Some of the major uses of ceramics are for the medical industry. Currently, ceramic is used for bone implants and biomedical pumps, and might someday be used in gene therapy and tissue engineering. Ceramic bone implants and replacements are very remarkable modern accomplishments, since they replicate the natural structural performance of the human skeleton. Successful bone implants also must limit any adverse effects to the rest of the body. ISO 13356:2015 – Implants for surgery – Ceramic materials based on yttria-stabilized tetragonal zirconia (Y-TZP) specifies guidelines and test methods for a biocompatible and biostable ceramic bone substitute material for surgical implant use.
Many ceramics are not suitable to act as bone substitutes, since they do not have the correct density and durability to work in unison with the rest of the body. The skeletal system is a collection of fulcrums and levers, opposing and responding to forces generated from gravity and human muscles. Our bones must be able to move with these forces and successfully resist any forceful effects, such as bending or torsion. For example, the human pelvis, which is often replaced artificially, has a unique bell shape to support our upright bipedal posture and movement and must be able to undergo a great deal of compression. Yttria-stabilized tetragonal zirconia (Y-TZP) is the ideal ceramic material for constructing artificial bones because of its high strength and toughness. Because of these qualities, unlike other ceramic bone materials, it can be used without any metal for support.
ISO 13356:2015 provides many testing guidelines for the ceramic material that is to be used as a medical implant. This involves exposing a sample of the material to extreme bending and cyclic fatigue. Each of these tests should result in a test report that establishes the method in which the tests were carried out and whether or not the samples passed those tests. Tests to determine the fracture stress of the material should also be undertaken by applying enough weight to a disc-shaped sample to cause it to break. To attain a measure of hardness of the sample, the standard recommends using the Vickers hardness method and creating a report in accordance with ISO 14705 – Fine ceramics – test method for hardness of monolithic ceramics at room temperature or ASTM C1327-15 – Standard Test Method for Vickers Indentation Hardness of Advanced Ceramics. This involves applying a test load of 9.81 N.
Even if the implant is structurally sound, its insertion still involves introducing a foreign material into the human body. If the material contains chemicals that can be harmful to the inside of an individual, then it should be avoided for use as an implant. However, according to the Introduction of the standard, “no known surgical implant material has ever been found to cause absolutely no adverse reactions in the human body”. Despite this, ISO 13356:2015 takes into account the long-term clinical experience and research indicating that ceramic materials based on Y-TZP demonstrate an acceptable amount of biological response if they are used in appropriate applications. The standard recommends testing for radioactivity, which is carried out with a high resolution single germanium semiconductor detector, connected to a multichannel analyzer (MCA). This should result in a report that lists all major isotopes present in the material.
