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A dental implant (also known as an endosteal implant or fixture) is a surgical device, used to replace one or more missing teeth by fusing to bone and supporting a crown, bridge of teeth, denture, facial prosthetic or to act as an orthodontic anchor. The term "dental implant" refers to that portion of the structure in the bone. Other functional components may be attached to make the device functional including an abutment, to pass through the gums, an abutment screw to secure the abutment to the implant and prosthetic teeth which attach to the abutment. Collectively, the fixture and components create the dental prosthetic.
The modern dental implant leverages the biologic process of osseointegration where a material (commercially pure titanium, titanium alloy and some ceramics) forms a structural and functional connection between living bone and the surface of a load-bearing artificial implant without causing the body to recognize it as a foreign material. Implantable fixtures to retain teeth are not a new concept. Prior to the discovery of osseointegration, various non-titanium implants were placed in the jawbones (or the periosteum surrounding them) but had high complication rates due to foreign body reactions and amplification of the foreign body response from movement when chewing.
The prerequisites to long term success of dental implants, are healthy bone and gingiva. When a tooth is lost the tissues of the mouth remodel and atrophy preventing the placement of dental implants or lowering long term survival. A group of operations, referred to as pre-prosthetic procedures, have been developed to rebuild the bone height, width, gingiva and connective tissue. In the presence of healthy tissues, a well integrated implant with appropriate biomechanical loads can have long term success rates of 92-96% for the fixture and 10-15 year lifespans for the prosthetic teeth.
Dental implants are used to retain artificial teeth, facial prosthetics or to act as anchor points for orthodontics by making use of osseointegration, the biologic principal where certain metals or ceramics create a connection to living bone to support physical loads, for decades, without failure.
There most basic use in in orthodontics. To move natural teeth through bone, light pressure is applied causing the periodontal ligament to remodel bone, allowing teeth to move in the direction of force. A dental implant, which lacks a periodontal ligament, will not cause bone remodeling under light forces, thereby acting as an orthodontic anchor point to which a spring or wire can be attached to generate the necessary forces.
The principal use of dental implants, however, are to stabilize artificial teeth. When a single tooth is missing, a dental prosthetic (the artificial tooth) can be made using 4 components. The implant fixture, which is placed in bone, a prosthetic abutment which traverses the gingiva and provides a surface for the body to create junctional epithelium, the abutment screw which secures the abutment to the implant fixture and the crown which is cemented to the abutment (diagram). Alternatively, the implant and abutment or the abutment and crown can be one piece.
Dental implants can also be used to retain multiple teeth. Multi-teeth retainers are broadly classified into fixed or removable. Fixed retainers, for small spans are referred to as implant-supported bridges or fixed partial dentures, where teeth attach to "implant abutments" (not to be confused with the "prosthetic abutment", mentioned above) and pontic teeth which span the space of missing teeth fusing to the implant abutments at each end. The entire bridge is one unit with a substructure made of metal or ceramic and a veneer of porcelain overlaid for esthetics. Where an entire arch of teeth is missing, the person is said to be edentulous and a full denture is required.
A fixed full denture can be created in the same manner as a fixed partial denture where a metal or ceramic substructure secures to the implant fixtures and porcelain veneers are added to the substructure for esthetics. Alternatively, a metal substructure can be made with an acrylic prosthetic overlaid, that has an appearance similar to a traditional denture but is fixed to dental implants with abutment screws. In both cases, the prosthetic is said to be fixed because it cannot be removed by the denture wearer.
It is not a requirement that the prosthetic be fixed in place. A removable denture can be created, where implants have a small button, ball, bar or magnetic abutment, that attaches to an analogous female adapters on the underside of the denture. In each solution, the denture can be removed with finger pressure by the denture wearer, and is referred to as an implant supported overdenture.
The same techniques have been used to retain facial prosthetics, like those used to replace facial structures lost due to cancer or battlefield injuries. The implants are placed in the facial bones and custom-made cribs are fabricated to retain the facial prosthetic.
|Dental implant common uses|
Planning for dental implants focuses on three core areas
There are few health conditions that absolutely preclude placing implants although there are certain conditions that can increase the risk of failure. Those with poor oral hygiene, heavy smokers and diabetics are all at greater risk for a variant of gum disease that affects implants called peri-implantitis, increasing the chance of long-term failures. Long-term steroid use, osteoporosis and other diseases that affect the bones can increase the risk of early failure of implants.
