Introduction to Dental Implantology
High rate of success achieved with osseointegrated dental implants allows a greater number of patients to enjoy the benefits of fixed rather than removable restorations. The main indications for implant restorations in the partially edentulous patient are the free-end distal extension where no posterior abutment is available and the long edentulous span. In both these situations, the conventional dental treatment plan would include a removable partial denture.
However, with the advent of implant abutments, the patient can benefit from fixed restorations. Additionally, in the short edentulous span, the single implant is a popular option
IMPLANT TYPES
There are three major subgroups of dental implants: subperiosteal, transosteal, and endosteal. The first two, subperiosteal and transosteal, are designed primarily to anchor dentures in the completely edentulous patient. The third, endosteal implants, are surgically placed within alveolar or basal bone and are most commonly used for the treatment of partially edentulous patients, either singly or in multiples. They can be further subdivided by shape into blade form (plate form) and root form (cylindrical). Blades are wedge shaped or rectangular in cross section and are generally 2.5 mm width, 8 to 15mm depth, and 15 to 30 mm length. Root forms are 3 to 6 mm in diameter and 8 to18 mm length, often with external threads (Fig.below).
Endosteal implants are also categorized as one stage or two stage. The one-stage implant is designed to be placed in the bone and to immediately project through mucosa into the oral cavity. The two-stage implant requires two surgical procedures. First, the implant is placed in bone to the level of the cortical plate and the oral mucosa is sutured over it; this is left for a prescribed healing period (usually 4-6 months ), depending on the quality of bone. Then, in a second surgery, the mucosa is reflected from the superior surface of the implant, and an extension collar or abutment that projects into the oral cavity is fastened to the implant.
PLATE IMPLANTS (BLADES)
Blades were the first dental implant to experience reasonable success in a large number of patients. All the original studies on blades used one-stage systems, but the success rates were considerably lower than those of current root-form implants. It has been suggested that many of the problems of blade implants can be traced to the high temperature at which the bone sites were prepared and the routine immediate loading of this type of implant. Both these practices have been linked to the fibrous encapsulation that occurred with many of the original blade implants. Consequently, submergible titanium blades are now available, and more recent blade studies have reported success rates above 80% for 5 years.
However, the drawbacks to blade implants remain-difficulty of preparing precision slots for blade placement compared to placing holes accurately for root-form implants and the disastrously large circumferential area of the jaw that can be affected when a blade fails.
ROOT-FORM IMPLANTS (CYLINDERS)
Cylindrical root-form dental implants are considered to be state-of-the-art implant dentistry. Advantages include adaptability to multiple intraoral locations, uniformly precise implant-site preparation, and comparatively low adverse consequences similar to that experienced when a tooth is lost. Most root forms are made of titanium or titanium alloy with or without hydroxyapatite coating, materials that are perceived to have the highest bio-functionality.
Both threaded and non threaded designs are available and are quite popular. Today many of the titanium implants are grit blasted or acid etched to roughen the surface and increase the surface area for bone contact.
TREATMENT PLANNING FOR THE IMPLANT PATIENT
Implant success reported from major research institutions is quite high. However, meticulous attention to the procedures of patient selection, diagnosis, and treatment planning is required to duplicate this success.
Feasible non implant alternatives should be included in the overall treatment discussions. Patients need to be evaluated preoperatively and assessed as to whether they will be able to tolerate the procedure.
The predictable risks and expected benefits should be weighed for each person. Although the placement of dental implants does entail some risks, they are relatively minor. Absolute contraindications, based on immediate surgical and anesthetic risks, are limited to individuals who are acutely ill, individuals with uncontrolled metabolic disease, and pregnant women (contraindications that apply to virtually all elective surgical procedures).
Local and systemic contraindications that threaten long-term implant retention must also be evaluated. Implants may be contraindicated in patients with abnormal bone metabolism, poor oral hygiene, and previous radiation to the implant site.
Most potential implant placement patients became edentulous or partially edentulous from caries and periodontal disease resulting from poor oral hygiene. Suspicion that inadequate hygiene will continue is a relative contraindication to implant placement. Patients must be motivated and educated in oral hygiene techniques as part of their preparation for implants. Some individuals, such as those suffering from paralysis of the arms, debilitating arthritis, cerebral palsy, and severe mental retardation, may not be able to improve their oral hygiene. Implants are contraindicated in these patients unless adequate oral hygiene will be provided by caregivers.
CLINICAL EVALUATION
Evaluation of the planned implant site begins with a thorough clinical examination. This examination will determine whether there is adequate bone and will identify anatomic structures that could interfere with ideal implant placement. Visual inspection and palpation allow the detection of flabby excess tissue, bony ridges, and sharp underlying osseous formations and undercuts that would limit implant insertion. However, clinical inspection alone may not be adequate if there is thick overlying soft tissue that is dense, immobile, and fibrous.
