Purpose: To determine the effect of crown height space, crown-to-implant ratio, and offset placement of a prosthesis on implant survival, crestal bone loss, and prosthetic complications; and to determine whether detrimental values for crown height space and crown-to-implant ratio exist for implant-supported restorations. Materials and Methods: Extra-short implants (length ≤ 6.5 mm) supporting a fixed denture in the posterior mandible and followed for at least 12 months were analyzed. Radiographic and clinical examinations were conducted to retrieve data about patients' dental and medical history, prosthetic complications, antagonist type, crown height space, crown-to-implant ratio, offset placement of the prosthesis, crestal bone loss, and implant failure. Results: Thirty-four patients (mean age, 60 ± 10 years) with 45 extra-short implants participated in this study. Patients were followed for up to 4 years (mean, 2 years) and no implants were lost. The mean crown-to-implant ratio was 2.4 (range, 1.5 to 3.69). Mean crown height space was 17.05 ± 3.05 mm, and 65.4% of the implants had a crown height space in the range of 15 to 20 mm. About 90% of the implants had a distal or mesial offset placement greater than 1 mm. The type of antagonist significantly affected marginal bone loss around extra-short implants: bone loss was greatest for implants opposing a partial denture (mean, 1.28 ± 1.09 mm) and was lower for implants opposing a natural dentition (mean, 0.73 ± 0.60 mm) or a complete denture (mean, 0.89 ± 0.60 mm). Analysis of marginal bone loss and the factors crown-to-implant ratio, crown height space, and offset placement according to antagonist dentition indicated a significant positive correlation only between bone loss and crown height space. Conclusions: When an increased crown-to-implant ratio is present, crown height space may influence crestal bone loss more significantly. Keywords: biomechanics, bone loss, crown height space, crown-to-implant ratio, short implants