An accurate diagnosis and proper treatment planning makes implant treatment a long term success. Clinical examination and mounted cast models give us information about the maxillomandibular relationship, available space, occlusion, interarch distance and relative parallelism. With the help of radiographic examination the critical landmarks such as the mandibular canal, maxillary sinus and roots of adjacent teeth are identified. Based on all these findings a workable treatment plan is formulated for the patient which solves the chief complaint of the patient, provides a good functional and esthetic result and is expected to function for a long duration of time.
The general rules that are followed during implant placement are:
- The minimum distance of implant to adjacent tooth should be 1.5 mm to 2 mm.
- The minimum distance between two implants should be 3 mm to 4 mm.
- Buccolingually, implant should be placed at 2 mm to 3 mm distance from the cervical height of contour.
- Coronoapically, implant should be placed at 2.5 mm to 3 mm from the buccogingival margin.
- At least 7 mm of interocclusal/interarch space should be available from shoulder of the implant to the occlusal surface of the opposing tooth.
- A buffer zone of 2-3 mm from the inferior alveolar nerve or the floor of sinus from implant apex should be maintained.
General rules followed during implant placement
Before we go into the details of the treatment planning let us first discuss the types of fixed and removable prosthesis.
Types of implant supported prosthesis:
Depending on various factors discussed in â€œMaking a diagnosis in implantologyâ€, prosthesis is designed for the patient. The surgical part of implant therapy is comparatively smaller part of the treatment. It must be remembered that number and position of the implants will be determined by the type of prosthesis that the patient will be restored with. Misch 1 describes the prosthetic restorations into five types. He has divided the prosthetic restoration in two groups: permanent restorations and removable restorations. He described them as FP (Fixed prosthesis) and RP (Removable prosthesis) as described in following table,
Types of implant supported prosthesis
|FP-1||Permanent prosthesis, which replaces only the crown, it looks like a natural tooth.|
|FP-2||Permanent prosthesis, which replaces the crown and part of the root, a natural contour of the crown can be observed in the half-occlusal third, but it is seen elongated or hyper outlined in the cervical or gingival third.|
|FP- 3||Permanent prosthesis, which replaces the crown, tissue and lost bone, the prosthesis uses denture teeth and acrylic gum.|
|RP-4||Removable prosthesis, overdenture fully supported with implants.|
|RP-5||Removable prosthesis, overdenture supported by soft tissue and implants.|
FP-1 fixed restoration is the most favourable replacement of tooth by implant form functional as well as esthetic point of view. Especially in the maxillary anterior area where the esthetic is a prime concern FP-1 fixed restoration provides very good results. In this situation the remaining alveolar bone is sufficient to allow the placement of implant in almost the same position as that occupied by the tooth.
Some problems associated with the anterior maxilla are width of crest is less as well as the interdental papillae are lost after extraction. To re-achieve the exact shape and contour bone augmentation and papilla reconstruction may be required.
In this case fixed restoration replaces the crown as well as portion of root of natural tooth. Usually sites of extraction showing a moderate bone loss as well as soft tissue loss can be restored by this kind of restoration. The incisal portion of the crown is in correct position but the cervical portion is overextended apically and lingually to the original tooth. It is done in cases with a high lip line where the cervical portion of the teeth is not displayed during smiling or speaking. The patient should be clearly told about this before going for implant placement.
