This case report demonstrates the construction of a complete restoration of the dentition by the surgical placement of endosseous titanium implants that support a fixed prosthesis in each jaw. The positioning of the implants and teeth in the prostheses are important factors for a successful long-term result. Distribution of the occlusal biting forces over as many implants as possible is important. Off-axial occlusal biting forces should be diverted to the anterior prostheses, where the forces are not as great and the posterior teeth are designed with flat occlusal surfaces that separate during excursionary chewing movements. Medial mandibular flexure caused by the contraction of the medial pterygoid muscle can be addressed by constructing the prosthesis in segments. This is so as not to have a rigid entity encased in flexing bone that may induce stress to the bone, leading to loss of implant integration and failure. Segmenting also insures an appropriate fit of the prosthesis with respect to casting and porcelain firing distortion. Lip support by means of a flange in the prosthesis may be necessary when there has been a large amount of bone loss from edentulous resorption. Cleaning and maintenance of the prostheses every 3 to 6 months is essential.
Tooth loss occurs by dental caries, periodontal disease, and trauma. In the past, the treatment for completely edentulous patients was complete denture removable prostheses. With time, dentures can allow or even contribute to bone resorption. Removing the osseous denture support can render a patient unable to comfortably wear any prosthesis in their later years. Missing teeth have been replaced using endosseous dental implants to support fixed or removable prostheses. Replacing a complete dentition with endosseous implant-supported fixed prostheses is possible. However, there are certain parameters of treatment that have become important. These parameters of treatment are mostly directed at control of occlusal force distribution to the supporting implants.1
The implants are made of titanium that reacts in nanoseconds with ambient oxygen to form a biocompatible oxide surface 600 to 1000 nm. Titanium oxide is very biocompatible. The surface of the implants are treated by sandblasting and acid etching or dual thermo-acid etching to create a range of surface roughness of 50 to 120 μm that allows direct bone apposition against the titanium oxide surface to provide bone-to-implant surface contact in a range of 20% to 70%. The surface roughness also engages the bone and provides mechanical retention. These 2 properties give the implant great stability and resistance to dislodging forces even though there can be less than 40% bone-to-implant contact. Cortical bone provides the most implant support. The forces applied to the prosthesis causes the most bone stress at the area where the implant emerges from the cortex.
A 39-year-old female presented for dental treatment. She was examined and evaluated. Her diagnosis was dental caries and chronic periodontitis. All of her remaining teeth were in hopeless condition and were in need of removal. Her medical history was not significant for dental treatment. A treatment plan was designed, explained, and accepted by the patient.
The treatment plan was as follows:
Construction of immediate complete dentures
Extraction of remaining teeth, establishment of appropriate centric relation, and vertical dimension of occlusion and delivery of removable complete dentures
Computerized tomography of edentulous jaws
Establishment of dental implant positions and grouping for occlusal support and force distribution
Construction of surgical guides for implant positioning and surgical installation
Surgical implant installation.
Healing phase for implant osseointegration
Abutment installation and provisional prosthetic construction
Substructure construction and fitting
Porcelain application, esthetic evaluation, and refinement of occlusion in centric and excursive movements and provisional cementation for evaluation of function
Maintenance of prosthesis and supporting peri-implant tissue
All surgical and prosthetic treatment was performed in office, with the patient sedated with sublingually incrementally administered 0.5 mg triazolam (Halcion). The teeth were extracted and, immediately, complete dentures were placed. An osseous healing period of 6 months was allowed for remodeling and development of supportive bone. The vertical dimension of occlusion in the dentures was established for function and patient comfort. This dimension, although not an exact dimension, was to be transferred to and maintained in the implant-supported prosthesis.2 Preimplant surgery panoramic and computerized tomographic (CT) scan radiography of the edentulous jaws were performed (Figure 1). A plan for implant positioning and sizing was made from the prepared imaging on compact disk made from the CT scan3 (Figure 2) (Materialise, Glen Burnie, Md). A clear plastic form simulating tooth position demonstrated that there was good lip support created by the proposed tooth arrangement without a prosthetic flange (Figure 3). Exact implant positions were determined using casts made from the patient's edentulous ridges and dimensional data gleaned from the CT scan.
