Abstract
Rational goals for denture construction are basically directed at the restoration of esthetics and masticatory function and the healthy preservation of the remaining natural tissues. Little concern has been given to the perfection and optimization of the phonetic quality of denture users. However, insertion of prosthodontic restorations may lead to speech defects. Most such defects are mild but, nevertheless, can be a source of concern to the patient. For the dental practitioner, there are few guidelines for designing a prosthetic restoration with maximum phonetic success. One of these guidelines involves the setting up of teeth within the neutral zone. The aim of this study was to evaluate, subjectively and objectively, the effect on speech of setting up teeth in the neutral zone. Three groups were examined: group I (control) included 10 completely dentulous subjects, group II included 10 completely edentulous patients with conventional dentures, and group III included the same 10 edentulous patients with neutral zone dentures. Subjective assessment included patient satisfaction. Objective assessment included duration taken for recitation of Al-Fateha and acoustic analysis. Subjectively, patients were more satisfied with their neutral zone dentures. Objectively, speech produced with the neutral zone dentures was closer to normal than speech with conventional dentures.
Keywords: Neutral zone, Speech, Complete dentures, Patient satisfaction, Consonants, Vowels
1. Introduction
Speech is a critical and essential human activity that affects personal communication and self-representation. Together with mechanics and esthetics, speech is a cardinal factor contributing to the ultimate success of a dental prosthesis. The central thesis of the neutral zone (NZ) approach is to position teeth and develop the external denture surfaces such that all of the forces exerted by oral and para-oral muscles are neutralized, and the denture is maintained in a state of equilibrium. Because all oral functions, including speech, involve the synergistic actions of these muscles, dentures constructed according to the NZ concept could provide more stability, retention, comfort, and better phonetic quality than conventionally constructed dentures.
The first part of this hypothesis has already been proven, as several studies have revealed that NZ dentures are more stable and comfortable than conventional dentures (Barrenas and Odman, 1989; Fahmi and Kharat, 1990; Wee et al., 2000). The aim of this study was to prove the last part of this hypothesis through subjective and objective investigations.
2. Materials and methods
2.1. Patients
Experimental groups II and III were comprised of 10 completely edentulous patients (8 males, 2 females; mean age: 52 years) selected from the outpatient clinic of Prosthodontic Department, Faculty of Oral and Dental Medicine, Cairo University. A control group (group I) of 10 completely dentate subjects (4 males, 6 females; same age range as groups II and III) was selected from volunteers from the Faculty of Oral and Dental Medicine, Cairo University. Inclusion criteria for both edentulous patients and dentulous subjects were as follows: subject was free from systemic diseases that could affect neuromuscular behaviors, had no speech disorders or hearing disabilities, and were not under any medication that could affect muscular activities. Edentulous patients had a class I maxillomandibular relationship, adequate interarch distance, and normal tongue behavior and size. Patients with xerostomia or excessive salivation and those with severe bony undercuts or bony exostosis were excluded.
2.2. Denture construction
Two types of dentures were constructed for each edentulous patient: one according to the conventional method (group II) and one according to the NZ technique (group III).
2.2.1. Group II
For group II, dentures were constructed in the conventional manner as follows. Maxillary and mandibular preliminary impressions were made in suitable stock trays with irreversible hydrocolloid impression material. The trays were immediately poured into stone plaster to obtain study casts on which maxillary and mandibular acrylic resin special trays were constructed. Border molding was done with putty-consistency rubber-base impression material. Final impressions were made with medium-consistency rubber-base impression material under light finger pressure.
Impressions were boxed and poured in dental stone to obtain master casts on which the occlusion blocks were constructed. The maxillary occlusal rim was adjusted for proper esthetics, occlusal plane, phonetics, and support of the upper lip. After adjustment and proper orientation of the occlusal plane, a maxillary face bow record was made to mount the maxillary cast on a semiadjustable articulator. The centric occluding relation was recorded by the wax wafer method to mount the mandibular cast. Artificial teeth were set, with the mandibular teeth set on the crest of the ridge.
