Dental Implants
A dental implant (also known as an endosseous implant or fixture) is a surgical component that interfaces with the bone of the jaw or skull to support a dental prosthesis such as a crown, bridge, denture, facial prosthesis or to act as an orthodontic anchor. The basis for modern dental implants is a biologic process called osseointegration where materials, such as titanium, form an intimate bond to bone. The implant fixture is first placed, so that it is likely to osseointegrate, then a dental prosthetic is added. A variable amount of healing time is required for osseointegration before either the dental prosthetic (a tooth, bridge or denture) is attached to the implant or an abutment is placed which will hold a dental prosthetic.
Success or failure of implants depends on the health of the person receiving it, drugs which impact the chances of osseointegration and the health of the tissues in the mouth. The amount of stress that will be put on the implant and fixture during normal function is also evaluated. Planning the position and number of implants is key to the long-term health of the prosthetic since biomechanical forces created during chewing can be significant. The position of implants is determined by the position and angle of adjacent teeth, lab simulations or by using computed tomography with CAD/CAM simulations and surgical guides called stents. The prerequisites to long-term success of osseointegrated dental implants are healthy bone and gingiva. Since both can atrophy after tooth extraction pre-prosthetic procedures, such as sinus lifts or gingival grafts, are sometimes required to recreate ideal bone and gingiva.
The final prosthetic can be either fixed, where a person cannot remove the denture or teeth from their mouth or removable, where they can remove the prosthetic. In each case an abutment is attached to the implant fixture. Where the prosthetic is fixed, the crown, bridge or denture is fixed to the abutment with either lag screws or dental cement. Where the prosthetic is removable, a corresponding adapter is placed in the prosthetic so that the two pieces can be secured together.
The risks and complications related to implant therapy are divided into those that occur during surgery (such as excessive bleeding or nerve injury), those that occur in the first six months (such as infection and failure to osseointegrate) and those that occur long-term (such as peri-implantitis and mechanical failures). In the presence of healthy tissues, a well integrated implant with appropriate biomechanical loads can have 5-year plus survival rates from 93 to 98 percent and 10 to 15 year lifespans for the prosthetic teeth.
Medical uses & Common uses of dental implants
Mouth with many implant supported teeth where it is difficult to distinguish the real teeth from the prosthetic teeth.
Individual teeth were replaced with implants where it is difficult to distinguish the real teeth from the prosthetic teeth.
Implant retained overdenture
Movement in a lower denture can be decreased by implants with ball and socket retention.
Implant retained fixed partial denture (FPD)
A bridge of teeth can be supported by two or more implants.
The primary use of dental implants are to support dental prosthetics. Modern dental implants make use of osseointegration, the biologic process where bone fuses tightly to the surface of specific materials such as titanium and some ceramics. The integration of implant and bone can support physical loads for decades without failure.
For individual tooth replacement, an implant abutment is first secured to the implant with an abutment screw. A crown (the dental prosthesis) is then connected to the abutment with dental cement, a small screw, or fused with the abutment as one piece during fabrication. Dental implants, in the same way, can also be used to retain a multiple tooth dental prosthesis either in the form of a fixed bridge or removable dentures.
An implant supported bridge (or fixed denture) is a group of teeth secured to dental implants so the prosthetic cannot be removed by the user. Bridges typically connect to more than one implant and may also connect to teeth as anchor points. Typically the number of teeth will outnumber the anchor points with the teeth that are directly over the implants referred to as abutments and those between abutments referred to as pontics. Implant supported bridges attach to implant abutments in the same way as a single tooth implant replacement. A fixed bridge may replace as few as two teeth (also known as a fixed partial denture) and may extend to replace an entire arch of teeth (also known as a fixed full denture). In both cases, the prosthesis is said to be fixed because it cannot be removed by the denture wearer.
A removable implant supported denture (also an implant supported overdenture is a type of dental prosthesis which is not permanently fixed in place. The dental prosthesis can be disconnected from the implant abutments with finger pressure by the wearer. To enable this, the abutment is shaped as a small connector (a button, ball, bar or magnet) which can be connected to analogous adapters in the underside of the dental prosthesis. Facial prosthetics, used to correct facial deformities (e.g. from cancer treatment or injuries) can utilise connections to implants placed in the facial bones. Depending on the situation the implant may be used to retain either a fixed or removable prosthetic that replaces part of the face.
