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How does immobilizing a limb in a cast or sling cause muscle and bone atrophy?

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atrophy One example of atrophy is the progressive loss of bone that occurs in osteoporosis (normal bone shown on left; osteoporotic bone shown on right).

Immobilizing a limb in a cast or sling causes muscle and bone atrophy primarily through simple disuse. When a limb is restricted, the resulting inactivity leads to a decrease in blood supply and diminished nutrition to the affected tissues.1

In the case of skeletal muscles, this lack of functional demand causes the muscle fibers to shrink. While this form of disuse atrophy is generally less severe than that caused by direct nerve injury, the underlying physiological changes are similar.1

Regarding bone, inactivity leads to a condition known as local osteoporosis, where the bone becomes lighter and more porous. Over time, the cortex of the long bones becomes thinned and experiences a decrease in mineral content. Although the exact mechanisms are not fully understood, this process is a direct consequence of the body adapting to the lack of mechanical stress and movement typically required to maintain healthy bone density and muscle mass.1