Aging and the Spinal Joints

During the third decade of life and thereafter, there is a significant decrease of water content within spinal joint (facet) cartilage. This is characterized by fibrillation or cracking of the facet joint cartilage. 

Some of the fibers of the small, but important multifidus muscle insert into the thick supportive capsule of the facet joint.  There is a pair of multifidus muscles at most levels of the spine.  Age related atrophy of the multifidus muscle could result in abnormal stress placed upon the facet joint complex, contributing to degeneration.

Aging and Spinal Muscles

Skeletal muscle fibers in humans are connected to nerves.  Since there are more muscle fibers than motor neurons, individual motor axons branch within muscles to connect with many different fibers that are typically distributed over a relatively large area within the muscle.  Fortunately, this unique arrangement between nerves and muscle fibers reduces the chance that damage to one or a few nerves will significantly alter a muscle's ability to contract and perform.

Each motor unit refers to a nerve branch and the muscle fibers, which it connects with.  The muscular motor unit (MU) undergoes a significant degree of evolutional change in the aging population.  Degeneration of muscle fibers and evidence of a loss of nerve connections to the muscles are found in the aging muscle.  Older muscles are more susceptible to contraction-induced mechanical injuries and are less capable of healing and regenerating after an injury.  The number of viable motor units within the spinal musculature decreases with increasing age, especially after mid life.  Muscle bulk and volume decreases with aging.

Carefully performed research studies on human and animals, have revealed the presence of progressive reduction of muscle fiber size with aging.  This generalized reduction of muscle fiber size is associated with a reduction of muscle mass of the elderly.  The loss of spinal muscle volume and size contributes to generalized postural abnormalities and increase stress upon the disc and facet joints.  There are slow twitch type 1 and fast twitch type 2 fibers in the muscles of the back.  Type 2 fibers are fast twitch and there is generally a change in proportion of the higher percentage of type 1 to type 2 fibers in older age.  Aging leads to decreases in chemical fuel (high-energy metabolites such as ADP, ATP, and phosphocreatine) in the muscles.  Impaired bioenergetics within muscle cells contributes to postural fatigue and other related back conditions. Selective loss of the muscle units becomes significant with 60-79 year old subjects and becomes more severe in the 80+ years old age category.