Hip replacements have a limited life before they too need replacing. This is known in the medical field as revision surgery and it is a source of much pain and inconvenience for the patient. Revision surgery is most often because the underlying bone has deteriorated. Bone needs stress to grow and to maintain strength, yet current hip replacement implants are too stiff - they constrain the affixed bone and prevent it from stretching. The bone around the implant starts to deteriorate and the bond between bone and implant fails.
Titanium, used in orthopaedic implants because it is strong, lightweight and biocompatible, transforms at temperatures above 883°C. The atoms rearrange from a densely-packed structure (called the alpha phase) to one that is loosely-packed (beta phase). Both alpha and beta phases are strong, but the beta phase is half as stiff and has elastic properties closer to that of bone. By adding elements such as tin, niobium and zirconium, our titanium alloy keeps its flexible beta structure at room temperature. If the alloy is then re-heated to 450°C, some alpha phase re-emerges, complicating the microstructure. This makes it more difficult for cracks to progress through the metal, making it less susceptible to fatigue failure. If the amount of alpha is controlled, a metal with good fatigue properties and low-stiffness is made.
To further reduce the stiffness of the implant and match it to that of bone, we use flexible scaffold structures. The scaffolding also allows the bone to grow into the implant, creating a strong bond. These complicated scaffolds are built from thin layers of titanium powder that are melted with a laser, then cooled to solidify. This manufacturing method is called Selective Laser Melting (SLM) and it is like 3D printing. Because SLM is good at producing one-off items, customised implants are possible; the patient’s CAT or MRI scans can be combined with Computer Aided Design to ensure a perfect fit.
My research at the University of Western Australia uses these new materials and advanced manufacturing techniques to try to overcome the main reason why orthopaedic implants fail. If successful, this work will help lead to more durable implants, reducing the need for revision surgery and allowing orthopaedic implants to be used on younger adult patients.
This video was created with friends and family for Gonzolabs "Dance Your Ph.D" 2011 Contest. It illustrates a central part of my research: the effect of the titanium alloy microstructure. The video was created using 2200 photographs because we didn't have a video camera, but also (and more importantly) because stop motion, even though tedious to shoot, is fun.
The music is "Mischa" by unsigned Perth indi-electropop outfit The Transients. Hear more of their music at thetransients.net
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