Although artificial hips allow many people to remain mobile, the things do wear out after a while, often requiring replacement surgeries. The need for such operations may soon become less frequent, however, thanks to research being conducted at the University of Arkansas.

Installation of an artificial hip involves using alloy prostheses to replace the femoral head (the "ball" at the top of the thigh bone) along with the acetabulum (the "socket" in the pelvis, which the ball fits into).

Ordinarily, in a natural hip joint, the interface between the ball and socket is kept lubricated with a viscous film of synovial fluid, which is produced and released by the hip's cartilage. When a hip is replaced, that cartilage is removed. While the body still does produce some synovial fluid, it isn't always enough to keep the alloy ball and socket adequately lubricated. The resulting wear on their surfaces can trigger an autoimmune response by the body which loosens the bonds between the prostheses and the adjacent bone, causing the artificial hip to fail.

Working with postdoctoral fellow Dipankar Choudhury, Prof. Min Zou decided to experiment with artificial femoral heads that had ball surfaces which were not perfectly smooth. Instead, using a picosecond laser, they were covered with an array of microscopic indentations that were either square, triangular or elliptically-shaped. This was intended to replicate the dimpled surface structure of natural cartilage.

The dimpled artificial femoral heads – along with a completely smooth one – were then tested in a rig that swung them back and forth like a pendulum, to replicate leg movement. An added mixture of bovine serum and water simulated the synovial fluid. Once the heads got swinging and that fluid had formed into a lubricating film, it was found that the heads with the square and triangular dimples produced a film that was 3.5 times thicker than that produced by the smooth head. Additionally, the square dimples formed that film in less than one second, which was much faster than any of the others.

"Also, hardly any scratches appeared on the post-experimental prosthesis with square-shaped dimple arrays, and only a few scratches were found on the prosthesis with triangular-shaped dimple arrays," says Zou. "We think that prostheses with square-shaped dimples could be a potential solution for durable, longer-lasting artificial hip joints."

A paper on the research was recently published in the Journal of the Mechanical Behavior of Biomedical Materials.