Study: Indentations on Artificial Femoral Heads Could Improve Implant Longevity

By simulating the textured surface of cartilage, researchers from the University of Arkansas and the Czech Republic have found that using prostheses with square micro-dimples could extend the life of hip implants.

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

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.

To address this problem, researchers at the University of Arkansas (including Min Zou, Ph.D, professor of mechanical engineering and Dipankar Choudhury, post-doctoral fellow in the Center for Advanced Surface Engineering), in collaboration with a research group from the Czech Republic, focused on the synovial fluid film that is produced and released by cartilage and forms between the interacting surfaces of the femoral head and the acetabulum.

Although cartilage is removed during hip-replacement surgery, the body produces enough synovial fluid to form a thin layer between the implants. The formation of this film after surgery and especially its thickness determine the long-term success of the implant.

“Lubrication plays a key role in the wear process of the implanted joint interface,” Zou said. “But this mechanism is challenging to understand because of the complex nature of the viscous fluid used for lubrication. Our study reveals promising results for understanding this mechanism better and significantly improving implants.”

Rather than manipulating synovial fluid, which most hip prosthesis research focuses on, Zou and Choudhury simulated the textured surface of cartilage to promote the secretion of synovial fluid and enhance thickness of the lubricating film.

Zou and Choudhury designed square-, triangular- and elliptical-shaped indentations on the surface of artificial femoral heads. These prostheses were made of chromium, cobalt and molybdenum and the indentations, fabricated with a picosecond laser, were 20 to 50 micrometers wide and 0.2 to 1 micrometer deep.

Collaborating with researchers in the Czech Republic, Zou and Choudhury joined the prosthetic parts and performed wear and friction tests with a pendulum that simulated the swinging motion of a human leg. When the pendulum returned to equilibrium position—after the lubricant film had fully developed—test results revealed significantly enhanced thickness of film.

Compared to the non-dimpled prostheses, all three shapes improved lubricant film thickness:

  • The prostheses with square- and triangular-shaped indentations demonstrated an average lubricant film thickness 3.5 times greater than the non-dimpled prosthesis.
  • 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. This clearly demonstrated that shape had a significant effect, according to the researchers.

“We think that prostheses with square-shaped dimples could be a potential solution for durable, longer-lasting artificial hip joints,” Zou said.

Each year in the U.S, orthopaedic surgeons perform more than 300,000 hip replacements, including roughly 36,000 revision surgeries when a replacement wears out. Hip implants generally last 15 to 20 years.

The hip market accounted for 40% of joint reconstruction sales in 2017 ($7,225.7MM), according to ORTHOWORLD estimates.

This study appears in the May 2018 edition of The Journal of the Mechanical Behavior of Biomedical Materials.



Rob Meyer is ORTHOWORLD’s Senior Editor. He can be reached by
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