Engineers are hoping to make life better for those patients with replacement joints by modeling how artificial hips are likely to rub them the wrong way.
The study conducted at the Rice University is published in the journal Biotribology.
The Brown School of Engineering lab of mechanical engineer Fred Higgs conducted a computational study that simulates and tracks how hips evolve, uniquely incorporating fluid dynamics and roughness of the joint surfaces. They also included factors clinicians typically use to predict how well implants will stand up over their expected 15-year lifetime.
The researchers felt the need to look beyond the limitations of previous mechanical studies and standard clinical practices that use simple walking as a baseline to evaluate artificial hips without incorporating higher-impact activities.
Christian said, "When we talk to surgeons, they tell us a lot of their decisions are based on their wealth of experience. But some have expressed a desire for better diagnostic tools to predict how long an implant is going to last. Fifteen years sounds like a long time but if you need to put an artificial hip into someone who's young and active, you want it to last longer so they don't have multiple surgeries."
The researchers wanted to know how long their best candidate hip joints would last. The model uses walking motion of real patients.
The results of physical stimulators can take months as they need to run millions of cycles to predict wear and failure points. Higgs' model looks to speed up and simplify the process by analyzing real motion capture data like that produced by the Fregly lab along with data from "instrumented" hip implants.
The new study incorporates four distinct modes of physics- fluid dynamics, contact mechanics, particle dynamics, and wear which play an important part in hip motion. According to the researchers, no previous studies considered all four simultaneously.
Another issue previous study did not consider was the changing makeup of the lubricant between bones. Natural joints contain synovial fluid which is an extracellular liquid secreted by the synovial membrane, connective tissue that lines the joint. Even when a hip is replaced, this membrane is preserved and continues to secrete the fluid.
Higgs said, "In healthy natural joints, the fluid generates enough pressure so that you don't have contact, so we all walk without pain. But an artificial hip joint generally undergoes partial contact, which increasingly wears and deteriorates your implanted joint over time. We call this kind of rubbing mixed lubrication."
This rubbing can increase the generation of wear debris, especially from the plastic material - an ultrahigh molecular weight polyethylene commonly used as the socket (the acetabular cup) in artificial joints. These particles of size 5 microns mix with the synovial fluid can sometimes escape the joint.
They can eventually loosen the implant or can cause the surrounding tissue to break down and often get carried to other parts of the body, where they can cause osteolysis.
The researchers noted that the metal sockets are more durable. However, if the metal shavings break off and accumulate, they can cause much more damaging effects than polyethylene particles.
The researchers noted future models will incorporate more novel materials being used in joint replacement.