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. An osteotomy or precision hole is carefully drilled into jawbone and the implant is installed in the osteotomy.
Implant surgery is typically performed as an outpatient under general anesthesia or with Local anesthesia by trained and certified clinicians including general dentists, oral surgeons, and periodontists. An increasing number of general or cosmetic dentists as well as prosthodontists are also placing implants in relatively simple cases. The most common treatment plan calls for several surgeries over a period of months, especially if bone augmentation (bone grafting) is needed to support implant placements. At the other end of the surgery scale, some patients can be implanted and restored in a single surgery, in a procedure labeled "immediate function" and "teeth in an hour."
A single implant procedure that involves an incision and "flapping" of the gum or gingiva (to expose the jawbone) takes about an hour, sometimes longer; multiple implants can be installed in a single surgical session lasting several hours. At the conclusion, the patient goes through a period of recovery, returns to consciousness and is sent home with a spouse or friend.
Healing and integration of the implant(s) with jawbone occurs over several months in a process called osseointegration. At the appropriate time, the restorative or cosmetic dentist or prosthodontist uses the implant(s) to anchor crowns or a prosthetic restoration containing several "teeth". Since the implants supporting the restoration are integrated, which means they are biomechanically stable and strong, the patient is immediately able to masticate (chew) normally.
In an immediate function procedure, the gingiva is not flapped (Flapless). Instead, the surgeon removes a small plug of gingiva directly over the drilling site. The site is drilled and the implant is installed. Then a crown is immediately added. Patients are cautioned to give their new "teeth in an hour" ample healing/integration time (weeks or months) before attempting normal mastication.
There are different approaches to place dental implants after tooth extraction. The approaches are:
- Immediate post-extraction implant placement.
- Delayed immediate post-extraction implant placement (2 weeks to 3 months after extraction).
- Late implantation (3 months after tooth extraction).
According to the timing of loading of dental implants, the procedure of loading could be classified into:
- Immediate loading procedure.
- Early loading (1 week to 12 weeks).
- Staged loading (3-6 months).
- Late loading (more than 6 months).
Most patients need the longer treatment plan, which has an excellent history going back many years. 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 accurately locate "vital structures" such as nerves and the position of critical anatomical features such as the mental foramen, which is the transit point in the jawbone for the nerve which innervates the lip and chin).
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 osseointegration 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 or prosthodontist, 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.
In all of these approaches, computer-based guidance has thrust itself onto the treatment stage. Not only will 3D digital imagery yield critical treatment guidance, the digital data can be used to manufacture precision drilling guides, virtually eliminating surgical errors.
Sinus lifting is a common surgical intervention. The trained general dentist, 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 crown to root 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 antibiotics and external antibiotic mouth rinses, 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.
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, trained general dentist, 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 Materialise' 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.
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 5 year success rate of implants to be between 90-95%. Patients who smoke experience significantly poorer success rates.
Failure of a dental implant is often related to failure to osseointegrate correctly. A dental implant is considered to be a failure if it is lost, mobile or shows peri-implant (after implant) bone loss of greater than 1.0 mm in the first year and greater than 0.2mm a year thereafter.
Dental implants are not susceptible to dental caries but they can develop a periodontal condition called peri-implantitis. The cause may be infection that was introduced during surgery; or failure by the patient to follow correct oral hygiene routines. In either case, inflammation in the bone surrounding the implant causes bone loss (recession) which ultimately may lead to failure, often evidenced by the ability to "spin" an implant.
Peri-implantitis is often dealt with pre-emptively by clinicians who prescribe a course of antibiotics in the days prior to surgery; and post-surgically with another course of antibiotics and special oral rinses. Since peri-implantitis is generally easy to see on standard panoramic and periapical X-rays, prudent clinicians who suspect the problem will take an X-ray soon after surgery, and again at staged intervals post-operatively.
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.