3 Histological Changes in the Aging Spine
Introduction
Back pain is one of the most common reasons for office visits to a physician. It accounts for 2% of all visits, surpassed only by routine examinations, diabetes, and hypertension.1 Back pain is a condition that predominantly affects the older population. Increased survival rates, better health care outcomes, and improved economic status will increase the number of older people in our society. At present, persons older than 65 years constitute 13% of our population. In 30 years, they will constitute 30% of the United States population, and by the year 2050 they will makeup 60% of the population.2 It is of paramount importance to recognize this trend of aging in the population and plan how best to fulfill the health needs of this growing part of our society.
Intervertebral Disk
The intervertebral disks are remnants of the notochord and are interspersed between adjacent vertebral bodies of the spine except between the fused bodies of the sacrum and the coccyx. The intervertebral disks are composed of a circular ring of more resilient annulus fibrosus, which holds a central core of gelatinous material called the nucleus pulposus (Figure 3-1). Biochemically, both the annulus fibrosus and the nucleus pulposus contain proteoglycans in addition to water. The amount of water varies and is responsible for their varied characteristics and, consequently, their functions. The intervertebral disks derive their nutrition by diffusion across vertebral endplates. As the rate of permeability decreases with aging, the health of the disk is threatened.
FIGURE 3-1 Intervertebral disk. Outer annulus fibrosus surrounding inner nucleus pulposus.
(Courtesy of Wolfgang Rauschning, MD.)
Due to decreased turgor and nutrition of the disks, radial and concentric fissures appear in the initial phases of degeneration. The normal avascular disks may develop microvascular capillaries at the periphery of the annulus fibrosus as a compensatory mechanism for decreased nutrition (Figure 3-2). However, this impaired neovascularization is detrimental, contributes to microedema, and exposes the disks to the body’s immune cells for the first time in adult life. Also, there is dissection of the microstructural organization of the annulus fibrosus. The radial fissures eventually enlarge and follow the path of least resistance posterolaterally in relation to the vertebral bodies and overlying the intervertebral foramina. In the late stages of disk degeneration, the nucleus pulposus tracks out over the intervertebral foramina and can compress the exiting spinal nerve, potentially causing symptoms of radiculopathy.
FIGURE 3-2 Neovascularization at periphery of an annular tear.
(Courtesy of Wolfgang Rauschning, MD.)