There are ten (five pairs) facet joints (also called apophyseal or zygoapophyseal joints) in the lumbar spine (Figure 9-1).² These diarthrodial joints consist of superior and inferior facets and a capsule. The facets are located on the vertebral arches. With a normal intact disc, the facet joints carry about 20% to 25% of the axial load, but this may reach 70% with degeneration of the disc. The facet joints also provide 40% of the torsional and shear strength.³ Injury, degeneration, or trauma to the motion segment (the facet joints and disc) may lead to spondylosis⁴ (degeneration of the intervertebral disc), spondylolysis⁵ (a defect in the pars interarticularis or the arch of the vertebra), spondylolisthesis⁵ (a forward displacement of one vertebra over another), or retrolisthesis (backward displacement of one vertebra on another). The superior facets, or articular processes, face medially and backward and, in general, are concave; the inferior facets face laterally and forward and are convex (Figure 9-2). There are, however, abnormalities, or tropisms, that can occur in the shape of the facets, especially at the L5–S1 level (Figures 9-3 and 9-4).⁶ In the lumbar spine, the transverse processes are virtually at the same level as the spinous processes.

These posterior facet joints direct the movement that occurs in the lumbar spine. Because of the shape of the facets, rotation in the lumbar spine is minimal and is accomplished only by a shearing force. Side flexion, extension, and flexion can occur in the lumbar spine, but the facet joints control the direction of movement. The close packed position of the facet joints in the lumbar spine is extension. Normally, the facet joints carry only a small amount of weight; however, with increased extension, they begin to have a greater weight-bearing function. The resting position is midway between flexion and extension. The capsular pattern is side flexion and rotation equally limited, followed by extension. However, if only one facet joint in the lumbar spine has a capsular restriction, the amount of observable restriction is minimal. The first sacral segment is usually included in discussions of the lumbar spine, and it is at this joint that the fixed segment of the sacrum joins with the mobile segments of the lumbar spine. In some cases, the S1 segment may be mobile. This occurrence is called lumbarization of S1, and it results in a sixth “lumbar” vertebra. At other times, the fifth lumbar segment may be fused to the sacrum or ilium, resulting in a sacralization of that vertebra. Sacralization results in four mobile lumbar vertebrae. These abnormalities are sometimes called transitional vertebra.⁷

The main ligaments of the lumbar spine are the same as those in the lower cervical and thoracic spine (excluding the ribs). These ligaments include the anterior and posterior longitudinal ligaments, the ligamentum flavum, the supraspinous and interspinous ligaments, and the intertransverse ligaments (Figure 9-5). In addition, there is an important ligament unique to the lumbar spine and pelvis—the iliolumbar ligament (Figure 9-6), which connects the transverse process of L5 to the posterior ilium.⁸ This ligament helps stabilize L5 with the ilium and helps prevent anterior displacement of L5.⁹

The intervertebral discs make up approximately 20% to 25% of the total length of the vertebral column. The function of the intervertebral disc is to act as a shock absorber distributing and absorbing some of the load applied to the spine, to hold the vertebrae together and allow movement between the bones, to separate the vertebra as part of a functional segmental unit acting in concert with the facet joints (Figure 9-7), and, by separating the vertebrae, to allow the free passage of the nerve roots out from the spinal cord through the intervertebral foramina. With age, the percentage of spinal length attributable to the discs decreases as a result of disc degeneration and loss of hydrophilic action in the disc.

The annulus fibrosus, the outer laminated portion of the disc, consists of three zones: 1) an outer zone made up of fibrocartilage (classified as Sharpey fibers) that attaches to the outer or peripheral aspect of the vertebral body and contains increasing numbers of cartilage cells in the fibrous strands with increasing depth, 2) an intermediate zone made up of another layer of fibrocartilage, and 3) an inner zone primarily made up of fibrocartilage and containing the largest number of cartilage cells.¹⁰ The annulus fibrosus contains twenty concentric, collar-like rings of collagenous fibers that crisscross each other to increase their strength and accommodate torsion movements.¹¹

The nucleus pulposus is well developed in both the cervical and the lumbar spines. At birth, it is made up of a hydrophilic mucoid tissue, which is gradually replaced by fibrocartilage. With increasing age, the nucleus pulposus increasingly resembles the annulus fibrosus. The water-binding capacity of the disc decreases with age, and degenerative changes (spondylosis) begin to occur after the second decade of life. Initially, the disc contains approximately 85% to 90% water, but the amount decreases to 65% with age.¹² In addition, the disc contains a high proportion of mucopolysaccharides, which cause the disc to act as an incompressible fluid. However, these mucopolysaccharides decrease with age and are replaced with collagen. The nucleus pulposus lies slightly posterior to the center of rotation of the disc in the lumbar spine.

The shape of the disc corresponds to that of the body to which it is attached. The disc adheres to the vertebral body by means of the cartilaginous end plate. The end plates consist of thin layers of cartilage covering the majority of the inferior and superior surfaces of the vertebral body. The cartilaginous end plates are approximately 1 mm thick and allow fluid to move between the disc and the vertebral body. The discs are primarily avascular with only the periphery receiving a blood supply. The remainder of the disc receives nutrition by diffusion, primarily through the cartilaginous end plate. Until the age of 8 years, the intervertebral discs have some vascularity; however, with age this vascularity decreases.

