Computer-based voice analysis system is a dynamic sound frequency spectrum analyzer that enables to acquire, analyze, edit, and playback speech signal and shows the dynamic relationship among various physical measures. At present, it is one of the most advanced equipments used in the phonological studies but is hardly used in the field of prosthodontics[11] . Patient’s phonetic function would recover to different extents after repair of complete denture. The base thickness of maxillary denture slightly influences phonetic final sound of standard mandarin while greatly influences phonetic first sound[12] . First sound is a major factor of determining lamprophony. Li et al[3] confirmed that following 1 week of denture acclimation, lamprophony was better in the titanium group than in the PMMA group.
Evidence exists that oral prosthesis range, thickness, and morphology directly influence patient’s phonetic function[13-14] . PMMA resin base is usually required to provide a thickness of 2.0-2.5 mm for assurance of intensity, while the thickness of titanium base is only 1-1.2 mm, which is 1/2 of PMMA base thickness. For this reason, titanium base slightly influences oral cavity and hardly limits tongue movement, contributing to patient’s pronunciation.
Pure titanium is a single metal with a fixed melting point, low casting shrinkage rate, and strong capacity of sealing with membrane. It can increase the retention force of complete denture. Pure titanium also exhibits good biocompatibility, high intensity and light quality, and would not induce sensitization, abnormality, and carcinomas[15-16] . A previous study demonstrated that because titanium base costs much, so most of patients select resin base or common base denture[15] . With improvement in people’s living quality and enhancement of consciousness of oral health care, most of patients begin to select titanium base denture with high performance, providing favourable condition for experiments.
Small base thickness relatively increases tongue-palate interspace, enlarges resonance cavity, expands range of motion of tongue, and contributes to rapid establishment of new touch sensation feedback pathway. At the same time, it relatively enlarges oral cavity air passage, increases airflow volume, contributing to the pronunciation of consonant sounds. Selection of sound to be examined is a key link in sound analysis[17] . This study selected some sounds according to effects of base morphology on pronunciation cavity, including stops /g, k/ with resistance region in back and superior tongue position, affricatives /j, q/ and fricative /x/ with resistance region between lingual surface and anterior alveolar ridge, affricatives /zh, ch/ and fricative /sh, r/ with resistance region between tongue tip and alveolar ridge. Bilabial fricatives /b, p/ are slightly influenced by the motion of tongue; fricatives /d, t/ with resistance region between tongue tip and alveolar ridge are pronounced depending on maxillary palatal rugae morphology[18] and are also slightly influenced by the space for tongue motion, so /b, p, d, t/ are not used. Sounds /z, c, s/ are made with the tip of the tongue up-raised and the blade of the tongue concaved, while sounds /zh, ch, sh/ are made with the tip of the tongue retroflexed and the blade of the tongue bulged. Sounds /z, c, s/ require small space for tongue motion in pronunciation and thereby resonance cavity is slightly influenced by base thickness compared with sounds /zh, ch, sh/. Therefore, sounds /zh, ch, sh/, rather than sounds /z, c, s/, are used.
CFA1, also called consonant resonance peak, is a high energy focusing field and presents as disordered stripes with frequencies focusing within some a range in spectrogram. The strongest peak position in each spectrogram is related to the region for pronunciation. Pronunciation region in front would cause small resonance cavity and high resonance frequency.
The anterior part of maxillary base of complete denture was thickened, producing influences on formation of consonant sound with resistance region between tongue tip and tongue blade. When pronouncing sounds /j, q, x/, tongue surface touches with artificial denture, and contact position and contact area directly influence phonetic sounds[19]. Anterior palatal rugae region of maxillary base is a region sensitive to phonetic sounds[11, 14] . Gao et al[12] found that there were more disordered stripes of consonant sound, as well as more obvious high frequency focusing field, in the metal base group (1 mm thickness) than in the plastic base group (2.0-2.5 mm thickness), as shown by spectrogram, which is shown by results from this study. The present study extracted CFA1 frequency value and found that CFA1 frequency values of sounds /x, sh, r, zh, ch, j, q/ were significantly lower in the titanium group than in the PMMA group. In addition, results from this study also demonstrated that CFA1 frequency value in the titanium group was more close to normal reference value. These findings indicate that compared with the PMMA group, the resistance region of tongue tip and tongue surface was in back, the blade of the tongue moves more freely, which corresponds to the physiological position of resistance between the tongue and the palate, thereby contributing to more natural and comfortable pronunciation in the PMMA group.
