According
to the American Academy of Orthopaedic Surgeons,
766,100 Americans underwent surgery for hip, knee
and shoulder replacements in 2002. During typical
procedures, surgeons remove an arthritic or damaged
joint and replace it with an artificial one. In about
1 to 2 percent of cases, the implant gets infected.
The most common culprit: Staphylococcus epidermidis .
Found on skin or in mucous membrane, S. epidermidis can
enter a surgical wound and adhere to an implant.
The bacteria multiply, causing a slimy layer, or
biofilm, to form around the implant. The slime is
tough stuff, acting as a physical and chemical barrier
that resists antibiotics. The result is additional
surgery to clean the implant or replace it outright.
Webster, along with former Purdue University colleagues
Gabriel Colon and Brian Ward, knew that abrading
or coating implants to produce microscopic bumps,
which create a sand-papery surface, aid in bone growth.
This helps anchor the implant in the body and extends
its life. Some artificial joints now sport these
microstructured surfaces.
But the team wondered if smaller peaks and craters – ones
that measure on the nanometer scale – would work
even better. And how would bacteria react? So they
experimented.
The engineers chose zinc and titanium oxides as
their materials. Zinc oxide is a well-known antimicrobial
agent. Titanium oxide, strong and light, is a commonly
used in implants. Engineers took nanoparticles of
these ceramics and pressed them into dime-sized discs.
They took microparticles of these same materials
and made more discs. Discs with nanostructured surfaces
had bumps that measured only .023 microns in diameter.
Discs with microstructured surfaces had bumps that
measured about 5 microns in diameter. Under a microscope,
the surface differences are extreme; the nanostructured
discs look like saw-toothed mountains, the microstructured
discs look like smooth plateaus.
The engineers put S. epidermidis on the
discs and waited an hour. Then they counted the bacteria.
The results were dramatic. Microstructured zinc oxide
discs were host to 1,000 times more bacteria than
the nanostructured zinc oxide discs. Similar, but
less striking, results were duplicated on titanium
oxide discs.
The engineers conducted similar experiments with
bone-forming cells and found that twice as many of
these cells grew and formed bone on nanostructured
discs. Other indicators of healthy bone growth, such
as collagen synthesis, were also stronger with nanostructured
discs.
“Surface area seems to be key,” Webster said. “With
the nanostructured surfaces we created, surface area
increased by 25 to 35 percent. We think that this
additional area, along with the unique surface energetics
of these nanomaterials, gave bone-forming cells more
places to adhere. But with bacteria, increased surface
area may work the other way, exposing the bugs to
more of the germ-fighting properties of the zinc
oxide.”
The National Science Foundation funded the work. |
