title: Predicting fatigue life of a PMMA based knee spacer using a multiaxial fatigue criterion.
abstract: OBJECTIVE
  Experimental tests have played a major role in the assessment of fatigue endurance of orthopedic prostheses; however, cyclic tests on devices entail high costs. Here, a multiaxial fatigue criterion coupled with computational simulations and material properties measurements has been employed to predict fatigue life of the tibial component of a polymeric PMMA spacer. The ultimate aim is to obtain valid information on fatigue behavior avoiding fatigue tests on the device.
  
  
  METHODS
  First, an accurate measurement of the static and fatigue properties of PMMA samples is performed. Then, numeric simulations of the fatigue behavior of the PMMA spacer reproducing the experimental test conditions according to ISO 14879-1 were carried out in order to calculate the stress field throughout the device. Finally, a Risk Index was calculated by using a proper multiaxial fatigue criterion for brittle materials (Kakuno-Kawada) for the assessment of the device fatigue behavior by predicting the F-N curves.
  
  
  RESULTS
  The numeric results were validated by comparing the predictions against experimental data already published by our group. The multiaxial fatigue criterion was able to predict the most critical point on the spacer upper surface and the fatigue behavior of the device that nicely matched the experimental curves.
  
  
  CONCLUSIONS
  This approach represents a valuable tool to investigate the mechanical reliability of implantable devices; nevertheless, the use of advanced and specific failure criteria coupled with accurate data of the device’s material is mandatory to represent a real alternative to the experimental approach in fatigue life prediction.??Key words: Acrylic bone cement, Fatigue endurance, Finite element analyses, Knee spacer.
authors.affiliation: Laboratory of Biological Structure Mechanics, Department of Structural Engineering, Politecnico di Milano, Italy.
authors.forename: Davide
authors.initials: D
authors.lastname: Carnelli
authors.forename: Tomaso
authors.initials: T
authors.lastname: Villa
authors.forename: Dario
authors.initials: D
authors.lastname: Gastaldi
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Pennati
mesh_terms: D000465:Algorithms; D016880:Anisotropy; D019245:Compressive Strength; D019544:Equipment Failure Analysis / Q000295:instrumentation* / Q000379:methods*; D007720:Knee Prosthesis; D011475:Prosthesis Failure