Thermal expansion is the tendency of matter to change in shape and volume in response to a change in temperature through heat transfer. Thermal expansion has been used in mechanical applications since early civilizations. Expansion and contraction of components by heat was widely used to fit metallic parts over one another, e.g. a metallic bushing can be fitted over a shaft by making its inner diameter slightly smaller than the diameter of the shaft, then heating it until it fits over the shaft, and allowing it to cool after it has been pushed over the shaft, thus achieving a “shrink fit”. Induction shrink fitting is common industrial method to preheat metal components between 150° C. and 300° C. thereby causing them to expand and allow for the insertion or removal of another component.
Modular orthopedic implants commonly use Morse tapers for attaching its components to provide to the operating surgeon the choice of different sizes in order to fit different anatomical shapes. However modularity have been frequently incriminated in release of metal wear debris, causing local inflammatory reactions ultimately leading to osteolysis. Pain and functional disability has frequently led to extensive surgical revisions of the implants and significant clinical and functional limitation.
It has been well established in-vivo as well as in-vitro that micromotion between the male and female components of a Morse taper was major culprit in initiating “fretting corrosion”. The later will damage the protective oxide layer of the contact surfaces between the taper components and initiate corrosive cascade. The presence of impurities and local tissue fluid will trigger galvanic corrosion by lowering the Ph of the interface milieu, which will release metal-hydrides ions causing further damage to the contact surface. It has been reported that manual impaction of the components was not sufficient to reduce micromotion between the components since cyclic loading of normal gait and other daily activities caused the components to cantilever and move.
Therefore, eliminating micromotion between the taper components at the level of the contact surface will significantly reduce the corrosion at multiple level of the corrosive cascade.