The effect of water uptake on the behaviour of hydrophilic cements in confined environments.

Physiological fluids will be in contact with the implant components from the first moments after a surgery. Therefore, the study of the effect of water on the properties of the bone cements that are part of the arthroplasty procedure is of critical importance to predict the long-term performance of the whole system. In our research group, we have developed a novel concept, the hydrophilic, partially degradable and bioactive cements which uptake considerably more water than standard bone cements. In this paper, we aimed to study the effect of water uptake (WU) by these cements on their behaviour. The tests were carried out in confined cavities, which represent more accurately the in vivo situation the cement will face (constrained by the bone and prosthesis surfaces). We observed that the equilibrium WU decreased up to 60% (as compared to non-confined situations), depending of the formulation. This decrease resulted in a latent tendency of the cements to swell, and the hindering of such swelling generated a swelling pressure against the constraining walls. The pressure, and consequent press-fitting effect, could be controlled by a number of mechanisms, and resulted in higher stability of the hydrophilic cements, expressed as an increase in the push-out force, required to extract the specimens from such constrained cavities. This effect was only observed in hydrophilic cements, not in commercial, hydrophobic ones used as controls. We conclude that such cements will provide an additional and very useful source of immediate adhesion in the short-term after surgery: water induced press fitting.