Bisphosphonate drugs and implants
The use of bone building drugs, like bisphosphonates and anti-RANKL drugs require special consideration with implants, because they've been associated with a disorder called Bisphosphonate-associated osteonecrosis of the jaw (BRONJ). The drugs change bone turnover, which is thought to put people at risk for death of bone when having minor oral surgery. At routine doses (for example, those used to treat routine osteoporosis) the effects of the drugs linger for months or years but the risk appears to be very low. Because of this duality, uncertainty exists in the dental community about how to best manage the risk of BRONJ when placing implants. A 2009 position paper by the American Association of Oral & Maxillofacial Surgeons, discussed that the risk of BRONJ from low dose oral therapy (or slow release injectable) as between 0.01% - 0.06% for any procedure done on the jaws (implant, extraction, etc...). The risk is higher with intravenous therapy, procedures on the lower jaw, people with other medical issues, those on steroids, those on more potent bisphosphonates and people who have taken the drug for more than 3 years. The position paper recommends against placing implants in people who are taking high dose/high frequency intravenous therapy for cancer care. Otherwise, implants can generally be placed.
The long-term success of implants is determined, in part, by the forces they have to support. As implants have no periodontal ligament, there is no sensation of pressure when biting so the forces created are higher. To offset this, the location of implants must distribute forces evenly across the prosthetics they support. Concentrated forces can result in fracture of the bridgework, implant components, or loss of bone adjacent the implant. The ultimate location of implants is based on both biologic (bone type, vital structures, health) and mechanical factors. Implants placed in thicker, stronger bone like that found in the front part of the bottom jaw have lower failure rates than implants placed in lower dentisity bone, such as the back part of the upper jaw. People who grind their teeth also increase the force on implants and increase the likelihood of early and late failures.
The design of implants, has to account for a lifetime of real-world use in a person's mouth. Regulators and the dental implant industry have created a series of tests to determine the long-term mechanical reliability of implants in a person's mouth by putting the implants in function until the point of failure.
When a more exacting plan is needed beyond clinical judgment, the dentist will make an acrylic guide (called a stent) prior to surgery which guides optimal positioning of the implant. Increasingly, dentists opt to get a CT scan of the jaws and any existing dentures, then plan the surgery on CAD/CAM software. The stent can then be made using stereolithography following computerized planning of a case from the CT scan. The use of CT scanning in complex cases also helps the surgeon identify and avoid vital structures such as the inferior alveolar nerve and the sinus.,
|Planning with CT scans, stents and models|
Most implant systems have five basic steps for placement of each implant:
1. Soft tissue reflection: An incision is made over the crest of bone, splitting the thicker attached gingiva roughly in half so that the final implant will have a thick band of tissue around it. The edges of tissue, each referred to as a flap are pushed back to expose the bone. Flapless surgery is an alternate technique, where a small punch of tissue (the diameter of the implant) is removed for implant placement rather than raising flaps.
2. Drilling at high speed: After reflecting the soft tissue, and using a surgical guide/stent as necessary, pilot holes are placed with precision drills at highly regulated speed to prevent burning or pressure necrosis of the bone.
3. Drilling at low speed: 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 or water spray keeps the temperature of the bone to below 47 degrees Celsius (approximately 117 degrees Fahrenheit).
4. Placement of the implant: The implant screw is placed and can be self-tapping, otherwise the prepared site is tapped with an implant analog. It is then 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).
5. Tissue adaptation: The gingiva is adapted around the entire implant to provide a thick band of healthy tissue around the healing abutment. In contrast, an implant can be "buried", where the top of the implant is sealed with a cover screw and the tissue is closed to completely over it. A second procedure would then be required to uncover the implant at a later date.
|Surgical implant placement|
Timing of implants after extraction of teeth
There are different approaches to placement dental implants after tooth extraction. The approaches are:
There are also various options for the loading of dental implants, classified into:
For an implant to become permanently stable, the body must grow bone to the surface of the implant (osseointegration). Based on this biologic process, it was thought that loading an implant during the osseointegration period would result in movement that would prevent osseointegration, and thus increase implant failure rates. As a result, 3–6 months of integrating time (depending on various factors) was allowed before placing the teeth on implants (restoring them).
