r fatty narrow compartments. Consequently, the number and position of fixtures cannot be finally decided upon until the perioperative period. Within the jaws it is generally advisable to start drilling for fixture sites close to the midline and then prepare the next ones as far posteriorly as possible, because the central and posterior sites strongly influence the out - come of the entire treatment. Only then can decisions be made on any interpositional fixtures to be placed.
In the totally edentulous mandible, placement of fixtures from one molar area to the other (if
the position of the mandibular canal allows) is not a recommended procedure. The mandible flexes somewhat during chewing, and rigid connection of such fixtures to a stiff bridge may cause microfractures in the perifixtural bone during mandibular flexing.
For oral purposes, one fixture can carry one crown only, two fixtures provide minimal support for a bridge in partial edentulism, and four fixtures are the minimum for a full - arch bridge, provided they are spaced well apart along a curve. Unpublished data from the Goteborg team showed no significant differences in 5 - to 12 - year survival rates for maxillary and mandibular bridges supported by four or six fixtures.
Inclination of fixture sites
The inclinations of the fixture sites depend on:
l. Local bone anatomy. The dominant principle is still that the fixtures should be totally embedded in bone. This may call for lingual or buccal positioning or a tilting of a fixture site to avoid concavities in the bone. If no bone grafts are placed into the floor of the maxillary sinus, distal tilting of the marginal parts of fixture sites may also be needed in the first premolar region to avoid penetration into the maxillary sinus. Moreover, such an inclination frequently allows advantage to be taken of the canine eminence.
2. Jaw relationships. Unless orthognathic surgery is performed before or possibly in combination with bone grafting at fixture installation, pseudoprognathism due to resorption generally calls for buccal inclination of maxillary and lingual tilting of mandibular fixture sites in total edentulism.
3. Design of the suptastructure. With proper inclination of fixture sites, penetration of bridge screw canals through buccal facings can be avoided. An overly palatal inclination may, on the other hand, result in a bulky bridge that interferes with phonation.
4. Desire for parallelism. If parallel fixture sites are prepared, the construction of the suprastructure may be facilitated. This aspect, however takes last priority.
Lengths of fixtures
The lengths of the fixtures should be determined only after all "high-speed" drilling has been finished. In particular, marginal countersinking may reduce the depth of a fixture site, then ftxtures shorter than originally anticipated must be chosen. The depth of a fixture site should be measured with a graded (ball-point) explorer to the lowest marginal bone edge.
Major extrabony protrusions of the apical parts of fixtures (eg, into the maxillary sinus or the nasal cavities) are not justified. Experience with surgical displacement of the inferior alveolar nerve to gai
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