Henning Schliephake from the George-Augusta University, Göttingen, Germany, discusses the state-of-the-art dental implants making their way to a dentist’s surgery near you
Dental implants are screws or cylinders, commonly made of titanium, which are inserted into the jaw bone in a surgical procedure. They act as artificial roots and can provide anchorage to crowns and bridges after the original root has been lost. Close contact between the adjacent bone and the implant surface is important for the artificial roots to remain fixed for the long-term. This bone-to-implant contact is accomplished by the growth of new bone on the titanium surface. The adsorption of biomolecules and blood components onto the titanium from the body fluids surrounding the implant is a crucial step in this process. This allows bone cells and their precursors to approach the surface and start growing bone tissue. Bone healing may be compromised by previous infection, radiotherapy or osteoporosis, and this can jeopardise the process and lead to implant failure.
In recent years, material scientists and clinicians have developed a number of approaches to improving the regenerative capacity of adjacent bone. One strategy is to create surface conditions that encourage quicker and more intense adsorption of biomolecules onto the implant surface, by altering the implants surface morphology and surface chemistry. Increasing the surface roughness using acid etching and sandblasting combined with integration of fluoride ions or calcium phosphate nanocrystals considerably increases the speed and strength of the bone-implant contact.
An alternative way of enhancing bone formation on the implant surface is using biological signals to attract bone cells to the surface and increase the speed they grow. Some of these signals are based on a short sequence of amino acids – the so called RGD motifs – that bind to specific receptors on the cell surface initiating cell migration and/or increasing cellular activity. The signals are held in place by various types of linker molecules that are anchored to the titanium surface.
Another way to accelerate bone growth is using signalling molecules or growth factors to accelerate cellular specialisation. As bone forming cells originate from so called precursor cells, using osteogenic signalling molecules such as bone morphogenic proteins on the implant surface could considerably increase the number of bone forming cells and therefore the amount and strength of the bone grown.
In native bone, many of these osteogenic signalling molecules are encased in the bone matrix, a highly structured assembly of minerals and proteins, which provides three-dimensional stability to the bone tissue. This reservoir of growth factors aids the healing process of bone, for example after a fracture. The most complex approaches among recent surface modifications of titanium implants have tapped into this source to produce matrix-engineered surfaces. Here bone matrix proteins such as collagen are arranged on the implant surface, together with molecules that bind growth factors in native bone, to provide a reservoir for osteogenic signals that are slowly released from the surface in a nature-like fashion after implant placement.
Many of these biological approaches have yet to work their way from the bench to the bedside. However, the huge potential of this research and improved biotechnological capabilities will eventually prove useful for the incorporation of many types of artificial materials into the human body – extending far beyond the integration of dental implants.