Diagrammatic representation of FP-1, FP-2 and FP-3 type of prosthesis
Advanced bone and soft tissue loss creates both functional as well as esthetic concerns. In this kind of restoration the prosthesis replaces both the tooth/teeth as well as the soft tissue structures. It is also indicated in those areas where the patient has high maxillary lip line during smiling or a low mandibular lip line during speaking and FP-2 prosthesis has esthetic concerns. A metal porcelain structure is made with gingival toned porcelain used in the gingival portion. Metal framework can also be used as superstructure for placing denture teeth and acrylic. This kind of prosthesis is usually called as hybrid prosthesis. This prosthesis is complicated to fabricate especially in edentulous cases where 6-10 implant supported prosthesis has to be given. Most important is the accuracy of fit of the metal framework on the implants. If there is improper fit, it may cause undesirable distribution of occlusal forces as well as lateral forces on some implants causing a mechanical failure. Other problem during fabrication metal ceramic prosthesis is increased risk of porosity and fracture. Because of its large framework more heat is accumulated and during the cooling cycles there are chances of porcelain fracture. The hybrid prosthesis is easy to fabricate as compared to metal ceramic prosthesis. It has other advantages also like less weight, ability to be repaired easily and less cost. But, in cases where hybrid prosthesis is opposing natural teeth there are more chances of fracture of denture teeth or acrylic due to high occlusal forces.
This type of removable prosthesis is totally supported by implants and/or teeth. In most commonly used design the implants are splinted by a metal bar or connector. The denture contains the attachment for this bar on its tissue surface. The prosthesis when inserted is rigid and totally rests on the metal framework. The placement of implant should be conducive for the prosthesis. In this case usually the implants are placed more lingually and apically as compared to FP-1 and FP-2. In mandible usually 5 implants are placed in the intermental region and in maxilla 6 implants are placed upto the anterio-inferior border of the sinus to support this kind of prosthesis.
It is implant as well as tissue supported prosthesis. It is usually planned in cases of advanced ridge resorption where totally implant supported prosthesis cannot be given. In mandible two or three implants are placed in the intermental region. These implants may or may not be connected to each other. The ball abutment attachment has been used widely in such cases. The problem with this attachment is that it becomes loose with due course of time. A new system known as â€œLocator attachmentâ€ has been introduced to overcome this problem. The details of this attachment are available in â€œDental implant components and Current concepts in implant designâ€ and â€œRehabilitation edentulous patients with implant therapyâ€.
Determination of number and positions of implants:
In case of missing single tooth or few teeth, the number and positions of implants can be easily determined. As a general rule for all partially edentulous situations in excess of the replacement of a single tooth, the minimum number of implants used should be two. Maximum number of implants to be used depend upon number of teeth to be replaced. Usually one implant should replace one tooth. As described above most of such the case require FP-1 or FP-2 restoration. Some cases with advanced bone and soft tissue loss may require FP-3 prosthesis. How to determine number and positions of the implants has been discussed in detail in clinical examination and Study of the cast models in â€œMaking a diagnosis in implantologyâ€.
It is comparatively easy to restore partially edentulous maxilla or mandible but, when full edentulous maxilla and/or mandible is rehabilitated with implants, we must take into account several factors which include:
- Anatomy of the residual bone
- Quantity and quality of the residual bone
- Type of prosthesis
- Number of implants
- Occlusal forces
- Inter dental arch relationship
Treatment planning for fully edentulous maxilla:
The amount of bone resorption in completely edentulous maxilla plays an important role in choosing the type of prosthesis. As the resorption of the residual alveolar ridge advances, the distance between the opposite residual ridges increases. When a patient presents a distance greater than 15 mm, the most indicated prosthesis is a removable type (overdenture), as we are able to compensate for the lost tissues using acrylic. If in such a case permanent restorations of metal porcelain type is used, it can result in the production of elongated teeth, which give an un-esthetic appearance.
In such cases the FP-3 type of prosthesis is designed. Here, hybrid prosthesis using an over contoured metal structure with acrylic and conventional denture teeth are fabricated. The crown, tissue and lost bone are replaced by uses denture teeth and acrylic gum. To effectively distribute the occlusal forces bilaterally, 6-10 implants are distributed according to the shape of ridge.
Misch 1 has elaborated the distribution of implant on a completely edentulous maxilla according to the shape of the residual alveolar ridge. The maxillary arch can be square, ovoid or conical in shape.
