[SEM study of failures in acid etched cast prosthodontics]

Eight bonded prosthodontic units (mean wearing time 14 months), with 7 retainers spontaneously debonded during clinical use and 9 retainers removed by the dentist, have been studied by SEM. Since only bond fractures at the acrylic/metal interface and mixed fractures could be detected, it may be concluded that late failure is governed by processes at the interface between metal and acrylic. The lost retainers showed overgrowth of mainly rod-shaped bacteria, although one of the artificially removed retainers that was clinically stable beforehand, also exhibited bacterial overgrowth. This suggests that sequential debonding is possible; hence bonded retainers impart a longterm caries risk to abutment teeth.