Dental Implants
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Implants come in all shapes and sizes. They may be used to replace a single missing tooth or rehabilitate an entire endentulous arch. The fully edentulous maxilla can be restored with 5-6 implants, whereas the fully edentulous mandible can be restored with 4-6 implants. The implant geometry or macro design refers to the overall shape (cylindrical, conical, hybrid, tapered etc.) and thread shape. Implants are often machined or press-fit but this results in smooth external surfaces that are not best suited for optimum function. Increased surface roughness has been shown to promote osseointegration. Sandblasting, acid etching, or coatings increase surface area and roughness to improve adaptation to bone. These changes refer to the microstructure of the implant. Sometimes additional surface coatings like hydroxyapatite are used to further enhance osseointegration and tissue acceptance.
Titanium is the material that offers the best biological integration with bone and gingival tissue. Titanium implants have a layer of titanium oxide on the outer surface that is responsible for osseointegration, with bone apposition adjacent to the implant surface by mechanisms similar to a bony fracture healing. Early loading of implants is sometimes implemented, but early loading may increase the chance of failure. 2 to 3 months is often allowed for proper osseointegration before the implant is definitively restored and placed under functional loading. Within weeks after placement woven bone is laid down at the bone-implant interface. As a few more weeks pass, the randomly oriented collagen fiber containing woven bone is replaced (remodeling) by lamellar bone. Functional forces transmitted through the bone are known to influence bony remodeling. A successful implant surface establishes a healthy long term connection with both soft and hard tissues. The soft-tissue interface between oral tissues and titanium can be established by keratinized and nonkeratinized mucosa, and is comparable to the junctional epithelial soft tissue interface to teeth. The epithelial attachment is composed of hemidesmosomes and a basal lamina.
A 3-4mm zone of connective tissue mimics the natural biological width seen in the natural dentition, with connective tissue fibers oriented parallel to the implant or abutment surface, but these connective tissue fibers do not directly insert into titanium. There is no periodontal ligament or supracrestal connective tissue inserting into the implant as in teeth, so no equivalent of a periodontal ligament can be formed on an implant surface. A lack of a periodontal ligament means that implants cannot dynamically “grow” with a developing patient’s skeleton. The implants will not continue to erupt like normal teeth, but instead act more like an ankylosed tooth. For this reason implants are not recommended until bony growth and maturation has completed.
Implants may be of benefit during orthodontic treatment because they lack a periodontal ligament and are therefore immobile, and easily retrievable once orthodontic treatment has been completed. The lack of periodontal ligament also means the implant lacks the same proprioceptive feedback that a patient would experience from their natural dentition. Though not identical, over time a patient might develop osseoperception which gives the patient a measure of tactile feedback regarding the implants in the bone.
With normal teeth, the cemento-enamel junction (CEJ) is the landmark used as reference to determine attachment level. With an implant, the reference often used is the shoulder of the implant abutment or the margin of the restoration (crown). Compared to the rough external surface of the implant, the smooth polished exterior surface of the implant abutment is often referred to as the “polished collar”, designed to facilitate hygiene by inhibiting plaque attachment/biofilm formation.
SINGLE STAGE VS TWO STAGE
An osteotomy describes the surgical preparation made to receive an implant. The osteotomy is largely influenced by the manufacturer specifications and is modified according to the quantity and quality of bone, and space available at the implant site. A two-stage (submerged) system involves the initial surgical placement of an implant, but the implant is “buried” to allow for unobstructed healing (osseointegration). A second surgical procedure is used to uncover the submerged implant and to place an abutment and/or start the restoration process.
A single-stage (one-piece/non-submerged) system avoids the second surgery, opting to either leave the implant itself exposed after surgery, or to use an interim healing abutment. The healing abutment is positioned to sit approximately 2mm above the height of the surrounding gingiva, but is not placed in direct occlusion to avoid excessive forces that could lead to implant failure. A radiograph is used to confirm proper placement. The healing abutment maintains favorable tissue contour to improve aesthetics once the final restoration (crown) is placed.
Patient implant assessment involves:
- Direct palpation of the prospective bony implant site.
- Radiographic imaging, with 3D imaging being the most useful.
- Occlusal analysis that includes contacts in maximal intercuspation, centric relation, and excursive movement, as well as opposing teeth and any restorative work associated.
When implants are placed care must be taken to avoid important surrounding structures. The anterior loop of the inferior alveolar nerve can extend up to 4mm mesial to the mental foramen. The inferior alveolar nerve canal drifts from a more lingual position to a more buccal position as it follows from the mandibular foramen to the mental foramen. The minimum interocclusal distance (restorative space) required is 6-8mm to allow for adequate room for prosthetic placement. Less interarch space is required for a screw retained crown when compared to a cement retained crown. Other routinely recommended space requirements for implants:
- At least 1mm of bone is required lingual and buccal to the implant, or from the implant to an adjacent tooth.
- At least 2mm between the implant and any other vital structure (inferior alveolar nerve canal, maxillary sinus floor, incisive canal, blood vessels, cortical plate etc.)
- At least 3mm between adjacent implants.
- At least 5mm between the implant and the radiographic position of the mental foramen if the position of the anterior loop of the inferior alveolar nerve is not visible on a radiograph.
If the minimum inter-implant space of 3mm is not followed it may lead to:
- The inability to clean the area properly.
- Bone loss between the two implants as the two healing zones touch.
- A papilla shape that appears unaesthetic (black triangle).
- Compromised aesthetics.
Bone quality is determined by assessing the cortical plate thickness and the trabecular/cancellous bone density. Type 1 bone is the most favorable for implant placement and longevity.
