Physiology, Tooth


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Barret Zimmerman


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Updated:
2/11/2019 8:38:22 PM

Introduction

Teeth serve multiple functions beyond mastication including shaping the kinetics of phonation, breathing, the maintenance of a patent airway, and serving as a foundation for the vertical dimensions of the face.[1][2] Certain teeth have specialized roles in types of chewing with the entire group functioning as a dynamic entity.[3] These critical roles are why the loss of teeth can be so devastating.[4][5]

Development

Although specific odontogenic (meaning of or to relating to the teeth) embryology is beyond the scope of this article, a few points are worth noting. There exist three main dentitions; a primary dentition (colloquially referred to as baby teeth), a transitional dentition, and an adult or permanent dentition. The primary dentition begins forming as early as the fourteenth week in utero and generally erupts fully by 3 years.[6][7][8] The adult dentition is generally fully erupted by 15 years, except the third molars (also known as wisdom teeth).[9][10] The age which teeth erupt can vary significantly from person to person, and congenitally missing teeth can occur.

Function

The primary function of teeth is mastication: cutting, mixing, and grinding ingested food to allow the tongue and oropharynx to shape it into a bolus which can be swallowed.[11][12] The teeth are generally conceptualized as a U-shape, with the bottom of the U representing the front teeth. The upper arch is the maxillary arch, with the teeth embedded in the maxilla, and the lower arch is the mandibular arch, with the teeth embedded in the mandible. The shape of the crown (externally visible part of the tooth) in combination with arch mechanics and its position in the mouth determine a tooth’s function in chewing. 

The anterior teeth are called incisors, a direct reflection of their role in cutting food into smaller pieces, without performing any grinding function. There are eight total incisors in the standard adult dentition, two central and two lateral incisors on each arch. Moving posteriorly, the next tooth is the canine, colloquially known as the fang tooth.[13]

The canine is known as the cornerstone of the dental arch, which, along with the first molar, are regarded as the most important teeth; this is because of their position and for the canine’s role in jaw dynamics and “canine guidance,” which describes its role in helping to control how the teeth slide off of each other. Also, it often has the longest root and is, therefore, fastened the hardest to the bone.

Next are the premolars, there are eight total, four in each arch. They are the first teeth that assist in grinding and mixing food.[14] The last and most posterior are the molar teeth. Adults have eight molars, a first and second molar on each side for a total of four per arch. Many adults also have third molars (also called wisdom teeth). These are often extracted, for reasons beyond the scope of this article, but they rarely contribute to mastication.[15][16]

Occlusion is the term for how the teeth all fit together and interlock. In a proper occlusion, the maxillary arch and teeth sit slightly anterior to the mandibular arch and teeth. In a transverse dimension, the maxillary teeth sit more lateral than the mandibular teeth. Correct occlusion is important for the primary function of teeth; a tight meshwork allows for efficient mastication. When this fitment is not ideal, it is called malocclusion, which is the attribute that orthodontists (dentists who specialize in tooth movement) work to correct through therapies like braces. Causes of malocclusion can be dental or skeletal. Skeletal abnormalities should provoke further thought on whether this could be part of a larger syndrome (Pierre Robin, DiGeorge, etc.)

To better understand how the teeth function, one must understand the mechanics of the temporomandibular joint (TMJ), which is where the mandible articulates with the skull, directly in front of the acoustic, auditory canal. Numerous muscles and ligaments guide and control it, but at the most basic level, the mandible has two possible movements, rotation, and translation. When the mouth first opens, the mandible will rotate, and then at a certain point slide forward to allow further opening, which is called translating. This translational movement occurs by two joint spaces within the joint and a cartilaginous disc between the two, which makes it a ginglymoarthrodial joint, meaning it both hinges and slides.[17][18]

These mechanics translate to the relationship of the arches to one another. The lower jaw can protrude (like a bulldog), and move side-to-side (i.e., laterally). Together, the range of these movements is called the envelope of occlusion.[19]This envelope should allow for idealized tooth mechanics which minimize shear forces and maximize vertical ones. Teeth sit in the jaw bone but do not directly touch the bone. They are embedded in what amounts to a shock absorber called the periodontal ligament. This system is designed to withstand and exert forces in a vertical vector (chewing), which they can do effectively with hundreds of pounds of vertical force.[20] The teeth are weakest in shear (lateral) forces. Therefore, all the anatomical and mechanistic concepts outlined above work in concert to allow the teeth to grind, mix, and cut food vertically while avoiding shear forces on them.

