Understanding the anatomy and biomechanics of the spine and its related structures is key to understanding patterns of injury and presenting findings in the athlete with back pain. The basic functional unit of the spine consists of two adjacent vertebrae and the interverterbral disc that separates them (see Figure 29.1). This functional unit is the building block for the entire spine, which as a whole can be further subdivided into anterior and posterior elements to gain a better appreciation of the key functional role that each plays.

The vertebrae of the thoracolumbar spine consist of an anteriorly oriented vertebral body, composed of both cortical and cancellous bone, and the posterior elements, which form the vertebral foramen or spinal canal. Posterior elements include the pedicles, lamina (including the pars interarticularis), superior and inferior articular processes, and the transverse and spinous processes. Vertebral body size and weight-bearing capacity increase with progression from the cervical to lumbar spine. Cancellous bone contributes most to the load-bearing capacity of the vertebrae (6). At the superior and inferior surface of each vertebral body is a cartilaginous endplate, which undergoes circumferential ossification to form a ring apophysis. Fibers of the outer intervertebral disc annulus attach to this apophysis, with the avascular nucleus pulposus receiving nourishment through the central endplate, an important factor in understanding certain injury patterns in the developing spine.

The intervertebral discs consist of an outer annulus fibrosis and inner nucleus pulposus. The fibrocartilaginous annulus, slightly thicker anteriorly than posteriorly, consists of a ring of concentrically aligned outer fibers attached to the vertebral endplate. It is composed of 60% to 70% water, which remains relatively constant with age, and receives sensory innervation posteriorly from the sinuvertebral nerves and laterally by branches of the ventral rami and grey rami communicans (7). The nucleus pulposus consists of a hydrophilic, proteoglycan matrix contained by the lamellar annulus, with a water content that approximates 90% in infancy and follows a linear decline into adult life (8). It possesses an inherent internal pressure that provides separation and shock absorption between the adjacent vertebrae. There is no direct blood supply to the nucleus in adults and no direct innervation. The intervertebral discs are susceptible to failure through desiccation associated with the aging process and exposure to acute and repetitive traumatic effects, including compression, rotation, and shear forces.

Posteriorly, the superior articular process of the vertebrae below and the inferior articular process of the vertebrae above join to form a facet joint. This zygapophyseal joint is a paired synovial joint surrounded by a redundant capsule that allows for flexion and extension movement while restricting lateral flexion and rotation. Between the articular processes lies a fibroadipose meniscus, or meniscal equivalent, that is well innervated with proprioceptive and nociceptive fibers. The vertebral arch, formed by the lamina, the pedicles, and posterior aspect of the vertebral body form the vertebral canal, containing the spinal
cord. The spinal cord terminates at the level of the first lumbar vertebra, becoming the conus medullaris.

The intervertebral foramen is formed by the pedicles above and below, the intervertebral disc and vertebral body anteriorly, and the lamina and anterior facet joint posteriorly. It contains the associated spinal nerve (see Table 29.2 for level) that occupies approximately 35% to 40% of the area, connective tissue, ligamentum flavum, arteries and veins, lymphatic vessels, and the sinuvertebral nerve. The L1–2 foramen has the largest cross-sectional diameter with L5-S1 having the smallest, while housing the largest nerve root (L5). This creates a level of vulnerability to injury from any relative change in the surrounding structures, such as disc protrusion, facet arthropathy, and local soft tissue swelling and edema.

The spinal nerve roots, carrying motor, sensory, and sympathetic fibers, are enclosed in a dural sheath that provides protection and nourishment to the root. The segmental organization of the spinal elements allows a “pattern” recognition of injury to a given level. Table 29.2 lists the more commonly affected lumbar nerve roots and the associated sensory, motor, and reflex changes. It is important to remember that a given nerve root may cross one to two vertebral levels before exiting the intervertebral foramen and may therefore be affected by pathology at a different level than predicted.

In addition to the elements of the columnar functional units, the static, erect spine is stabilized by a variety of other soft tissue structures, including muscular, ligamentous, and fascial supports. The intact spine consists of four physiological curves, a cervical and lumbar lordosis and thoracic and sacral kyphosis, which provide a spring-like mechanism to absorb shock and improve load bearing. Four muscle groups, the erector spinae, multifidus, intersegmental, and quadratus lumborum, compose the thoracolumbar paraspinal muscle mass. The erector spinae group, the predominant extensors of the lumbar spine, lies lateral to the multifidi, consists of longissimus, iliocostalis, and spinalis groups, and attaches from the pelvis to the thoracic and lumbar spinal elements and ribs. The more medial multifidae originate at the lumbar lamina and spinous processes and attach at the sacrum, acting to extend the spine, rotate to the opposite side, and control deceleration of forceful spinal flexion. The intersegmental muscles, intertransversalis and interspinalis, attach to adjacent transverse and spinous processes, and laterally bend to the ipsilateral side and extend the spine, respectively. The quadratus lumborum, a powerful lateral trunk flexor, attaches to the 12th rib, iliac crest, and transverse processes of the lumbar vertebrae.

In addition, various other muscle groups exert effects on the thoracolumbar spine, and include the glutei, hamstrings, quadriceps, latissimus dorsi, iliopsoas, and the anterior abdominals—rectus abdominus and abdominal obliques. Further support is provided by the thoracolumbar fascia that encloses the erector spinae and quadratus lumborum muscles. Additional stability is afforded by the ligaments acting on the bony spine, including the ligamentum flavum, supraspinatus ligament, interspinous ligament, and the anterior and posterior longitudinal ligament (see Figure 29.2).

Coordinating the various structural elements of the spine to produce a functional outcome is a complex, highly integrated system of neurophysiological and biomechanical processes. A three-system theory, consisting of neural, passive, and active components,
has been proposed to help explain this complex interaction (9). The neural component initiates the precise movement and coordinates all motion factors. The passive component consists of the static structures discussed previously, including discs, facet joints and capsules, and ligaments. The active component includes the various muscles and ligaments acting on the passive structures. The effective interaction of the three components constitutes the kinetic or dynamic spine. Any breakdown in the function of one or more components can produce a mechanical irritation that triggers a pain response and altered function.

Pain is generated when nociceptive fibers are stimulated by mechanical or chemical means. A number of structures in the back possess such fibers and are therefore potential pain generators. These include the disc annulus, anterior and posterior longitudinal ligaments, facet joint capsule, and various other muscular and other ligamentous groups. Trauma, either micro or macro, activates a cascade of events leading to the release of the chemical mediators of pain, including bradykinins, prostaglandins, leukotrienes, and histamines. In addition, there are psychosocial factors that
impact an individual’s subjective pain experience and may play a significant role in the athlete with back pain.

A directed history and physical examination, cognizant of key elements needed to rule in or out specific disorders, is key to developing an accurate differential diagnosis and selecting appropriate further diagnostics and treatment interventions. The AHRQ guidelines for acute low back problems in adults provides a useful starting point for developing an initial approach to the athlete with LBP, recognizing the limitations inherent in the guideline, including age restriction and chronicity.