D. Plamondon

Measurement of transient and residual stresses during polymerization of bone cement for cemented hip implants

The initial fixation of a cemented hip implant relies on the strength of the interface between the stem, bone cement and adjacent bone. Bone cement is used as grouting material to fix the prosthesis to the bone. The curing process of bone cement is an exothermic reaction where bone cement undergoes volumetric changes that will generate transient stresses resulting in residual stresses once polymerization is completed. However, the precise magnitude of these stresses is still not well documented in the literature. The objective of this study is to develop an experiment for the direct measurement of the transient and residual radial stresses at the stem-cement interface generated during cement polymerization. The idealized femoral-cemented implant consists of a stem placed inside a hollow cylindrical bone filled with bone cement. A sub-miniature load cell is inserted inside the stem to make a direct measurement of the radial compressive forces at the stem-cement interface, which are then converted to radial stresses. A thermocouple measures the temperature evolution during the polymerization process. The results show the evolution of stress generation corresponding to volumetric changes in the cement. The effect of initial temperature of the stem and bone as well as the cement-bone interface condition (adhesion or no adhesion) on residual radial stresses is investigated. A maximum peak temperature of 70 degrees C corresponds to a peak in transient stress during cement curing. Maximum radial residual stresses of 0.6 MPa in compression are measured for the preheated stem.

Design of a progressively expandable stent using finite elements

Currently, a minimally invasive surgery called stenting is extensively used to increase the lumen of partially obstructed arteries. Unfortunately, restenosis, a postoperative phenomenon in which the lumen of the artery is reduced due to a traumatism of the artery, is still a concern. The most popular solution that has been adopted by stent manufacturers comprises drug-eluting stents. This paper presents a new stent concept in which the treatment of restenosis is carried out from a completely different angle. Indeed, instead of traumatizing the artery, and then trying to control restenosis with drugs, the new stent minimizes the traumatism of the artery by expanding itself, not instantaneously, but progressively, and in a controlled manner. To achieve this, a nitinol stent over which a series of polymer rings are installed tries to reach a fully deployed configuration, but the polymer rings, which act as a retainer, become soft over time due to creeping. Thus, after the initial deployment in the artery, the stent continues its expansion autonomously over an extended period of time (a few weeks). It is believed that the artery has enough time to adapt to the expansion, leading to minimum traumatism. This paper presents the stent design.

Measurements of the Residual Stresses due to Cement Polymerization for Cemented Hip Implants

The initial fixation of the cemented hip prosthesis relies on the resistance of the interface between the metallic stem of the implant, the PMMA, and the adjacent bone. During the operation, the bone cement still in a liquid form is inserted between the implant and the bone. During polymerisation of the cement, residual stresses are generated in the bulk cement. An experiment has been devised to reproduce the in vivo behaviour of a cemented hip prosthesis, and to develop a technique to measure the residual stresses of the bone cement at the stem-cement interface. An idealized prosthesis (19-mm diameter) was placed inside a synthetic bone (outer diameter of 40 mm, inside diameter of 30 mm). The bone cement was poured between the stem and the bone. A sub-miniature load cell was inserted inside the idealized stem to measure directly the radial stresses generated by the cement curing on the hip stem. The tests are conducted at body temperature of 37°C to simulate the in-vivo conditions.