not require sacrificing the crowns of neighboring teeth for its anchorage. This use most closely approaches the dream of "re-placing a tooth with a tooth", a misleading ambition for several reasons extensively explained elsewhere.

The other use is to share and balance occlusal stresses on a multi-unit fixed prosthesis with other weaker abutments. Because dental disease leading to tooth loss usually affects more than one tooth, this latter use is by far the more common.

The criteria for both uses differ slightly, but both require that existing periodontal conditions around neighboring teeth be cured or stabilized before implantation.

When a single-unit fixed restoration is desired, the teeth bordering the span must be healthy and firmly supported by bone (3). These teeth will be included with some form of stabilizing device for the implant, such as lingual extension rests (4) from the single crown or splinting. If the teeth have poor bone support (5), implantation is not necessarily contraindicated, but the single-unit fixed restoration is. The implant can be used to stabilize the loose teeth by providing more support in a multi-unit fixed bridge (6).

Esthetics also influence whether or not a single-unit restoration is advisable. It is more difficult to fashion a complimentary restoration for a single central incisor than to pleasingly restore both central incisors simultaneously—with one crown supported by an implant (7), and the other by the prepared remaining central incisor (8). A lateral incisor, with its bilateral counterpart three teeth away, poses fewer matching problems and would more easily give satisfactory results.

If a large diastema (9) existed before tooth loss, a single-unit restoration filling the gap would be awkward and unnatural looking. If the restoration were small and maintained the diastema, it would be very difficult to stabilize. It would be better to use the implant to anchor a two or three-unit fixed bridge, with two crowns closing the diastema (10).

Bone in a state of flux is also an unwise implant site for a single-tooth restoration. Lateral extension rests from the single-unit or splinting do not provide as much stability as does inclusion in a multi-unit restoration. Therefore, if a single-unit restoration is contemplated in an extraction site, implantation should be postponed at lease three months to allow firm healing of the socket. Space should be maintained with a conventional appliance to prevent drifting or tipping. However, in those situations where the width of the single tooth implant can engage good bone

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