However, later research suggests that the initial stability of the implant in bone is a more important determinant of success of implant integration, rather than a certain period of healing time. As a result, the time allowed to heal is typically based on the density of bone the implant is placed in and the number of implants splinted together, rather than a uniform amount of time. When implants can withstand high torque (35Ncm) and are splinted to other implants, there are no meaningful differences in long-term implant survival or bone loss between implants loaded immediately, at 3 months, or at 6 months. The corollary is that single implants, even in solid bone, require a period of no-load to minimize the risk of initial failure,
One-stage, two-stage surgery
After an implant is placed, the internal components are covered with either a healing abutment, or a cover screw. A healing abutment passes through the mucosa, and the surrounding mucosa is adapted around it. A cover screw is flush with the surface of the dental implant, and is designed to be completely covered by mucosa. After an integration period, a second surgery is required to reflect the mucosa and place a healing abutment.
In the early stages of implant development (1970−1990), implant systems used a two-stage approach, believing that it improved the odds of initial implant survival. Subsequent research proved that no difference in implant survival existed between one-stage and two-stage surgeries, and the choice of whether or not to 'bury' the implant in the first stage of surgery became a concern of soft tissue (gingiva) management,.
When tissue is deficient or mutilated by the loss of teeth, implants are placed and allowed to osseointegrate, then the gingiva is surgically moved around the healing abutments. The down-side of a two-stage technique is the need for additional surgery and compromise of circulation to the tissue due to repeated surgeries. The choice of one vs two-stages, now centers around how best to reconstruct the soft tissues around lost teeth.
An increasingly common strategy to preserve bone and reduce treatment times includes the placement of a dental implant into a recent extraction site. One the one hand, it shortens treatment time and can improve esthetics because the soft tissue envelope is preserved. On the other hand, implants may have a slightly higher rate of initial failure. Conclusions on this topic are difficult to draw, however, because few studies have compared immediate vs delayed implants in a scientifically rigorous manner.
In order for an implant to osseointegrate, it needs to be surrounded by a healthy quantity of bone. In order for it to survive long-term, it needs to have a thick healthy soft tissue (gingiva envelope around it. It is common for either the bone or soft tissue to be so deficient that the surgeon needs to reconstruct it either before or during implant placement.
Hard tissue (bone) reconstruction
Bone grafting is necessary when there is a lack of bone. While there are always new implant types and techniques to allow compromise, a general treatment goal is to have a minimum of 10 mm in bone height, and 6 mm in width. Alternatively, bone defects are graded from A to D (A=10+ mm of bone, B=7–9 mm, C=4–6 mm and D=0–3 mm) where an implant's likelihood of osseointegrating is related to the grade of bone.
To achieve an adequate width and height of bone, various bone grafting techniques have been developed. The most frequently used is called guided bone graft augmentation where a defect is filled with either natural (harvested or autograft) bone or allograft (donor bone or synthetic bone substitute), covered with a semi-permeable membrane and allowed to heal. During the healing phase, natural bone replaces the graft forming a new bony base for the implant.
Three common procedures are:
Other, more invasive procedures, also exist for larger bone defects including mobilization of the inferior alveolar nerve to allow placement of a fixture, onlay bone grafting using the iliac crest or another large source of bone and microvascular bone graft where the blood supply to the bone is transplanted with the source bone and reconnected to the local blood supply. The final decision about which bone grafting technique is best is based on an assessment of the degree of vertical and horizontal bone loss that exists, each of which is classified into mild (2-3 mm loss), moderate (4-6 mm loss) or severe (greater than 6 mm loss).
|Bone grafting types|
Soft tissue (gingiva) reconstruction
The gingiva surrounding a tooth has a 2–3 mm band of bright pink, very strong attached mucosa, then a darker, larger area of unattached mucosa that folds into the cheeks. When replacing a tooth with an implant, a band of strong, attached gingiva is needed to keep the implant healthy in the long-term. This is especially important with implants because the blood supply is more precarious in the gingiva surrounding an implant, and is theoretically more susceptible to injury because of a longer attachment to the implant than on a tooth (a longer biologic width).
When an adequate band of attached tissue is absent, it can be recreated with a soft tissue graft. There are various methods to accomplish this by either taking a roll of tissue adjacent an implant (palatal roll), transplanting gingiva from the palate, transplanting the underlying connective tissue from the palate or creating a finger of tissue based on a blood vessel in the palate (vascularized interpositional periosteal-connective tissue (VIP-CT) flap).
Additionally, for an implant to look esthetic, a band of full, plump gingiva is needed to fill in the space on either side of implant.
