Aims: the aim of the job is the evaluation of the pressure values between bone cement and the stem of hip prosthesis in post-operating medium-term conditions.

Methods: the experimental activity was developed in two phases. During the first one the hygroscopic behaviour of some cements in various application conditions was characterized. In particular some cylindrical specimens were realized using two types of cements: the Surgical Simplex P by HowmedicaA8 and the CMW 1 Radiopaque by DePuyA8. The specimens were obtained both with vacuum techniques and manual techniques. The specimens was instrumented with resistive strain gages and placed in a 37A1C water thermostatic bath. In the second part of the activity models of stems realized with aluminium tubular elements (diameter 10 mm) were instrumented with strain gages in order to measure the circumferential strains from which is possible to evaluate the pressure acting on the stems. The models was then inserted in cylindrical cavities representing the femoral cavities (diameter 15 mm) in which the same cements used in the first test were injected with the same operative conditions. Such specimens were inserted in the thermostatic bath to estimate the effects of the hygroscopic phenomena acting on the stems. Cements temperatures during the polymerization was also evaluated by means of special sensors applied on the stems. For all the phases the strains were acquired for an 8 weeks period.

Result: The activity permitted to analyze the hygroscopic expansion coefficients of the considered bone cements and to estimate the pressure values on the prosthetic stems. The pressure values evaluation have shown that, after an initial phase, during which we assist at a depression due to the volumetric shrinkage due to the polymerization of the resin, it follows a compression action on the stem due to absorption of humidity coming from the biological fluid by cements.