Calcium phosphate and chlorhexidine - releasing, high strength light-cured composites which promotes hydroxyapatite and antibacterial co-precipitation

Aljabo, Anas (Eastman Dental Institute, University College London)
Knowles, Jonathan C (Eastman Dental Institute, University College London)
Young, Anne (Eastman Dental Institute, University College London)

Introduction

Dental composites have been used for over 50 years as restorative materials. Compared with dental amalgam, these trigger less safety concerns and provide improved aesthetics. Over the years there has been significant increase in mechanical properties of commercial composites enabling a reduction in failure due to fracture and wear. Polymerisation shrinkage and lack of anti-bacterial activity, however, are continuing issues as they enable micro gap formation between the tooth and restoration followed by bacterial microleakage and continuing tooth demineralisation. To prevent this, a wide range of calcium phosphates (CaP)(eg hydroxyapatite (HA) and amorphous calcium phosphate) and antibacterial agents (eg chlorhexidine (CHX)) have been added to dental composites. Component release, to enable tooth remineralisation or antibacterial action, however, has either been restricted or reduced material strengths. Recently these issues were addressed by using reactive acidic and basic mono and tricalcium phosphate fillers (MCPM/ β-TCP) together with CHX (1). These CaP promote water sorption induced swelling to compensate composite shrinkage. The strengths (~ 100MPa) and CHX release of these novel composites were both relatively high, but the formulations had compromised optical properties. The aim of this study was therefore to develop light-curable, even higher strength formulations that would still release CaP and CHX. Furthermore, it was thought such materials should promote surface hydroxyapatite precipitation in simulated body fluid (SBF) through supersaturation. This hydroxyapatite might additionally co-precipitate with CHX as it is known to absorb this antibacterial from surrounding solutions (2).

Materials and Methods

Light curable monomers were prepared by mixing urethane dimethacrylate : triethylene glycol dimethacrylate : hydroxyethyl methacrylate : camphorquinone : dimethylparatoluidine in the weight ratio 68:25:5:1:1. This was combined with glass particles containing fibers (20wt%), CaP (0, 10, 20 or 40wt%) and CHX (10 wt%) at a PLR of 4. Polymerised discs (10 mm diameter, 1 mm thick) were made to assess the following: • Conversion and shrinkage using FTIR and density measurements. • Surface chemical changes upon immersion in water or SBF using Raman mapping, SEM, EDX, XRD. • Mass of any surface HA through careful removal and gravimetric determination. • CHX release into water or trapped in any surface HA layer using UV spectroscopy. • Biaxial flexural strength and modulus after one month in SBF.

Results

Monomer conversion and polymerization shrinkage of all composites was ~ 70% and ~3 vol. %, irrespective of CaP content. SEM images of composites stored dry or in water showed only scratches and small pores (Figure 1(a)). Any samples containing CaP and stored in SBF for one day or more, however, were covered with HA spheres (Figure 1(b-d)). After 4, 2 and 1 week, in SBF, samples containing 10, 20 and 40% CaP were approximately 90% covered with HA respectively. From EDX the ratio of Ca/P in the precipitate was 1.67 when focussed solely upon the precipitate as expected for HA. Raman spectra before and after immersion in SBF are illustrated in Figure 2. For the dry surface, sharp CHX peak, a glass peak and polymer peaks were evident. Phosphate peaks due to MCPM and β-TCP were also present. After 1 week immersion in SBF or water, the peaks attributed to MCPM disappeared. Those due β-TCP remained after water immersion but were masked by the very intense HA peak for composites immersed in SBF. Formulations with CaP stored in SBF all showed low-crystallinity HA peaks on XRD patterns. The total mass of HA scrapped from the disc surfaces at 12 weeks was between 3 and 15 mg, and was shown to be proportional to the calcium phosphate concentration in the sample. The early gradients for CHX release in water and SBF are provided in Table 1. Linear regression shows these gradients are proportional to CaP contents but in addition doubled in water compared to that in SBF. The CHX trapped in the removed HA layer after 12 weeks in SBF was also proportional to CaP content in the samples. Biaxial flexural strength and modulus both decreased with raising CaP level after storing for 1 month in SBF.

Discussion and Conclusion

Storing composite discs in SBF induced the precipitation of HA on the surface of the composites. Higher levels of CaP elevated CHX release in water due to increased water sorption. Significantly lower release of CHX, however, was shown in SBF. HA precipitates were shown to trap this antibacterial at the material interface providing a highly antibacterial interface. Even with high levels of reactive calcium phosphate these materials were shown to still have good flexural strength after 4 weeks immersion in SBF. This study has therefore overcome the various issues with previous antibacterial, remineralising composites described in the introduction.

Table 1: Gradients of CHX release versus the square root of time in water and SBF, CHX entrapment in the HA layer at 12 weeks and 4 week flexural strength and modulus for formulations with 0, 10, 20 and 40% CaP. Gradients and intercepts of the data versus CaP wt.% are also provided with R2 values.

Figure 1: SEM images for composite (a) 0% CaP, (b) 10% CaP, (c) 20% CaP and (d) 40% CaP immersed in SBF for 1 week.

Figure 2: Raman spectra for composite with 20% CaP dry, 7 days in water and 7 days in SBF.

References

1. Mehdawi I, Pratten J, Spratt DA, Knowles JC, Young AM. High strength re-mineralizing, antibacterial dental composites with reactive calcium phosphates. Dental Materials. 2013;29(4):473-84. 2. Misra D. Interaction of chlorhexidine digluconate with and adsorption of chlorhexidine on hydroxyapatite. Journal of Biomedical Materials Research. 1994;28(11):1375-81.

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