2.2.2. Group III
For Group III, the mandibular trial denture base was modified onto an acrylic plate with metal pins projecting upward. Two occlusal finger rests (stops) were built up on the acrylic plate on the premolar regions of both sides with a green stick tracing compound. The stops were molded in the mouth of each patient to the correct vertical dimension of occlusion. The NZ impression was made by applying tissue conditioning material (Viscogel ®, Dentsply, Germany) on the acrylic plate and finger rests, contouring it to an approximate rim, then instructing the patient to perform the following series of actions for 10 min: swallow mildly and take frequent sips of water, talk aloud, pronounce the vowels, recite Al-Fateha (standard passage from the holy Quran), count from 60 to 70, smile, grin, lick, and purse the lips. The NZ impression (Fig. 1) was boxed to obtain tongue and facial matrices, according to which the mandibular artificial teeth were set (Fig. 2).
Figure 1.
The mandibular acrylic plate, with the neutral zone impression, on the master cast.
Figure 2.
The mandibular artificial teeth set in the neutral zone that has been well defined by the facial matrices and the tongue matrix.
After waxing up, all dentures in both groups were evaluated while being worn. Any necessary adjustments were made. Trial dentures were processed as usual in heat-cured acrylic resin at 75 °C for 1.5 h and at 100 °C for an additional 0.5 h.
2.3. Denture assessment
Subjective assessment for groups II and III was done by evaluating patient satisfaction with a questionnaire (Bergman et al., 1996; Ambard et al., 2002), which was developed in consideration of the most important aspects used to evaluate prostheses: esthetics, function, retention, stability, comfort, and speech. Patients were asked to rank each denture type as 1 (not satisfied), 2 (satisfied), or 3 (very well satisfied).
Objective assessment involved (i) recording the duration taken for recitation of Al-Fateha (which represented automatic, continuous, uninterrupted speech) with a stop watch and (ii) acoustic analysis using computerized speech lab (CSL) (Kaymodel 4300, USA). For each patient, 2 measurements were taken for each type of denture (one immediately after insertion and another 3 weeks later). Subjects in the control group were subjected to the same protocol of objective assessment only once.
Consonant assessment included the fricative duration of the /s/, /∫/, and /f/ sounds (where /∫/ stands for the “sh” sound), as well as the voice onset time (VOT) of the /k/ and /t/ sounds. Vowel assessment included vowel duration and formant frequencies (F1, F2, F3) of the following vowels; /a/, /i/, and /u/. Each subject/patient was asked to pronounce certain Arabic words containing each of the mentioned speech sounds. Each word was pronounced in a carrier phrase to ensure stable stress pattern. The mean of three pronunciations was calculated.
2.4. Statistical analysis
All samples were calculated with spectrographic analysis. Repeated measures analysis of variance (ANOVA) was used to compare means. Bonferroni’s test for pair-wise comparisons was used to determine significant differences between means when ANOVA was significant. The significance level was set at P ⩽ 0.05.
3. Results
Results of patient satisfaction are shown in Fig. 3. All patients, without exception, reported being more comfortable with their NZ dentures. Most patients reported that they chewed and spoke well with both dentures; however, some experienced further improvement in their chewing ability and speech when they wore their NZ dentures.
Figure 3.
Patient satisfaction.
Results of objective assessment are shown in Tables 1–5. Objectively, any decrease in duration of the evaluated parameters indicated improvement in speech. For Al-Fateha recitation (Table 1), there were significant differences between both denture groups and the control group at immediate insertion. However, 3 weeks later, there was still a significant difference between groups I and II, while the difference was statistically insignificant between groups I and III.
Table 1.
Effect of group on recitation of Al-Fateha.
| Group |
Group I |
Group II |
Group III |
P-value | |||
|---|---|---|---|---|---|---|---|
| Period | Mean (in seconds) | SD | Mean (in seconds) | SD | Mean (in seconds) | SD | |
| At insertion | 18.65b | 1.2 | 21.3a | 1 | 20.62a | 0.7 | 0.001⁎ |
| After 3 weeks | 20.27a | 0.8 | 19.09b | 1.3 | 0.025⁎ | ||
SD: standard deviation.
Values with the same letter (a,a) are statistically insignificant at P < 0.05.
Values with different letters (a,b) are statistically significant at P < 0.05.
Significant at P ⩽ 0.05.
Table 2.
Effect of group on fricative duration.
| Group | Group I |
Group II |
Group III |
P-value | |||
|---|---|---|---|---|---|---|---|
| Period | Mean (in seconds) | SD | Mean (in seconds) | SD | Mean (in seconds) | SD | |
| /s/ sound | |||||||
| At insertion | 0.109b | 0.008 | 0.141a | 0.009 | 0.135a | 0.01 | <0.001⁎ |
| After 3 weeks | 0.130a | 0.009 | 0.122a | 0.011 | <0.001⁎ | ||
| /∫/ sound | |||||||
| At insertion | 0.116b | 0.01 | 0.137a | 0.01 | 0.133a | 0.01 | <0.001⁎ |
| After 3 weeks | 0.130a | 0.01 | 0.122b | 0.01 | <0.001⁎ | ||
| /f/ sound | |||||||
| At insertion | 0.136b | 0.006 | 0.145a | 0.006 | 0.143a | 0.008 | 0.028⁎ |
| After 3 weeks | 0.142a | 0.007 | 0.136b | 0.009 | 0.037⁎ | ||
SD: standard deviation.
Values with the same letter (a,a) are statistically insignificant at P < 0.05.
Values with different letters (a,b) are statistically significant at P < 0.05.
Significant at P ⩽ 0.05.
Table 3.
Effect of group on voice onset time (VOT) duration.
| Group | Group I |
Group II |
Group III |
P-value | |||
|---|---|---|---|---|---|---|---|
| Period | Mean (in seconds) | SD | Mean (in seconds) | SD | Mean (in seconds) | SD | |
| /t/ sound | |||||||
| At insertion | 0.040a | 0.005 | 0.048a | 0.007 | 0.046a | 0.008 | 0.067 |
| After 3 weeks | 0.043a | 0.005 | 0.043a | 0.005 | 0.292 | ||
| /k/ sound | |||||||
| At insertion | 0.044a | 0.005 | 0.049a | 0.008 | 0.047a | 0.006 | 0.175 |
| After 3 weeks | 0.046a | 0.005 | 0.044a | 0.005 | 0.557 | ||
SD: standard deviation.
Values with different letters (a,b) are statistically significant at P < 0.05.
Values with the same letter(a,a) are statistically insignificant at P < 0.05.
∗Significant at P ⩽ 0.05.
Table 4.
Effect of group on vowel duration.
| Group |
Group I |
Group II |
Group III |
P-value | |||
|---|---|---|---|---|---|---|---|
| Period | Mean (in seconds) | SD | Mean (in seconds) | SD | Mean (in seconds) | SD | |
| /a/ vowel | |||||||
| At insertion | 0.174b | 0.008 | 0.205a | 0.01 | 0.202a | 0.01 | <0.001⁎ |
| After 3 weeks | 0.195a | 0.009 | 0.191a | 0.01 | <0.001⁎ | ||
| /i/ vowel | |||||||
| At insertion | 0.178b | 0.006 | 0.206a | 0.009 | 0.203a | 0.01 | <0.001⁎ |
| After 3 weeks | 0.197a | 0.01 | 0.187a | 0.009 | <0.001⁎ | ||
| /u/ vowel | |||||||
| At insertion | 0.177a | 0.008 | 0.196a | 0.009 | 0.182b | 0.008 | 0.002⁎ |
| After 3 weeks | 0.188a | 0.01 | 0.176b | 0.06 | 0.033⁎ | ||
SD: standard deviation.
Values with the same letter (a,a) are statistically insignificant at P < 0.05.
Values with different letters (a,b) are statistically significant at P < 0.05.
Significant at P ⩽ 0.05.
Table 5.
Effect of group on formant frequencies.
| Group | Group I |
Group II |
Group III |
P-value | ||||
|---|---|---|---|---|---|---|---|---|
| Period | Mean (in hertz) | SD | Mean (in hertz) | Mean (in hertz) | SD | |||
| /a/ vowel | ||||||||
| F1 | At insertion | 569a | 80.8 | 544.9a | 44.2 | 582.9a | 56 | 0.053 |
| After 3 weeks | 526.6a | 55.6 | 566.3a | 74.5 | 0.303 | |||
| F2 | At insertion | 1931.9a | 147.8 | 1690.1a | 192.9 | 1536.1a | 303 | 0.051 |
| After 3 weeks | 1775.8a | 169.1 | 1747.4a | 179 | 0.063 | |||
| F3 | At insertion | 2548.9a | 294 | 2472.9a | 306 | 2331.4a | 342 | 0.600 |
| After 3 weeks | 2593a | 98 | 2578.2a | 233.6 | 0.655 | |||
| /i/ vowel | ||||||||
| F1 | At insertion | 364.1a | 63.3 | 328.6a | 27.9 | 342.1a | 56.8 | 0.054 |
| After 3 weeks | 316.3a | 51 | 337.8a | 54.3 | 0.122 | |||
| F2 | At insertion | 1980.5a | 514.8 | 2212.8a | 158.9 | 1933.5a | 559.1 | 0.052 |
| After 3 weeks | 2017.3a | 449.7 | 1925.8a | 653.4 | 0.187 | |||
| F3 | At insertion | 2815.6a | 192.3 | 2596.6a | 232.9 | 2626.9a | 242 | 0.315 |
| After 3 weeks | 2672.5a | 248.2 | 2518.2a | 261.5 | 0.059 | |||
| /u/ vowel | ||||||||
| F1 | At insertion | 340.8a | 66.2 | 387.4a | 42.9 | 377.9a | 48.4 | 0.128 |
| After 3 weeks | 308.4a | 49.5 | 347.3a | 52.6 | 0.072 | |||
| F2 | At insertion | 1017.6a | 333.2 | 1006a | 148.7 | 939.3a | 158.3 | 0.396 |
| After 3 weeks | 722.5a | 207.3 | 1130.9a | 267.5 | 0.065 | |||
| F3 | At insertion | 2428.9a | 593.7 | 2311.4a | 328.8 | 2436.9a | 264.2 | 0.294 |
| After 3 weeks | 2036.7a | 549.9 | 2264.5a | 352.6 | 0.081 | |||
SD: standard deviation.
Values with different letters (a,b) are statistically significant at P < 0.05.
Values with the same letter (a,a) are statistically insignificant at P < 0.05.
∗Significant at P ⩽ 0.05.
Significant differences of fricative duration (Table 2) and vowel duration (Table 4) were found between the control group and denture groups for all tested sounds and vowels at immediate insertion. After 3 weeks, there were no significant differences between the control and NZ group, except for the /s/ sound and /a/ vowel. There were no significant differences among all 3 groups in terms of the VOT duration (Table 3) or formant frequencies (Table 5) of the evaluated sounds and vowels.
4. Discussion
In the current study, only the mandibular denture was modified for the NZ procedure, similar to other research (Ohkubo et al. 2000; Wee et al., 2000; Makzoumé 2004). However, the maxillary occlusal rim was also adjusted for proper esthetics, occlusal plane, phonetics, and support of the upper lip. The NZ procedure is more beneficial to the mandibular denture, because many other factors favor the retention and stability of the maxillary denture compared to the mandibular denture (Zarb et al., 1990).
