INTRODUCTION:

Speech is a very sophisticated, autonomous and unconscious activity. Its production involves neural, muscular, mechanical, aerodynamic, acoustic and auditory factors. It is a combination of phonation and articulation. Oral motor functions, such as mastication and speech production, share many common features. They are intimately related because the mouth, lower jaw, lips, teeth, and tongue are used for both activities. Because a major portion of speech articulation takes place within the oral cavity, complete dentures may alter those structures and will inevitably mediate a disturbance in speech production (1). The disturbance may be minor or more substantial in importance, depending on individual responses (2). Fortunately, the phonetic problems that arise when speaking with new dentures rarely pose serious difficulties, Because most patients’ ability to adapt is good, initially experienced speech disturbances will be transient. Nevertheless, the treatment objective is to make complete dentures conform to the individual patient’s existing neuromuscular patterns, rather than rely too much on the patient’s ability to adapt.

The purpose of this article is to describe some basic background factors for speech production and to provide knowledge of possible speech problems induced by the fabrication and wearing of complete dentures (3).

Mechanism of speech production:

Controlling the airstream that is initiated in the lungs and passes through the larynx and vocal cords produces all speech sounds. Subtle adjustments in air flow contribute to variations of pitch and intensity of the voice. The structural controls for speech sounds are the various articulators or valves made in the pharynx and the oral and nasal cavities (4,5). Speech articulation takes place when any approximation or movement of the articulators constricts, impedes, or diverts the airstream to produce a single sound (6) .All speech sounds are nearly emitted from the mouth, the airway into the nose is closed off from the oropharynx during speech (7,8,9). Closure is performed by an upward lift of the soft palate. A rapid, continuous movement of the entire length of the soft palate takes place during speech. Intimacy of pharyngeal wall contact, as well as magnitude of movement by the soft palate, varies with the nature and sequence of the speech sounds. The Normal functioning of speech is mainly inveigled by five aspects which are as follows:

· Motor: Consists of lungs and associated muscle and which supplies air.

· Vibrator: Consists of vocal cord and which offers pitch to the voice.

· Resonator: Consists of oral, nasal, pharyngeal cavities and para-nasal sinuses, which creates a tone and is specific for each individual.

· Enunciators and Articulators: Consisting of lips, tongue, soft palate, hard palate and teeth, which form musculoskeletal valves to control the amount of air passage

· Initiator: motor speech area of brain and nerve pathways, which convey motor speech impulses to speech organs (5,10,11).

As the outgoing air passes through the mouth, the tongue, lips, and mandibular oscillations modify it. The tongue has a critical impact on speech production and needs optimal mobility to lift, protrude, flatten, form a groove (s), and contact adjacent tissues freely. Jaw and tooth relationships enable the tongue to articulate against the maxillary teeth or alveolus, permit the maxillary teeth and lower lip to make easy contact, and allow the lips to touch (12).Speech production includes large numbers and sequences of innate and learned motor acts produced in rapid sequences of 12 to 16 sounds per second in a rhythmic behavior. It has been hypothesized that the premotor and motor cortex are required for processing of skills as they become automatic. For the precise movements executed in speech production, the pyramidal motor system has the primary role. A precise coordination between different articulators is essential for the final sound production (13,14).

Complete loss of teeth can cause persistent speech disorders by altering dental articulation areas that will severely reduce the quality of speech; particularly the alteration of frontal maxillary morphology leads to impairment of speech production (4,15,16(. It appears that adaptation to complete dentures may be explained by feedback mechanisms related to speech motor programming. Initially, a complete denture wearer attempts to overcome problems related to the new prosthesis by the help of auditory and orosensory feedbacks during function (17-18).