The concept of the envelope of occlusion helps us understand how the teeth should function physiologically. When one bites together so that the teeth touch in a normal position (a good cue to help patients do this is to ask them to bite on their back teeth), and then slides the lower jaw forward slowly, one will note that the front teeth slide against each other, but the back teeth open and no longer touch. This is called anterior guidance and is an important secondary function of the anterior teeth (the incisors and canines). This function minimizes shear forces on the posterior teeth. From the same starting position with normal occlusion, if one, again keeping the teeth together, slides the lower jaw to one side, one will note that the teeth on the ipsilateral side touch, but the teeth on the contralateral side do not. In some people, only the canines touch here. This is again a function to help prevent shear forces in excursive, or sideways, movements.[21]

Beyond mastication, teeth have an essential role in phonation. If one makes a fricative sound, like saying the letter F, one will note that the maxillary incisors rest against or close to the vermillion border of the lower lip.

Finally, teeth, like bones, also participate in mineral exchange.  The inner layers of the tooth, closest to the circulatory supply and pulp are most active in this, although the enamel may exchange minerals with saliva.[22]

Clinical Significance

The eruption and composition of teeth is a process influenced by body metabolism, endocrine function, genetics, and local factors.[23][24][25] This process can be illustrated by the pathological disruption of any of these factors, resulting in odontogenic issues. For example, genetic diseases like osteogenesis imperfecta result in abnormally shaped and formed teeth, Gardner syndrome often manifests with multiple osteomas (extra teeth-like structures), and alterations in hormone levels, vitamin and mineral levels, and even ingestion of systemic levels of some drugs (i.e., tetracycline), can permanently alter the teeth.[26][27][28][29]

As described above, the teeth have a functional role in speech: loss of these teeth can affect the formation of the fricative sound, and by extension, many other sounds. Another important concept is called the vertical dimension of occlusion. Without teeth, or by extreme wear and destruction of teeth, the jaws will over-close. This results in very aged appearance accentuated nasolabial folds, and generally gives the appearance of a patient older than their chronological age, which speaks to the cosmetic function of teeth. As the soft tissues lose support, it can also affect phonation, bolus creation and swallowing, and can lead to a structural change of other bony parts in the face.[2][30][31][32]


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Physiology, Tooth - Questions

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Loss of which teeth will result in difficulty with phonation, particularly the fricative sounds?



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The teeth involved in anterior guidance are which group and what is the reason behind the anterior guidance?



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A patient has suffered a traumatic facial injury and has lost several teeth. If the most anterior teeth are lost, what is their name and function in phonation?



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A biomedical engineer asks for help designing a direct laryngoscopy blade that would minimize damage to the dentition. If one had to place a load on the teeth, how would this force ideally be applied?"



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Physiology, Tooth - References

References

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Traser L,Birkholz P,Flügge TV,Kamberger R,Burdumy M,Richter B,Korvink JG,Echternach M, Relevance of the Implementation of Teeth in Three-Dimensional Vocal Tract Models. Journal of speech, language, and hearing research : JSLHR. 2017 Sep 18     [PubMed]
Shrestha D,Rajbhandari P, Prevalence and Associated Risk Factors of Tooth Wear. JNMA; journal of the Nepal Medical Association. 2018 Jul-Aug     [PubMed]
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Adeyemo WL, Do pathologies associated with impacted lower third molars justify prophylactic removal? A critical review of the literature. Oral surgery, oral medicine, oral pathology, oral radiology, and endodontics. 2006 Oct     [PubMed]
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Nickel JC,Iwasaki LR,Gonzalez YM,Gallo LM,Yao H, Mechanobehavior and Ontogenesis of the Temporomandibular Joint. Journal of dental research. 2018 Oct     [PubMed]
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