Validation of the Small Punch Test Technique to Measure the Mechanical Properties of Acrylic Bone Cement

Introduction The main function of acrylic bone cement is to transfer loads from the prosthesis to the cancellous bone or increase the load carrying capacity of the surgical prosthesis. If the imposed stresses are greater than the load carrying capability, then cement fracture may occur resulting in failure of the joint replacement. The fracture of the cement mantle and subsequent premature loosening are directly related to the strength of the cement mantle. The small punch test method covers the determination of mechanical behaviour of acrylic bone cement by small punch testing of miniature disk specimens. The test method has been established for characterising the mechanical properties of metals and polymeric materials for over the past two decades. The small punch test procedure has also been used to determine the mechanical properties of polymeric biomaterials, in particular ultra high molecular weight polyethylene (UHMWPE) and to a lesser degree, acrylic bone cement. The objective of this study is establish the efficacy of the small punch test as a means of quantifying the mechanical properties of acrylic bone cement, in particular retrieved bone cement. To verify the accuracy of the small punch technique, the elastic modulus of the bone cement will be determined using four point bending technique as described in ISO standard, 5833 . Materials and Methods Palacos R® acrylic bone cement was used for this study, it was mixed using a third generation mixing device under the application of a pressure of 76 kPa below ambient. Specimens for the small punch test were prepared from the injection nozzle of the mixing system 24 hours after full-polymerisation was reached. Each sample was then machined into discs, each with a thickness of 0.5 mm and a diameter of 6.4 mm. The specimens were then stored under four different conditions for seven days prior to testing; dry air at 22 C, wet at 22 C, dry air at 37 C, and wet at 37 C. The small punch test was conducted using Lloyds tensile testing machine (Lloyds, UK) at a constant displacement rate of 0.5 mm min. The design of the small punch test device used by Kurtz et al was used in this study, but modified in order to conduct the test while the sample was immersed in distilled water . Testing on all samples was conducted at ambient and body temperatures (22.0 ± 0.4 C and 37.1 ± 0.2 C, respectively). The loaddeformation plot was used to determine the mechanical behaviour of the acrylic bone cement. The elastic modulus was determined using standard engineering mechanics principles for a circular plate, concentrated load at centre, simply supported. As a benchmark the flexural properties were also determined using the four point bending technique as defined in the ISO standard, 5833. Similar storage and test conditions were adopted for the four point bend samples. Re