Strength Characterization of Ceramic Femoral Heads of Total Hip Arthroplasties using a Fluid Pressure Controlled Destructive Proof Test and Finite Element Analysis
Authors:   , Heffernan John Michael , Kurtz Steven M. 
Published: July 2021

Modern ceramic femoral head components of total hip implants offer improved tribology and wear rates in comparison to traditional bearing materials. However, their brittle nature leaves them susceptible to fracture. As a result, manufacturers test their product lines with experiments designed to prove the mechanical failure strength of these components. These destructive proof tests apply a load to the ceramic samples in a manner that models in vivo conditions. The fracture strength is therefore determined to ensure the implants will withstand physiological loads. Finite element analyses (FEAs) are used to model these experiments and determine the mechanical effects on the heads. The standard method used in these experiments is adequate for new ceramic components, but is undesirable for testing surgically retrieved samples. This test has been recognized to damage the surface of the head's tapered cone which complicates finite element and fracture analysis. The goal of this thesis was to develop a destructive proof test that would minimize taper surface damage by loading ceramic heads in a physiological manner with water pressure, and to create an FEA model of the testing conditions. This concept originated in a patent held by CeramTec GmbH, and was built using a high pressure system that pressurized the inside of ceramic samples until failure. Specifically, this pressure was localized along the region that would be in contact with a metal femoral stem during implantation. The pressure applied to the heads was digitally recorded in ADMET's MTestQuattro software, and was used to select a constant rate of pressure increase. In all, two new alumina components from CeramTec's BIOLOX Forte product line were tested. No retrieval heads were tested, as this experiment was used as a pilot for determining the functionality of the designed experiment. Also, five FEA models were produced to simulate this experiment. Two modeled the conditions experienced by the two new alumina samples, whereas the other models were used for a material comparison. The comparison models were created by applying the maximum possible system pressure to heads made of the three most common ceramic materials: alumina, zirconia (Y-TZP), and zirconia toughened alumina (ZTA). The alumina samples fractured under pressures of 191.86 MPa applied at a rate of 1.96 MPa per second, and 192.60 MPa applied at 1.80 MPa per second. Based on the FEAs, the heads experienced maximum tensile principal stresses of 167.69 MPa and 168.33 MPa respectively. From these stress values, the critical depth for an edge crack to cause material failure along the pressurized region was calculated as 92.41 [mu]m and 91.70 [mu]m. Despite having a significantly lower burst strength than zirconia or ZTA, the two alumina samples required 93% of the test system's pressure capacity to cause failure. The results of the three remaining FEAs displayed similar magnitudes of stress between the modeled materials, and for this reason, no other ceramics were tested. Although there were limitations to this study, the test and FEA performed as required. During the testing procedure, one primary source of error was introduced by the equipment that was used to fixate the ceramic samples. Due to this, the stress values calculated for the fractured heads in the FEAs were likely underestimated. However, the designed test was capable of fracturing ceramic heads in a controlled and repeatable manner without causing significant damage to the surface of the taper. As such, this test should be considered a viable means for evaluating the strength of both new and retrieved ceramic heads.