Synthesis and characterisation of core–shell structures for orthopaedic surgery

https://doi.org/10.1016/j.jbiomech.2007.05.002Get rights and content

Abstract

This paperwork deals with the obtaining and characterisation of new acrylic cements for bone surgery. The final mixture of cement contains derivatives of methacryloyloxyethyl phosphate, methacrylic acid or 2-acrylamido-2-methyl-1-propane sulphonic acid. The idea of using these monomers is sustained by their ability to form ionic bonds with barium, which is responsible for X-ray reflection and by the biocompatibility of these structures.

The strategy consists in the obtaining of core–shell structures through heterogeneous polymerisation, which are used for final cement's manufacture. The orthopaedic cements were characterised by SEM, EDX, compression resistance and cytotoxicity assays.

Introduction

Acrylic bone cements mainly consist in poly(methyl methacrylate) (PMMA) and have been used in orthopaedic surgery and dentistry for more than 40 years (Vallo et al., 1999; Fini et al., 2000; Pascual et al., 1999a, Pascual et al., 1999b; Hooy-Corstjens et al., 2004; Ginebra et al., 2002).

In orthopaedic surgery, bone cements serve as a mechanical interlock between the metallic prosthesis and the bone. PMMA cements are usually composed of preformed PMMA beads mixed with methyl methacrylate (MMA) monomer or a mixture of MMA and butylmethacrylate (BMA). The polymerisation process occurs as a result of the reaction between benzoyl peroxide (BPO) in the polymer powder and N,N-dimethyl-p-toluidine (DMpT) in the monomer. The acrylics can be polymerised by free-radical chain reaction process. Prior to the application, the powder and the liquid parts are mixed until soft dough is obtained and then they are applied to the desired bone cavity. Acrylic bone cement hardens within the following minutes, due to a rapid polymerisation reaction.

A recent study of our team, Zaharia et al. (2007), tackled the problem of obtaining inorganic–organic hybrid acrylic cements for bone surgery, using in the final mixture derivatives of methacryloyloxyethyl phosphate (MOEP).

In this way, we obtained biocompatible materials capable of inducing hydroxyapatite formation and having good radio-opacity (Tamada et al., 1999; Kato et al., 1996, Kato et al., 1997; Tretinnikov et al., 1994; Stancu et al., 2004; Kamei et al., 1997; Dalas et al., 1991).

MOEP monomer was selected due to its ability to chemically bind barium ions (Ba2+) and to reflect X-rays, as well as due to biocompatibility and osteoconductive properties of the resulting polymers. The research study had two major directions (Zaharia et al., 2007):

  • (a)

    the obtaining of p(MMA-co-MOEP) copolymers by various methods, which allow products’ separation as very fine powders followed by superficial Ba2+ treatment through Ba(OH)2 neutralisation; these copolymers were used in the final mixtures and

  • (b)

    preliminary obtaining of the Ba2+ salt of the phosphoric monomer (MOEP–Ba) and its use as reactive filling agent in the final cement.

The cements obtained presented good compression resistance comparable or even superior to those of the standard commercial product and also improved X-ray opacities (Zaharia et al., 2007).

Nevertheless, a certain lack of homogeneity of the biomaterial was noticed. This aspect was observed by X-rays (agglomeration of the particles treated with Ba2+), although it was proved the chemical bond of Ba2+ on the phosphate group. This situation could have happened due to the existence of a limited compatibility between the particles treated with Ba2+ and the organic matrix, during bone cement hardening.

To avoid this limitation, in the present work some core–shell structures were preformed; here, the core consists in the salt of Ba2+-treated monomer, and the shell—in PMMA chemically bonded to the contact area. These types of structures were used in the final cement. The monomers used as Ba2+ salts and acting as core for polymerisation were: MOEP, methacrylic acid (AM), and 2-acrylamido-2-methyl-1-propane sulphonic acid (AMPSA), due to the possibility of ionic bonds’ formation with Ba2+, and to the biocompatibility of the corresponding polymers (Konar and Kim, 1999; Donini et al., 2002; Clausen and Bernkop-Schnurch, 2001; Pradny et al., 2005; Yin et al., 2003; Robinson and Peppas, 2002; Sotiropoulou et al., 2004; Piletsky et al., 1998; Chen et al., 2005; Mohan et al., 1992).

Section snippets

Materials

The monomers MOEP, AMPSA, and AM (Merck) (Fig. 1) were used as such. The agent for Ba2+ treatment was barium carbonate (BaCO3) (Fluka). MMA (Merck) was purified by in vacuo distillation (T=47 °C, p=100 mmHg). Absolute ethanol (EtOH) (Fischer) was used as reaction medium for MMA polymerisation in the presence of Ba2+-treated monomers. The initiation system for heterogeneous polymerisation, ascorbic acid (AA) and tertbutyl perbenzoate (PBTB) (Merck) were used as such. PMMA (additive for the final

Results and discussion

The first step of the research study pursued the morphological and compositional analysis of the surface of Ba2+-treated monomers.

SEM revealed the lack of dimensional uniformity due to the formation of different Ba2+ derivatives (Zaharia et al., 2007). EDX showed the presence of a high-Ba2+ density on the surface of the Ba2+ treated monomers (Fig. 2, Fig. 3).

The next step of this study consisted in the heterogeneous polymerisation of MMA in the presence of Ba2+ treated monomers.

The conversions

Conclusions

The main conclusions of the present paper are:

  • (a)

    A preliminary study concerning the replacement of PMMA pearls used in reactive acrylic cements based on core–shell particles was performed. The shell of the cements is made out of PMMA and the core consists in different salts of polymerisable unsaturated organic acids.

  • (b)

    We have tested all the cylindrical samples obtained by compression resistance measurements and we have compared all the results obtained, among the standard cement and our cements. In

Conflict of interest

There is no conflict of interest.

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