Читать книгу Pathy's Principles and Practice of Geriatric Medicine - Группа авторов - Страница 75

The most prominent change to the skeletal system with age is calcium loss from bone.⁷⁴ Bone mass starts to decline gradually after the age of 30. Macroscopically, trabecular bone loss is greater than cortical bone loss. Microscopically, bone mainly consists of an organic matrix and inorganic calcium salts. The balance between osteoblast and osteoclast activity shapes the bone matrix. Osteoblasts produce matrix proteins, mainly type 1 collagen, while osteoclasts resorb old or damaged bone matrix.^75‐76 The production‐resorption balance is maintained by Receptor activator of nuclear factor kappa‐b(RANK)RANK ligand interaction. Binding of RANK with RANK ligands results in resorption. Osteoprotegerin is excreted from osteoblasts, which blocks this binding and preserves the intact bone matrix when resorption is unnecessary. Osteoblastic activity decreases while osteoclastic activity increases with age. Calcium salts start to accumulate on the newly synthesized bone matrix. Parathyroid hormone (PTH) and vitamin D are major actors for bone mineralization with calcium by activating or inhibiting osteoclasts. Vitamin D causes mineralization, while PTH causes resorption of calcium from bone. Vitamin D deficiency contributes to calcium loss in older adults.^78‐78 Sclerostin, a recently found protein, inhibits the canonical Wnt signalling of osteoblasts. Thus, it blocks osteoblastic bone formation. Sclerostin levels increase with age. In addition, oestrogen decreases in older women, and testosterone decreases in older men, causing a lack of anabolising effect on bone mass.⁷⁹ All of these mechanisms contribute to osteoporosis pathogenesis. Bisphosphonates and denosumab are drugs that block bone resorption, while teriparatide acts anabolically for new bone synthesis. Antibodies against sclerostin are under investigation for use in osteoporosis medication.

Cartilage changes also occur in older adults. Although mechanical stress on joint cartilage over the years causes damage, older adults who had sedentary lives suffer from cartilage deformation in big joints due to inactivity. Deformation of the cartilaginous tissue of intervertebral discs causes a reduction in height in older adults. The water, type 2 collagen, elastin, and hyaluronic acid content of cartilage tissue decrease with age as well.⁸⁰ Tendons become weaker and lose resistance against power with age. A decrease in water content, along with elastin and collagen destruction, are responsible for tendon damage with age.⁸¹