A surgical guide was constructed on the casts of the edentulous jaw arches. The guides used the hard palate for a landmark and support during the procedures, thus providing a stable guide base for the placement of the implants.
The surgery was rehearsed with the staff to minimize operative time. Twelve maxillary implants (Implant Innovations Inc., Palm Beach Gardens, Fla) were installed in a 2-hour appointment. At a later appointment, 12 mandibular implants were installed in 2 1/2 hours. An apically positioned gingival flap was made in the mandible to increase the zone of attached gingiva, which protects and supports the epithelial peri-implant tissue. The zone of attached gingiva was adequate in the maxilla and did not need augmentation.
The implants were placed so as to maximize osseous support and to insure delivery of axial forces to the implants. Osteotomes were used to compress osteotomy sites with less dense bone to increase the bone-to-implant contact. A panoramic radiograph was taken (Figure 4). The patient was prophylactically medicated with amoxicillin 875 mg bid and a 0.12% chlorhexidine oral rinse. Postoperative pain was controlled with hydrocodone and acetaminophen (Vicodin ES). Healing on each surgical occasion was uneventful.
An osseous healing time of 4 months was allowed to develop integration of supporting bone to the implants. At 1 postoperative month, the distal-most maxillary right implant failed to integrate and was replaced with a larger diameter rescue implant, which successfully integrated (Innova, Toronto, Ontario).
At the next phase, abutments were placed and secured with screws into each implant. Each implant abutment was placed at 32 N cm of torque, according to the manufacturers instructions.
Working casts of the prepared abutments were made from maxillary and mandibular full-arch polyvinyl siloxane impressions (Imprint, Dentsply, Philadelphia, Pa). The working casts were related to the temporomandibular joint and mounted on a nonarcon, semiadjustable articulator (Hanau). Noble alloy (Pd-Au) substructures for porcelain fused to metal maxillary and mandibular fixed prostheses were to be constructed using a lost-wax technique (Figure 5).
The centric relation and vertical dimension of occlusion parameters were transferred to the articulator from the patient using sectioned segments of the complete dentures (Figure 6). Records of the patient's protrusive and excursive mandibular movements were recorded and transferred to the articulator (Figure 7). Provisional acrylic fixed prostheses were constructed to confirm the function and patient comfort of the vertical dimension as well as the esthetics (Figures 8 and 9).
Maxillary and mandibular substructures were waxed, cast, fitted, and remounted to confirm the parameters of occlusion (Figure 10). Porcelain was formed and fired to the substructure to give an anterior guidance-type occlusion, where the anterior teeth bear the jaw forces and remove the posterior teeth from contact during chewing or grinding jaw movements (Figures 11 and 12).
The following is the occlusal scheme:
Splinting of posterior units
Mandibular flexure concerns
No cusps, rounded posterior mandibular teeth
Short lingual maxillary cusps
The final prostheses were fitted and the occlusion adjusted and cemented with zinc oxide and eugenol (Temerex, Dentsply) to provide an esthetic functional dentition (Figures 13 through 18).
The patient returns for routine cleaning and maintenance every 6 months.
Dental implants will bear axial forces well but may not tolerate off-axial loads. A tolerance of a maximum of 150 μm of micromovement of dental implants has been mentioned in the literature but has not been well studied and has not yet been correlated to a specific amount of force.4 Design of a tooth arrangement and shape for the purpose of load transfer to the implants in a specific direction and location is desirable (Figures 17 and 18). The axial loads may be delivered to the posterior implants, but the off-axial excursive forces should be delivered to the anterior implants, where the forces are not as great.5 Up to 500 N of force can be delivered by human jaws. The amount of off-axial forces tolerated by a particular implant in a particular density of bone has not been studied. An appropriate occlusal scheme to minimize off-axial forces to the prosthesis and the supporting implants is necessary.6 This can be accomplished by keeping the off-axial forces in the anterior segment. Here they are minimal. The forces should be distributed over as many supporting implants as possible. The forces of occlusion are lower where the tooth contact is the farthest away from the temporomandibular joint fulcrum. An occlusal design that allows direct axial forces and minimizes off-axial forces is appropriate. The shape of the mandibular tooth coronas should be flat and rounded convexly toward the gingival. The facial-lingual tooth dimension should be kept to a minimum. The lingual cusps of the maxillary prosthetic teeth should be relatively flat to occlude with the mandibular teeth.7 A porcelain, pointed tooth cusp that occludes with an opposing flat dentition may be prone to fracture. The facial maxillary cusps can be present but short and rounded to fulfill an esthetic requirement. However, it may be better for these facial cusps not to have contact with opposing teeth during excursive mandibular movements. A disclusion occurs by the contact of the anterior lateral and cuspid teeth with their antagonist lateral and cuspid. Patients may have differing angles of disclusion and may require the first premolar and cuspid to enact the disclusion, depending on the relation of the jaws and jaw-arch shape.
