Dental implant as replacement for missing teeth is currently the preferred choice of most dentists and patients. However, for some patients, the bone in the jaw is insufficient to support an implant and requires bone grafting, a method of augmenting the deficiency with bone either taken from another part of the patient’s body or from an alternative source, such as human cadavers, animals or synthetic. The question of how long to wait after bone grafting before dental implant surgery can be done was a subject of intense debate in Singapore recently after a dentist was charged with placing an implant seven weeks after bone grafting.
Last week, I participated in a forum to discuss this topic. On the panel with me was a periodontist and a senior dentist who was a pioneer in dental implantology in Singapore. While it is tempting to find a certain magical number of weeks or months of bone graft healing that will ensure success, bone physiology, unfortunately does not quite work that way. There are many guidelines issued by various organizations but there are many caveats that accompany them as well.
We need to understand the basic science of how normal native bone functions and what happens to the bone graft when it is placed onto the native bone. We also need to understand the process of osseointegration ie the way a dental implant bonds with the jaw bone.
Bone is a living tissue and there is a remodeling process that goes on all time. On average 0.7% of our bone cells were changed daily, which means that every 142 days, the bone cells are completely changed. This process depends on a balance between the osteoblasts and the osteoclasts, which are responsible for building new bone and removing old bone respectively. This is a different process from that which occurs when there is healing bone after an injury, be it a fracture, drilling of the bone in preparation of a dental implant or placement of a bone graft. Bone modeling in response to injury is not predicated on the osteoclast removing bone first. The injury initiates bleeding and consequently the clotting process. In the case of a bone graft, the fibrin hold the graft particles together as the new blood vessels form and penetrate the graft, bringing oxygen and nutrients to support the graft. The clotting process also releases signaling molecules that bring in more cells into the site to produce new bone cells. These bone cells produce bone which in the early stages is soft but gradually hardens over time.
Osseointegration of implants
When placing an implant, the jaw bone is prepared by drilling a hole to fit the implant. This drilling causes bleeding and a blood clot is quickly formed between the bone and the implant surface. Fibrin in the clot connects the implant to the bone and bone cells travel along these fibrin strands and lay down bone as they move from the bone to the implant surface. Similarly, the new bone undergoes a period of consolidation whereby mineralization hardens the new bone formed between the bone and implant surface.
Waiting vs No waiting
It is estimated that this process of bone healing takes about four to six months. However, there is no randomized controlled study done to show that placement of implants into bone grafted sites after less than four months will result in failure. This is further complicated by the fact that there are protocols whereby implant insertion can be done at the same time as bone graftingand yield a high success rate. The fact that there is signification variation in the timing of implant placement post bone grafting that ranges from zero waiting to a waiting time of up to a year shows that the process is an dynamic spectrum and not a “one or the other” scenario.
In addition, there are many other variables at play. The type of bone graft makes a difference. Bone that is harvested from the patient’s own body and directly grafted on the recipient site heals better and faster than bone substitutes such as synthetic bone or bone mineral processed from animal sources. Furthermore, the form of the bone ie whether it is in the form of a block or in particles, also has an impact on the healing process of the graft, the former needing a longer time than the latter. The recipient site is yet another variable as sites with a better blood supply will yield faster consolidation of the graft than those that are poorly perfused. The patient’s systemic condition also affects the process, as older patients tend to take a longer time to heal than a younger one.
"Don't do this at home"
To further confound the issue, the surgeon factor is an important factor that is often neglected. When we go to the circus and we see all the acrobats doing all kinds of amazing maneuvers, we are often reminded that we should not be attempting these at home. The fact that these acrobats can trapeze from one pole to another fifty feet above the ground does not mean that the rest of us can do the same thing. We can study and read all the books in the world about these acts but without the long hours of practice and years of experience, it is not possible to put on such a performance. For surgery, it is the same. There is no substitute for actual practice. Just because a meta analysis has been published showing that a particular protocol has a 99% success rate does not mean that anyone practicing that protocol will get a 99% success rate as well.
Role of Guidelines
Given that there are so many factors that can affect the outcome of an implant in a bone grafted site, we cannot rely purely on guidelines and protocols. As much as possible, treatment we carry out for patients should be substantiated by scientific evidence. However, the absence of evidence is not evidence of absence. The fact that there is no scientific evidence that something works does not constitute as evidence that it cannot work. It is through a combination of constantly undating ourselves with the current state of evidence as well as meticulous practice of skills supported by such evidence that optimal results can be attained.