Continued from Part One.
The MOM surfaces are cast and then machined to size and finally polished to a mirror finish to create a perfect fit. The idea is, a perfect fit means the surfaces should never touch each other, according to Dr. Timothy Wright, Kirby Chair of Orthopedic Biomechanics: “Just like the cylinder in an engine block,” he said.
But controversy has also arisen about the way surgeons fit these devices. The femoral head may be polished to a mirror surface, but then the ball surface is hammered into the thigh bone with a mallet, raising the question: how perfect is that fit going to be on assembly of the ball into the socket after such a process?
Nor is the problem confined to full hip replacements. The “replacement light” option of applying a metal coating to the patient’s existing femur and a metal lining to the pelvic socket has suffered similar problems of metallic ion contamination of the blood, suggesting heavy-handed surgery work on the full hip replacements is, at best, only part of the problem.
The use of MOM hip replacement devices has been dramatically reduced with DuPuy’s ASR model, along with others, since being removed from the market in September 2010. The worry for the industry is toxicity caused by MOM joints may cause a backlash against use of metal components and hasten development of ceramic and plastic alternatives. Already new ceramic materials are far less brittle than earlier types, opening up the prospect of extremely close tolerance viable ceramic joints in place of metal on plastic designs.
Joint manufacturers and the medical profession knew of these problems as far back as 2006 and have since done little more than monitor rejections and failures; so one has to say, if the repercussion is a dilution of a metals-dominant role, the medical industry will have done the metals industry a considerable disservice.