Dental Implants

Artificial tooth root replacement and is used in prosthetic dentistry. There are several types. The most widely accepted and successful is the osseointegrated implant, based on the discovery by Swedish Professor Per-Ingvar Branemark that titanium could be successfully incorporated into bone when osteoblasts grow on and into the rough surface of the implanted titanium. This forms a structural and functional connection between the living bone and the implant. A variation on the implant procedure is the implant-supported bridge, or implant-supported denture.

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 Branemark 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 Branemark, 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 Branemark observed that bone had grown into such close approximity with the titanium that it effectively adhered to the metal. Branemark 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 centre on knee and hip surgery, Branemark 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 Branemark, 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 Branemark published many studies on the use of titanium in dental implantology 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 Branemark System implants have now been placed and hundreds of other companies produce dental implants.

Procedure

A typical implant consists of a titanium screw (resembling a tooth root) with a roughened surface. This surface is treated either by plasma spraying, etching or sandblasting to increase the integration potential of the implant.

Implant surgery is typically performed as an outpatient under general anesthesia by trained and certified clinicians such as an oral surgeon or periodontist. A single implant procedure takes about one hour, sometimes longer; multiple implants can be done in a single surgical session lasting several hours. At the conclusion of surgery, the patient goes through a period of recovery, returns to consciousness and is sent home with a spouse or friend.

Before surgery, with the patient fully awake or during an earlier office visit, a prudent clinician planning mandibular implants will conduct a neurosensory examination to rule out altered sensation, thus setting a base line on nerve function. Also prior to surgery, a panoramic X-ray will be taken using a metal ball of known dimension so that calibrated measurements can be made from the image (to locate "vital structures" such as nerves and the position of critical anatomical features such as the mental foramen).

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 or IAN within the mandible). A zone of safety, usually 2 mm, is the standard of care for avoiding vital structures like the IAN. When computed tomography (3D X-ray imaging) is used preoperatively to accurately pinpoint vital structures, the zone of safety may be reduced to 1 mm through the use of computer-aided design of surgical guides.

Drilling into jawbone usually occurs in several separate steps. The pilot hole is expanded by using progressively wider drills (typically between three and seven successive drilling steps, depending on implant width and length). Care is taken not to damage the osteoblast or bone 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 (overloaded bone can die, a condition called osteonecrosis, which may lead to failure of the implant to fully integrate or bond with the jawbone). Typically in most implant systems, the osteotomy or drilled hole is about 1mm deeper than the implant being placed, due to the shape of the drill tip. Surgeons must take the added length into consideration when drilling in the vicinity of vital structures.

Once properly torqued into the bone, a cover screw is placed on the implant, then the gingiva or gum is sutured over the site and allowed to heal for several months for integration to occur between the titanium surface of the implant and jawbone.

After several months the implant is uncovered in another surgical procedure, usually under local anesthetic by the restorative dentist, 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 fabricated 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.

Complementary procedures

Sinus lifting is a common surgical intervention. The oral surgeon or periodontist thickens the inadequate part of atrophic maxilla towards the sinus with the help of bone transplantation or bone expletive substance and as a result creates a better quality bone site for the implantation.

Bone grafting will be necessary in cases where there is a lack of adequate maxillary or mandibular bone in terms of front to back (lip to tongue) depth or thickness; top to bottom height; and left to right width. Sufficient bone is needed in three dimensions to securely integrate with the root-like implant. Improved bone height -- which is very difficult to achieve -- is particularly important to assure ample anchorage of the implant's root-like shape because it has to support the mechanical stress of chewing, just like a natural tooth. If an implant is too shallow, chewing may cause a dangerous jawbone crack or full fracture.

Typically, implantologists try to place implants at least as deeply into bone as the crown or tooth will be above the bone. This is called a 1:1 ratio. This ratio establishes the target for bone grafting in most cases. If 1:1 or better cannot be achieved, the patient is usually advised that only a short implant can be placed and to not expect a long period of usability.

A wide range of grafting materials and substances may be used during the process of bone grafting / bone replacement. They include the patient's own bone (autograft), which may be harvested from the hip (iliac crest) or from spare jawbone; processed bone from cadavers (allograft); bovine bone or coral (xenograft); or artificially produced bonelike substances (calcium sulfate with names like Regeneform; and hydroxyapatite or HA, which is the primary form of calcium found in bone). The HA is effective as a substrate for osteoblasts to grow on. Some implants are coated with HA for this reason.

Bone graft surgery has its own standard of care. In a typical procedure, the clinician creates a large flap of the gingiva or gum to fully expose the jawbone at the graft site, performs one or several types of block and onlay grafts in and on existing bone, then installs a membrane designed to repel unwanted infection-causing microbiota found in the oral cavity. Then the gingiva is carefully sutured over the site. Together with a course of internal and external antibiotics, the graft site is allowed to heal (several months).

The clinician typically takes a new panoramic x-ray to confirm graft success in width and height, and assumes that positive signs in these two dimensions safely predicts success in the third dimension, depth. Where more precision is needed, usually when mandibular implants are being planned, a 3D or cone beam X-ray may be called for at this point to enable accurate measurement of bone and location of nerves and vital structures for proper treatment planning. The same X-ray data set can be employed for the preparation of computer-designed placement guides.

Correctly performed, a bone graft produces live vascular bone which is very much like natural jawbone and is therefore suitable as a foundation for implants.

Considerations

For dental implant 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 bone graft procedure discussed earlier. Sometimes, this procedure is 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 final implant restoration, 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 implant itself (fracture) or bone loss, a "melting" or resorption of the surrounding bone.

The restorative 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. The restoring dentist consults with the oral surgeon or periodontist to co-treat the patient. Usually, physical models or impressions of the patient's jawbones and teeth are made by the restorative dentist at the surgeon's request, and are used as physical aids to treatment planning. If not supplied, the surgeon makes his own or relies upon advanced computer-assisted tomography or a cone beam CAT scan to achieve the proper treatment plan.

Computer simulation software based on CAT scan data allows virtual implant surgical placement based on a barium impregnated prototype of the final prosthesis. This predicts vital anatomy, bone quality, implant characteristics, the need for bone grafting, and maximizing the implant bone surface area for the treatment case creating a high level of predictability. Computer CAD/CAM milled or stereo lithography based drill guides can be developed for the implant surgeon to facilitate proper implant placement based on the final prosthesis occlusion and aesthetics.

Treatment planning software can also be used to demonstrate "try-ins" to the patient on a computer screen. Software products like Materialize' SimPlant (simulated implant) use the digital data from a CAT scan (such as an iCAT or a NewTom) to provide extremely accurate simulations that are easily understood by patients. When options have been fully discussed between patient and surgeon, the same software can be used to produce precision drill guides.

Success rates

Dental implant 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 10 year success rate of implants to be 95%. Patients who smoke experience significantly poorer success rates.

Risk of failure is increased in smokers. For this reason implants are frequently placed only after a patient has stopped smoking as the treatment is very expensive. More rarely, an implant may fail because of poor positioning at the time of surgery, or may be overloaded initially causing failure to integrate. If smoking and positioning problems exist prior to implant surgery, clinicians often advise patients that a bridge or partial denture rather than an implant may be a better solution.

Contraindications

There are no absolute contraindications to implant dentistry, however there are some systemic, behavioral and anatomic considerations that should be considered.

Guarantees

for prosthesis: 1 years
for fillings: 2 years
for inlays: 3 years
for crowns and bridges: 5 years
for implants: 5-10 years!