A review on the polymer properties of Hydrophilic, partially Degradable and Bioactive acrylic Cements (HDBC)

Abstract

Acrylic bone cements were developed around 50 years ago for the fixation of hip prostheses during arthroplasty. Over the intervening years, a series of drawbacks have been disclosed that have fostered intensive research on the development of novel or alternative formulations to the standard acrylic cements. Here, we will review the development and characterization of a novel class of cements, the Hydrophilic, partially Degradable and Bioactive Cements (HDBCs), an example of multifunctional cements. They were developed to have improved biocompatibility and initial fixation to the prosthesis and to induce the growth of bone on the surface of the cement and within pores generated by the degradation of the solid component. HDBCs have higher water uptake than typical acrylic cements, leading to press-fitting inside constrained cavities. They are tougher, albeit less stiff and strong than hydrophobic cements, and their mechanical properties may be easily adjusted by small changes in composition. Last, the simultaneous bioactive and degradable character of HDBCs have been shown to allow in vitro growth of calcium phosphates into pores within the bulk of the cement.

Introduction

Acrylic bone cements were used in the late 50s for fixing hip prostheses during total hip arthroplasty [1]. They were composed of cold-polymerized poly(methylmethacrylate) (PMMA), which reached the final setting in situ and could thus serve as an elastic buffer, maintaining the prosthesis in place and transferring the load from the prosthesis to the bone.

Since then, little has changed in the composition of commercially available formulations. All of them are composed of two phases. The solid is composed of PMMA or copolymers thereof, initiator (benzoyl peroxide, BPO) and an opacifier (ZrO₂ or BaSO₄); the liquid component is methylmethacrylate (MMA) monomer or a mixture with butyl methacrylate, an activator (dimethyl-p-toluidine, DMT) and a stabilizer to avoid premature polymerization. Colorant (chlorophyllin) and antibiotics may also be present in some formulations. These cements are one of the most traditional classes of biomaterials in use, and are simultaneously, the object of intensive research in universities and companies worldwide.

Bone cements are well suited to their function and have an excellent performance record. Despite several modifications, proposed as alternatives to the original formulations, none have been successfully introduced in the market. Therefore, acrylic bone cements are still the gold standard in arthroplasty.

This does not mean bone cements are free of drawbacks that limit their performance. On the contrary, problems such as thermal or chemical necrosis of the bone, high porosity, low interfacial strength between cement and bone and between cement and prosthesis, residual shrinkage stresses, infection and inflammation, among other complications, may occur [2], [3], ultimately leading to aseptic loosening of the implant, the major cause of failure of hip arthroplasties [2], [4]. Therefore, the search for modified acrylic formulations with improved mechanical and biological properties and better overall performance has been keen in recent years. The alterations to the conventional formulation have included fiber reinforcement (which was intended to improve mechanical properties, but presented serious drawbacks regarding handling), addition of bioactive fillers (which simultaneously improve mechanical properties and allow direct bonding to bone), replacement of radiopaque agents, toughened cements (by the addition of rubber particles or hydrophilic moieties), development of novel activators to replace DMT, partially degradable formulations (developed to improve the drug release profile of the cements, at the cost of compromised mechanical properties), crosslinked cements (developed to decrease chain mobility and improve mechanical properties) and two-solution formulations (as opposed to formulations with one solid and one liquid component). Accordingly, there are several review papers dealing with both the performance of commercially available cements and the properties of novel formulations.

Lewis [3] and Saha and Pal [5] reviewed all the data on mechanical properties of both commercial and experimental bone cements while Harper and Bonfield [6] focused on the tensile properties of commercially available cements. Kühn [7] presented a complete listing of all commercially available bone cements in Germany, comparing their composition, handling and mechanical properties (besides some other physical properties). Deb [2] and Serbetci and Hasirci [8] reviewed the latest developments in bone cement research, presenting the main properties, and advantages and disadvantages of the alternative formulations mentioned above. Harper [9] concentrated on bioactive cements, which are the major focus of attention in terms of novel formulations of bone cements. This set of publications also contains useful information about the basics of bone cements, and the reader is directed to them for more introductory information about the history, handling and mechanical properties, the behavior of the interfaces and the occurrence of infections, among other topics.

The present review covers the in vitro characterization of a novel class of acrylic bone cements, developed mainly in the last 5 years, which has not been reviewed previously. This formulation is a good example of multifunctional cements, that is, cements with modifications aimed to address several of the drawbacks at once. This is in contrast to the formulations mentioned above, which in general only try to address one or two of the problems encountered in acrylic bone cements.