Ageing and moisture uptake in polymethyl methacrylate (PMMA) bone cements

Abstract

Bone cements are extensively employed in orthopaedics for joint arthroplasty, however implant failure in the form of aseptic loosening is known to occur after long-term use. The exact mechanism causing this is not well understood, however it is thought to arise from a combination of fatigue and chemical degradation resulting from the hostile in vivo environment. In this study, two commercial bone cements were aged in an isotonic fluid at physiological temperatures and changes in moisture uptake, microstructure and mechanical and fatigue properties were studied. Initial penetration of water into the cement followed Fickian diffusion and was thought to be caused by vacancies created by leaching monomer. An increase in weight of approximately 2% was experienced after 30 days ageing and was accompanied by hydrolysis of poly(methyl methacrylate) (PMMA) in the outermost layers of the cement. This molecular change and the plasticising effect of water resulted in reduced mechanical and fatigue properties over time. Cement ageing is therefore thought to be a key contributor in the long-term failure of cemented joint replacements. The results from this study have highlighted the need to develop cements capable of withstanding long-term degradation and for more accurate test methods, which fully account for physiological ageing.

Keywords: Biodegradation; Bone cement; Diffusion; Fatigue; Mechanical properties.

Graphical abstract

Fig. 1

Change in weight due to ageing in air at 23 °C and Ringer's solution at 37 °C over time. Both cements experienced a loss in weight when aged in air and an increase when aged in Ringer's solution. Palacos R experienced greater moisture uptake over 60 days when compared to Cemex cement.

Fig. 2

Surface porosity for Cemex Isoplastic (a) before and (b) after ageing in Ringer's solution at 37 °C for 60 days, and for Palacos R (c) before and (d) after ageing in Ringer's solution at 37 °C for 60 days. There was no increase in surface porosity as a result of ageing in Ringer's solution for both cements. White arrows demonstrate surface imperfections before ageing and black arrows demonstrate radiopacifier particles.

Fig. 3

Crack growth rates over different stress intensities for Cemex Isoplastic and Palacos R before and after ageing in Ringer's solution for 60 days at 37 °C. Ageing Cemex cement in Ringer's solution increased the crack growth rates, however further analysis was required to establish changes in the Palacos cement.

Fig. 4

SEM images of fracture surfaces for Cemex Isoplastic after (a) 24 h in air at 23 °C and (b) 60 days in Ringer's solution at 37 °C; and Palacos R after (c) 24 h in air at 23 °C and (d) 60 days in Ringer's solution at 37 °C. Porosity was observed before and after ageing and both cements demonstrated increased fracture surface roughness.

Fig. 5

FTIR spectra using ATR for Cemex Isoplastic and Palacos R after 24 h in air at 23 °C and Ringer's solution at 37 °C for 30 and 60 days. Several changes were observed for both cements indicating chemical changes as a result of ageing.

Fig. 6

FTIR spectra using KBr for Cemex Isoplastic and Palacos R after 24 h in air at 23 °C and Ringer's solution at 37 °C for 30 and 60 days. No chemical changes were observed when scanning a cross-section of the cement samples.

Fig. 7

The proposed mechanisms by which hydrolysis of ester groups of (a) MMA and (b) PMMA occurs.