The most common soft tissue complication is called a black-triangle, where the papilla (the small triangular piece of tissue between two teeth) shrinks back and leaves a triangular void between the implant and the adjacent teeth. Dentists can only expect 2–4 mm of papilla height over the underlying bone. A black triangle can be expected if the distance between where the teeth touch and bone is any greater.
|Gingival grafting types|
The prosthetic phase begins once the implant is well integrated (or has a reasonable assurance that it will integrate) and an abutment is in place to bring it through the mucosa. Even in the event of early loading, most practitioners will place temporary teeth until osseointegration is confirmed. The prosthetic phase of restoring an implant requires an equal amount of technical expertise as the surgical because of the biomechanical considerations, especially when multiple teeth are to be restored. The dentist will work to restore the vertical dimension of occlusion, the esthetics of the smile, and the structural integrity of the teeth to evenly distribute the forces of the implants.
Prosthetic procedures for single teeth, bridges and fixed dentures
An abutment is selected depending on the application. In many single crown and fixed partial denture scenarios (bridgework), custom abutments are used. An impression of the top of the implant is made with the adjacent teeth and gingiva. A dental lab then simultaneously fabricates an abutment and crown. The abutment is seated on the implant, a screw passes through the abutment to secure the it to an internal thread on the implant (lag-screw). There are variations on this, such as when the abutment and implant body are one piece or when a stock (prefabricated) abutment is used. Custom abutments can be made by hand, as a cast metal piece or custom milled from metal or zirconia. Both have similar long-term outcomes.
The platform between the implant and the abutment can be flat (buttress) or conical fit. In conical fit abutments, the collar of the abutment sits inside the implant which allows a stronger junction between implant and abutment and a better seal against bacteria into the implant body. To improve the gingival seal around the abutment collar, a narrowed collar on the abutment is used, referred to as platform switching. The combination of conical fits and platform switching gives marginally better long term periodontal conditions compared to flat-top abutments.
Regardless of the abutment material or technique, an impression of the abutment is then taken and a crown secured to the abutment with dental cement. Another variation on abutment/crown model is when the crown and abutment are one piece and the lag-screw traverses both to secure the one-piece structure to the internal thread on the implant.
Prosthetic procedures for removable dentures
When a removable denture is worn, retainers to hold the denture in place can be either custom made or "off-the-shelf" (stock) abutments. With custom retainers, an impression of the implants is taken and a dental lab creates a custom metal bar with attachments to hold the denture in place. Significant retention can be created with multiple attachments and the use of semi-precision attachments (such as a small diameter pin that pushes through the denture and into the bar) which allows for little or no movement in the denture, but it remains removable. The downside of these types of removable solutions is their bulk and high cost compared to stock abutments. Additionally, a similar number of implants, angled in such a way to distribute occlusal forces can safely hold a fixed denture in place with comparable costs and number of procedures giving the denture wearer a fixed solution.,
Alternatively, stock abutments are used to retain dentures using a male-adapter attached to the implant and a female adapter in the denture. Two common types of adapters are the ball-and-socket style retainer and the button-style adapter. These types of stock abutments allow movement of the denture, but enough retention to improve the quality of life for denture wearers at a lower cost than custom solutions. Regardless of the type of adapter, the female potion of the adapter that is housed in the denture will require periodic replacement.
After placement, implants need to be cleaned (similar to natural teeth) with a Teflon instrument to remove any plaque. Because of the more precarious blood supply to the gingiva, care should be taken with dental floss. Implants will lose bone at a rate similar to natural teeth in the mouth (e.g. if someone suffers from periodontal disease, an implant can be affected by a similar disorder) but will otherwise last. The porcelain on crowns should be expected to discolour, fracture or require repair approximately every 10 years, although there is significant variation in the service life of dental crowns based on the position in the mouth, the forces being applied form opposing teeth and the restoration material. Where implants are used to retain a complete denture, depending on the type of attachment, connections need to be changed or refreshed every 1–2 years.
Placement of dental implants is a surgical procedure and carries the normal risks of surgery including infection, excessive bleeding and necrosis of the flap of tissue around the implant. Because the surgeon is blind to the location of the tip of the drill when it is in the bone, nearby anatomic structures can also be injured such as the inferior alveolar nerve, the maxillary sinus and blood vessels. An inability to place the implant in bone to provide stability of the implant (referred to as primary stability of the implant) increases the risk of failure to osseointegration.
