The neutral zone for mandibular complete dentures : a clinical trial
Geerts, Greta Aimée Virginie Maria
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Rehabilitation of edentulous jaws without the option of osseointegrating implants will remain the only treatment option within reach of many older patients for the foreseeable future. Many routine prosthodontic procedures are based on dogmas, because no high-level scientific evidence exists to either accept or reject them, among these is the “neutral zone” (NZ) concept. In spite of paucity of evidence using approved patient-based outcome instruments, it is generally agreed that the NZ should be respected when constructing complete dentures. The purpose of this research project was to determine how shapes of conventional and NZ mandibular dentures differ, and if the two different types of dentures impact differently on oral health–related quality of life by using an accepted oral health-related quality of life instrument as a patient-based outcome. Thirty nine edentulous patients were selected for this prospective, randomised, cross-over, single-blinded clinical trial. Two sets of complete dentures were made for each patient. One denture set was made following conventional biometric guidelines for determining the position of the mandibular posterior denture teeth in relation to the ridge; another set was made following a functional impression of the potential denture space. Each set of dentures was worn for at least two months. A similar number of types of dentures were delivered first. Widths of residual ridges and mandibular denture arches were measured using digital measuring software. Position of denture teeth was related to the ridge. Denture dimensions were compared by means of analysis of variance using the mixed procedure. Using formula of parabola, arch-widths were compared using paired t-tests. Pre- and post-treatment patient feedback was obtained by means of the 20-item Oral Health Impact Profile (OHIP-20) and a preference score. Treatment effect size (ES) was established based on the OHIP-20 scores. Relevant associations among denture dimensions, OHIP-20 scores, preference, age, gender, marital status, education, income, period of edentulousness, and quality of denture-bearing tissue were done using the generalised linear model and correlation analysis. For all statistical analysis, level of significance was determined at p<0.05. The mean age of the sample was 62.3 years. Twenty four patients were female. Mean period of edentulousness was 31 years and mean number of denture sets worn prior to the trial was 2.5. Except for the canine region, NZ dentures were statistically wider than anatomic dentures. The difference in mean widths between the two types of dentures was larger for female patients. Older patients had smaller differences in denture dimensions. More unfavourable denture-bearing tissue was associated with a larger difference in the two types of dentures. Both types of mandibular dentures significantly improved the OHRQoL of patients. Both types of dentures had a high treatment ES. The OHIP-20 instrument could not distinguish a statistical difference in impact on OHRQoL between the two treatment options. There was a minute difference in treatment ES between the two types of treatment. The only domain representing a small clinical benefit between NZ and anatomic dentures was “physical pain”, with the NZ dentures scoring better. There was no correlation between pre- and post-treatment scores for both types of dentures. No significant associations were found between post-treatment OHIP- 20 scores on the one hand and tissue scores, gender, age, education, marital status, period of edentulousness and denture dimension differences on the other hand. Based on OHIP-20 scores, there was a significant association between denture preference and NZ dentures, but not for the other preferences. No significant associations were found between denture preferences on the one hand and tissue scores, gender, age, period of edentulousness and denture dimension differences on the other hand. Even though no significant relationship was found between preference and gender, the majority of female patients preferred the NZ denture and the majority of male patients did not express a preference. Providing new complete dentures improved OHRQoL of edentulous patients. The majority of female patients preferred the NZ compared over the ANA denture. The NZ technique appeared to have a higher positive impact on OHRQoL of female patients compared to male patients.