In orthodontics, small diameter dental implants, referred to as Temporary Anchorage Devices (or TADs) can assist tooth movement by creating anchor points from which forces can be generated.[10] For teeth to move, a force must be applied to them in the direction of the desired movement. The force stimulates cells in the periodontal ligament to cause bone remodeling, removing bone in the direction of travel of the tooth and adding it to the space created. In order to generate a force on a tooth, an anchor point (something that will not move) is needed. Since implants do not have a periodontal ligament, and bone remodelling will not be stimulated when tension is applied, they are ideal anchor points in orthodontics. Typically, implants designed for orthodontic movement are small and do not fully osseointegrate, allowing easy removal following treatment.
Technique & Planning
Techniques used to plan implants
To help the surgeon position the implants a guide is made (usually out of acrylic) to show the desired position and angulation of the implants.
To help the surgeon position the implants a guide is made (usually out of acrylic) to show the desired position and angulation of the implants.
Sometimes the final position and restoration of the teeth will be simulated on plaster models to help determine the number and position of implants needed.
Sometimes the final position and restoration of the teeth will be simulated on plaster models to help determine the number and position of implants needed.
CT scans can be loaded to CAD/CAM software to create a simulation of the desired treatment. Virtual implants are then placed and a stent created on a 3D printer from the data.
CT scans can be loaded to CAD/CAM software to create a simulation of the desired treatment. Virtual implants are then placed and a stent created on a 3D printer from the data.
General considerations
Planning for dental implants focuses on the general health condition of the patient, the local health condition of the mucous membranes and the jaws and the shape, size, and position of the bones of the jaws, adjacent and opposing teeth. 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
The use of bone building drugs, like bisphosphonates and anti-RANKL drugs require special consideration with implants, because they have 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 and Maxillofacial Surgeons, discussed that the risk of BRONJ from low dose oral therapy (or slow release injectable) as between 0.01 and 0.06 percent 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 three years. The position paper recommends against placing implants in people who are taking high dose or high frequency intravenous therapy for cancer care. Otherwise, implants can generally be placed and the use of bisphosphonates does not appear to have an impact on implant survival.
Biomechanical considerations
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 density 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 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 where the implant is struck repeatedly with increasing forces (similar in magnitude to biting) until it fails.
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.
Main surgical procedures
Basic implant surgical procedure
The area of the mouth that is missing a tooth is identified.
An area with a single missing tooth
An incision is made across the area and the flap of gingiva is opened to show the bone of the jaw.
An incision is made across the gingiva, and the flap of tissue is reflected to show the bone of the jaw.
A series of slow-speed drills create and gradually enlarge a site in the jaw for the implant to be placed. The hole is called an osteotomy.
Once the bone is exposed, a series of drills create and gradually enlarge a site (called an osteotomy) for the implant to be placed.
The implant fixture is turned into the osteotomy. Ideally, it is completely covered by bone and has no movement within the bone.
The implant fixture is turned into the osteotomy. Ideally, it is completely covered by bone and has no movement within the bone.
A healing abutment is attached to the implant fixture and the gingiva flap is sutured around the healing abutment.
A healing abutment is attached to the implant fixture, and the flap of gingiva is sutured around the healing abutment.
Placing the implant
Most implant systems have five basic steps for placement of each implant:
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.
Drilling at high speed: After reflecting the soft tissue, and using a surgical guide or stent as necessary, pilot holes are placed with precision drills at highly regulated speed to prevent burning or pressure necrosis of the bone.
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 low.
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 with a torque controlled wrench[19] 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).
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 cover it. A second procedure would then be required to uncover the implant at a later date.
Timing of implants after extraction of teeth
There are different approaches to placement dental implants after tooth extraction. The approaches are:
Immediate post-extraction implant placement.
Delayed immediate post-extraction implant placement (two weeks to three months after extraction).
Late implantation (three months or more after tooth extraction).
There are also various options for when to attach teeth to dental implants,classified into:
Healing time
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, three to six 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 (35 Ncm) and are splinted to other implants, there are no meaningful differences in long-term implant survival or bone loss between implants loaded immediately, at three months, or at six 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 versus 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 suggests 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.(pp9–12) The choice of one or two-stages, now centers around how best to reconstruct the soft tissues around lost teeth.
Immediate placement
An increasingly common strategy to preserve bone and reduce treatment times includes the placement of a dental implant into a recent extraction site. On 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 and delayed implants in a scientifically rigorous manner.