Sometimes, preoperative planning in dental implantology, based on sufficient alveolar height, cannot be verified due to transversal deficiencies. A total of 102 bony mandibles and 95 maxillae were analysed after classification of atrophy, simulating implant insertion at 518 standardised edentulous cross sections with regard to anterior/posterior, mandible/maxilla and class of atrophy. Furthermore, the relation of alveolar height to possible implant length in 86 patients was evaluated retrospectively. Implant length reduction compared with alveolar height was necessary in 10% (mandibles) and 7.5% (maxillae) of the bony-jaw sections and 52.5% (mandibles) and 41.5% (maxillae) of the patients' implant regions. In this respect, the class of atrophy of the bony jaws was more important than the region of simulation. However, the highest differences were observed between mandibles and maxillae, both clinically and experimentally: simulation was possible in all mandibles and 42.5% of the maxillae, but clinical implantation was only possible in 86% of 62 mandibles and 0% of 24 maxillae, mostly due to reduced alveolar height. Alveolar ridge width primarily affected the possible implant lengths. Nevertheless, in four (two mandibular and two maxillary cases) of 58 patients (7%) with sufficient height, a surgical procedure that had already been started had to be stopped. It is expected that cross-sectional radiographical techniques of implantation planning, including ridge-width determination, will gain importance in the future.