The treatment planning for implant dentistry is primarily driven by the existing bone volume in the edentulous sites. As a result, longer and narrow implants are selected for anterior implants or shorter and wider implants were placed in the posterior region of the mouth. The second guideline for implant treatment planning has developed based on esthetics. In this scheme, implant positions were primarily controlled by the teeth being replaced.

The primary causes of complications in the implant dentistry are related to biomechanics. The 3rd rule for implant treatment planning is determined by occlusion. SLOCK® implant system has developed a treatment plan sequence to decrease the risk of biomechanical overload, consisting following:

1. Prosthetic guided surgical treatment.
2. Patient force factors.
3. Available bone in the edentulous sites.
4. Implant size.
5. Bone density in the edentulous sites.
6. Key implant positions and number.
7. Implant design.

As a general rule, implant-supported prostheses independent from natural adjacent teeth are designed whenever possible. Implant abutments may be splinted, and unsplinted natural tooth crown. Independent implant prostheses may also reduce or eliminate pontics, while simultaneously increasing the number of abutment supported by fixture and distributing forces more effectively. The increase in fixture number decreases the risk of complications and exhibit greater long term success rates of the prosthesis and greater survival rates of the adjacent teeth. In addition, the distribution of occlusal forces is optimized when implant treatment planning is established with implant selections related to key positions and number.

Key Implant Positions
The most critical factor of implant surgical phase is the position of fixture placement. There are 4 general guidelines to determine key implant positions.

1. Arch dynamics
2. Canine-molar rule
3. No cantilevers
4. No two adjacent pontics

Key Arch Positions and Geometrically Stable Form
An arch may be divided into three segments, similar to a triangle and square. The anterior teeth are one triangle or semicircle segment, the canines are independent pillar of the anterior segment, and the two central and two lateral incisors are side walls or parts of circle. The premolars and molars on each side form a square segment. In other words, geometrically stable form has inherent biomechanical advantage by itself. Therefore, when two or more segments of an arch are connected, the biomechanical effect is greater to lateral force, and, as a benefit, an stable arch dynamics is created from the most distal terminal abutments to the most anterior abutments.
When multiple adjacent missing teeth extend beyond one of the geometric segments, a key implant position needs to be situated within each segment. Therefore if the patient is edentulous from first premolar to first premolar, the key implant positions include the terminal abutments (the two first premolars), the two canines, and either of the central incisor positions. These implant positions follow the rules of: (1)no cantilever, (2)no two adjacent pontics, (3)the canine position, and (4)at least one implant in each edentulous segment of an arch.

A fixed restoration replacing a canine is greater risk than nearly any other restoration in the mouth. The maxillary or mandibular adjacent incisor is one of the weakest teeth in the mouth, and the first premolar is often one of the weakest posterior teeth. Therefore if a patient desires a fixed prosthesis, implants are required whenever the adjacent teeth are missing in either arch, the key implant position is the canine and 2nd key implant position is the premolar. These positions result in an anterior cantilever to replace the lateral incisor. However, because the lateral incisor is the smallest tooth in the arch and in the anterior region has the least bite force, the cantilever is of limited negative impact. And the canine implant is usually larger than a lateral incisor implant for the esthetic requirements of the restoration. This further reduces effect of the cantilever. In addition, the occlusion is modified so that slight occlusal contact is present on the lateral incisor pontic in the centric occlusion and excursions of the mandible. When there are multiple missing teeth and the canine edentulous site is a pier abutment position, the canine position is a key implant position to help disoccluding the posterior teeth in mandibular excursions. As a result, when four or more adjacent teeth are missing, including a canine and at least one adjacent posterior premolar tooth, the key implant positions are the terminal abutments, the canine position, and additional pier abutments, which limit the pontics spans to no more than two teeth.
The first molar is also a key implant position when three adjacent posterior teeth are missing. The bite force doubles in the molar position compared with the premolar position in both the maxilla and mandible. In addition, the edentulous span of a missing first molar is 10 to 12mm, compared with a 7-mm span for a premolar. As a result, when three or more adjacent teeth are missing, including a first molar, the key implant positions include the terminal abutments and the first molar position. For example, in a patient missing the second premolar, first molar, and second molars, three key implant positions are needed to restore the full contour of the missing molars teeth: the second premolar and second molar terminal abutments and the first molar pier abutment. When one implant replaces a molar (for a span of less than 13mm), the implant should be at least 5 mm in diameter. When a smaller-diameter implant is selected, the molar may be considered the size of two premolars.

No cantilevers
The first rule for ideal key implant positions is that no cantilever should be designed in the prosthesis. Cantilevers are force magnifiers to the implant, abutment screws, cement, and implant-bone interface. Cantilevers on the fixed prostheses by implant have higher complication rate than abutments with terminal implants. Therefore the ideal key implant positions include the terminal abutment positions when adjacent teeth are missing.
The length of the cantilever is directly related to the amount of the additional force placed on the abutment of the prosthesis. When the cantilever length is increased, the moment force on the abutment is increased. As a result, any part of implant system is at an increased risk of biomechanical failure (e.g., porcelain fracture, uncemented prosthesis, abutment screw loosening, crestal bone loss, implant component or body fracture, implant failure). To enforce the rule of no cantilever, the key implant positions indicate one implant per missing tooth. The ideal treatment plan should eliminate cantilevers. When a three- to four teeth are missing, the key implant positions include the two terminal abutments, one on each end of the prosthesis. However, in some clinical conditions, a cantilever is the most prudent treatment option. For example, in an edentulous mandible, available bone in the posterior regions may be sufficient for implant placement, without advanced procedures (e.g., nerve repositioning, iliac crest bone graft). An alternative treatment plan may be to cantilever pontics from anterior implants.
The fact that, on occasion, a cantilever may be acceptable when force factors are low and bone density is favorable does, 2014.06.02 오후 3:41:13 | 5040 hit(s) 0 comment(s)
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