The spine is divided into four sections: there are 7 cervical, 12 thoracic, and 5 lumbar vertebral sections and the fused sacrum (Hainline, 1995) (Fig. 46-1). The bony vertebral bodies and intervertebral discs provide weight bearing and shock absorption. The posterior elements—vertebral arches, transverse and spinous processes, and facet joints—protect the spinal cord and nerve roots. The facets, ligaments, and paraspinal muscles provide stability and balance (Wipf & Deyo, 1995; Brady, 1996). Pain may arise from any of these structures. The combined anterior and posterior elements are referred to as a functional unit.

The lumbar vertebra is a static structure that is made up of the anterior vertebral body and posterior elements (spinous process, articular facets, and transverse process). There is a strong potential for pain at the vertebral body and facet articular cartilage secondary to increased pain sensitivity. The most common etiologies are degenerative joint disease and fracture of any part of the vertebra (body, compression fraction secondary to osteoporosis, pars interarticularis stress fractures).

The intervertebral disc plays a major role in LBP. It is composed of the outer annulus fibrosis, made up of multiple layers of collagen fibers, and the inner nucleus pulposus, made up of a colloidal gel of proteoglycans and collagen. It is 80% fluid (Hainline, 1995; Cyriax, 1991; Al-Magdassy, 1996). Its functions are absorbing shock, providing resistance to compression, and allowing flexibility of the vertebral column (Hainline, 1995). A compromised disc lends itself to abnormal motion and disc herniation, with potential radiculopathy and pain. The neurologic components of spinal anatomy are discussed below.

Degenerative disc disease is usually secondary to an annular tear, with subsequent leakage of nuclear material and dehydration of the disc (Hainline, 1995). Large annular tears may lead to prolapse or herniation of the nuclear material and subsequent nerve root compression. With aging, the percentage of water decreases and the disc becomes less able to function normally, decreasing the risk of disc herniation in the older population.

The superior and inferior posterior lumbar facets are synovial articulations situated between each vertebral body (Margo, 1994; Wipf & Deyo, 1995; Hainline, 1995; Cyriax, 1991). They play a potential role, along with the intervertebral discs, which play a major role, in lower back dysfunction and pain.

Ligaments play a major role in stability and restriction of motion in the lumbar spine. The key ligamentous structures include the anterior and posterior longitudinal ligaments, which help reinforce the annulus. As the ligament descends toward the sacrum, it narrows, increasing the risk of a herniated nucleus pulposus. The ligamentum flavum and the interspinous and supraspinous ligaments all provide further support to spinal movement (Hainline, 1995; Al-Magdassy, 1996).

Muscles can be divided into anterior and posterior muscle groups. Most posterior is the erector spinae, which is made up of the lateral iliocostals, the intermediate longissimus, and the medial spinalis. The middle layer is the thick multifidus, and the deep posterior muscles include the rotators, interspinalis, and intertransversarii (Fig. 46-2). The anterior muscle group includes the abdominal muscles, which play a major role in lumbar stabilization. They include the psoas (which is a primary hip flexor), the outer external obliques, the intermediate internal obliques, and the inner transversus abdominis. The remaining anterior muscles include the midline rectus abdominis and the lateral quadratus lumborum, which functions as a lateral trunk flexor (see Fig. 46-2). All these muscles play a role in trunk motion and stabilization.

The anatomy of LBP is a combination of fact and theory. Studies have shown that pain can be reproduced by injecting hypertonic saline into the ligamentum flavum in facet joint capsules. Supra- and infraspinous ligaments are innervated by afferent branches of the posterior primary rami, and muscle spasms are thought to be a motor reflex to pain. The role of muscle injury as it relates to back pain has not been fully elucidated. Strains and sprains, however, remain the leading diagnosis.

Sciatic pain requires both mechanical and inflammatory stimuli. The sciatic nerve arises from L4–S3 nerve roots (common peroneal and fibial nerves), passes over the lower edge of the sciatic notch, and lies just beneath the piriformis muscle and just above the obturator internus muscle (see Fig. 46-2). The nerve innervates the gluteus medius and minimus as well
as the tensor fasciae latae before passing beneath the piriformis muscle. After passing through the main branch of the sciatic nerve, it innervates the hamstring and the muscles of the lower leg, and provides sensory innervation of the leg and foot (Borenstein et al, 1995).

Although there are no true diagnostic criteria for LBP, Table 46-1 shows the multitude of LBP syndromes. This section focuses on the more common etiologies, or specific diagnoses of secondary back pain. The provider must be able to differentiate between benign causes, such as lumbar strain and disc disease, and the more ominous and life-threatening disorders that need quick referral, such as cauda equina syndrome, malignancy, and vertebral fracture with neurologic compromise. A thorough history and a detailed physical exam, including a neurologic exam, are warranted.

LBP can be divided into three clinical courses: acute, subacute, and chronic. It is useful to differentiate these three categories when approaching treatment protocols. The definitions are as follows:

It is also important to categorize back pain along with its etiology for correct treatment and accurate referral.