How do bones respond to disuse such as constant bed rest?
When a bone is subjected to disuse, such as during constant bed rest, it undergoes several changes in its structure and composition, known as disuse osteoporosis. Here's how bones respond to disuse:
Bone Loss:
- Reduced Mechanical Stress: Lack of weight-bearing activities and gravitational loading during bed rest significantly reduces the mechanical stress on bones. This decreased stress leads to a reduction in bone formation and an increase in bone resorption, resulting in bone loss.
- Imbalance in Bone Turnover: Disuse disrupts the delicate balance between bone formation (osteogenesis) and bone resorption (osteoclasis). Osteoblastic activity decreases, while osteoclastic activity remains unchanged or even increases, leading to a negative bone balance and further bone loss.
- Alterations in Bone Structure: Prolonged disuse causes thinning of the cortical bone (the outer layer of bone) and a reduction in trabecular bone (the cancellous bone inside the cortex). The bone becomes weaker and more susceptible to fractures.
Bone Adaptation:
- Adaptation to Reduced Loading: Over time, bones adapt to the reduced loading conditions. They remodel and redistribute their mass and strength to match the new mechanical demands. However, this adaptation is not as efficient as the constant remodeling that occurs with normal weight-bearing activities.
- Compromised Bone Quality: The newly formed bone during disuse has reduced mineralization and impaired material properties. It is weaker and less dense compared to bone formed under normal loading conditions.
Other Effects:
- Muscle Atrophy: Disuse also leads to muscle atrophy, which further contributes to bone loss since muscles and bones are closely connected and interdependent. Muscle weakness can reduce the mechanical stimulus on bones and increase the risk of falls and fractures.
- Calcium Loss: Reduced weight-bearing activities decrease calcium absorption and increase calcium excretion, resulting in a negative calcium balance and loss of calcium from the bones.
- Impaired Bone Marrow Function: Disuse affects bone marrow function, including hematopoiesis (blood cell production) and immune cell activity. Prolonged bed rest can disrupt the production and maturation of blood cells, leading to anemia and other hematological disorders.
To mitigate the adverse effects of disuse on bones, it's important to engage in regular physical activity and weight-bearing exercises, maintain adequate nutrition, and receive proper medical care during periods of prolonged bed rest or immobilization.