After a while, only the patient will be aware of remaining articulatory difficulties, which often are related to certain specific sounds. The listener (dentist) is, however, not able to detect any speech production disturbances. At this stage, there is still sensory stimulation from orofacial afferents to central areas. Finally, if the process of adaptation proceeds, the patient will not be aware of any articulatory difficulties or distortional sounds caused by the prosthesis. New speech production central engrams have been established, and adaptation and habituation to the complete denture occur (19,20,21). The patient emotional behavior in relation to a prosthetic treatment can also influence the speech patterns (22). Patients, who psychologically accept their dentures, shortly after they start wearing them, and have fewer speech difficulties than those who don't accept the dentures (23). As the patient uses his custom-made prosthesis with accurate biomechanical and physiological properties, these complaints decrease and elimination of these problems can be noted over time (24,25(.

Problems with speech are often the most difficult to solve since speech is perceptual; the patient hears his speech sounds differently than do those with whom he converses. When the patient has problems with certain sounds, the first place to look is the old dentures. If changes have been made, the dentures should be modified, either by reshaping the existing resin base or by adding wax to areas of under contour (26,27,28). When there is no apparent difference, the patient must be informed that a period of time is required to adapt to the dentures since the dentures cannot adapt to them. No matter how well made the new dentures are, if his speech does not appear to him to be normal, the patient will reject the dentures. Fortunately, the problem that cannot be resolved is rare. Most texts on complete dentures examine the possible etiology of speech problems in great depth and serve as resource documents for the clinician (29,30).

The speech articulatory organs include the tongue, palate, alveolar processes, gums, teeth, and lips. The teeth, alveolus, and palate are static components of speech articulation whereas tongue, lip, and velum are dynamic components. Therefore, phonetics must be considered with mechanics and esthetics as the cardinal factors contributing to the success of the dental prosthesis (1, 31, 4(. Removable complete dentures can partly solve this problem. However, they disturb speech production themselves as they restrict the flexib (32,33,34,35).

The role of teeth and oral structures in speech production:

As the teeth are being arranged for esthetics, it is not the speech sound itself that is critical but rather, the interrelationships of the tongue, teeth, denture base, and lips. Speech production made by patients at the try-in appointment can never be as accurate as when the processed acrylic resin denture base has been substituted for the trial bases, and the patient has become accustomed to the new dentures (36,37). Vowels are voiced sounds; that is, the vocal cords are activated by vibration in their production. They are the free emission of a speech sound through the mouth and require subglottic pressure for the production. The vowels in English are: a, e, i, o, and u, which require minimum articulation and are classified according to the tongue position in the oral cavity and the position of the lips (38). Consonants are produced as a result of the airstream being impeded, diverted, or interrupted before it is released, such as p, g, m, b, s, t, r, and z. Consonants may be either voiced sounds or breathed sounds, which are produced without vocal cord vibration (39,40). Most consonants may be classified in pairs that are formed in the same manner, except that one is breathed and the other voiced. For example, p is breathed; b is voiced (41). Consonants also are divided into groups, depending on their characteristic production and use of different articulators and valves. Plosive consonants, such as p and t, are consonant sounds produced by stopping the airflow in the vocal tract and releasing the air in an explosive way (42,43). On the other hand, fricatives, such as s and z, also are called sibilants and are characterized by their sharp and whistling sound quality created when air is squeezed through the nearly obstructed articulators (44). The consonants which are relevant to clinician can be classified according to the anatomic structures involved in their productions:

Bilabial Sounds:

The sounds b, p, and m are made by contact of the lips. In b and p, air pressure is built up behind the lips and released with or without a voice sound. Insufficient support of the lips by the teeth or the denture base can cause these sounds to be defective (45,46). Therefore the anteroposterior position of the anterior teeth and thickness of the labial flange can affect the production of these sounds (40,47). Likewise, an incorrect vertical dimension of occlusion (VDO) or teeth positioning hindering proper lip closure might influence these sounds.