RADIOGRAPHIC EVALUATION
Radiographic evaluation is also necessary. The best initial film is the panoramic view. However, there can be variations in magnification (5% to 35%); a small radiopaque reference object should therefore be placed near the proposed implant placement site during the exposure. Measurement of this image on the actual radiograph will enable the practitioner to correct for any magnification error. A ball bearing placed in wax on a denture or impression putty works well. Some new panoramic radiography machines have standardized enlargement ratios, which makes correction markers less necessary.
The widths of the posterior mandible and maxilla are determined primarily by clinical examination. Bone width not revealed on a panoramic film can be evaluated in the anterior maxilla and mandible with a cephalometric film. The location of the inferior alveolar canal and maxillary sinus can be determined by specialized CT scans, although high radiation exposure and considerable expense may limit their routine use.
DIAGNOSTIC CASTS
Accurately mounted diagnostic casts are essential for treatment planning. They are used to study the remaining dentition, evaluate the residual bone, and analyze maxilla-mandibular relationships. They can be helpful to the surgeon for fixture placement. A diagnostic waxing is done on the cast or on a duplicate. Proposed fixture installation sites are checked for proper alignment, direction, location, and relation to the remaining dentition.
The waxing helps determine the most esthetic placement of the teeth to be restored and the potential for functional speech disturbances. After adjustments and the diagnostic waxing are completed, a resin template can be made from the cast to guide the surgeon during implant placement. Diagnostic waxing and surgical templates are essential when planning implants as part of a full-mouth reconstruction or when restoring the anterior esthetic zone.
RESTORATIVE CONSIDERATIONS
Implant and Restoration Size. The choice of implant and its superior-inferior placement location are modified by the diameter of the intended restoration and can be adjusted for different sizes of teeth.
For example, the typical root diameter of a maxillary central incisor is 8.0 mm; the average implant diameter is 4.0 mm. Therefore, a distance of 2.0 to 3.0 mm is needed to make the transition gradually from 4.0 to 8.0 mm. If this is done over too short a distance, the restoration will be over contoured or look unnatural. By contrast, many mandibular centrals and laterals are smaller than 4.0 mm at the cemento-enamel junction. Therefore, an esthetic restoration on a 4.0-mm implant is impossible.
Smaller-diameter implants (about 3.0 mm) have been developed to allow esthetic restoration in these areas. It is also possible to use a larger implant (5.0 to 6.0 mm) for molar restorations with adequate bone. Restoration size must always be considered during the treatment-planning stage so that a properly sized implant will be placed in the ideal location.
Single Tooth Implant. Treatment planning for the single tooth restoration, particularly in the anterior esthetic zone, is one of the most challenging problems faced by the implant restorative dentist. Placement of the implant for both esthetics and biomechanical loading (to minimize screw loosening) is especially critical. In addition, at the treatment planning stage, the decision to place an implant with an anti rotational feature built into the system (e.g., a spline or a hexagon) is essential.
IMPLANT PLACEMENT
Placement procedures for all implant systems require atraumatic preparation of the recipient site. Thermal injury to bone is minimized by using a low-speed, high-torque handpiece, along with copious irrigation. The irrigation is either externally or internally applied and directed through channels in the drill. Manufacturer recommendations relating to the type of irrigation and speed of the drilling equipment should be followed. Threaded implants often require final thread preparation in the bone at very low speeds.
The implant recipient site is prepared with a series of gradually enlarged burs. All implant systems have an initial small-diameter drill used to mark the implant site. The implant site is located using the surgical template, which may also assist in directing angulation of the implant. The center of the implant recipient site is marked with the initial drill, and a pilot hole is prepared. A paralleling pin is then placed in the preparation to check alignment and angulation. At this point, a final determination is made regarding the adequacy of the recipient site for implant placement. Although implant placement is a surgical procedure, it is influenced by critical restorative parameters. The stent communicates the range of acceptable implant positions and angulations.
At this step, if it is apparent that supporting bone will not allow proper positioning of the implant, further osseous augmentation may be necessary, either simultaneously with implant placement, or as a separate procedure with implant placement delayed until proper osseous support is available. After the desired depth and diameter of the recipient site are achieved, the implant is placed. For titanium implants, an uncontaminated surface oxide layer is required for osseointegration.
Hydroxyapatite-coated implants are also sensitive to contamination. Non threaded implants are positioned in the recipient site and gently tapped into place with a mallet and seating instrument. Threaded implants are screwed into place, which also requires cutting the screw threads in the recipient site. Self-tapping implants are available for use in the maxilla, where the bone is soft enough to make prethreading unnecessary. After all implants are placed, tension-free closure prevents wound dehiscence.
POSTOPERATIVE EVALUATION
A radiograph should be taken postoperatively to evaluate the position of the implant in relation to adjacent structures (e.g., the sinus and the inferior alveolar canal) and other implants. Any significant problems noticed at this time should be corrected.
