Microscopic indentations increase the thickness of a lubricating film on an artificial hip’s surface. (Credit: University of Arkansas)

Researchers have developed a method that could extend the life of an artificial hip by adding an array of microscopic indentations that increase the thickness of a lubricating film on its surface.

Each year in the United States, orthopedic surgeons perform more than 300,000 hip-replacements, including roughly 36,000 revision surgeries when a replacement wears out. A primary reason for artificial joint failure — and thus the need for revision surgery — is a loosening of the bond between the implant and bone, which in many cases is caused by the body’s autoimmune response to particles and wear on the surface of the implant, as well as the toxicity of metallic ions released into the body because of this wear.

Rather than manipulating synovial fluid, they simulated the textured surface of cartilage, its so-called “converging gaps,” or natural dimples, that promote the secretion of synovial fluid and enhance thickness of the lubricating film. They designed square-, triangular- and elliptical-shaped indentations on the surface of artificial femoral heads. (Magnified images resemble the surface of a golf ball.) These prostheses were made of chromium, cobalt and molybdenum, the standard metallic alloy for hip-replacement implants. The indentations, fabricated with a picosecond laser by a collaborator in Japan, were 20–50 µm wide and 0.2–1 µm deep.

The prosthesis with square-shaped indentations formed its lubricant film less than a second after the pendulum started moving, much faster than all other prostheses, including those with indentations made of other shapes.

They also recently developed processes for fabricating smaller micro-dimples of various complex shapes, which are expected to further decrease friction and wear of the prosthetic joints.