Although all implants used in this study were withdrawn from the market about 20 years before, the longitudinal clinical outcomes over decades will help to better understand potential factors leading to implant failure or complications and assess the safe and predictable use of dental implant. Our analyses revealed a 25-year cumulative survival rate of 89.8% after the prosthesis setting, which seems comparable to the result of a recent study [7]. Although approx. 62% of the patients and implants in our original cohort dropped out during the follow-up period, according to Table 1, a majority of patients who underwent implant treatment were middle- and old-aged (82.6% of the patients were 40–69 years old) and thus some of the patients could not continue maintenance for varying reasons over a 25-year follow-up. The other 38% of the patients are healthy and likely to visit their dentists for maintenance, and they were included in the 25-year cumulative survival rate [7]. Therefore, the true long-term survival rate might have been lower than we reported herein due to bias from the patients who dropped out.

In addition, only four implants of one patient were inserted into a re-constructed site from the iliac bone; no other bone augmentation procedure such as bone graft, guided bone regeneration (GBR), and sinus floor elevation were conducted. The principle of guided tissue regeneration (GTR) was introduced in 1982 [10], and GBR was introduced in 1988 [11]. The technique of sinus floor elevation was initially introduced in 1980 [12]. These complex augmentation procedures had not been common at that time [2], especially in private practices in Japan, and thus, they were not used for any of the patients in the present study. There is thus some degree of bias regarding the numbers of implant and the lengths of the implants according to the implant position. The number of implants applied to the maxilla anterior region was only two, since getting the esthetic result with implant prostheses was uncertain in those days. And the number of implants under 10 mm long was greater at posterior sites compared to anterior sites due to the sinus and inferior alveolar nerve.