Bone remodeling unit

Effects on Bone PTH increases bone resorption and thereby increases Ca dehvery to the extracellular fluid. The apparent cellular target for PTH is the osteoblast. PTH also recruits osteoclast precursors to form new bone remodeling units see below). 

Bone remodelling, which continues throughout adult life, is necessary for the maintenance of normal bone structure and requires that bone formation and resorption should be balanced. Bone remodelling occurs in focal or discrete packets know as bone multicellular unit (BMU). In this process, both bone formation and resorption occur at the same place so that there is no change in the shape of the bone. After a certain amount of bone is removed as a result of osteoclastic resorption and the osteoclasts have moved away from the site, a reversal phase takes place in which a cement line is laid down. Osteoblasts then synthesize matrix, which becomes mineralised. The BMU remodeling sequence normally takes about 3 months to produce a bone structure unit (Fig. 2). 

Under normal circumstances, the skeleton undergoes a dynamic process of bone remodeling. Bone tissue responds to stress and injury through continuous replacement and repair. This process is completed by the basic multicellular unit, which includes both osteoblasts and osteoclasts. Osteoclasts are involved with resorption or breakdown of bone and continuously create microscopic cavities in bone tissue. Osteoblasts are involved in bone formation and continuously mineralize new bone in the cavities created by osteoclasts. Until peak bone mass is achieved between the ages of 25 and 35, bone formation exceeds bone resorption for an overall increase in bone mass. Trabecular bone is more susceptible to bone remodeling in part owing to its larger surface area. 

Both cortical and trabecular bone are continuously remodeled through the formation of a bone-modeling unit (BMU), or cutter-cone this process involves activation of osteoclasts, leading to resorption of bone by osteoclasts and formation of new bone by osteoblasts on the site of the old, resorbed bone (Fig. 7) (Martin and Burr, 1989). Under normal physiological conditions (i.e., in the absence of either growth or disease) the dynamics of bone remodeling maintain bone homeostasis throughout a person s lifetime. 

The principal result of the effect of glucocorticoids on the bone and its metabolism in vivo is in the reduction of number of bone cells, drop in quantity of remodeling units, reduction of the absorption of calcium in the bowel, and development... 

An estimated 75 million people are affected by osteoporosis to some degree in the United States, Europe, and Japan. Osteoporosis is a systematic skeletal disease characterized by bone mass and microarchitectural deterioration with a consequent increase in bone fragility and susceptibility to fracture. Operationally, osteoporosis can be defined as a certain level of bone mineral density. The definition of osteoporosis is somewhat arbitrary and is based on epidemiological data relating fracture incidence to bone mass. Uncertainty also is introduced due to variability in bone densitometry measurements. Other clinical measures to assess the skeleton include collagen cross-links (measure of bone resorption) and levels of bone-specific alkaline phosphatase and osteocalcin (bone formation). A list of biochemical markers of bone remodeling is provided in Table 37-3. Measurement of total serum alkaline phosphatase level and urinary hydroxyproline or calcium levels is of limited value. 

Bone metabolism is characterized by two opposing activities bone formation and bone resorption (Martin, 2002). Once formed, the bones in adulfs are confinuously remodeled, and the remodeling rate is between 2% and 10% of the skeletal mass per year. Bone mass depends on the balance between resorption and formation within the remodeling unit. As osteoporosis is characterized by a decrease in bone mass with deterioration in the architecture of bones, it is the result of an imbalance between bone formation and resorption. 

Rodents lack lamellar bone and have a limited capacity for bone remodelling. It has been proposed (Heaney, 1981) that remodelling (i.e., turnover of local bone units) in the human is necessary to prevent bone fatigue . Slow remodelling (which is initiated by osteoclastic resorption of bone units under the stimulus of parathyroid hormone) has been associated with sites of osteoporotic fractures. It is of interest, therefore, that fractures have not yet been associated with a slow rate of remodelling in... 

Cortical bone, also called compact or lamellar bone, is remodelled from woven bone by means of vascular channels that invade the embryonic bone from its periosteal and endosteal surfaces. It forms the internal and external tables of flat bones and the external surfaces of long bones. The primary structural unit is an osteon, also known as a Haversian system, a cylindrical shaped lamellar bone surrounding longitudinally oriented vascular channels (the Haversian canals). Horizontally oriented canals (Volkmann canals) connect adjacent osteons. The mechanical strength of cortical bone results from the tight packing of the osteons. 

Ramanathan and Ackerman (1999) have shown that solid-state 31P NMR imaging can be used to measure quantitatively the mass of hydroxyapatite in the presence of bone hence to follow non-invasively the resorption and remodeling of calcium phosphate implants in vivo. A three-dimensional projection reconstruction technique has been used to record NMR images in the presence of a fixed amplitude field gradient, the direction of which was varied uniformly over the unit sphere. Chemical selection was achieved using differences in T1 relaxation time of neighbouring protons as the synthetic hydroxyapatite has a shorter T1 (1.8 s at 4.7 T) compared to bone (approximately 15 s at 4.7 T in vivo, 42 s ex vivo). The experimental results demonstrated that a linear relationship exists between image intensity and HAp density. 

In the equations of adaptive elasticity, denotes volume fraction of bone material. By considering bone matrix as a porous elastic material, material is added to or removed from the pores as C changes, maintaining the dimensions of bone. An important assumption made here, is that at constant temperature and zero body force, for all values of Q, there exists a unique zero-strain state which is taken as the reference. The remodeling rate, at which mass of the porous structure is increased or decreased per unit volume, is denoted by e. The stress-strain relationship and the numerical example can be found in previous papers [10]. 

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