The expansion procedure presents an additional bicortical anchorage in the oro-vestibular direction. In hard bone, this might be a disadvantage and lead to asymmetrical expansion. Manufacturers’ recommendations for hard bone should be strongly considered.

Regarding resonance frequency analysis, the values are related to bone quality and quantity as well as the exposed implant height above the alveolar crest, which depends on the type of implant and insertion technique. Our results (primary stability in the maxilla 66.9 ± 8.9 ISQ units and in the mandible 72.5 ± 11.1 ISQ units; secondary stability in the maxilla 66.4 ± 10.0 ISQ units and in the mandible 73.0 ± 9.7 ISQ units) are comparable with the results from Becker and co-authors (standard-length implants): primary stability 72.1 ISQ units and secondary stability 72.6 ISQ units. These values are marginally lower than those of short implants inserted only in the posterior mandible (79.0 ISQ units). Other authors measured in the posterior maxilla 68.2 ISQ units (6-mm implants). Altogether, our mean results (Fig. 2a,b) represent high stability values. Huré and co-authors measured in their animal study the following stability values (expandable implant of ≥ 10-mm length): for primary stability, 53.6 ± 3.0 ISQ units, and for secondary stability (3 months after insertion), 59 ± 4.5 ISQ units. The evaluation of stability values during the osseointegration period was not possible in our trial due to submerged healing. The question, whether the level of implant stability achieved at insertion can be maintained during the early healing period, remains. This should be analysed separately for all bone types in front of the known lowest stability values at 3–4 weeks after placement for all bone types and the recent attempts of immediate or early (6 weeks) functional loading of other short implant systems.