The different constituents of bone function together as a unit, to facilitate normal growth, adapt to changing stress, repair microinjury, regulate mineral homeostasis, and respond to injury.
Changes in bone size and shape are called modeling. Modeling is the way in which bone responds to loading. It is exemplified by normal growth, as the developing bone shapes to a contour that allows it to withstand loading. Bone will also model during periods of abnormal use. The modeling process occurs by Wolff’s Law, which states that bone shape and orientation will adapt to use. The response is formation at sites of compression and resorption at sites of tension. When modeling occurs slowly, the deposited bone is lamellar; where modeling occurs rapidly the deposited bone is woven. Modeling differs remodeling as bone can directly begin forming or resorbing (not the same activation-resorption-formation sequence as outlined below).
During remodeling, bone is removed from one area and replaced in another. Remodeling plays a crucial role in normal cortical growth, as woven bone is replaced by osteonal bone. This process is also important in repairing ‘wear and tear’ (microcracks), and replacing old bone with new bone. The sequence of remodeling is always the same: activation, osteoclastic resorption, reversal, formation. Collectively, osteoblasts and osteoclasts combine to form the ‘basic metabolic unit’ of bone. Activation begins as osteoblasts are stimulated to contract and remove the lamina limitans (activation factors include PTH and others). Reversal is the inhibition of resorption and initiation of formation. During formation, the space left by osteoclasts becomes filled by osteoid deposited by osteoblasts. An osteonal remodeling unit is constantly changing. The osteon has a central cavity shaped like an ice-cream cone with osteoclasts at the top leading edge (called the ‘cutting cone’). Further away from the top, the cavity left by the cutting cone becomes progressively filled by new bone deposited in concentric lamellae (giving the remodeling unit a coned appearance in longitudinal section). There are progressively more centripetal lamellae as sections are further from the site of osteoclastic resorption. When osteoclastic activity ceases, the top of the remodeling unit becomes filled and becomes a mature secondary osteon.
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