebral arch extends posteriorly from the vertebral body to

create an opening known as the spinal canal or vertebral foramen,29 through which nerve

roots extend downward from the spinal cord.30 Figure 9-3. The vertebral arch attaches to

the vertebral body by paired pedicles, like struts. Bony processes extend outward from the

sides of the vertebral arch as the transverse processes,31 which serve as the attachment for

muscle, and complete the structure as the lamina.32 The term pedicle may sound familiar to

many trial lawyers because of the various lawsuits involving pedicle screws. Because this

bony part projects rearward, they are accessible to physicians for “pedicle fixation,” the

placement of screws passing through the pedicle and into the vertebral body.33 Figure 9-3.

The vertebral arch terminates as the spinous process, which can be felt as the bony

bumps vertically arranged underneath the skin of the back.34 Each vertebra has two pairs

of vertebral joints, the facets, that connect adjacent vertebrae and allow forward, back-