Remodelling cortical bone

Zimmerman, M. C Meunier, A., Katz, J. L., and Christel, R (1990). The evaluation of cortical bone remodeling with a new ultrasonic technique. IEEE Trans Biomedical Engineering 37, 433-41. [196]... 

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. 

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. 

Trabecular (cancellous) bone lies between cortical bone surfaces and consists of a network of honeycombed interstices containing haematopoietic elements and bony trabeculae. The trabeculae are predominantly oriented perpendicular to external forces to provide structural support. Trabecular bone continually undergoes remodelling on the internal endosteal surfaces. 

Sintered HAp particles are remodeled by the host tissue when implanted into bone. Implantation into the cortical bone of the femur of sheep has revealed that stoichio-metrically pure HAp resorbs by several microns after 18 months with dissolution occurring mainly at the grain boundaries (Benhayoune et al. 2000). The tensile strength of bone from the tibia of a rabbit onto a HAp cylinder is 0.85 MPa after 3 months (Edwards et al. 1997). 

The bone Is composed of two distinct tissue structures cortical (compact) bone, and trabecular (cancellous) bone (3). Eighty percent of the skeleton is composed of cortical bone (e.g., long bones such as the humerus, radius, and ulna) (4,5), which is a relatively dense tissue (80-90% calcified) (4) that provides structure and support (3). Bone marrow cavities, flat bones, and the ends of long bones are all composed of trabecular bone, which Is considerably more porous (5-20% calcified) (4,5). To maintain healthy, well-mineralized bone, a continuous process of bone resorption (loss of ionic calcium from bone) and formation occurs along the bone surface. Cortical bone Is remodeled at the rate of 3% per year, whereas 25% of trabecular bone, which has considerably higher surface area, is remodeled annually (3). In terms of calcium turnover in bone, approximately 500 mg are removed and replaced on a daily basis. 

Vincentelli, R., and Grigorov, M. (1985) The effect of haversian remodeling on the tensile properties of human cortical bone. J. Biomeck, 18, 201-207. 

The pattern of age-related bone loss in dogs has been extensively investigated, notably by Jee and co-workers (1976), and has been found to resemble that in man. The morphology of bone is similar, and about 80% of bone mass is made up of cortical bone in both species. Female dogs also exhibit a faster rate of bone loss than do males. However, dog bone exhibits a much more rapid rate of bone remodelling. An iliac trabecular bone turnover rate of nearly 200%/yr has been reported for 2-year-old beagles (Jee et al. 1978). Also, dogs lose bone four times as fast as humans do (Jee et al., 1976), yet their fracture rate is lower. This difference in susceptibility to fractures has been attributed to a relatively... 

At the same time, Suzuki and co-workers implanted films of a highly piezoelectric polymer thin strips of PVDF were applied to the femur and mandible of a macaque monkey [56). After 6 weeks, osteogenesis was observed around the film, and a slight remodeling of the cortical bone was evident under it, while the callus remained spongy bone. 

More recently, the use of thick monomorph and bimorph PVDF films has been reported [57]. Piezoelectric polymer samples were implanted around the femoral diaphy-sis of rabbit and a comparative study on effects showed a great anwuni of callus and an important remodeling of the cortical bone for the bimorph PVDF. 

Cortical tissue is the dense part of bone. As a living entity, this material is able to maintain and adapt its stracture to external physical stimuli [1], The seat of bone remodeling mechanisms corresponds to cyhndrical stractuial elements called osteons. Each osteon is surrounded by a thin layer (cement line) and is centered on Haversian canal which runs primarily in the bone longitudinal axis. The Haversian canals contain the vasculature, the nerves and interstitial fluid. There are also Volkmann canals which are similar to Haversian canals except that they run along the transverse direction of the bone. At a smaller scale, other extravascular pores exist in the solid matrix of the bone forming the la-cuno-canalicular system. This porous network irrigates the mechano-sensitive osteocytes which are believed to play an important role in bone adaptation as stated in recent experimental studies [2,3,4]. 

On the macroscale structure, two distinct bone types can be identified, namely, cancellous and cortical bone. Cortical bone is the dense bone tissue, with low porosity, that forms the shell surrounding bones, whereas the interior part of bone, enclosed by the cortical shell, is filled with cancellous bone that is metabolically more active, remodeled more often and composed of trabecular struts that form the porous structure filled with marrow (Rho et al., 1998). 

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