Role of Mechanics in Medical Implant Industry

It is interesting to note that although mechanicians have been working on orthopaedic implants in large numbers and with great enthusiasm, the fundamental design of the artificial hip has not changed since that originally forwarded by Charnley in the 1960s.   

It is also perhaps interesting, in light of the recent discussion on why solid mechanics and fluid mechanics are such distinct fields, that mechanicians working on natural materials and tissues (associated with the term "biomechanics") and mechanicians working on implant materials (associated with the term "biomaterials") also work independently and there is relatively little cross-talk between the two communities.   This is perhaps another example where better communication across disciplines might just lead to the breakthrough that changes modern medicine. 

Michelle makes a very good point. Based on consulting I have done for some of the major orthopaedic implant manufacturers, I would say that mechanicians have only a marginal practical influence on the design of orthopaedic implants. In my estimation, surgeons, regulating bodies (e.g. FDA in the US), and insurers are the primary influences. Many implant designs are quite primitive, especially for the spine. The number of revision surgeries for disc replacements (to alleviate unbearable pain and other complications resulting from migration of the implant) is considerable, in part due to the difficulty of surgical placement but also due to design flaws. Given that disc revision surgery can be terribly dangerous due to the anterior approach and difficulties associated with scar tissue that forms over the major bloodways in that area after the implant is installed, designing a better disc replacement is imperative. Yet progress is very slow and seems to occur as a succession of minor modifications to existing designs, which is the safe approach for manufactures seeking approval of their devices. A radical new design can be financially catastrophic if a company invests heavily but does not receive FDA approval, or receives approval from the FDA but not from insurers.

The FE analyses I conducted (using TrueGrid and LS-DYNA) were primarily of spinal devices (although some knee and hip) and were used by manufacturers to apply for FDA approval. The analyses were cadaver specimen-specific including bones meshed to surfaces extracted from CT scans, ligaments, cartilage and muscle forces applied at muscle attachment sites for a variety of normal loading conditions (e.g. standing, running). However, it was really only the stress analysis of the implant that was of great interest to the manufacturers without too much consideration given to its performance (although recently knee and hip studies sponsored by implant manufacturers have been focusing more on post-operative bone remodelling and facture of trabecular bone induced by the implant design). One dynamic stabilization device of the spine that I analyzed had a design flaw which led to it offering only slightly more range of motion than a fusion despite the intent. However, it was already at the stage of being submitted to the FDA when I analyzed it. My main point is that I believe it is quite difficult for mechanicians to have significant practical impact on orthopaedic implant design because of the bottlenecks that exist, and that these bottlenecks are the primary reason designs tend to change very slowly. I think a proper appreciation of the market forces is necessary to make inroads in this area. Teaming with surgeons may be the best course.