Square shaped arch:
In this shape the central and lateral incisors do not present a marked facial cantilever, compared to canines. Less occlusal forces are distributed in the anterior region and implants at canine region can well distribute the forces. So, Misch indicates a minimum of two implants in the canine position.
Implant placement for square arch form
Ovoid shaped arch:
In case of ovoid arch more forces are distributed in the anterior maxilla. A minimum of three implants in the premaxilla, two in position of the canines and one in position of central or lateral incisors is indicated here.
Implant placement for ovoid arch form
Conical shaped arch:
This is the most challenging situation because high occlusal force distribution in the anterior part of maxilla. In this case, a minimum of four implants in the front section are indicated, two in canine position and two in position of central incisors.
Implant placement for conical arch form
Treatment planning for fully edentulous mandible:
As discussed in case of maxilla mandible also has three types of arch forms:
- Square shaped arch
- Ovoid shaped arch
- Conical shaped arch
Usually in case of advanced resorption RP-4 and RP-5 prosthesis are designed. If implants can be placed in posterior areas of the mandible, FP-3 prosthesis can also be designed for the patient. In case of removable prosthesis most important measurement is the anteroposterior spread of the implants. As a general rule, when 5 implants are placed in the anterior mandible between the foramina, the cantilever should not exceed 2.5 times the anteroposterior (A-P) spread with all other force factors being low 2. This anteroposterior spread differs in different arch forms. The square arch form provides the shortest anteroposterior spread, with an anteroposterior dimension often of 2 to 5 mm. Conversely, the tapering arch form will result in the largest anteroposterior spread, with an anteroposterior distance greater than 8 mm. The ovoid, which is the most common, will often have an anteroposterior distance of 6 to 8 mm 1.
Factors to be considered during formulation of treatment plan:
Decisions have to be made regarding treatment plan according to the patientâ€™s clinical presentation, to deliver a prosthesis which is expected to last long and fulfils patientâ€™s desires. Patient may be partially or fully edentulous. In both the cases the basic principles of implant therapy have to be followed. Following is the description of various factors which have to be considered during making of a treatment plan,
To retain or extract a natural tooth:
On examination the teeth are examined for their health status. According to Micsh 2 if a natural tooth has prognosis for more than ten years, it is included in the treatment plan and if it is in the range of 5-10 years, an independent implant supported prosthesis is advised. If the prognosis of the tooth is < 5 years, extraction of the tooth and bone graft placement is advised so that implant placement after healing can be done. A tooth with good periodontal support can be utilized as an abutment for tooth and implant supported prosthesis. Teeth with poor long term prognosis such as repeated failures with endodontic treatment, root canal anatomy does not allow the tooth to be restored by post and core treatment, tooth associated with peri-apical pathology can be considered for extraction.
Immediate implant placement:
Implant can be immediately placed in an extraction socket. Several studies have documented the high success rate of immediate implant placement 3, 4. However, in posterior maxilla immediate implant placement is frequently complicated by the absence of adequate quality and quantity of the bone and presence of maxillary sinus. Augmentation of sinus floor is indicated to achieve adequate volume of the bone for proper implant stabilization.
Immediate implant placement in the maxillary and mandibular anterior region has esthetic concerns. New implant materials and designs have helped us in achieving this goal, but an accurate surgical procedure as well as implant placement is must to achieve desired results. Detailed description on this topic is available in â€œImmediate implant treatmentâ€.
Number and location of missing teeth:
The number and location of missing tooth/teeth play an important role in determination of treatment plan. It generally accepted that implant therapy in esthetic zone is a challenging task. A lot of factors have to be considered while placing implant in esthetic zone which include volume of the bone available, soft tissue profile, angulation of implant for best esthetic results, and buccolingual placement of implant.
Implant placement in maxillary and mandibular posterior areas may be challenging when adequate bone is not present. In such cases bone augmentation is required to provide adequate support to implant. Maxillary bone is porous as compared to mandibular bone because of which achieving primary stability is difficult especially when bone available is minimal. Maxillary sinus uplift procedure is carried out to increase the volume of bone to achieve adequate bone support for the implants.