VOT results from this study further confirm the theory that base thickness influences the resistance region of consonant sound. VOT is defined as the length of time between the release of a stop consonant and the beginning of vocal fold vibration[8] .
VOT plays an important role in understanding the pronunciation of short consonant fricatives and affricatives. The longer distance of resistance region of fricatives and affricatives from lips produces longer sound channel that air passes through, leading to longer pronunciation time. Pronunciation time serves as the judgement criterion for resistance region.
Results from this study demonstrated that the VOT values of lingual root fricatives /g, k/ were significantly lower in the titanium group than in the PMMA group, and the VOT values in the titanium group was more close to normal reference value. These findings confirm that lingual root resistance region in the titanium group was in front compared with the PMMA group. The theory of denture repair says that posterior border of maxillary denture should be just cover the line for making sound /a/ on the juncture of soft and hard palates; posterior border of maxillary denture should be thin, and increased thickness of posterior border easily influences the correct pronunciation of consonant sounds /g, k/[1, 20] . VOT values of affricatives /zh, ch, j/ made with resistance region in tongue tip/tongue blade and anterior alveolar ridge were significantly greater in the titanium group than in the PMMA group and were more close to normal reference value. These findings indicate that resistance region in the tongue tip and tongue blade was in back in the titanium group compared with the PMMA group, which is in accordance with CFA1 results. Results from this study demonstrated that tiny difference in titanium or resin base thickness in the soft palate region would cause significant changes of phonological parameters of consonant sounds, such as VOT, and titanium base is more suitable for physiological location of resistance region between the tongue and the palate.
Results from this study demonstrated that in the spectrograms of lingual root fricatives /g, k/, redundant spike 1 cm before normal spike was observed in 33 (78.6%) out of 42 patients who wore PMMA denture base. These 33 patients were asked to repeat lingual root fricatives and results revealed that the frequency of redundant spike of fricatives /g, k/ averaged 84.5%. However, such a feature was found in the spectrogram of normal people[8] . Thicker posterior border of maxillary denture causing resistance in back in pronouncing consonant sound /g, k/likely contributes to appearance of redundant spike. During pronouncing lingual root sounds, lingual root elevates and touches with posterior border of base. But resin base provides thicker posterior border than titanium base, which causes resistance in advance between the lingual root and the soft palate, thus patients would have a rapid adjustment during the process of transient pronunciation, contributing to the second resistance, i.e., real initiation spike of affricative. This adjustment would be accomplished within several milliseconds, but this would bring discomfortableness to patients. For this reason, patients wearing titanium base yield better pronunciation than those wearing PMMA base. Such a phenomenon cannot be obviously distinguished by subjective hearing, but it can be clearly embodied by spectrogram. The concept and cause of redundant spike mentioned in this study are based on analysis of phonological theory. Morphological analysis should be performed in further experiment in conjunction with EPG or X-ray photography. Through the use of CSL computer-based voice analysis system and computer PPAAT technique, the present study displayed phonetic sound, which is conventionally distinguished by subjective healing, by spectrogram and phonological parameters. This transforms subjective qualitative analysis into objective quantitative analysis and greatly avoids the bias caused by anthropic factor. In the future clinical experiments, sample size can be enlarged to further reduce the errors and to make research results more objective and accurate.
Taken together, selection of titanium base denture with low base thickness in the tongue and palate region would help to recover patient’s phonetic function. PMMA complete denture base is not favorable for recovery of patient’s phonetic function to some extent owing to large base thickness. Extraction and analysis of phonological parameters, such as CFA1 and VOT, using CSL computer-based voice analysis system, would provide objective evidence for lamprophony analysis after wearing complete denture. Maxillary denture with thinner base in tongue and palate regions better promotes the recovery of patient’s phonetic function.
