TMJsurgeon.com Surgery and alloplast materials. Part I

An alloplast is an artificial material used to replace body tissues. An artificial joint is an alloplast. In modern orthopedic surgery, the development of the artificial hip was pioneered by English surgeon Dr. (Sir) John Charnley in the 60′s and a colleague from the same facility pioneered the development of the artificial knee. The history and politics of all of these developments are as fascinating as the study of the people who saw the need and committed their professional lives to the bioscience. Politically, medico-legally, and socially, artificial joint replacements face many challenges. Charnley himself was labeled a quack in England and was delegated by the health service there to practice in an old tuberculosis hospital, out of the mainstream. His patients were the sickest and most impaired. Charnley, though, was one of those rare individuals who saw a need and the scientific and developmental talent….not to mention the will… to pull everything together. In today’s world, it is not likely the risk of the tort system and healthcare politics would ever allow a Charnley to exist, much less create a significant advancement in orthopedic medicine.

The Charnley hip development process served as the prototype to prosthetics of today, primarily due to two important concepts: function of a metal artificial “ball component” against a strong molecular weight, dense “ plastic” cup or socket replacement and a way to “fixate” the prosthesis to withstand the body’s weight over time.. In efforts to always try to improve things and create a better mousetrap, changes in format are often the rule. Theories of different biomaterials and their ability to provide better use and lifespan, manufacturing techniques to reduce material wear…. are always being researched. His first designs failed due to material wear, fracture of the metal neck that was cast and part of the metal ball component, and because of fixation problems. Failures though led to better biophysics and biomechanical understanding improving his next generation of prosthetics.   His pioneering work still stands today as the basics for bioengineering understanding of the modern hip prosthesis. 

If one makes a critical review of today’s modern “failures” …….findings can be  surprisingly consistent…there has been a variation of a system in some way that it violates the biomechanical principles that Charnley’s early failure taught his researchers.  Often they can be traced back to significant variations of one of Charnley’s premises….that any prosthetic system must be consistent of two dissimilar substances….particularly the cup or socket component performs best long term  if it is the high density “plastic” component or similar type substance against the metal ball component.  This is because substances of similar makeup and particularly metal against metal have shown microscopic wear and element breakdown……”metallosis” or microscopic breakdon of metal against metal creates localized reactions in joints that combined with other factors lead to loosening….but there is now published literature which reveals release of metallic ions at least into surrounding tissues.  This is not to say that the other systems do not wear also…..they do, but the localized reaction seems to be more of an inert event in surrounding tissues.  Somewhat a lesser of two evils concept when a surgeon makes a choice as to what type or design to employ.

In the 1960s, the learning curve was to take an eventual 100% impairing and crippling human condition and improve it significantly, offering humans an alternative to a life  of permanent pain, and eventual use of  crutches or a wheel chair. Charnley’s pioneering work and basic improvements of systems he advocated served a tremendous need.  The field of maxillofacial surgery faces significant similar needs as well, in selected circimstance.

Two Current total joint replacement systems of the human jaw joint use materials similar to knees and hips…the high weight bearing “plastic” fossa and the metal condyle replacement system. TMJ Concepts, Inc and Biomet, Inc. are the developers of these systems. Oral and maxillofacial surgery has had its “Charnley moments’ developmentally and professionally as we will see in the next blog.