Implant only supported or tooth and implant supported prosthesis:
It was believed historically that if a tooth and an implant were used as abutments in the same prosthesis, the implant would be subjected to an increased bending moment because of differences in their mobility patterns. Because of these forces the success rate for a tooth-implant supported prosthesis (TISP) was proposed to be less compared with implant-only supported prosthesis (ISP) 5-9. In patients with para-functional habits like bruxism occlusal forces are even more deleterious to the implant. But there are multiple benefits of using tooth with implant to support prosthesis. These include,
- Splinting teeth to implants provides more treatment options especially, when anatomic limitations restrict insertion of additional implants (eg, maxillary sinus, mental foramen), when the bone is lacking for implant placement and when patient refusal to undergo a bone augmentation procedure.
- Teeth provide proprioception so a natural tooth like perception can be enhanced
- Additional support for the total load on the dentition.
- Reduction of the number of implant abutments needed for a restoration and hence reduced cost.
- A need for cantilever may be avoided by using tooth as a support.
Because of the bio-mechanical problems associated with tooth-implant supported prosthesis, many authors have advocated the use of nonrigid connector or telescopic crowns to reduce the bending moments on the implant 10-18. But, problem associated with this treatment is the intrusion of tooth. Surveys indicated intrusion occurred, on average, in 3% to 5.2% of the cases 19, 20. Other option proposed is the use of conventional fixed partial denture.
In spite of all the complexities involved, the tooth-implant supported prosthesis can be used if the following factors are kept in mind before planning the treatment,
- Only those teeth which have good periodontal health and dense bone support should be used.
- Rigid connectors should be used most preferably in one piece casting.
- The parallelism between the implant and the tooth is important for efficiently distribute the occlusal forces.
- Permanent cementation should be used.
- Only short span bridge should be planned whenever possible.
- This kind of prosthesis should be avoided in patients with para-functional habits.
- Patients with high caries index are not good candidates for this treatment.
- Occlusal harmony with the opposite arch is must and there should be an efficient load distribution in the opposite arch.
- Endodontically treated teeth with insufficient occlusal structure or teeth with un-satisfactory treatment should not be included in the treatment plan.
Visibility of the residual ridge crest:
Visibility of the residual ridge crest is usually associated with amount of bone loss. The visibility is more common with minimal bone loss. In advanced bone loss (division C and division D), the residual ridge is not usually visible and the treatment planning is changed accordingly. In case of minimal bone resorption there is tooth-only defect and a metal ceramic prosthesis may be designed. But, in case of advanced bone loss there is composite defect which requires replacement of tooth as well as soft tissue. The detailed description of type of prosthesis has been discussed later.
Lack of vertical dimensions of occlusion (VDO):
Due to loss of posterior teeth the posterior stops are lost which can lead to bite collapse. In such situations, the vertical dimension at rest position is measured and interim prosthesis is given to open the bite in harmony with the temporomandibular joint functions. This situation is further complicated by the supraeruption of the teeth in the opposite arch which further reduces the interarch distance. If after opening the bite to its desired vertical dimensions, interarch space is still insufficient to place implants; intentional root canal treatment of supraerupted tooth/teeth and position of crown/bridge in desired vertical position is recommended.
Planning for implants becomes further complicated by presence of conditions like combination syndrome. Combination syndrome is condition in which edentulous maxilla is opposed by natural mandibular anterior teeth, presenting with loss of bone from the anterior portion of the maxillary ridge, overgrowth of the tuberosities, papillary hyperplasia of the hard palateâ€™s mucosa, extrusion of the lower anterior teeth, and loss of alveolar bone and ridge height beneath the mandibular removable dental prosthesis basesâ€”also called anterior hyperfunction syndrome 21. The condition requires surgical intervention to correct the maxillary tuberosity by bone resection as well as endodontic and prosthetic intervention to modify the mandibular teeth to make them conducive for implant placement.