Usually, the intervertebral disc has no nerve supply, although the peripheral posterior aspect of the annulus fibrosus may be innervated by a few nerve fibers from the sinuvertebral nerve.^13,14 The lateral aspects of the disc are innervated peripherally by the branches of the anterior rami and gray rami communicants. The pain-sensitive structures around the intervertebral disc are the anterior longitudinal ligament, posterior longitudinal ligament, vertebral body, nerve root, and cartilage of the facet joint.

With the movement of fluid vertically through the cartilaginous end plate, the pressure on the disc decreases as the patient assumes the natural lordotic posture in the lumbar spine. Direct vertical pressure on the disc can cause the disc to push fluid into the vertebral body. If the pressure is great enough, defects may occur in the cartilaginous end plate, resulting in Schmorl nodules, which are herniations of the nucleus pulposus into the vertebral body. These are found in 20% to 30% of individuals.¹⁵ Normally, an adult is 1 to 2 cm (0.4 to 0.8 inch) taller in the morning than in the evening (20% diurnal variation).^3,16 This change results from fluid movement in and out of the disc during the day through the cartilaginous end plate. This fluid shift acts as a pressure safety valve to protect the disc.

If there is an injury to the disc, four problems can result, all of which can cause symptoms.¹⁷ There may be a protrusion of the disc, in which the disc bulges posteriorly without rupture of the annulus fibrosus. In the case of a disc prolapse, only the outermost fibers of the annulus fibrosus contain the nucleus. With a disc extrusion, the annulus fibrosus is perforated, and discal material (part of the nucleus pulposus) moves into the epidural space. The fourth problem is a sequestrated disc, or a formation of discal fragments from the annulus fibrosus and nucleus pulposus outside the disc proper (Figure 9-8).¹⁸ These injuries can result in pressure on the spinal cord itself (upper lumbar spine) leading to a myelopathy, pressure on the cauda equina leading to cauda equina syndrome (saddle anesthesia [Figure 9-9], bowel/bladder dysfunction),¹⁹ or pressure on the nerve roots (most common). The amount of pressure on the neurological tissues determines the severity of the neurological deficit.²⁰ The pressure may be the result of the disc injury itself or in combination with the inflammatory response of the injury. Saal has outlined favorable, unfavorable, and neutral factors for positive-outcome prognostic factors for nonoperative lumbar disc herniation (Table 9-1).¹⁷

Within the lumbar spine, different postures can increase the pressure on the intervertebral disc (Figure 9-10). This information is based on the work of Nachemson and colleagues,^21,22 who performed studies of intradiscal pressure changes in the L3 disc with changes in posture. The pressure in the standing position is classified as the norm, and the values given are increases or decreases above or below this norm that occur with the change in posture.

In the lumbar spine, the nerve roots exit through relatively large intervertebral foramina, and as in the thoracic spine, each one is named for the vertebra above it (in the cervical spine, the nerve roots are named for the vertebra below). For example, the L4 nerve root exits between the L4 and L5 vertebrae. Because of the course of the nerve root as it exits, the L4 disc (between L4 and L5) only rarely compresses the L4 nerve root; it is more likely to compress the L5 nerve root (Figure 9-11).

In general, the L5–S1 segment is the most common site of problems in the vertebral column because this level bears more weight than any other vertebral level. The center of gravity passes directly through this vertebra, which is of benefit because it may decrease the shearing stresses to this segment. There is a transition from the mobile segment, L5, to the stable or fixed segment of the sacrum (S1), which can increase the stress on this area. Because the angle between L5 and S1 is greater than those between the other vertebrae, this joint has a greater chance of having stress applied to it. Another factor that increases the amount of stress on this area is the relatively greater amount of movement that occurs at this level compared with other levels of the lumbar spine.

Problems of the lumbar spine are difficult to diagnose; in fact, diagnosing pain due to a disc is primarily diagnosis of exclusion.23 Most of the examination commonly revolves around differentiating symptoms of a herniated disc (or space occupying lesion), which refers radicular symptoms into the leg from other conditions (e.g., inflammatory reaction, sprains, strains, facet syndrome) more likely to cause localized pain.²⁴ If there are no radicular symptoms below the knee, it often becomes difficult for the examiner to determine where in the spine the problem is, or for that matter, whether the problem is truly in the lumbar spine or coming from problems in the pelvic joints, primarily the sacroiliac joints, or the hips. Waddell pointed out that in only about 15% of cases can a definitive diagnosis as to the pathology of back pain be made.³ Hall broke low back pain into four categories—two of which are back pain dominant and two of which are leg pain dominant (Table 9-2).²⁵ Pattern 1 suggests disc involvement, whereas pattern 2 suggests facet joint involvement. Pattern 3 suggests nerve root involvement (primarily by a disc or some other space occupying lesion or an injury accompanied by inflammatory swelling), and pattern 4 suggests neurogenic intermittent claudication (pressure on the cauda equina). Thus, only by taking a careful history, followed by a detailed examination, is the examiner able to determine the cause of the problem.^26–28 Even then, some doubt may remain.

In addition to the questions listed under the “Patient History” section in Chapter 1, the examiner should obtain the following information from the patient: