Influence of the height of the mandibular ridge on the masticatory function during the functional adaptation with new complete dentures

Abstract The period of functional adaptation to a new conventional complete denture embraces many transitory issues, and this period is directly related to the rehabilitation success. Objective This clinical trial evaluated the influence of the height of mandibular ridge on the masticatory function of complete denture (CD) wearers during the adaptation period. Methodology A total of 28 individuals wearing new CDs (NR, n=14, normal mandibular ridges, 64±12.5 years, 9 female; RR, n=14, resorbed mandibular ridges, 69±6.8 years, 9 female) were assessed at 24 hours, 30 days, three months and six months after the insertion of the CDs for masticatory performance (MP, sieves method), satisfaction with CDs (questionnaire) and maximum occlusal bite force (MOBF) (gnatodynamometer). The classification of the mandibular ridges followed the Kapur index. Data of MP and MOBF were analyzed by two-way ANOVA and satisfaction with CDs was analyzed by Generalized Estimating Equations (GEE), α=.05. Results Participants with NR presented better masticatory performance (p=.000 - NR 30.25±9.93%, RR 12.41±7.17%), general satisfaction (p=.047), retention of mandibular denture (p=.001), chewing ability (p=.037), and comfort of wearing a mandibular denture (p=.000). Regardless of the mandibular ridge, MP (p=.000) was higher at three (21.26±12.07%) and six months (24.25±12.26%) in comparison to 24 hours (18.09±10.89%), the MOBF (p=.000) was higher at three months (78.50±6.49 N) compared to 24 hours (57.34±5.55 N) and 30 days (62.72±5.97 N), and the comfort of wearing a mandibular denture (p=.002) at three months (1.61 ± 0.07) was greater than 24 hours (1.29±0.10) and 30 days (1.36±10). Conclusions The study suggests that the participants with NR have higher MP and satisfaction with their CD, regardless of the follow-up period after the insertion of the new CD. After subjects received the CD, a period of 3 months was necessary for achieving better achievement MOBF, MP, and self-perceived comfort with the mandibular denture, regardless of the height of the mandibular ridge.


Introduction
Edentulism has been frequently observed worldwide for centuries. 1 The rehabilitation with conventional complete denture (CD) is the first option for edentulous individuals because it is economically more viable than implant-supported dentures, thus a significant option for rehabilitation. 2,3 Some studies have shown that treatment with conventional CD provides a high level of satisfaction among their users (65-90%), 3 providing these patients with improved masticatory and phonetic functions, increased bite force, and quality of life. 4 The period of functional adaptation begins after the patients receive the new conventional CDs. In this period, many problems, although transitory, may occur and they directly influence the rehabilitation success. 5 Mandibular dentures are usually the greatest cause of discomfort and low satisfaction among denture wearers due to their poor retention and stability, hampering one's adaptation. 6 A hypothesis for this hampered adaptation is related to a lower mandibular area and the process of resorption of the alveolar ridge after extraction or tooth loss, which occurs with greater intensity in the mandible. 7 Studies have already shown that advanced age and the use of drugs with xerostomic effect are factors that can influence the patients' adaptation and consequently the number of returns after the insertion of these dentures. 8 Thus, the post-insertion period of CDs should not be neglected by the professional, since it is directly related to the success or failure of treatment.
During functional adaptation with the new conventional CD, the masticatory performance (MP) is a relevant aspect, since it is an significant and deficient function for conventional CDs wearers.

Several factors may influence the MP of conventional
CDs wearers, such as age, sex, height and shape of the residual alveolar ridges, retention and stability of the complete denture, occlusal bite force, neuromuscular control, activity of mandibular muscles, salivary flow, occlusal scheme, posterior tooth forms, and previous experience with dentures. [9][10][11] The masticatory performance of CD wearers is 10 to 20% in comparison to subjects with natural teeth. [12][13][14][15] Another aspect of interest among conventional CDs wearers is the maximum occlusal bite force (MOBF) strength that these individuals can exert, which is related to their chewing ability. 16 The MOBF is an objective evaluation of denture quality and performance, and it is closely related to masticatory function. 17 A CD wearer can exert only 15% of the force of a subject with natural teeth. 11 In the functional adaptation with new CDs, the MOBF increases progressively after the insertion of the new denture. 18 The process of mandibular alveolar ridge resorption may directly influence the MOBF. 19 Patients' satisfaction with their new CD is extremely important to evaluate the treatment success, since the individual's well-being indicates success in treatment. 20 Studies have shown that different factors can influence patients' satisfaction, such as the quality of the prosthesis, personality and psychological factors of the patients, salivary flow, age, sex, interaction between professional and patient, previous treatment experiences, cultural factors, and conditions of patients' oral tissues, including the height of the mandibular ridge. 1,21,22 The literature does not yet establish a relationship between the height of the mandibular ridge and physiological parameters and satisfaction of CDs wearers in the period after their installation. Only one study 23 has presented, with subjective evaluation, that the degree of resorption of the mandibular alveolar ridge is a determining factor in the satisfaction of patients with new complete denture. However, the evaluation of objective variables was not carried out for this purpose, and other studies still seem to indicate a certain controversy about this problem. 24,25 Thus, the aim of this study was to evaluate the influence of the height of the mandibular ridge in the MF, MOBF, and patient satisfaction at different stages after the insertion of a new set of conventional CDs.
The primary outcome of this study is measurements of MP, six months after the insertion of the CD.

Participants
Potential participants were completely edentulous requiring replacement of the bimaxillary conventional complete denture who asked for treatment at the Participants who fulfilled these criteria were assigned to the study groups (normal mandibular ridges or NR and resorbed mandibular ridges or RR) based on the classification described by the Kapur index 27 adapted by Gonçalves, et al. 16 (2014).
According to the Kapur index, 27 the mandibular ridges were classified considering their size and form; location of the inserted muscles and tissues and resilience of fibromucous. The size and form of the ridges were scored with a 4 point scale: 1 ("low or flat"), 2 ("V-shaped"), 3 ("shaped between U and V"), and 4 ("U-shaped"). The location of the inserted muscles and tissues were scored with a 3-point scale: 1 (high attachment), 2 (low attachment) and 3 (medium attachment). The resiliency of fibromucous of the ridge were scored with a 3 point scale: 1 (flabby), 2 (resilient) and 3 (firm). The mandibular ridges can scope a maximum score of 10 points. In this study, all participants were classified according to these scores and the individuals who presented scores equal to or higher than 7 were classified as having normal mandibular ridges (Group NR), whereas the resorbed mandibular ridges group presented scores lower than 7 (Group RR). The classification of the height of the mandibular ridges was performed by a single previously calibrated rater.
Individuals who fulfilled the characteristics of the inclusion and exclusion criteria were invited to participate in the study immediately after the insertion of the new conventional complete denture. A written consent form was obtained from the participants prior to the enrollment.
In this study, all dentures were fabricated according to the same clinical and laboratory procedures. The dentures were fabricated according to the conventional method described by Cunha, et al. 28 (2013). A preliminary impression was obtained using stock the CDs at anytime, if required, during the follow-up period. All the raters were blind for the alveolar ridge classification for the study variables.

Masticatory performance (MP)
The MP was evaluated using the sieve method and some almonds were used as a natural test food. 27,32 The participants were instructed to chew five almonds for 20 masticatory cycles. Chewing of the almonds was performed as usual, without swallowing any fragment, under the rater's supervision. The  Masticatory performance (MP) The two-way ANOVA detected that the mandibular ridge height (p=0.000) and the follow-up periods were significant factors for the outcome variable (p=0.000). There was not significant effect of the interaction between mandibular ridge x follow-up periods (p=0.052).
Participants with NR (30.25±9.93%) had better MP than RR (12.41±7.17%), regardless of the follow-up period. Moreover, Figure 2 shows that, regardless of the mandibular ridge height, a significant improvement of MP (p=0.000) was observed at three months (21.26±12.07%) and six months (24.25±12.26%), in comparison to the 24 hours after the insertion of the complete dentures (18.09±10.89%). the group and follow-up period, and the p values for the factors "ridge," "period," and the interaction "ridge x period." It was observed that the "mandibular ridge" factor was significant for questions about the general satisfaction (p=0.047), retention of the mandibular CD (p=0.001), chewing ability (p=0.037) and the comfort of wearing a mandibular denture (p=0.000). Regardless of the period, participants with NR significantly improved these aspects compared to RR. For the question related to the comfort with the mandibular denture (Q8), the factor "period" was also significant (p=0.002). It means that participants with NR had more comfort with their mandibular dentures compared to RR, and a three-month period was necessary for this achievement, regardless of the height of the mandibular ridge. The height of the mandibular ridges and follow up periods were not significant for denture aesthetics, ability to speak and comfort of wearing a maxillary denture.  Table 3 shows that at 30-day follow-up period, participants with NR (2.00±0.00) had a higher satisfaction with the retention of the maxillary CD than participants with RR (1.64±0.16).

Maximum occlusal bite force (MOBF)
The two-way ANOVA detected that the follow-up periods were significant for the outcome variable (p=0.000). The resorption height of the mandibular ridge (p=0.082) and the interaction mandibular ridge x follow-up periods (p=0.566) was not significant.

Discussion
The null hypothesis of this study was rejected since the height of the mandibular ridges and the follow-up periods after the new dentures insertion significantly influenced the outcomes of the patient with their dentures. We tested the hypothesis that the height of the mandibular ridges would influence the adaptation period with new CDs. Thus, we evaluated MP, MOBF, and patients' satisfaction with their dentures.
Objective masticatory function (MP) is measured by determining the individual's capacity to grind a test food after a fixed number of chewing cycles. 12,32,39 It was observed that participants with NR presented better MP compared to participants with RR.
Gonçalves, et al. 16 (2014)     impairment in chewing function of these individuals. Pero,et al. 34 (2019)  This study demonstrated that the height of the mandibular ridge influenced the self-perception of general satisfaction, the retention of the mandibular complete denture, the chewing ability and the comfort of wearing a mandibular denture as better for participants with normal mandibular ridges, regardless of the follow-up period. The height of the mandibular ridge is strongly related to the stability and retention problems of the mandibular complete denture, 21-23 causing discomfort during the function.
Moreover, the mucosa overlying the resorbed ridges is usually more susceptible to ulcerations, and thus, the mastication process and the comfort of wearing a mandibular denture may be painful and limited for complete dentures wearers with resorbed denturebearing areas. 40,41 Our results are in agreement with Yamaga, et al. 22 (2013). These authors found that patients with NR were more satisfied with the conventional mandibular CD in comparison to the RR ones. Fenlon and Sherriff 23 (2008) observed that patients with RR mostly complained about the stability and retention of their mandibular CDs. Comfort, stability, and retention of the CD are determining factors for the chewing ability. 42 In our study, the results of the satisfaction questionnaire showed that the participants with RR had poor self-perceived capacity to chew and discomfort with their mandibular CDs.
The results of our study demonstrated that participants at the three months follow-up had better self-perception about their comfort of wearing a mandibular denture than participants at 24 hours and 30 days, regardless of the height of the mandibular ridge. Regis, et al. 43 (2013) concluded that six months were necessary for patients be satisfied with their new conventional CDs and to relieve any initial complaints, however, the participants did not receive returns for CD adjustments during the three-month and six-month evaluations, which may justify the results contrasting with our study. In our study, after the insertion of the new CDs, adjustments recalls were scheduled if asked by the participants, which may have resulted in a shorter time to feel comfortable with their complete dentures and a faster functional adaptation process.
In this study we also observed that, within 30 days after the insertion of conventional complete dentures, patients with NR had better satisfaction with the retention of the maxillary denture than patients with RR. A hypothesis for this finding is that, at 30 days, patients with RR did not have coordinated mandibular movements, resulting in diffused occlusal forces, possibly altering the retention of the maxillary CD.
Previous studies support this hypothesis, 44 in which the authors state that when a CD is subjected to occlusal forces, the supporting tissues may deform and, consequently, the complete denture displaces, which may affect patients' self-perception regarding the retention of the CD. In the following periods of this study, there was no difference regarding the retention of the maxillary CD, indicating that patients could possibly be more coordinated, distributing the occlusal forces better, resulting in better self-perception due to the retention of their maxillary CDs.
The results of this study showed that participants at three and six months after the insertion of the new CDs had better MP in comparison to the 24 hours-period, regardless of the height of the mandibular ridge.
These results agree with Eberhard, et al. 45 (2018)  months. 47 Moreover, it is noticeable that, in this study, participants at three months felt more comfortable with the mandibular denture, which could explain the higher MOBF over the same period.
The limitations of this longitudinal clinical trial include the sample characteristics. The results should not be extrapolated to patients with other conditions (for example; low salivary rate or patients who had never used a complete denture). It could be hypothesized that patients with low salivary rate would present poorest MP, since saliva plays a significant role during the motor activities like chewing and swallowing. 39,48 Furthermore, different results might be addressed if participants had been previously experienced complete dentures. 1,21,22 Few participants were lost during the follow-up, and the groups were equal in number of male and female individuals. Moreover, the groups were homogeneous regarding age and only patients with normal salivary flow were included to avoid bias.