Labiodental Sounds:

The labiodental sounds f and v are made between the upper incisors and the labiolingual center to the posterior third of the lower lip. If the upper anterior teeth are set too high up (too short), the v sound will be more like an f. If they are set too far down (too long), the f will sound more like a v However, the most important information to be sought while the patient makes these sounds is the relationship of the incisal edges to the lower lip (48,15). The dentist should stand alongside the patient and look at the lower lip and the upper anterior teeth. If the upper teeth touch the labial side of the lower lip while these sounds are made, the upper teeth are too far forward, or the lower anterior teeth are too far back in the mouth (49,50). In this situation, the relationship of the inside of the lower lip to the labial surfaces of the teeth should be observed while the patient is speaking. If the lower lip drops away from the lower teeth during speech, the lower anterior teeth are most probably too far back in the mouth. If, on the other hand, imprints of the labial surfaces of the lower anterior teeth are made in the mucous membrane of the lower lip or if the lower lip tends to raise the lower denture, the lower teeth are probably too far forward, and this means that the upper teeth also are too far forward. If the upper anterior teeth are set too far back in the mouth, they will contact the lingual side of the lower lip when f and v sounds are made (51,52). This may occur also if the lower anterior teeth are too far forward in relation to the lower residual ridge. Observing from the side and slightly above the patient will provide the necessary information for determining which changes should be made (47,53).

Linguoalveolar sounds:

Alveolar sounds (e.g., t, d, s, z, n, and l) are made with the valve formed by contact of the tip of the tongue with the most anterior part of the palate (the alveolus) or the lingual side of the anterior teeth (54,44).The sibilants sounds s, z, sh, ch, and j are alveolar sounds because the tongue and alveolus form the controlling valve. The important observation when these sounds are produced is the relationship of the anterior teeth to each other. The upper and lower incisors should not touch but only approach end to end (54) . The S sound requires near contact of the upper and lower incisors so that the air stream is allowed to escape through a slight opening between the teeth. In abnormal protrusive and retrusive jaw relationships, some difficulty may be experienced in the formation of this sound, and it will probably necessitate adjustment of the upper and lower anterior teeth antero posteriorly, so that approximation can be brought about successfully.

From a dental point of view, the s sound is the most interesting one because its articulation is mainly influenced by the teeth and palatal part of the maxillary prosthesis (55).

Clinical experience suggests that s and t can cause most problems in a prosthodontic context. In nearly all languages of the world, s is a common speech sound (52,56).

On the other hand, the interindividual variation in articulatory details may be great because of individual variation in teeth, palate, lower jaw, and tongue shape and size. Given this variation, different speakers have to shape the detailed s gestures differently to achieve a similar s quality (40). The following phonetic properties, however, are common to all s sounds.

Linguo dental sounds:

Dental sounds (th) are made with the tip of the tongue extending slightly between the upper and lower anterior teeth. This sound is made closer to the alveolus (the ridge) than to the tip of the teeth (57,58,59). Careful observation of the amount of tongue that can be seen with the words this, that, these, and those will provide information as to the labiolingual position of the anterior teeth. If about 3 mm of the tip of the tongue is not visible, the anterior teeth are probably too far forward (except in patients with a Class II type 1 malocclusion), or there may be an excessive vertical overlap that does not allow sufficient space for the tongue to protrude between the anterior teeth. If more than 6 mm of the tongue extends out between the teeth when such th sounds are made, the teeth are probably too far lingual (60,61).

Palatolingual sounds:

The palatolingual sounds are produced by tongue, hard palate or soft palate. these truly palatal sounds present less of a problem for dentures. The velar sounds (k, g, and ng) have no effect on dentures, except when the posterior palatal seal extension encroaches on the soft palate.

Components of speech:

Speech is divided speech into 6 components:

1. Respiration.

2. Phonation.

3. Resonation.

4. Articulation.

5. Neural integration

6. the ability to hear sounds (56).

Most important factors affecting phonation:

1. Vertical Dimension:

Speech is used as an aid in the various ways to record maxilla-mandibular relation.Landa (1947) recommended various phonetic tests to determine proper vertical dimension using such sound as s, c, z. Silverman (1956) and Burnnet (1994) stated that sibilant sound “s” as a mean for determining the correct vertical dimension. The closest speaking space calculates the vertical relation when the mandible and its muscles are involve in physiologic function of speech (37,62,50).