Subjectively, the patient satisfaction results supported the initial hypothesis. Few patients complained about their speech immediately after they wore their conventional dentures; however, none complained when they wore their NZ dentures. The key reason for this finding could be that we set up the teeth as “the main static articulators” in the neutral “zone of equilibrium,” which allowed for a harmonious relationship to be achieved between the denture and the tongue, which is “the most powerful dynamic articulator”. The continuous uninterrupted speech (Al-Fateha recitation) results recorded immediately after insertion emphasize the importance of patient adaptation. The results obtained after 3 weeks of denture use indicate that speech in the NZ group was closer to normal than that in the conventional group, and patients adapted faster to their NZ dentures.
Results of VOT duration and formant frequencies revealed insignificant differences between all 3 groups, both at the time of insertion and after 3 weeks. Palato-lingual consonants depend on tongue movement and its contact with the palate anteriorly (as in /t/) and posteriorly (as in /k/). This insignificant difference could be attributed to the proper palatal contours and thickness inherent in both dentures, which allowed sufficient tongue movement and proper articulation of these sounds.
On the other hand, vowel production depends mainly on resonation, because it is accomplished by alterations in the shape/character of the oral cavity as a resonating chamber. Tongue height and position, lip protrusion or rounding, and the degree of mouth opening are all factors that alter the shape of the oral cavity to produce the acoustic requirements for each vowel. Therefore, the insignificant difference indicates that changes in the shape and volume of the oral cavity after introducing either type of dentures were very subtle and too minute to affect resonation. This result may be attributed to the proper flange thickness and contour, as well as proper palatal thickness and contour in both dentures. Moreover, the result shows that the tongue space in both denture groups was sufficient to allow the tongue to attain the different positions and heights necessary for production of these vowels.
Statistical analysis of vowel duration revealed significant improvement for the /i/ and /u/ vowels in the NZ group, but no significant change in duration of the /a/ vowel in either denture group. This finding coincides with that reached by Boghosian and Spangenberg (1961), who reported that, after insertion of a prosthesis, greatest improvement in the duration of vowel sounds occurs in the front and back of the mouth with high or half-high vowel sounds such as /i/ and /u/.
Significant differences between the control and denture groups were observed for all fricatives (/s/, /∫/, and /f/), highlighting the importance of “patient adaptation” as an influential factor on their pronunciation. However, after 3 weeks, durations of the /∫/ and /f/ sounds improved in the NZ group only, but were insignificantly different from the control group. This result further proves that patients adapted faster to their NZ dentures. Because artificial teeth play an important role in pronunciation of both /∫/ and /f/ sounds, positioning teeth in the NZ may have shortened the time span of the adaptation period. On the other hand, there was still a significant difference between the control and denture groups for the /s/ sound. This finding is consistent with previous results showing that lingua-alveolar fricatives (such as /s/ and /z/) are more sensitive in their pronunciation than other fricatives, because they require adequate spreading of the air through the teeth, high precision movement of the tongue, and controlled jaw positioning (Lundquist et al., 1992; Runte et al., 2001; Molly et al., 2008). Palmer (1979) reported that the /s/ sound demands considerable compensatory physiologic maneuvering from the patient, especially if he/she has been edentulous for a long time. Lingualveolar fricatives, therefore, require a longer adaptation period (Molly et al., 2008).
Within the limitations of this study, the following conclusions can be drawn. Subjectively, patients favored and were more satisfied with their NZ dentures than with their conventional dentures. Articulation was more affected than resonation in both denture groups. Consonants (especially fricatives) were more affected than vowels. Acoustically, speech produced with the NZ denture was generally closer to normal than that with the conventional denture. However, some speech sounds (such as the /s/ sound) appeared to require a longer adaptation period than others. Therefore, it is recommended that the evaluation period be extended (i.e., to months instead of weeks) in future studies to obtain a more conclusive assessment of speech production.
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