Bone loss may preclude the appropriate construction of a prosthesis. Bone augmentation for implant support or prosthetic flanges for support of the lips may be necessary.8 If the time between tooth loss and implant placement is reduced to minutes or days, there may be reduced bone loss, making prosthesis construction less complex. Traumatic injury may result in bone loss or irreversible damage that precludes easy prosthetic construction. Various corrective surgical procedures have been in use to augment or correct available bone to accept dental implants for support of a fixed or removable prosthesis. If there has been much bone loss over time, the support of the lips may have been compromised. A prosthesis that restores this lip support may be necessary. If the time of tooth extraction has been less than 1 year, there may not be sufficient bone loss to create an esthetic or functional problem. There can be as much as 30% alveolar bone loss 3 months after tooth extraction. A flanged prosthesis may be necessary to provide adequate lip support if this occurs. These prostheses may be fixed or removable, depending on its design or the ability of the patient to adequately keep it plaque free. A preoperative construction of a provisional appliance or guide can demonstrate the need for lip support of the final prosthesis. A patient with procumbent maxilla or bimaxillary protruding jaws may not require a flanged prosthesis (Figure 17).
The final prosthesis in this case was constructed of porcelain fused to metal. Keeping the cast—metal-frame substructure of the porcelain prosthesis to as few units as possible keeps the cast-metal processing warpage to a minimum. The metal must be thick enough to maintain rigidity for support of the porcelain and the porcelain must be thick enough to provide an esthetic result.9
Long-span metal castings can change dimension when cooled from the high heat of casting and porcelain bonding. This may alter the accuracy of the fit of the metal substructure. The fit of the metal substructure should be passive, that is, it should not bind or transmit wedging or compressive forces to the implant abutments and therefore to the supporting implants. With a shorter span length of the prosthesis, there is less change of the dimension of the substructure during the casting and the application of porcelain processes.
The mandible bends slightly at the inferior ramus toward the medial during wide opening as a result of the contraction of the medial pterygoid muscle. The splinted implant-prosthesis entity is rigid in the flexing mandible. The medial flexure has been measured to range between 0.049 and 0.137 mm.10 This flexure, though small, has been known to cause symptoms in patients with rigid, fixed appliances in the mandible. This can cause a stress area in the bone against the rigidly splinted implants and may lead to a loss of integration and failure of the implants and prosthesis. Some displacement of the implants beyond the mentioned 150 μm tolerance may occur and then cause a loss of integration by separation of the bone from the implant surface, microhemorrhage, and fibrous tissue in-growth. The splinted implant grouping should be such as to allow mandibular flexure without stressing the bone-implant interface. Usually the separation in the fixed prosthesis can be made at the canine or premolar region to allow for mandibular flexure without detriment to the bone-implant interface.11
Implants are best placed with 3 mm of separation, to allow blood supply to the gingiva and interimplant bone.12–15 In the anterior esthetic area, this may present a problem for the patient if there is a 1-for-1 implant-to-tooth ratio. This esthetic problem is what appears as black triangles formed by the embrasures of porcelain teeth and the gingival base. This may be resolved by omitting an implant at, for example, the lateral incisor site. The lateral incisor can be replaced with an ovate-type pontic, making the appropriate dimensions in the porcelain to allow an esthetic result. Additionally, in patients with a procumbent maxilla or a lip line that exposes the tooth embrasures, salivary bubbles may be produced from the triangles during certain word pronunciations. The size of the triangle created may allow the salivary surface tension to form a bubble-producing film. This usually cannot be corrected by augmenting the gingival papilla to fill the triangle. Porcelain can be added to the embrasure areas to minimize the size of the triangle, thus preventing the saliva from bubbling through. However, a larger triangle may allow easy cleansing in these areas that are not accessible to flossing but may allow food particles to catch in some areas. The use of water-irrigation devices is usually not recommended because the force of the water stream may tear the epithelial attachment at the implant percutaneous area. Simple tooth brushing is usually appropriate.
