Implants calcium deposition

Table 9.22 Calcium deposition (mg) next to control and heat-treated Ti-6A1-4V implants (Hazan et al 1993)...

Biocompatibility describes the interaction between host and graft antigenicity is a key feature in this relationship as it is a major determinant of the reaction of tissues to the implanted material (Waite and Broomell, 2012). It usually leads to calcium deposition as end point of a foreign-body reaction. Calcium alters dramatically the characteristics of the material, ultimately undermining its functionality. Biocompatibility is involved not only in acute reactions but also in the long-term degradation, which affects deeply the operative life of the implant. 

Biologic materials for cardiovascular application are derived from bovine or porcine sources (Liao et al., 1992). They lend to have a better tolerability requiring less pharmacologic adjuncts such as anticoagulants therapy as in most of the cases an antiplatelet regimen is sufficient however, they lend to degrade over time either because of reabsorption or due to calcium deposition that alters the mechanical properties of the tissue (Butany and Leask, 2001). The pace of this process may vary widely according to individual differences and site of implant. 

Mineralization is a phenomenon where the body deposits calcium salts, such as hydroxy apatite on/within the device (intrinsic) or in the fibrotic encapsulation (extrinsic). Note that all implanted devices have been reported to be subject to mineralization, regardless of the materials used. At present there is really no good, predictive accelerated test method, although implant in adolescent rats appears to be about as good as is available. Explants are evaluated by SEM and EDS for calcium deposition. 

Other coating processes involving fluoridated apatite have been investigated to improve the long-term adhesion and promote osteointegration of cementless titanium-based metal implants pulsed laser deposition, electron beam deposition and ion beam sputter deposition techniques, and sol-gel methods, for example. They lead to fluor-containing calcium phosphates (apatites in most cases) with different compositions and crystallinity states. 

Biomaterial scientists and engineers are currently investigating novel formulations and modifications of existing materials that elicit specific, timely, and desirable responses from surrounding cells and tissues to support the osseointegration of the next generation of orthopedic and dental biomaterials (Ratner, 1992). Enhanced deposition of mineralized matrix at the bone-implant interface provides crucial mechanical stability to implants. Proactive orthopedic and dental biomaterials could consist of novel formulations that selectively enhance osteoblast function (such as adhesion, proliferation and formation of calcium-containing mineral) while, at the same time, minimize other cell (such as fibroblast) functions that may decrease implant efficacy (e.g., fibroblast participation in callus formation and fibrous encapsulation of implants in vivo). 

To the best of our knowledge, only a few studies have attempted to prevent the calcification of HEMA-based hydrogels. It has been reported that introduction of carboxylate anions can either prevent or enhance calcification. Cemy et al. (21) found that copolymers of HEMA with 4 wt% methaciylic acid (MAAc) did not calcify under subcutaneous implantation in a rat for 14 months. A similar result was also observed after the implantation of HEMA/MAAc copolymers in the animal urinary tract (22). Other studies carried out in vitro showed that the presence of carboxylate anions significantly reduced the deposition of calcium phosphate (23) and calcium oxalate (24) on acrylic polymers and certain biopolymers. These findings obviously suggest an inhibitory effect of carboxylate anions on calcification. However, by contrast,... 

It has been demonstrated that the release of citric acid from PHEMA hydrogels hinders the formation of calcium phosphates, especially hydroxyapatites. Because of this inhibitory effect, the calcium phosphate phases formed during in vitro calcification were mainly present as non-apatite phases, possibly MCPM and DCPD. The porous morphology of the outer surface of the spherical calcium phosphate deposits could be due to the dissolution of precipitates in the presence of citric acid. The results obtained after subcutaneous implantation ofPHEMA and PHEMA containing citric acid in rats confirmed the resistance of PHEMA-citric acid to calcification. The calcium phosphate deposits which formed in vivo consisted mainly of Ca2+ and OH deficient hydroxyapatites. However, it is not yet known whether or not the differences between the calcium phosphate phases found in vivo and in vitro arise from the presence of proteins/peptides in the in vivo calcifying medium. 

Addition of compounds with appropriate functionality to serve as nucleation sites for calcium phosphate growth to polymers can potentially improve the biocompatibility of the latter and thus the long-term stability of implant devices (Drelich and Field, 2007). Zinc stearate was added to poly(ethylene) to form poly(ethylene)-stearate blends with increased surface porosity potentially able to improve mechanical stability of the implant through enhanced osseointegration, improved rates and quality of bone-implant fusion and enhanced soft tissue wound healing via stimulation of angiogenesis. While immersion of these blends in supersaturated calcium phosphate solutions triggered deposition of a porous layer, the deposition rate was very slow, around 100 nm/day. 

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