After doing all these procedures if the required interarch space is not achieved, the soft tissue reduction or bone reduction of the ridge can be performed most preferably at the time of implant placement to reduce number of surgeries.
Planning for cantilever:
As already explained, when planning a treatment for edentulous mandible, cantilever prosthesis can be designed depending upon the total implant support as well as the shape of the ridge. Long-term clinical studies have demonstrated that implant-supported full-arch reconstructions with bilateral cantilevers in the mandible exhibited high survival rates 22-24. However, it has been suggested that certain cantilever lengths may decrease the survival of the prostheses 25.
Conical/tapered shape of the ridge is more suitable for distal cantilever as the anterior implants are able to offset the distal cantilever force. The anteroposterior length of the cantilever can be determined by calculating the distance from the center of the most anterior implant to the line joining the distal aspects of most distal implants. The length of the cantilever should not be more that two and one half times of the measured anteroposterior distance between the anterior and most posterior implants.
Overloading of the implants should be avoided as it can be attributed to clinical complications such as screw loosening or fracture, fracture of the veneering material, prosthesis and implant fracture as well as implant failure 26-29.
Use of surgical guide during implant placement:
The fabrication of a surgical guide during implant placement is a very important step in implant therapy. The diagnostic wax-up provides us the information regarding the placement of teeth and hence the implants. This information has to be transferred translated via some form of guide to the surgeon during the surgical phase of treatment. For this purpose a surgical guide is used which fabricated using information obtained from all diagnostic procedures previously performed 30-31. With the help of surgical template, not only position of fixture/fixtures but also their correct angulation is achieved.
Indications for surgical template:
- Partially edentulous cases where the alignment of the implants is difficult.
- Full mouth rehabilitation cases where multiple implants are planned.
- Implant placement in areas where important anatomical structures (e.g. maxillary sinus, inferior alveolar nerve) play an important role.
- Used as a carrier when diagnostic markers such as metallic balls have to be placed in mouth during radiographic procedures to identify the potential implant sites.
There are two types of surgical templates: Same-arch where the template is constructed in the same arch in which fixtures are to be placed and opposing-arch where the template is constructed in the arch opposite the one in which fixtures are to be placed.
Procedure of making surgical template for the same arch:
After the impressions of the maxillary and mandibular arches are made, the diagnostic casts are obtained and articulated on an articulator. After articulation diagnostic wax-up is done and again an impression is made of complete arch. The impression is poured and again a cast is obtained with all the teeth in their proposed positions. Using cold cured or heat cured resin a template is fabricated on the cast. The template is relocated on the original cast and drill holes are prepared through the cingula of the anterior teeth or the occlusal surfaces of posterior teeth. These holes are made according to the diameter of the drills used, so that these allow their passage during the surgery.
Procedure of making surgical template for the opposite arch:
The initial procedure upto the articulation of diagnostic casts is same for both procedures. After articulation the surgical template is fabricated on the opposite arch to that planned for implant placement. The template is constructed using heat-curing acrylic resin or vacuum-molded acrylic resin. The position and angulation for the fixture (ie, through cingula or occlusal surface) is determined. Now, an orthodontic wire is taken and is attached to the template by means of acrylic resin directed in such a way that the opposite end of the wire points at the position of the fixture in the arch where implant placement has to be done. For this, the tooth where the implant has to be placed is removed so that the wire touches the ridge of the arch. During the fabrication of template the inter-arch distance with the wire should be sufficient to accommodate the hand-piece with drill during surgical procedure.
An accurate treatment planning is the corner stone of successful implant therapy. All the above factors play an important role during decision making regarding size, position and angulation of the implants. It should be remembered that the final prosthesis plays a vital role in determination of implant positions. So, a prosthetic wax-up is very useful in treatment planning for an implant case. Weather to retain a natural tooth or to extract it, to give tooth and implant supported prosthesis, to give cantilever etc. have to be considered during decision making.