Hydroxyapatite Bone Grafts
We have used the inorganic material hydroxyapatite as part of the construction of implantable prosthetics for a little over a year at this point in time. Whilst it is an inorganic material, it also makes up over 70% of the structure for bones and teeth found in the human body. Both hydroxyapatite and varieties of artificial hydroxyapatite have been used to 'disguise' an implant from the body's immune system.
Pennsylvania State University researcher Henry J. Donahue, Professor of Orthopaedics and Rehabilitation, Penn State College of Medicine, has found another, not too dissimilar use for the material. As a spray-on layer to increase the chances of a bone graft implant meshing with existing bone.
The major problem with bone grafts, comes in the implant site preparation process. Natural bone grafts need to be sterilized and processed with chemicals and radiation before implantation into the body to ensure that disease is not transmitted by the graft. However, normally natural bones have a rough surface, easy for new bone growth to latch onto. But the sterilization procedure smooths away all the ridges and holes that make up this rough surface, leaving quite a clean, smooth surface in their wake. This smooth surface actively discourages new bone formation around the implant.
So, what was required was a method of bringing back the roughness, once the bone had been fully sterilised, so it looked like it did before the procedure took place, but was still totally free of any possibility of infection. This is what the professor and his team set out to do, using materials naturally found in bone - hydroxyapatite.
"We created a method for resurfacing bone that had been processed, and resurfacing that bone so that it is now nearly as osteogenic as unprocessed bone -- meaning it works nearly as well as bone that hadn't been processed at all," Professor Donahue stated.
The research team tried several methods of applying the material, from immersion to painting it on, to dabbing and spraying, trying to recreate the texture of natural bone. Ultimately they settled on physical vapour deposition to create a very realistic effect. Tiny particles of hydroxyapatite in a mist solution are sprayed over the implant, falling onto it in a rough, random pattern, and quickly building up into clumps of different densities dozens of nanometres thick.
A range of thickness was tested, exposed to cultures of bone to see what thickness created the best bonding process. If the hydroxyapatite coating was not thick enough -- or there was none -- the graft implant worked, but did not integrate as well as if there were a few nanometres more layered onto the surface. If the hydroxyapatite was too thick, the graft implant again worked, but did not integrate as well as the researchers had seen was possible. In the end, a thickness of just one hundred nanometres was found to be optimal.
Whilst the method was designed for bone graft implants, due to the way it functions, acting as a bonding layer on top of an otherwise smooth surface, Professor Donahue believes it will work just as well for a normal prosthetic, binding the previously smooth surface of the prosthetic into the bone, so that new bone forms around it, anchoring it far more firmly in place than has until now, been possible.