Dental implant

Overview
A dental implant is an artificial tooth root replacement and is used in prosthetic dentistry. There are several types of dental implants; the most widely accepted and successful is the osseointegrated implant, based on the discovery by Swedish Professor Per-Ingvar Brånemark that titanium can be successfully fused 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.

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 Per Brånemark started working with titanium. While 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 under the nose 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 proximity 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 centre 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 implantology until in 1978 he entered into a commercial partnership with the Swedish defense 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.

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. 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 relative 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:
 * 1) Immediate post-extraction implant placement.
 * 2) Delayed immediate post-extraction implant placement (2 weeks to 3 months after extraction).
 * 3) Late implantation (3 months after tooth extraction).

According to the timing of loading of dental implants, the procedure of loading could be classified into:
 * 1) Immediate loading procedure.
 * 2) Early loading (1 week to 12 weeks).
 * 3) Staged loading (3-6 months).
 * 4) 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.

Complementary procedures
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.

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, 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.

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 5 year success rate of implants to be between 90-95%. Patients who smoke experience significantly poorer success rates.

Failure
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.

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 mandibular canal also called 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, followed by precision treatment planning with computer-designed surgical guides.

Patients should be referred to an appropriate cone beam imaging center if 3D is not available in the implantologist's practice. Cone beam systems (brand names include iCAT, NewTom, Accu-i-Tomo and TeraRecon) have been widely available in developed nations since 2001 or earlier. Many hospitals have dental modules for their multi-slice CT systems, since the mid 1980's, for example the current GE Lightspeed and Philips Brilliance systems. And, companies who supply in-office X-ray systems are beginning to offer value-priced cone beam systems, such as the Planmeca Promax 3D, as clinicians fully recognize their utility as the gold standard and standard of care for certain cases in everyday use.

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 and may be accompanied by unconscious drooling.

Lack of sufficient alveolar bone is another contraindication to the procedure. Typically, a preoperative in-office panoramic X-ray is taken to establish (with allowances for image distortion, a known problem with panoramic X-rays) in two dimensions (height and width) the amount of available bone. A bone graft or augmentation procedure, sometimes called guided bone regeneration may be performed and allowed to heal several months before implantation surgery. A new panoramic X-ray will help determine if the graft was successful.

This is an important step inasmuch as improved bone height is much more difficult to achieve than increased bone depth. For mandibular grafts, a 3D or cone beam X-ray enables measurement of bone height (top to bottom), width (left and right) and depth (front to back) to an accuracy of 0.1mm. The precision of cone beam has stimulated a new industry that produces computer-designed surgical guides based on the cone beam X-ray's digital data. These surgery aids are employed by implantologists to precisely locate and drill into the mandible and maxilla, and to avoid vital structures.

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 implant placement.

There is new information about intravenous and oral bisphosphonates (taken for certain forms of breast cancer and osteoporosis, respectively) which may put patients at a higher risk of developing a delayed healing syndrome called osteonecrosis. Implants are contraindicated for some patients who take intravenous bisphosphonates.

The many millions of patients who take an oral bisphosphonate (such as Actonel, Fosamax and Boniva) may be advised to stop the administration prior to implant surgery, then resume several months later. But this protocol may not be necessary. As of January, 2008, an oral bisphosphonate study reported in the February 2008 Journal of Oral and Maxillofacial Surgery, reviewing 115 cases that included 468 implants, concluded "There is no evidence of bisphosphonate-associated osteonecrosis of the jaw in any of the patients evaluated in the clinic and those contacted by phone or e-mail reported no symptoms." (JOMS, Volume 66, Issue 2, Ppgs 223-230).

The American Dental Association had addressed bisphosphonates in an article entitled "Bisphosphonate medications and your oral health," (JADA, Vol. 137, page 1048, July 2006.) In an Overview, the ADA stated "The risk of developing BON [bisphosphonate-associated osteonecrosis of the jaw] in patients on oral bisphosphonate therapy appears to be very low...". The ADA Council on Scientific Affairs also employed a panel of experts who issued recommendations [for clinicians] for treatment of patients on oral bisphosphonates, published in June, 2006. The overview may be read online at ada.org but it has now been superseded by a huge study -- encompassing over 700,000 cases -- entitled "Bisphosphonate Use and the Risk of Adverse Jaw Outcomes." Like the 2008 JOMS study, the ADA study exonerates oral bisphosphonates as a contraindication to dental implants. (JADA, January 2008, 139:23-30).

Bruxism (tooth clenching or grinding) is another contraindication. 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 implants 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.

Postoperatively, after implants have been placed, there are physical contraindications that prompt rapid action by the implantology team. Excessive or severe pain lasting more than three days is a warning sign, as is excessive bleeding. Constant numbness of the gingiva (gum), lip and chin -- usually noticed after surgical anesthesia wears off -- is another warning sign. In the latter case, which may be accompanied by severe constant pain, the standard of care calls for diagnosis to determine if the surgical procedure insulted the IAN. A 3D cone beam X-ray provides the necessary data, but even before this step a prudent implantologist may back out or completely remove an implant in an effort to restore nerve function because delay is usually ineffective. Depending upon the evidence visible with a 3D X-ray, patients may be referred to a specialist in nerve repair. In all cases, speed in diagnosis and treatment are necessary.

The market
There are over 100 dental implant companies available on the U.S. market. Notable companies include Bicon Dental Implants, BioHorizons, Nobel Biocare, Straumann, OsteoCare, Osteo-Ti, 3i, Zimmer, Astra Tech, Ankylos, Implant Direct, Intra-Lock, Lifecore, Bio-Lok and Diskimplant.

Specialists such as oral and maxillofacial surgeons, periodontists, and more recently, prosthodontist play a role in the placement of implant fixtures, however these procedures are not beyond the scope of general dentists. Regardless of who places the implant, it is most appropriate for either a prosthodontist or general dentist to initiate and coordinate implant service, since they can best assess the merits of this treatment against other prosthetic options. However, the implant surgeon (the person performing the implant surgery) is ultimately responsible for correct placement in a co-treatment environment.

Cost
The price of dental implants varies greatly between countries. In the United Kingdom, a single tooth implant with crown generally costs between £2000-£2800 depending on the type of crown used. Similarly, in the United States, implants average around $2500, in addition to the cost of the crown. The cost of full mouth reconstructions with implants begin around $12000 per arch, and can approach $50000, depending on the complexity of the case. In Israel, single implants begin around 5000 NIS, comparable to Turkey and Egypt where they begin around 700-800$.