2-Year Prosthodontic Outcomes of Mandibular Overdentures on Four Titanium-Zirconium Mini-Implants

Objectives: This prospective study is part of a 2x2 factorial RCT that assessed the incidence of prosthetic complications after the insertion of mandibular overdentures retained by four mini-implants.
Methods: Treatments were provided between April and November 2021. 74 patients received four mini implants to retain a mandibular overdenture as opposed to a conventional maxillary denture. The one-piece titanium-zirconium Straumann® Mini-Implant System and the Optiloc® Retentive System (Institut Straumann AG) were used in this study. Patients were randomized to two surgical protocols (flapped or flapless) and two loading protocols (immediate or delayed). A chairside modification of the mandibular denture into an overdenture and intraoral incorporation of the matrix housings were performed using self-curing PMMA resin. Outcomes were assessed at 3-, 6-month, 1- and 2-year after loading in scheduled follow-up visits. The occurrence of any prosthodontic maintenance event, the type of clinical procedure, and the date of the event were registered at all time points, as well as in additional unscheduled visits. The 2-year prosthetic survival and success rates, and incidence rates of prosthodontics events were registered. Kaplan-Meier survival analysis and Cox regression were used to assess time-dependent endpoints.
Results: There were 182 prosthetic events 147 visits, and 88 (59.9%) were unscheduled. The most frequent events were the need for minor adjustments of the overdenture base, matrix replacements, and reline. A higher risk of adjustments was observed for flapless surgery (p=0.011), and the patient's age was a predictor of matrix replacement (p=0.031). There were no differences in the incidence of other prosthetic complications concerning gender, loading protocol, or ridge form.
Conclusions: The most frequent prosthetic complications were minor adjustments or repairs of overdenture’s base during follow-up, and the incidences were higher in the initial post-treatment period (up to six months), especially in cases of flapped surgery. Overdenture reline and matrix replacements tend to occur after one year of overdenture use. Although the 2-year findings were deemed favorable, more extended follow-up periods should be considered to assess the clinical performance of this novel mini-implant system.