On the Machining of Joint Implant UHMWPE Inserts

Abstract

The modern orthopaedic implants for applications in hips, knees, shoulders, and spines are
composed of hard metal alloys or ceramics. The tribo-logical sub-component is composed of soft
materials with good tribological properties – e.g. UHMWPE (Ultra High Molecule Weight
Polyethylene). The UHMWPE implants need to be machined into their final shape after the
polymerization and consolidation into a blank profile or near-net shaped implant.
So machining is a crucial technology that can generate an accurate and precise shape of the implant
that should comply with the joints' function. However, the machining technology can affect the
topography and integrity of the surface, and its resistance to wear. The technology, cutting tools, and
cutting conditions can impact the physical and mechanical properties of the entire implant, limiting its
life span and creating a need to be replaced.
The basic machining technologies are turning and milling (each can be used as roughing or finishing).
There are many ways to machine these surfaces. Many problems such as low rigidity of the product,
poor thermal properties of the mate-rial, high melt viscosities, and sticking of the material to the
cutting edge (production of built-up edges) have been solved. UHMWPE can be damaged by excessive
heat, feed rate, cutting force, and tool micro-geometry. The shapes and dimensions for the customized
implants vary broadly for the humans this complicates the machining technology. No standard
programs can be used repeatedly so each joint must be designed and produced individually. However,
it results in the longer implant life and a better comfort of patients.