Regarding the width of keratinized mucosa, many studies and a review have indicated that the presence of a sufficient width of keratinized mucosa is necessary for maintaining healthy peri-implants [26,27,28,29]. In the present study, when 2 mm of keratinized mucosa was used as the adequate width, the p value was 0.053 (data not shown). This also showed the tendency of the availability of keratinized mucosa around implants, and it may indicate that at least 2 mm of keratinized mucosa is preferable for the long-term success and survival of implants.

Our analysis showed that 16 of 223 implants were lost during the observation period. Among the six factors examined, only the implant position affected the cumulative implant survival rate and the main reason for implant failure was peri-implantitis (14/16 failed implants). However, the implant position did not affect the incidence of peri-implantitis. Compared to the mandible, the bone quality of the maxilla is lower [30] and the loading force is tilted to the implant axis. These factors might have acted as an exacerbating factor of peri-implantitis, resulting in the lower survival rate of the implants in the maxilla compared to the mandible.

Prosthetic complications occur due to the accumulation of mechanical damage to the implant, implant components, and supra-structures, resulting in the need for repairs and reconstructions of the implant prostheses, which may require time-consuming procedures and additional financial resources. The present investigation was a retrospective and multicenter study, and there were many differences in design patterns, materials, connections, and the attachment of supra-structures. It was therefore difficult to subdivide and review the factors that may affect the prosthetic survival rate, and only gender and type of prosthesis could be analyzed in this study.

The implant-supported fixed prostheses showed the highest complication-free survival rate in our study. It was reported that the veneering material’s chipping/fracture is the most common type of prosthetic complication for fixed prostheses [31, 32]. Pjetursson et al. reported that veneer fracture was observed in 13.5% of fixed prostheses after at least 5-year functioning [33]. In the present study, approx. 76% of the fixed prostheses were not veneered (metal occlusal surface), resulting in the lower complication rate after 25 years of functioning.