Thermally induced strains and total shrinkage of the polymethyl-methacrylate cement in simplified models of total hip arthroplasty

An evaluation of transient and stabilized strains in the cement mantle during polymerization was carried out in simplified cemented total hip arthroplasty (THA) model. A mathematical approach combined with a simple finite element simulation was used to compare measured and calculated stabilized strain values and to provide the Von Mises stresses at the stem/cement interface due to shrinkage related to temperature decrease after exothermal reaction. A second similar model was carried out to measure stem/cement/mold interfacial shear strength and dimensional changes of the cement mantle to obtain total shrinkage due to temperature decrease plus cement polymerization. The results indicated that positive strain peaks found during the exothermic stage of polymerization have the potential to produce pre-loading cracking. After the initial expansion, it was observed a progressive strain decrease pattern down to stabilized values that takes place near 2 h after the cementation. Even though there is a great deal of dispersion in the measured stabilized strain values, in average those values match quite well with the numerical simulations, indicating 4,7 MPa von Mises interfacial stress due to thermal shrinkage. The total cement shrinkage leads to a negative radial stress of 11 MPa and 14 MPa von Mises interfacial stress. Finally, total shrinkage has the potential to enhance gaps in the cement/mold interface.

► We analyze the strains resulting from the polymerization of PMMA used in THA.
► Strains were monitored “in situ” during polymerization of PMMA cement mantle models.
► Strains were monitored to distinguish the portions of thermal and total shrinkage.
► Mechanical tests were also performed to obtain the interfacial strength of the model.
► Shrinkage provides residual stresses at stem/cement interface and cement/bone gaps.