Purpose: To conduct a retrospective study on the marginal bone level (MBL) of reduced diameter implants (RDIs) to analyze them in the context of various surgical and prosthetic treatment strategies using heterogeneous data from a private practice. Materials and Methods: A total of 123 patients were treated with 326 implants. Of those implants, 247 of them were RDIs, and the remaining 79 implants were standard-diameter implants (SDIs) as patient-related controls. The mean observation time was 24.4 months, and the maximum observation time 76.0 months. The peri-implant bone level of the implants was evaluated, while considering the diameter, time of implant placement, time of loading, extent of augmentation, and localization of the implants. The data were evaluated after restructuring using a mixed model analysis. Results: No significant difference was found between the use of RDIs or SDIs in the analyzed indications. Furthermore, no significant difference was found for the implant placement time, loading time, or the use of two-stage augmentations regarding the stability of the peri-implant bone level. Conclusions: Reduced-diameter implants are a sufficient treatment option in horizontally deficient bone conditions. The use of RDIs in the posterior region shows promising results; 3.5-mm- diameter implants may be indicated considering the individual patient situation. The use of a mixed model analysis for the evaluation of heterogeneous practice data can lead to a significant increase in the number of retrospective studies and data integration from practices, forming a sound basis for evidence-based dentistry.