The patient's tolerance of the surgical placement of multiple implants is another consideration. Typically American Society of Anesthesiologists (ASA) Class I and II rated patients are acceptable candidates for implant placement. Sedation or general anesthesia may be a consideration.
Psychological factors may be considered. Some patients may object to the endosseous nature of the implants. Others have objected to the feel of the implants. Teeth have a periodontal ligament that allows the tooth to intrude about 150 to 250 μm. Integrated implants may intrude 2 to 7 μm, which may give them a rigid feeling.16 Although many dental implant patients claim to have a sensibility of the implants with time, they may be sensing neuronal perception from the gingiva or temporomandibular joint.
Once the patient has been deemed a candidate for implant treatment, the available bone should be evaluated. Radiographs that depict the osseous structures can be made. Periapical, panoramic, or CT scans are useful. Avoiding anatomical structures, such as the mandibular neurovascular bundle, is a goal. The maxillary sinus may have expanded by pneumatization due to edentulism, and the remaining alveolar bone may be inadequate for implant placement. This bone deficit may be corrected by an augmenting sinus lift from a lateral (Caldwell-Luc) or inferior (ridge crest) approach. A CT scan is very useful for determining available bone and for implant positioning and spacing. These are accurate to a tenth of a millimeter. Some companies can construct bone models, surgical guides, and provisional prostheses from the CT scan, making the surgical placement and prosthetic treatment less stressful and time consuming (Materialise).
The surgical placement of the implants can be parallel or diverging or tripodal. There has been discussion as to which arrangement is appropriate.17 Prosthetic complications may arise in a diverging or tripod arrangement.
There has been a dearth of totally artificial fixed-implant-supported dentition reports. Long-term results are not known. Based on the longevity of smaller prostheses, there can be an intuitive extrapolation that would indicate an excellent prognosis. This patient has been postoperative for over 1 year and is functioning well without sequellae.
This case report demonstrates that totally artificial, endosseous implant-supported, fixed dentition, in-office treatment is possible with only local anesthesia and mild sedation for pain and anxiety control.
Complete maxillary and mandibular fixed dentition supported by endosseous implants is possible, but distribution of occlusal forces is probably an important factor for long-term survival of the prostheses. Axial occlusal forces can be tolerated by the supporting implants, but off-axial forces should be transferred to the anterior region where the forces are not as great. Lateral excursion movements of the mandible should be borne by the implants in the anterior region of the jaws. Splinting of the supporting implants may be necessary to provide long-term resistance to off-axial occlusal forces. Implant positioning should be 3 mm apart, but esthetic demands may necessitate divergence from this dimension. Parallel or diverging implant positioning may be used in various osseous anatomies to provide an optimal or simplified prosthetic construction. Mandibular flexure should be considered in prosthetic design. Prosthetic material properties may require alterations in the design of the prostheses. Bone loss in the anterior maxilla and mandible may necessitate a prosthesis with a flange for esthetic and functional support of the lips. In-office surgical and prosthetic installation is possible with oral sedation techniques. Cleaning and maintenance of the prostheses should be ongoing, every 3 to 6 months, as with a natural dentition.
The author acknowledges the kind redaction of Danielle Green DMD and Martie Flanagan BSRN.
Dennis Flanagan, DDS, is in private practice. Address correspondence to Dr Flanagan at 1671 West Main Street, Willimantic, CT 06225 (firstname.lastname@example.org)