Purpose: Inadequate bone width in atrophic edentulous jaws is a challenge for successful oral rehabilitation with endosseous dental implants. The aim of this clinical study was to evaluate the effectiveness of a new method for ridge expansion with sagittal splitting using a new surgical device (Crest-Control Bone Splitting System, Meisinger) and to determine whether it is necessary to fill the expansion area with bone substitute to maintain the expanded bone volume. Materials and Methods: During a 3-year period, a prospective study was performed in 32 patients (59% women, 41% men). All participants needed implants in the horizontally atrophied edentulous mandible and were treated in a private practice and a hospital. The only inclusion criteria were a mandibular ridge width between 3 and 4 mm and ridge height of at least 11 mm. Expansion with horizontal splitting of the ridge was performed simultaneously with implant placement. In 17 of the 32 arches, selected alternately, the expanded ridges were filled with a biphasic calcium phosphate (hydroxyapatite 60% and beta-tricalcium phosphate 40%) synthetic bone substitute (SBS 60/40). The other 15 expanded arches were left unfilled. All areas were covered with a resorbable collagen membrane (Bio-Gide, Geistlich). Results were analyzed with the Mann-Whitney and Kruskal-Wallis tests (α = .05). Results: There was a significant difference (α = .02) between the patients who received SBS 60/40 (17 cases) and those who did not (15 cases). The ridges that received SBS 60/40 after expansion showed no bone resorption. Conclusion: Horizontal expansion of the ridge is easily reproducible. In this study, in very narrow ridges, a lack of bone substitute resulted in significant resorption of 3- to 4-mm-wide crests (5%), even after expansion. A bone substitute should be placed to maintain the alveolar bone walls after expansion. Keywords: bone expansion, bone grafting, narrow ridge, synthetic bone