Teeth Alignment and Adult Speech: What Moving Teeth Can and Cannot Change

Speech production involves shaping a column of air using the tongue, lips, palate and teeth. For most sounds, the teeth play a minor role. For a small number, they are essential — and for those specific sounds, tooth position genuinely matters.

The difficulty with the popular version of this topic is that it implies a simple relationship: crooked teeth cause speech problems, straightening them resolves the problems. The clinical reality involves two separate components — an anatomical one and a learned motor one — and treatment usually needs to address both.

Speech sounds are classified by where and how the airflow is obstructed. Only a few categories involve the teeth.

Sibilants: /s/ and /z/. These are produced by channelling a narrow jet of air along a groove in the tongue and directing it against the edges of the upper incisors. The turbulence created at the incisal edges produces the characteristic hiss. This requires a precise relationship — the tongue tip close to but not touching the alveolar ridge behind the upper front teeth, and a small, controlled gap between the upper and lower incisors. Anything that disrupts that geometry affects the sound.

Post-alveolar sounds: /ʃ/ as in "ship", /tʃ/ as in "church". Similar principles, with the constriction slightly further back.

Labiodental sounds: /f/ and /v/. Produced by placing the lower lip against the upper incisal edges. Requires the upper incisors to be in a position the lower lip can reach.

Interdental sounds: /θ/ and /ð/ as in "think" and "this". The tongue tip contacts or protrudes slightly between the incisors.

Alveolar stops: /t/, /d/, /n/, /l/. The tongue tip contacts the ridge behind the upper front teeth. The teeth themselves matter less here, but the position of that ridge relative to the tongue at rest matters.

Notably, vowels, /k/, /g/, /m/, /b/, /p/, /h/ and several others involve the teeth barely or not at all. This is why most malocclusion produces no perceptible speech effect whatsoever.

Anterior open bite — where the front teeth do not meet when the back teeth are together — is the malocclusion most strongly associated with speech effects. The persistent gap allows air to escape at the front during sibilants, producing a lisp, and the tongue often habitually occupies the space. This is the clearest anatomical case.

Large diastema or spacing between the upper incisors allows air to escape laterally or through the gap, sometimes producing a whistling quality on /s/. Our article on how quickly aligners can close a gap covers treatment.

Severe overjet places the upper incisors forward of where the lower lip naturally sits, which can affect /f/ and /v/ and alter the sibilant channel. Our article on treating overjet with aligners covers this.

Deep bite — where upper incisors substantially overlap the lower — can restrict the space available for the tongue tip. Our article on clear aligners for deep bites covers the treatment.

Missing front teeth remove the surface against which several sounds are formed, and the effect can be marked.

Crowding and rotations rarely produce any noticeable speech effect. This is worth saying plainly, because it is the most common reason adults seek alignment.

Here is why moving teeth is often not sufficient on its own.

An adult who has had an open bite since childhood learned to speak with that anatomy. The tongue movement pattern used for /s/ was established at around three or four years of age and has since been repeated hundreds of thousands of times. It is an automated motor programme, executed without conscious attention, and it was optimised for the mouth as it was.

Close the open bite and you have removed the anatomical obstacle. You have not altered the motor programme. The tongue continues to do what it has always done, and the lisp frequently persists — sometimes sounding different, but still present.

This is well recognised in clinical practice. Orthodontic correction improves speech in some adults spontaneously, particularly where the anatomical obstacle was severe and recent. In others it does not, and what is needed is speech and language therapy to retrain the pattern, which is usually far more effective once the anatomy permits the correct movement.

There is a related point about causation running in the other direction. A tongue that habitually rests between the front teeth, or thrusts forward during swallowing, can itself contribute to an open bite and can cause it to reopen after orthodontic correction. Where a tongue habit is present, addressing it is part of the plan rather than an afterthought — otherwise the relapse risk is substantial.

A different and much more common scenario: people who had no speech difficulty developing a temporary lisp when they start wearing aligners.

This happens because the aligner adds a layer of material — typically under a millimetre thick — over the palatal surfaces and incisal edges. The tongue has been calibrated to those surfaces for decades. Changing them by even a fraction of a millimetre disturbs the sibilant channel.

The adjustment is usually quick. Most people notice a change on the first day or two and adapt within a week, and many report that subsequent trays cause no difficulty at all because the adaptation has generalised. Reading aloud for a few minutes daily accelerates it noticeably, because it provides the deliberate practice the motor system needs.

The effect tends to be more pronounced with attachments on the front teeth, and with thicker aligner materials. Our article on aligner materials and the science of constant force covers the material differences.

For people whose work depends on their voice, the practical advice is to start a new tray in the evening rather than before a day of presentations. Our article on aligners for teachers and lecturers covers the professional considerations, and our article on aligners for models and actors addresses similar concerns.

Fixed braces produce a similar but often longer adjustment period, particularly where brackets are placed on the inner surfaces of the teeth. Our article on comparing aligners with fixed braces covers the broader differences.

Retainers matter here too. A bonded retainer on the inner surface of the upper front teeth sits directly in the sibilant channel and can produce a persistent minor speech change in a small number of people — usually an alternative retainer design can be considered if it does not settle. Removable retainers worn at night rarely cause difficulty, since speech is not required while asleep.

Where speech is the primary concern, a useful assessment involves several elements.

A dental examination establishes the malocclusion, the resting tongue position, the swallowing pattern and whether there are missing teeth. It is worth recording speech, because patients often cannot hear their own pattern accurately.

Where the effect is significant, joint assessment with a speech and language therapist is valuable. The therapist can determine which sounds are affected, whether the pattern is anatomically constrained or habitual, and whether retraining is likely to succeed before, during or after orthodontic treatment. Sequencing matters — retraining a pattern that the anatomy cannot accommodate is unlikely to succeed, but waiting until orthodontics is finished may allow the old pattern to become entrenched in the new anatomy.

It is also important to exclude non-dental causes. Neurological conditions, hearing impairment, structural differences in the palate, and tongue-tie all affect speech and are not addressed by moving teeth.

• Only a limited set of sounds depend on tooth position: sibilants, labiodentals, interdentals and some alveolar sounds.

• Anterior open bite, large spacing, severe overjet and missing front teeth are the malocclusions most likely to affect speech.

• Crowding and rotations rarely produce a perceptible speech effect.

• Speech is a learned motor pattern; correcting the anatomy does not automatically change the pattern.

• Speech and language therapy alongside orthodontic treatment is often what produces the change.

• Temporary lisping when starting aligners is common and usually settles within a week.

If you have concerns about how tooth position affects your speech, an assessment can establish what is anatomical and what is habitual. You can contact our team to arrange an appointment, and treatment may involve clear aligner therapy.

This article is provided for general information only and does not constitute dental or medical advice. Speech difficulties have many possible causes, including non-dental ones, and should be assessed by appropriate professionals. The suitability of orthodontic treatment can only be determined through clinical examination, and outcomes vary between individuals.

Next review due: 9 September 2027