Abstract
INTRODUCTION
Ceramic hip components are known for their superior material properties and longevity. In comparison to other materials commonly used, ceramics have a very low friction coefficient and a high fracture load. However, even though in-vivo fractures of ceramic ball heads are a relatively rare occurrence compared to other reasons for revision, they are of concern to the surgeon using ceramic components. The goal of this work was to evaluate the most probable causes for fracture and to quantify the influence of the metal taper contamination and shell deformation, respectively.
METHODS
An experimental set-up imitating the in-vivo loading situation was used to analyze different scenarios that may lead to the fracture of the ball heads, such as dynamic loading, edge loading and the metal taper contamination.
58 ceramic ball heads made of pure alumina were loaded until fracture under various conditions. Parameters under investigation were the inclination of the insert, the loading velocity, and the contamination of the interface between taper and ball head.
RESULTS
The behavior of the ball heads for the different scenarios showed a large variation. If the inclination of the insert equaled 45°, it is not possible to break the ceramic ball head prior to the failure of the metal taper due to high plastic deformation. In case of edge loading, due to the reduction of load transfer area, the load required to fracture dropped significantly. The loading rate had no measurable influence on this value. The largest effect on the fracture load had a contamination with osseous tissue and a damage of the metal taper. The fracture load decreases to approximately 20% compared to the value measured without the contamination.
DISCUSSION
Contamination of the interface with osseous tissue or damages on the metal taper lead to a minimum fracture load in the range of the maximum forces ever measured in vivo. According to these findings, diligence is recommended during the implantation of the ceramic hip components in order to avoid disturbances or contamination of this interface. Because the reduction of the friction and the damage or contamination of the ceramic/metal interface results in a reduction of the fracture load, the presence of any material on the component tapers should be avoided.