Patients are given mild analgesics and 0.12% chlorhexidine gluconate rinses for 2 weeks after surgery to keep bacterial populations to a minimum ring healing. Weekly evaluations are recommended until soft tissue healing is complete (2 to 3 weeks). If possible, complete or removable partial dentures should not be worn for 1 week after surgery. The resin over the implant can then be reduced by 2.0 or 3.0 mm and replaced with a soft liner, so that the denture can be worn without injuring the healing implant site.
IMPLANT RESTORATIVE OPTIONS
Distal-extension Implant Restoration. Implant support offers major advantages in the treatment of partially edentulous patients in whom no terminal abutment is available. In this situation, the conventional dental treatment plan would include a re movable partial denture. However, with the implant alternative, patients can avoid the discomfort and inconvenience of a removable prosthesis. There are two distal-extension restorative options. One option is to place an implant distal to the most posterior natural abutment and fabricate a fixed prosthesis connecting the implant with the natural tooth. However, there are problems associated with implants connected to natural teeth. The other option is to place two or more implants posterior to the most distal natural tooth and fabricate a completely implant-supported restoration.
If the crown-to-implant ratio is favorable, two implants to support a three-unit fixed partial denture may be considered. If implants are short and crowns are long, one implant to replace each missing tooth is highly recommended. If doubt remains, more implants are used when heavier forces are expected (e.g., the posterior part of the mouth in patients with evidence of parafunctional activity). Fewer implants are used when lighter forces are expected (e.g., those opposing a complete denture or those supporting a prosthesis in the anterior part of the mouth).
Long Edentulous Span Restoration. Similar options can be used when treating a long edentulous span. The clinician may choose to have multiple implants placed between the remaining natural teeth and to fabricate a fully implant-supported restoration. As an alternative, one or two implants can be placed in the long edentulous span and the final restoration connected to natural teeth. When it is necessary to connect implants and the natural teeth, protecting the teeth with telescopic copings is recommended. In this manner, prosthesis retrievability can be maintained. In addition, some long edentulous spans require the reconstruction of soft and hard tissue as well as teeth. In these instances, using resin teeth processed to a metal substructure rather than a conventional metal ceramic restoration is recommended. Soft tissue esthetics can be more easily and accurately mimicked with heat-processed resin and large defects. This type of restoration has been called a hybrid because it combines the principles of conventional fixed and removable prosthodontics. For smaller defects, pink porcelain can be used to compensate for missing soft tissue.
Single-tooth Implant Restoration. The use of single implants in restoring missing teeth is an attractive option for the patient and the dentist. However, it requires careful implant placement and precise control of all prosthetic components. Single-tooth restorations supported by implants may be indicated in the following situations:
1. An otherwise intact dentition
2. A dentition with spaces that would be more difficult to treat with conventional fixed prosthodontics
3. Distally missing teeth when cantilevers or removable partial dentures are not indicated
4. A prosthesis that needs to closely mimic the missing natural tooth
The requirements for single-tooth implant crowns are as follows:
1. Esthetics
2. Antirotation-to avoid prosthetic component loosening
3. Simplicity-to minimize the amount of components used
4. Accessibility-to maintain optimum oral health
5.Variability-to allow the clinician to control the height, diameter, and angulation of the implant restoration
Several systems have been developed to comply with these demands. Common indications include congenitally missing maxillary lateral incisors and teeth in which endodontic treatment was unsuccessful. Screw loosening has most commonly been associated with the terminally positioned single molar implant crown.
Matching the soft tissue contours of adjacent natural teeth remains the most difficult challenge for completing the anterior single-tooth restoration. These contours can be reliably created with provisional restorations. One technique, which combines soft tissue contouring and provisional placement. When the tissue has matured around the provisional restoration, a final impression can be taken to complete the definitive restoration. Impressions can also be made at the time of Stage I surgery so that a provisional can be delivered at Stage II to facilitate more ideal soft tissue contours.
The best soft tissue esthetics are still generally achieved when inter-dental papillae are present before the surgery. If soft tissue contours are deficient before surgery, the patient should expect some compromise in the final soft tissue result.
Fixed Restoration in the Completely Edentulous Arch. For completely edentulous patients who require non removable restorations, there are two implant options: a hybrid prosthesis and a fixed metal-ceramic rehabilitation The hybrid prosthesis is a cast alloy framework with processed denture resin and teeth. It requires a minimum of five implants in the mandible and six in the maxilla. One major determining factor for selecting this option is the amount of bone and soft tissue lost. For patients who have had moderate bone loss, the prosthesis restores both bone and soft tissue contours.
