In rats, bone screws made out of silk can fasten broken bones together just as well as metal and polymer ones. 

 

For bones to heal and bond, doctors have been screwing in metal alloys (the gold standard), but compared to the surrounding bone, they’re stiff and unyielding. The stiff implant ends up carrying most of the load, but bones don’t get stronger without physical stress. This phenomenon is called stress shielding, and over time, it leads to bone degradation. Additionally, metal increases infection risk, and its inability to degrade in the body usually means a second operation is needed to remove the pieces. Or worse, if the metal corrodes and has to be taken out before the fracture has even healed.

 

While there are biodegradable alternatives -- poly-lactic-co-glycolic acid fixation systems, to be exact -- these synthetic polymers can trigger inflammation. And unlike metal screws, they aren’t “self-tapping,” which means you end up having to drill a hole in the bone first in order to fashion a helical ridge inside to hold the screw in place.

 

For a robust biocompatible solution, a team led by Samuel Lin of Beth Israel Deaconess Medical Center and David Kaplan from Tufts turned to cocoons from Bombyx mori silkworms. 

 

Silk has a known low stiffness similar to that of bone -- but it’s tough enough to carve its own threads into bone as it’s screw in. According to Lin, silk materials maintain structural stability under very high temperatures, withstand extreme conditions, and can be readily sterilized. 

 

The team dissolved silk in alcohol, poured the solution into molds shaped like surgical plates and screws, and baked them, New Scientist details. Then they implanted 28 of their silk bone screws into the hind limbs of six rats. During the eight-week investigation period, the team found that the screws promoted bone remodeling, without causing inflammation. They don’t show up in X-rays, obscuring the view, and the body will just absorb it over time. 

 

Another advantage to silk: It can stabilize and deliver bioactive component, so these screws "could actually deliver antibiotics to prevent infection, pharmaceuticals to enhance bone regrowth and other therapeutics to support healing,” Kaplan explains. 

 

The work was published in Nature Communications this week. 

 

[BIDMC release via New Scientist]

 

Image: Gabriel Perrone