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2 Acta of Bioengineering and Biomechanics

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Fracture properties of an acrylic bone cement

Bialoblocka-Juszczyk E., Baleani M., Cristofolini L., Viceconti M.

Acta of Bioengineering and Biomechanics

2008

Vol. 10, nr 1

21-26

This study investigated experimentally the fracture properties, i.e., the fatigue strength, the resistance to crack propagation and the fracture toughness, of an acrylic bone cement (CemexĆĘ RX). The mean endurance limit was determined following the staircase method. The endurance limit was estimated at 9.2 MPa. The fatigue crack propagation rate was measured according to the ASTM E647 standard. The equation of the line fitting the crack growth per cycle (da/dN) versus the stress-intensity factor range (ŁGK), in a logˇVlog graph, was used to calculate the empirical constants of ParisˇŚ law for the selected bone cement: da/dN (m/cycle) = 3.56ˇP10ˇV7ˇPŁGK (MPaˇPm1/2)5.79. This power-law relationship described well (R2 = 0.96) the growth rate in the stable crack growth region, i.e., in the mid ŁGK range. The fracture toughness KIC of the bone cement was determined according to the ASTM E399 standard. The KIC mean value was 1.38 MPaˇPm.. These experimental results provide the set of necessary inputs for numerical studies aimed to investigate the damage accumulation process in the mantle fixing cemented prostheses.

In vitro history to evaluate the effects of daily activities on cemented hip implants

Cristofolini L., Savigni P., Teutonico A. S., Viceconti M.

Acta of Bioengineering and Biomechanics

2003

Vol. 5, nr 2

77-87

The loads acting on the hip joint during daily activities contribute to the failure of the fixation of cemented hip stems. An in vitro analysis of newly designed prostheses is necessary prior to in vivo clinical trials. In vitro pre-clinical testing procedures up to now have consisted in simulating only one or two conditions. The goal of this work was to define a procedure to assess the long-term effect of the most stressing activities on the integrity of the cement mantle. Thus, a cyclic load of constant amplitude is not acceptable. All activities inducing high loads need to be included, so as to replicate the most critical scenario from a fatigue point of view. The following activities were included in the load history: stair climbing and descending, car entry and exit, bathtub entry and exit, and stumbling. Load values and direction were assigned to each activity, based on the literature. A typical week was defined for a patient, based on statistics from the literature. An in vitro simulation running for 2 weeks was able to replicate the load peaks occurring in 24 years of patient activity. Stem-cement elastic micromotion and permanent migration arc continuously recorded at 5 locations. The cement mantle is inspected by means of dye penetrants after test completion to quantify the fatigue damage in the cement mantle. The load history was successfully applied to two different designs and is therefore ready for future applications to evaluate the long-term performance of the new prostheses and the effects of daily activities on implant outcomes.