The metal-ceramic rehabilitation also requires five implants in the mandible and six in the maxilla. It can be made esthetically pleasing only if minimal bone loss has occurred and is best suited for patients who have recently lost their natural teeth (within 5 years). For patients with severe bone loss, there is probably only one option: a removable restoration.
The main advantage of a completely fixed restoration, whether it is hybrid or metal-ceramic, is that it is completely retained by the patient at all times. Therefore, patients experience the psychologic benefit of having a restoration that closely resembles their original natural teeth. In addition, movement within the system is minimized, and the components tend to wear out less quickly. Because the prosthesis is screw retained, the dentist can remove it, allowing access for cleaning and repairs. A potential disadvantage is that the implants must be precisely placed, especially in the maxillary anterior esthetic zone. Implants placed in embrasure spaces can lead to disastrous esthetic results and can impede access for hygiene. With a hybrid prosthesis, the clinician must decide between leaving enough space for hygiene access and minimizing space for optimum esthetics. Some patients may be concerned by the amount of metal shown in a hybrid prosthesis. However, from a conversational distance, a properly made prosthesis will be hardly noticeable.
Esthetic and phonetic problems in the maxillary arch can often be avoided by not placing implants near the midline and restoring the incisor teeth with pontics. This approach to implant placement improves the restorative outcome considerably .
CONNECTING IMPLANTS TO NATURAL TEETH
It has been suggested" that connecting a single osseointegrated implant to one natural tooth with a fixed partial denture can create excessive forces because of the relative immobility of the osseointegrated implant compared to the functional mobility of a natural tooth. During function, the tooth moves within the limits of its periodontal ligament, which can create stress at the neck of the implant up to two times the implied load on the prosthesis
Potential problems with this type of restoration include
(1) breakdown of the osseointegration,
(2) cement failure on the natural abutment,
(3) screw or abutment loosening, and
(4) failure of the implant prosthetic component.
This situation is encountered clinically when the most posterior abutment is lost in the dental arch and a fixed prosthesis is needed to connect a single implant to the natural tooth. If possible, a totally implant-supported fixed partial denture with two or more implants should be provided. However, anatomic limitations of the maxillary sinus or the mandibular canal often limit restorative efforts directed at a single fixture site. When connecting an implant to a natural tooth is necessary, multiple implant or natural tooth abutments should be used. A semi-precision attachment (keyway) in the prosthesis between the implant and the natural tooth may solve potential problems. However, under most circumstances, when a load is applied to the pontic, the additional movement at the attachment actually increases the cantilever effect on the implant abutment. In practice, the only advantage of a semi-precision attachment may be that it allows a screw-retained implant abutment crown to be removed for periodic evaluation.
When circumstances dictate using a natural tooth abutment, a telescopic coping should be considered. This is permanently cemented to the natural tooth and can prevent decay if loosening occurs. Provisional cement is used to attach the prosthesis to the coping. If it leaches out of the implant crown, the natural tooth will still be protected.
PROSTHETIC FAILURE
Additional implant prosthetic complications include fracture of the implant components or the prosthesis. Fracture of implant components is usually attributed to fatigue from biomechanical overload. Failure of the implant prosthesis is usually traceable to less than ideal laboratory procedures or prosthesis design .
SUMMARY
Implant-supported prostheses, using cylindrical osseointegrated fixtures placed by a two-stage surgical technique, should be considered in the treatment of any partially edentulous patient. They are a reliable solution to many situations that are difficult to treat by conventional measures: the patient who cannot wear removable appliances, the patient with a long edentulous span or other circumstance (e.g., short roots) that diminishes the prognosis for an FPD, and the patient with a single missing tooth but sound adjacent teeth.
Success with implant prosthodontics requires the same attention to detail and careful planning as conventional fixed prosthodontics. Often a team approach is recommended, with a surgeon placing the implant and a restorative dentist designing the prosthesis. The critical stage is optimum placement of the implant(s). The surgeon s main concern is that it be well within the available bone and away from vital structures (e.g., the inferior dental canal). The restorative dentist s main concern is that the positioning and angulation of each fixture allow optimum occlusion, esthetics, and tissue health as well as minimum stresses at the implant-bone interface. Information obtained from a clinical examination, radiographs, and a diagnostic waxing on articulated casts is crucial to planning. Surgery is guided by a template made from the diagnostic waxing. Depending on the implant site, a two-stage surgical technique requires 3 to 6 months for bone to heal against the implant. In a second surgery, the implant is uncovered and implant abutments are screwed into place. Subsequently, a screw-retained prosthesis is fabricated to restore function and appearance.
Several implant systems are available, each with a variety of components for restorative management (e.g., an antirotational feature incorporated in an implant for single tooth replacement). Problems unique to implant prosthodontics include screw loosening and bone loss from premature loading or repeated overloading. Occlusal considerations, prosthesis fit, plaque control, and follow-up care are all primary concerns to the professionals who deal with implants and conventionally supported prostheses.