During implant placement a countersinking step may be employed. This involves a final osteotomy to flare the coronal portion of bone to allow for the placement of the body of the implant. Countersinking is used to compensate for the more dense cortical bone or to allow for a more flared implant design. Tapping the ostomy refers to the use of a threaded bur during the osteotomy to create a spiral furrow on the wall of the preparation that corresponds to and accepts the implant threading. This reduced the amount of torque required during implant placement. Self-tapping implants do not require this step. A screw-shaped implant is the most popular, allowing for increased initial stability.
Radiographs are frequently used before, during, and after implant placement to:
- Assess bone quality, space, and position of surrounding structures.
- Confirm proper implant placement.
- Confirm proper abutment placement following the final torque application.
- Confirm the fit of any other component.
- Review long ongoing/long term health.
Three dimensional imaging can be attained using cone beam computed tomography (CBCT) and linear tomography, though the latter produces an image of significantly lower quality. Two-dimensional images (periapical, occlusal, lateral cephalometric, panoramic) are easier to obtain and generally require lower doses of radiation. Crowns which are implanted-supported may be held to the abutment with either a screw (screw-retained) or cement.
A screw retained crown requires an access hole which does affect the structural integrity of the final restoration. This access needs to be covered once the restoration is placed and can compromise the aesthetics of the restoration. However, when compared to a cement retained crown, the screw-retained crown is relatively easy to retrieve, avoids potentially messy cementation, and permits the use of a shortened crown abutment.
Screws are tightened using a torque wrench (toque controller) according to predetermined manufacturer specification. The force applied to the screw is called preload. Anti-rotational elements between the implant and the abutment prevent loosening or rotation of the abutment relative to the implant. External anti-rotational elements consist of a permanent extension from the implant that fits into the a receptacle in the abutment.
Internal anti-rotational elements consist of a permanent extension from the abutment that fits into the a receptacle in the implant. Anti-rotational elements can also be used to prevent movement between implants and the permanent single unit restoration (crown). Multi-unit restorations would not require anti-rotational features.
Impressions are required to transfer an impression coping to the study model. A closed tray impression refers to the impression made with an impression tray that does not have an access hole cut over the implant. After the impression tray and set impression material is removed, the impression coping is left in the mouth, still attached to the implant or abutment. The coping is subsequently removed, attached to an implant analogue (replica) and seated in the impression material, before the cast is poured. This process is called an indirect transfer impression technique.
A pick-up impression (open tray) refers to the impression made with an impression tray that does have an access hole cut over the implant. Initially the impression coping is attached to the abutment by a retention screw. An impression is taken using an impression tray with a window that allows for access to the impression coping once the material has set. The screw is released and the impression and impression coping are removed together. This impression technique is useful when taking an impression of multiple implants that are not perfectly parallel. An implant level impression means the impression coping was attached to the implant at the time of the impression. An abutment level impression means the impression coping was attached to the implant abutment (which is attached to the implant) at the time of the impression.
IMPLANT HEALTH
In health, the area surrounding a dental implant contains predominantly coccoid, aerobic bacterial species with a low number of gram-negative anaerobic species. There is an absence of visual signs of inflammation, pocket formation (bone loss) and bleeding on probing. Greater than ≥90-95% success rates can be expected for titanium implants in healthy patients with optimum bone quality and good at home care. Peri-implant disease refers to the inflammatory changes in the tissue surrounding a load-bearing implant. The soft tissue surrounding an implant is called mucosa (not gingiva).
Peri-implant mucositis is a reversible inflammatory response of the soft tissues surrounding an implant without the loss of supporting bone. A microbial shift leads to higher proportions of anaerobic gram-negative rods and spirochetes, and the area is characterized by bleeding on probing and visual signs of inflammation. There is no bone loss present. Peri-implant mucositis can be reversed with measures aimed at reducing or eliminating the pathognomonic bacteria and plaque buildup.
Peri-implantitis is an inflammatory reaction with loss of supporting bone in the tissues surrounding an implant. Peri-implant disease is associated with poor plaque control and is linked to a history of severe periodontitis. If left untreated, progressive loss of bony support may eventually lead to implant failure.
Hygiene is very important to long term implant success. It is not recommended to clean a titanium implant surface with a conventional ultrasonic scaler tip since the relatively soft titanium-oxide surface of the implant can easily be damaged. Specially designed plastic ultrasonic tips may be used. Failure rates in totally edentulous patients are significantly higher than that seen in partially edentulous patients. Failure rates are also significantly higher in edentulous maxillas compared to edentulous mandibles. Advanced age is not a contraindication to implant placement, but any medical condition that significantly inhibits healing does. Smoking is not a firm contraindication but a patient needs to be warned smoking increases the likelihood of implant failure.
Osseointegration can fail due to:
- Occlusal overload.
- Peri-implantitis caused by bacterial byproducts.
- Overloading or micromotion during the healing phase.
- Excessive cantilevering of the prosthesis.
- Failure to achieve adequate primary stability.
The following would be relative contraindications to implant placement. Implants may still be placed with caution:
- Diabetes (poorly controlled).
- Bone metabolic disease (e.g., osteoporosis).
- Radiation therapy (head and neck), depending on the area and dosage.
- Immunosuppressive medication, immunocompromising disease (e.g., HIV, AIDS) or any other condition that affects the patient’s ability to heal.
- Smoking or tobacco use.
- Parafunctional habits.
- Current infection (e.g. endodontic).
- Periodontal disease, depending on the extent.
The following would likely be absolute contraindications to implant placement:
- Bisphosphonate therapy (especially IV) that predisposes patients to bisphosphonate-related osteonecrosis of the jaw (BRONJ).
- Psychiatric syndromes (e.g., schizophrenia, paranoia).
- Mental instability (neurotic or hysterical patients) or poor cooperation.
- Irrational fears (phobias).
- Unrealistic expectations.
- Substance abuse (e.g. alcohol, drugs).
