One-year prospective clinical study comparing patient satisfaction and masticatory performance of mandibular overdentures supported by one versus two implants

Abstract There is a lack of clinical evidence on mandibular overdentures (MOD) supported by a single implant. Objective: To compare patient satisfaction and masticatory performance in MOD supported by one versus two implants in a two-group parallel randomized clinical trial. Material and Methods: Twenty-one patients wearing new maxillary and mandibular complete dentures (CDs) were randomly divided to receive one (GI, n = 11) or two (GII, n = 10) implants in the mandibular arch. Four months after implant placement, o-ring abutments were installed in the implants, and matrix attachments were placed in the lower complete dentures. Patient satisfaction with their dentures and masticatory performance were compared at baseline, 3, 6, and 12 months after the procedure. Data on patient satisfaction were analyzed using the Friedman test and the Mann-Whitney U test. Data on masticatory performance were analyzed using one-way repeated measures analysis of variance (ANOVA) and Student's t test (α=0.05). Results: Both groups exhibited a significant increase in overall patient satisfaction in all periods evaluated (p<0.05), except for GI after 12 months, which had values similar to baseline (p=0.74). Satisfaction levels of GI and GII were similar at baseline, 3 and 6 months, but GII showed higher satisfaction levels (p=0.01) than GI at 12 months. GI and GII exhibited a significant increase (p<0.05) in masticatory performance for all periods relative to baseline. However, GII had higher masticatory performance with dentures than GI, regardless of the period (p<0.05). Conclusion: MOD supported by two implants demonstrated better patient satisfaction in the follow-up at 12 months and better masticatory performance than MOD supported by one implant.


Introduction
The lack of retention and stability of mandibular complete dentures (CDs) often result in several problems for edentulous patients, as mastication issues, decreased quality of life and self-confidence, reduced satisfaction, and more limited social contact 1 .
According to MacEntee 2 (2003), complete dentures supported by implants can offer comfort, optimism, and relief for these patients.
It is known that mandibular overdentures (MOD) supported by two implants exhibit good long-term results and are commonly proposed for treatment of edentulous mandibles 3 . However, no robust evidence has been found to support this treatment as a single standard of care for patients with an edentulous mandible 4 , since the costs are prohibitive for most edentulous patients 5,6 and there is a difficulty in placing the implants in reduced buccolingual dimensions of the bone without using bone graft procedures 7 .
This modality is originally recommended for older edentulous patients whom experience discomfort and functional difficulties with their conventional mandibular dentures 10 . One implant is inserted in the midline symphysis region of the mandible after adequate presurgical diagnoses and is used for anchorage of the mandibular overdenture 12 . This less invasive and less costly intervention could allow more people to benefit from this treatment, even with general health concerns 13 .
In a recent systematic review and meta-analysis, Srinivasan, et al. 6 (2016) affirmed that the existing scientific evidence on MOD supported by a single implant is still scarce. Consequently, before recommending this treatment for edentulous mandibles, long-term observations are needed, and larger functional and patient-centered outcome studies are suggested 6 .
Patient satisfaction is one factor that influences the success of overdenture treatment 7 . Previous studies have demonstrated that MOD supported by a single implant significantly improves patient satisfaction in comparison to a conventional mandibular CD 5,15 , but scientific evidence regarding satisfaction of the MOD supported by one versus two implants is scarce in the literature 5,9,14 individuals did not meet the inclusion criteria ( Figure   1). Then, 30 individuals were invited to participate in this clinical trial, and each signed an informed consent form prior to enrollment.  During polymerization of the resin, the patient was asked to keep his/her denture in centric occlusion using moderate pressure, so that the denture base was in intimate contact with the supporting tissues 11 (Figures 4 and 5). Prosthodontic procedures were performed by graduate students under supervision of two professors. Recall appointments were scheduled for 1 week after the procedure, and patients were then referred for follow-up evaluations 3, 6, and 12 months after the delivery of the MOD. Satisfaction with their dentures (maxillary CD and MOD) and masticatory performance were assessed in each follow-up visit by a different examiner.

Study outcomes
Satisfaction with dentures was assessed by a questionnaire ( Figure 6) used in previous trials 19,20 .
The questionnaire considered eight items, as follows: comfort in wearing mandibular denture; comfort in wearing maxillary denture; retention of maxillary denture; retention of mandibular denture; aesthetics;   Table 1 shows baseline demographic and clinical characteristics for each group and Figure 1 shows        compared to each other (Table 2), GI had similar levels of satisfaction to those of GII at 3 and 6 months.

Results
The only difference was observed at 12 months, when satisfaction was higher among GII participants (p=0.01). Table 3   reports showed a lower masticatory performance compared to 6 months (p=0.46) and results were similar to those from 3 months (p=0.511). When groups were compared to each other (Table 4), GI participants' reports showed a lower masticatory performance (p<0.05) compared to GII participants in all periods evaluated.

Discussion
Considering the findings of this study, the null hypothesis was rejected because there were significant differences after one year between and within both groups in most parameters evaluated.
Based on results obtained from GI at 3 and 6 months, the overall satisfaction with the dentures for this group was expected to remain high for an entire year. However, at 12 months, participants in this group experienced a decrease in satisfaction compared to prior periods and to GII. These findings contrast with other prospective studies 5,9,14 . Tavakolizadeh, et al. 14 (2015) observed an improvement in satisfaction among patients with MOD supported by one or two implants after six and twelve months when these patients were compared to others with conventional CDs. The authors also reported that both groups had similar results in the periods evaluated. In a five-year prospective study, Bryant, Walton and MacEntee 9 (2015) also found similar satisfaction levels among subjects treated with MOD supported by one or two implants, and the results of these two groups were always higher than baseline levels among participants with conventional CDs. Kronstrom, et al. 5 (2017), in a prospective study of five years, stated that patient satisfaction scores in MOD supported by one or two implants increased significantly when compared to baseline values and remained high for both groups, with no significant differences.
In our study, the lower overall satisfaction levels of GI participants at 12 months may be associated to a perception of a lower retention (p=0.006) of their MOD in comparison to the GII, as shown in Table 3.
This difference was not observed in other evaluated periods.
The satisfaction of patients with their dentures may be also influenced by the attachment system for Most studies that observed continuity of high levels of patient satisfaction used a ball attachment for implantretained overdentures 5,9,14 . Although the retention force of o-ring system was not measured in this study, it may be suggested, based on literature data 23,24 , that the other previous studies 5,9,14 used retention systems with higher retention force than that one used in this study and, consequently, showed better results for subjects treated with MOD supported by one implant.
It is also important to mention that new maxillary and mandibular CDs were produced for all participants to normalize the aesthetic and functional parameters  of their dentures. The production of new CDs is a step that has also been used in other studies 1,5,15  Consequently, muscle activity was more devoted to and directed towards masticatory function and less effort was required to stabilize or retain the prosthesis 8 . This could also be a possible explanation for the best results found in GII for masticatory performance.
A total average for an implant survival rate of 93.5% was obtained (90.9% for GI and 95% for GII). The needs for prosthetic maintenance were higher if compared to other studies 9,10,12 . Some unscheduled appointments were needed and associated to the repair of fractured overdentures, as two midline denture fractures were found for GI and in one for GII. Gonda, et al. 28