Immediate post-operative risks
Failure to Integrate
An implant is tested between 8 and 24 weeks to determine if it's integrated. There is significant debate as to which factors best determine whether or not an implant has successfully integrated, but common tests are;
Dental implant success is related to operator skill, quality and quantity of the bone available at the site, and the patient's oral hygiene but the most important factor is primary implant stability. While there is significant variation in the rate that implants fail to integrate (due to individual risk factors), the approximate values are 1-6%,
The long-term complications that result from restoring teeth with implants relate, directly, to the risk factors of the patient and the technology. There are the risks associated with esthetics including a high smile line, poor gingival quality and missing papillae, difficulty in matching the form of natural teeth that may have unequal points of contact or uncommon shapes, bone that is missing, atrophied or otherwise shaped in an unsuitable manner, unrealistic expectations of the patient or poor oral hygiene. The risks can be related to biomechanical factors, where the geometry of the implants does not support the teeth in the same way the natural teeth did such as when there are cantilevered extensions, fewer implants than roots or teeth that are longer than the implants that support them (a poor crown-to-root ratio). Similarly, grinding of the teeth, lack of bone or low diameter implants increase the biomechanical risk. Finally there are technological risks, where the implants themselves can fail due to fracture or a loss of retention to the teeth they're intended to support.
From these theoretical risks, derive the real world complications.
Loss of integration to bone
Long-term failures are due to either loss of bone around the tooth and/or gingiva due to peri-implantitis or a mechanical failure of the implant. Because there is no dental enamel on an implant, it does not fail due to cavities like natural teeth. While large-scale, long-term studies are scarce, one long-term retrospective study found overall, implant failure was 8.16% in the maxilla and 4.93% in the mandible overall. The risk of the prosthetic (the teeth supported by the implants) failing is roughly four-times that of the implant themselves failure. In early implants, all crowns where attached to the teeth with screws, but more recent advancements have allowed placement of crowns on the abutments with dental cement (akin to placing a crown on a tooth). This has created the potential for cement, that escapes from under the crown during cementation to get caught in the gingiva and create a peri-implantitis (see picture below). While the complication can occur, there does not appear to be any additional peri-implantitis in cement-retained crowns compared to screw-retained crowns overall.
Complications associated with single crown implants (5-year)
Complications associated with fixed complete dentures
The most common complication being fracture or wear of the tooth structure, especially beyond 10 years.
Complications associated with removable dentures (overdentures)
|Gallery of complications|
|Common implant types|
A typical implant consists of a titanium screw (resembling a tooth root) with a roughened or smooth surface. The majority of dental implants are made out of commercially pure titanium, which is available in 4 grades depending upon the amount of carbon, nitrogen, oxygen and iron contained. Cold work hardened CP4 (maximum impurity limits of N .05%, C .10%, H .015%, Fe .50%, O .40%) is the most commonly used titanium for implants. Grade 5 titanium, Titanium 6AL-4V, (signifying the Titanium alloy containing 6% Aluminium and 4% Vanadium alloy) is slightly harder than CP4 and used in the industry mostly for abutment screws and abutments. Most modern dental implants also have a textured surface (through etching, anodic oxidation or various-media blasting) to increase the surface area and osseointegration potential of the implant.
Implant dentistry is the second oldest discipline in dentistry (oral surgery is considered to be the first). Root form implants have been used for thousands of years. 4000 years ago, the ancient Chinese used carved bamboo pegs, tapped into the bone, to replace lost teeth. 2000 years ago the ancient Egyptians used similarly shaped pegs made of precious metals. Some Egyptian mummies were found to have transplanted human teeth, and in other instances, teeth made of ivory.
A metal tooth, made of wrought iron, was found in the skull of a man who died around 100 AD, and was buried in a Gallo-Roman graveyard near present-day Essonne, France. It had been pounded into the socket, and subsequent bone formation around the implant suggests that it had integrated into the jaw.
One of the most remarkable finds was by Dr. and Mrs. Wilson Popenoe in 1931, at an archaeological site in Honduras dating back to 600 AD. It was the lower mandible of a young Mayan woman, with three missing incisors replaced by pieces of shell, shaped to resemble teeth. Bone growth around two of the implants, and the formation of calculus, indicates that they were functional as well as esthetic. The fragment is currently part of the Osteological Collection of the Peabody Museum of Archaeology and Ethnology at Harvard University.
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 the 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 further studies into this phenomenon, using both animal and human subjects, which all confirmed this unique property of titanium.
Although Brånemark had originally considered that the first work should centre on knee and hip surgery, he 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 his first titanium dental implant into a human volunteer.
Over the next fourteen years Brånemark published studies on the use of titanium in dental implantology. The hybrid bridge on Brånemark implants was a concept of good clinical results. 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. Initially, dental implants where intended for those who needed to retain complete dentures, but the technology and markets evolved into the more modern uses in the 1980s. As the technology developed, other companies such as Straumann that where already producing osteosynthesis plates then developed their own technology for dental implants.