Stress shielding

Although the use of implants has been documented since the 1950s, little attention has been directed to how mechanical forces at the implant interface affect tissue metabolism and the fate of the implant. Some interest has been expressed concerning the use of balloons for tissue expansion, pressure-induced necrosis (cell death) of skin and fat cells, and bone resorption as a result of stress shielding by hip and knee implants. These effects may need closer examination in light of the recent findings that most tissues are normally stretched in tension and that any interruption of this tension adversely affects homeostasis via perturbations in normal mechanochemi-cal transduction and could lead to implant failure. 

Stress shielding Bone is protected from stress by the stiff implant. 

One of the main problems of metallic orthopedic implants is that they are much stiffer than the natural tissues they replace (see Table 1). As described above, a difference in stiffness between adjacent materials can lead to a number of problems, including loosening as a result of micromovement and bone resorption due to stress shielding. Adequate fixation of an orthopedic implant to the... 

D.R. Sumner, J.O. Galante, Determinants of stress shielding— design versus materials versus interface, Clin. Orthop. Relat. Res. 274 (1992) 202-212. 

High T. (which can lead to stress shielding, see Section 35.3)... 

Kennady, M.C., Tncker, M.R., Lester, G.E., Bnckley, M.J. Stress shielding effect of rigid internal fixation plates on mandibular bone grafts. A photon absorption densitometry and quantitative computerized tomographic evaluation. Int. J. Oral Maxillofac. Surg. 18, 307-310 (1989)... 

Huiskes, R., Weinans, H., and van Rietbergen, R. (1992), The relationship between stress shielding and bone resorption around total hip stems and the effects of flexible materials, Clin. Orthop. 274 124-134. 

Stress Shielding. Beyond the traditional biocompatibility issues, hard tissue biomaterials must also be designed to minimize a phenomenon known as stress shielding. Due to the response of bone remodeling to the loading environment, as described by Wolffs law, it is important to maintain the stress levels in bone as close to the preimplant state as possible. When an implant is in parallel with bone, such as in a bone plate or a hip stem, the engineered material takes a portion of the load— which then reduces the load, and as a result, the stress, in the remaining bone. When the implant and bone are sufficiently well bonded, it can be assumed that the materials deform to the same extent and therefore experience the same strain. In this isostrain condition, the stress in one of the components of a two-phase composite can be calculated from the equation ... 

An ideal implant would match the modulus of bone and occupy no greater cross-sectional area than the tissue replaced while meeting all the other design requirements of the implant. Since such a constraint generally cannot be met by current materials or designs, it is necessary to construct an implant that will minimize—if not entirely eliminate—stress shielding. 

One of the keys to the chemical biocompadbility of stahdess steel and cobalt chromium was the formation of a passivation laya in vivo, dius minimizing the amount of corrosion that occurs to the implant. However, as indicated in Table 13.4, while the strength of these two metals reduced the chance for failure within the implant, their elastic moduli are an order of magnitude higher than that seen in healthy cortical bone. This resulted in the occurrence of stress shielding and concomitant bone loss in many patients with large implants. 

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