PMMA/HAp

Besides having an inert material for fibroblastic cells observed at the bone-cement interface, PMMA is still the current standard for cement-held prostheses. Vallo and his group measured the flexural, compressive, and fracture properties of PMMA bone cements that incorporated various amounts of HAp. They found that addition of up to 15 wt% HAp increases the flexural modulus and fracture toughness [89]. It was also observed that, with increasing HAp incorporation into PMMA, the biological responses of the bone cement increased and thus increased osteoblast adhesion and response [88]. Kwon and coworkers reported that a PMMA/HAp composite with 30 wt% of HAp increased the interfacial shear strength at the bone-implant interface 6 weeks after implantation in rabbits [101]. Several other research studies revealed that, depending on the type of bone cement, the addition of HAp can improve the mechanical properties of bone cements [102]. 

Moursi et al. [103] studied the response of osteoblasts to PMMA/HAp materials. They found that osteoblasts attached equally well to PMMA/HAp. In contrast, the increase of osteoblasts on PMMA/HAp was significantly improved, compared to PMMA, after 8 days in culmre. 

In another work, Kwon and coworkers conducted studies that showed that a PMMA/HAp composite (30 wt% of HAp) increased the interfacial shear strength at the bone-implant interface 6 weeks after implantation in rabbits [123]. Several other studies also found that the addition of HAp can enhance the mechanical properties of bone cements, although the extent of improvement varied, depending on the type of bone cement [124]. 

In contrast, subsequent proliferation of osteoblasts on PMMA/HAp was significantly enhanced (compared to PMMA) after 8 days in culture. 

The porosity features of ceramic materials prepared by directed ice crystallization are also useful for the development of biomaterials. Since the size of the pores can be varied by changing the crystallization conditions, the materials are suitable for the synthesis of ceramic scaffolds. The oriented character of the porosity leads to an anisotropy in the mechanical properties of such ceramics that is similar to the anisotropy of natural bones [132, 174—178]. The most significant progress in this direction was achieved by the development of hydroxyapatite (HAP) ceramics with a compression strength close to that of natural bone [179], and also by the development of tough AI2O3-PMMA layered nacre-like composites [180]. 

In addition, to improve adhesion between polymer matrix and HAp, several copolymer such as PMMA, PBMA, and PHEMA were grafted onto the surface of HAp using isocyanatoethyl methacrylate (ICEM) or hexamethylene diisocyanate (HMDI)/hydroxyethyl methacrylate (HEMA) [100]. These three methacrylate polymers were coupled to HAp particles via covalent bonding with isocyanate groups. 

Glass, glass-ceramic or HAP with PMMA Tricalcium phosphate/HAP with PE... 

Although PMMA is still the current standard for cementheld prostheses, it is an inert material for fibroblastic cells observed at the bone-cement interface. It forms a strong bond with the implant, but the bond between the cement and the bone is considered to be weak, with fibroblastic cells observed at the implant site. Incorporation of HAp increases the biological response to the cement from tissue around the implant site, thus giving increased bone apposition. Research revealed that the addition of up to 40 wt% of HAp to PMMA cement has been shown to increase the fracture toughness, and that the addition of up to 15 wt% of HAp led to an increase in flexural modulus, while the tensile and compressive strengths remained constant [113]. 

Scheme 1 Two schemes for grafting PMMA, PBMA or PHEMA to the surface of HAp by using either ICEM or HMDI-HEMA. Reprinted from [122] with permission from Wiley InterSdence...

Kim and coworkers [118] proposed bioactlve bone cement (BBC), composed of natural bone powder (HAp), chitosan (CS) and commercially available PMMA-based bone cement. Investigators obtained three types of BBCs with different composition ratios BBC I, BBC n and BBC HI with 10 wt% of CS and 40, 50 and 60 wt% of HAp, respectively. Observation of the interfacial area between the host bone and the bone cement indicated that the BBC II composite has numerous pores that could be expected to afford space for bone ingrowth. However, after 4 weeks, the gaps between the host bone and the BBC II became narrower, and PMMA exhibited undesirable cleavage at the interfacial area simultaneously, histological examinations of the interfaces at 4 weeks post-implantation demonstrated more new bone formations in the BBC H implant than in pure PMMA. In addition, the exothermic effects in the BBCs were considerably lower than that of pure PMMA. 

The development of mineral-organic composite materials offers the possibility of combining the favorable properties of bioceramics such as the HAP, alumina or titanium dioxide with the molding capacity of biocompatible polymers (polymethylmethacrylate) PMMA [KHO 92], poly(L-lactic) acid PLLA [ROD 95], poly (ethylene) PE pOW 91]). It is also conceivable to attain a value of the modulus of elasticity near to that of the bone. 

We can differentiate composites as bioresorbable or non-bioresorbable. The nonbioresorbable composites are the result of the combination of a non-bioresorbable calcium phosphate (HAP) with a non-bioresorbable polymer (PMMA, PE). In this case, we have to avoid the covering of ceramic grants of the sttrface, so as to preserve their biological activity. 

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