Physiological loading

Abstract Synthetic polymers and biopolymers are extensively used within the field of tissue engineering. Some common examples of these materials include polylactic acid, polyglycolic acid, collagen, elastin, and various forms of polysaccharides. In terms of application, these materials are primarily used in the construction of scaffolds that aid in the local delivery of cells and growth factors, and in many cases fulfill a mechanical role in supporting physiologic loads that would otherwise be supported by a healthy tissue. In this review we will examine the development of scaffolds derived from biopolymers and their use with various cell types in the context of tissue engineering the nucleus pulposus of the intervertebral disc. 

Traditional materials utilized for orthopedic and dental applications have been selected based on their mechanical properties and ability to remain inert in vivo this selection process has provided materials that satifisfy physiological loading conditions but do not... 

Traditional materials for orthopedic and dental applications have been selected based on their mechanical properties and ability to remain inert in vivo this selection process has provided materials that satisfied physiological loading conditions but did not duplicate the mechanical, chemical, and architectural properties of bone. Most importantly, to date, failure of conventional orthopedic and dental implant materials is often due to insufficient bonding to juxtaposed bone (that is, insufficient osseointegration). 

Plates are available in a wide variety of shapes and are intended to facilitate fixation of bone fragments. They range from the very rigid, intended to produce primary bone healing, to the relatively flexible, intended to facilitate physiological loading of bone. 

Recently, a cellular, structural biomaterial comprised of 15 to 25% tantalum (75 to 85% porous) has been developed. The average pore size is about 550 p,m, and the pores are fully interconnected. The porous tantalum is a bulk material (i.e., not a coating) and is fabricated via a proprietary chemical vapor infiltration process in which pure tantalum is uniformly precipitated onto a reticulated vitreous carbon skeleton. The porous tantalum possesses sufficient compressive strength for most physiological loads, and tantalum exhibits excellent biocompatibility [Black, 1994]. This porous tantalum can be mechanically attached or diffusion bonded to substrate materials such as Ti alloy. Current commercial applications included polyethylene-porous tantalum acetabular components for total hip joint replacement and repair of defects in the acetabulum. 

Despite brittle characteristics, ceramic components enjoy several outstanding tribological properties, including their hardness (Table 6.2), which contributes to wear resistance and scratch resistance. Ceramic surfaces are also more hydrophilic than the CoCr surfaces of a femoral head, as illustrated in Figure 6.7 by the water droplets. The improved wettability of ceramics contributes to lower friction than CoCr when articulated against UHMWPE under physiologic loading and lubrication conditions (Morlock et al. 2002). 

None of the theoretical approaches outlined above considered adequately the cyclic nature of physiological loading and motion In synovial joints. Medley et al (6) Included such effects In their studies of ankle joint lubrication and they found that a reasonable approximation to the Instantaneous film thickness at any instant In the loading cycle could be obtained by means of an extended... 

In complex in vivo environments, it is difficult to clarify the exact effect of a specific force or to delineate the role of a specific signaling pathway in mechanotransduction processes due to the interference of myriad chemical factors and the presence of other mechanical forces. Therefore, investigations on cellular responses to mechanical stimulation have relied heavily on the use of in vitro systems. Table 33.1 summarizes devices that are commonly used to subject cultured cells to flow, stretch, and pressure [52]. These devices expose a large number of cells to well-defined mechanical stimuli that replicate physiological loading. Techniques for applying localized forces to individual cells have been developed. The reader is referred to other resources on the subject [2]. 

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