— a feat requiring a great deal of skill and ingenuity on the part of the operator.

1. A subperiosteal implant must always be seated on cortical bone, the denser the better. After tooth loss, the occlusal surface of the posterior maxillary quadrant is almost always characterized by ex-posed cancellous bone. Although the bone flanking the sinus is compact, it is typically eggshell-thin when the sinus is large. Posteriorly, the hamular notch area and the palatal side of the alveolar crest are composed of dense compact bone that bears up well under surface pressure. From canine pillar to canine pillar the bone is suitable for a subperiosteal implant. Premaxillary palatal bone and labial bone up to the anterior nasal spine are as dense as most areas of the mandible, except the mandibular symphysis.

2. A subperiosteal implant is always individually designed, never prefabricated! It must be designed on a stone model cast from a direct bone impression. All areas that might be included in the implant must be exposed for the impression by incising and retracting their overlying soft tissues well away from the sites.

3. The bars or struts of a subperiosteal implant must wrap around or lodge against distinct morphological features for adequate bracing against lateral forces.

4. A subperiosteal implant must be planned only for those areas where the overlying tissues are firmly attached to bone. Tight re-sealing of the tissues to bone is essential to bind the implant to the site.

5. The implant must be light-weight. Economy in design is essential. The larger and heavier the implant, the more it is affected by gravity — a particularly important consideration in the maxillae.

6. The bars or struts should either be narrow or fenestrated to minimally interrupt living tissues and permit short connective tissue strands to extend almost directly from the mucoperiosteum to the bone. Narrowness and fenestration also lighten the implant.

The endosteal and subperiosteal implants may be combined in any way. A successful fixed prosthesis should be anchored firmly anteriorly and posteriorly on both sides of the arch. The type of anchor matters less than its ability to withstand vertical and lateral forces, and this ability derives from the implant's appropriateness to the site. Thus it is not unusual, particularly in the maxillae, to see endosteal bladevents combined with one or more forms of subperiosteal implants, or both implant types used with prepared natural teeth.

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