Dental Implantation: History
The Mayan civilization has been shown to have used the earliest known examples of endosseous implants (implants embedded into bone), dating back over 1,350 years before the famous Per Brånemark started working with titanium. Whilst excavating Mayan burial sites in Honduras in 1931 archaeologists found a fragment of mandible of Mayan origin, dating from about 600 AD. This mandible, which is considered to be that of a woman in her twenties, had three tooth shaped pieces of shell placed into the sockets of three missing lower incisor teeth. For forty years the archaeological world considered that these shells were placed after death in a manner also observed in the ancient Egyptians. However in 1970 a Brazilian dental academic, Professor Amadeo Bobbio studied the mandibular specimen and took a series of radiographs. He noted compact bone formation around two of the implants which led him to conclude that the implants were placed during life.
In the 1950s research was being conducted at Cambridge University in England to study blood flow in vivo. These workers devised a method of constructing a chamber of titanium which was then embedded into the soft tissue of the ears of rabbits. In 1952 the Swedish orthopaedic surgeon, P I Brånemark, was interested in studying bone healing and regeneration, and adopted the Cambridge designed 'rabbit ear chamber' for use in the rabbit femur. Following several months of study he attempted to retrieve these expensive chambers from the rabbits and found that he was unable to remove them. Per Brånemark observed that bone had grown into such close approximity with the titanium that it effectively adhered to the metal. Brånemark carried out many further studies into this phenomenon, using both animal and human subjects, which all confirmed this unique property of titanium.
Although he had originally considered that the first work should center on knee and hip surgery, Brånemark finally decided that the mouth was more accessible for continued clinical observations and the high rate of edentulism in the general population offered more subjects for widespread study. He termed the clinically observed adherence of bone with titanium as 'osseointegration'. In 1965 Brånemark, who was by then the Professor of Anatomy at Gothenburg University in Sweden, placed the first titanium dental implant into a human volunteer who was a Swede named Gösta Larrson.
Over the next fourteen years Brånemark published many studies on the use of titanium in dental implantation until in 1978 he entered into a commercial partnership with the Swedish defence company, Bofors AB for the development and marketing of his dental implants. With Bofors (later to become Nobel Industries) as the parent company, Nobelpharma AB (later to be renamed Nobel Biocare) was founded in 1981 to focus on dental implantology. To the present day over 7 million Brånemark System implants have now been placed and hundreds of other companies produce dental implants.
Dental Implantation: Procedure
A typical implant consists of a titanium screw, with a roughened surface. This surface is treated either by plasma spraying, etching or sandblasting to increase the integration potential of the implant. At edentulous (without teeth) jaw sites, a pilot hole is bored into the recipient bone, taking care to avoid vital structures (in particular the inferior alveolar nerve within the mandible).
This pilot hole is then expanded by using progressively wider drills. Care is taken not to damage the osteoblast cells by overheating. A cooling saline spray keeps the temperature of the bone to below 47 degrees Celsius (approximately 117 degrees Fahrenheit). The implant screw can be self-tapping, and is screwed into place at a precise torque so as not to overload the surrounding bone. Once in the bone, a cover screw is placed and the operation site is allowed to heal for a few months for integration to occur.
After some months the implant is uncovered and a healing abutment and temporary crown is placed onto the implant. This encourages the gum to grow in the right scalloped shape to approximate a natural tooth's gums and allows assessment of the final aesthetics of the restored tooth. Once this has occurred a permanent crown will be constructed and placed on the implant.
An increasingly common strategy to preserve bone and reduce treatment times includes the placement of a dental implant into a recent extraction site. In addition, immediate loading is becoming more common as success rates for this procedure are now acceptable. This can cut months off the treatment time and in some cases a prosthetic tooth can be attached to the implants at the same time as the surgery to place the dental implants.
Dental Implantation: Considerations
For the dental implantation procedure to work, there must be enough bone in the jaw, and the bone has to be strong enough to hold and support the implant. If there is not enough bone, more may need to be added with a procedure called bone augmentation. In addition, natural teeth and supporting tissues near where the implant will be placed must be in good health.
In all cases, what must be addressed is the functional aspect of the dental implantation, the final occlusion. How much force per area is being placed on the bone implant interface? Implant loads from chewing and parafunction can exceed the physio biomechanic tolerance of the implant bone interface and/or the titanium material itself, causing failure. This can be failure of the dental implantation (fracture) or bone loss, a "melting" of the surrounding bone.
The dental implantation dentist must first determine what type of prosthesis will be fabricated. Only then can the specific implant requirements including number, length, diameter, and thread pattern be determined. In other words, the case must be reversed engineered by the restoring dentist prior to the surgery. If bone volume or density is inadequate, a bone graft procedure must be considered first.
Dental Implantation: Success Rates
Dental implantation success is related to operator skill, quality and quantity of the bone available at the site, and also to the patient's oral hygiene. Various studies have found the 5 year success rate of implants to be between 75-95%. Patients who smoke experience significantly poorer success rates.
Dental Implantation: Failure
Failure of a dental implantation is usually related to failure to osseointegrate correctly. A dental implantation is considered to be a failure if it is lost, mobile or shows peri-implant bone loss of greater than one mm in the first year after implanting and greater than 0.2mm a year after that. dental implantations are not susceptible to dental caries but they can develop a periodontal condition called peri-implantitis where correct oral hygiene routines have not been followed. Risk of failure is increased in smokers. For this reason dental implantations are frequently placed only after a patient has stopped smoking as the treatment is very expensive. More rarely, a dental implantation may fail because of poor positioning at the time of surgery, or may be overloaded initially causing failure to integrate.
Dental Implantation: Contraindications
There are no absolute contraindications to implant dentistry, however there are some systemic, behavioral and anatomic considerations that should be considered.
Particularly for mandibular (lower jaw) implants, especially in the vicinity of the mental foramen (MF), there must be sufficient alveolar bone above the inferior alveolar canal or IAC (which acts as the conduit for the neurovascular bundle carrying the inferior alveolar nerve or IAN). The standard of care for mandibular implants calls for 3D or cone beam X-ray imaging (computer assisted tomography) because 3D enables precise measurements to 0.1mm by the implantologist, followed by precision treatment planning with surgical guides. Patients should be referred to an appropriate cone beam imaging center if 3D is not available in the implantologist's practice.
Failure to precisely locate the IAN and MF invites surgical insult by the drills and the implant itself. Such insult may cause irreparable damage to the nerve, often felt as a paresthesia (numbness) or dysesthesia (painful numbness) of the gum, lip and chin. This condition may persist for life.
Uncontrolled type II diabetes is a significant relative contraindication as healing following any type of surgical procedure is delayed due to poor peripheral blood circulation. Anatomic considerations include the volume and height of bone available. Often an ancillary procedure known as a block graft or sinus augmentation are needed to provide enough bone for successful dental implantation.
There is new information about bisphosphonates (taken for osteoporosis and certain forms of breast cancer) which put patients at a higher risk of developing a delayed healing syndrome called osteonecrosis. Dental implantations may be contraindicated in patients who take this class of drug.
Bruxism (tooth clenching or grinding) is another contraindication for dental implantation. The forces generated during bruxism are particularly detrimental to implants while bone is healing; micromovements in the implant positioning are associated with increased rates of implant failure. Bruxism continues to pose a threat to dental implantation throughout the life of the recipient. Natural teeth contain a periodontal ligament allowing each tooth to move and absorb shock in response to vertical and horizontal forces. Once replaced by dental implants, this ligament is lost and teeth are immovably anchored directly into the jaw bone. This problem can be minimized by wearing a custom made mouthguard (such an NTI appliance) at night.