The sounds like “m” and “S” are useful in determining the vertical dimension, when the sound “m” is pronounced there will be passive contact between the upper and the lower lip, which aid in obtaining the correct vertical dimension. ‘Morrison’ suggested the use of the word, ‘sixty six’ & ‘Mississippi’ to determine closest speaking space. when the patient pronounces the letter “S” the height of occlusal rims which are located in the mouth should be adjusted until the minimum space exists between the maxillary and mandibular occlusal rims (63,64,65).

2. Denture thickness:

A denture, which has a thick border at posterior palatal seal area, will be irritate the dorsum of the tongue resulting in faulty articulation of speech. In some cases, the denture may become unseated further blighting the speech. Careful evaluation in these situations will be reveal the dentures rise and fall with the tongue movements during speech (5). Seifert E, Runte C, Riebandt M, Lamprecht - Dinnesen A, Bollmann F (2000) concluded that variations of thickness and or volume of dentures and of the vertical and horizontal dimension of occlusion may result in unpredictable audible changes to the voice. Patients should be informed about possible effects of modified or new dentures on their voice (66,67). Many studies as Ravishankar and Singh and Mahroos reported that the lingopalatal (anterior) sounds as c, d, t, n, s, z, and r are formed by the contact of the tip of the tongue with the most anterior part of the palate called the alveolus or the lingual side of anterior teeth. If the denture base in the rugae area is too thick, the "t" in “tend” sounds like "d" and the "d" sounds like "t" (41).

One of the reasons for inappropriate articulation of speech is decreased air volume and loss of tongue space in the oral cavity, which occurs usually due to thick dentures. Most significant is the thickness of denture base covering centre of the palate, since creation of palatolingual sounds involves contact of tongue with palate and alveolar process of teeth. With the consonants “T” and “D”, tongue makes firm contact with the anterior part of the hard palate and suddenly drawn downwards to produce an explosive sound. When pronouncing letters “S”, “C”, “Z”, “R” and “L” sounds, contact occurs between tongue and most anterior part of hard palate. In case of “S”, “C” and “Z” sounds a slit like channel is formed between tongue and palate through which air escapes. If denture base is made too thick in these areas, the air flow will be obstructed that eventually causing impairment in the sounds (68,69).

3. The Occlusal Plane:

The labiodental sounds like “f”, “v” are helpful in determining the antero-posterior positioning of the upper incisors and the occlusal plane. because If the upper anterior teeth are arranged below the occlusal plane the letter “f” will be pronounced like “v” and if the upper anterior teeth are too short of occlusal plane the letter “v” will more likely pronounce as “f”. If the occlusal plane is set too high the correct positioning of the lower lip may be difficult, if on the other hand the occlusal plane is too low, the lip will overlap the labial surfaces of the upper teeth to a greater extent than is need (70,51).

4. Relationships of the upper anterior teeth with lower anteriors:

The upper and lower incisors must have close contact so that the air stream is allowed to escape through a slight opening between the teeth and the patient can pronounce the letter "S" (66,71,33). In abnormal jaw relationships, it is difficult for patient to form this sound, and it will probably necessitate adjustment of the upper and lower anterior teeth antero posteriorly, so that approximation can be brought about successfully. Silverman (1967) stated that the Whistle and Swish sounds are produced during speech due to air abnormally passing over the tongue and through the inter-incisal space. These sounds may be caused due to decreased overjet (9,42).

CONCLUSION:

The dentures should restore not only the esthetics and mastication for patients, but also the accurate functions of speech. Speech adaptation to new complete dentures normally takes place within 2 to 4 weeks after insertion. If all factors such as tooth position, occlusal plane, vertical dimension, thickness of palatal vault are correct and satisfactory, and the speech problem cannot be resolved by dental methods, persists in spite of providing the patient with phonetically correct dentures, then the patient should be referred to a speech pathologist or the dentist must contemplate the patient’s level of education.

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Received on 25.06.2018 Modified on 15.07.2018

Research J. Pharm. and Tech 2018; 11(11): 5173-5178.

DOI: 10.5958/0974-360X.2018.00945.9