Tooth eruption
Encyclopedia
Tooth development
Tooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. Although many diverse species have teeth, non-human tooth development is largely the same as in humans...
in which the teeth
Tooth
Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...
enter the mouth
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
and become visible. It is currently believed that the periodontal ligament
Periodontal ligament
The periodontal fiber or periodontal ligament, commonly abbreviated as the PDL, is a group of specialized connective tissue fibers that essentially attach a tooth to the alveolar bone within which it sits...
s play an important role in tooth eruption. The first human teeth to appear, the deciduous (primary) teeth
Deciduous teeth
Deciduous teeth, otherwise known as reborner teeth, baby teeth, temporary teeth and primary teeth, are the first set of teeth in the growth development of humans and many other mammals. In some Asian countries they are referred to as fall teeth as they will eventually fall out, while in almost all...
(also known as baby or milk teeth), erupt into the mouth from around 6 months until 2 years of age, in a process known as "teething
Teething
Teething is the process by which an infant's first teeth sequentially appear by emerging through the gums. Teething may start as early as three months or as late, in some cases, as twelve months. The typical time frame for the first teeth to appear is somewhere between six and nine months...
". These teeth are the only ones in the mouth until a person is about 6 years old. At that time, the first permanent tooth
Permanent teeth
Permanent teeth are the second set of teeth formed in humans. There are thirty-two permanent teeth, consisting of six maxillary and six mandibular molars, four maxillary and four mandibular premolars, two maxillary and two mandibular canines, four maxillary and four mandibular incisors.The first...
erupts and begins a time in which there is a combination of primary and permanent teeth. This stage, known as the mixed stage, lasts until the last primary tooth is lost. Then, the remaining permanent teeth erupt into the mouth.
Theories
Although researchers agree that tooth eruption is a complex process, there is little agreement on the identity of the mechanism that controls eruption. There have been many theoriesTheory
The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, , meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action...
over time that have been eventually disproven. One of the theories is that the tooth is pushed upward into the mouth by the growth of the tooth's root. Others advocated that a tooth is pushed upward by the growth of the bone around the tooth. In addition, some believed teeth were pushed upward by vascular pressure or by an anatomical feature called the cushioned hammock. The cushioned hammock theory, first proposed by Harry Sicher, was taught widely from the 1930s to the 1950s. This theory postulated that a ligament
Ligament
In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...
below a tooth, which Sicher observed under a microscope on a histologic slide, was responsible for eruption. Later, the "ligament" Sicher observed was determined to be merely an artifact created in the process of preparing the slide.
The most widely held current theory is that while several forces might be involved in eruption, the periodontal ligaments provide the main impetus for the process. Theorists hypothesize that the periodontal ligaments promote eruption through the shrinking and cross-linking of their collagen fibers and the contraction of their fibroblasts.
There is good evidence from experimental animals that a traction force is unlikely to be involved in tooth eruption: Animals treated with lathyrogens that interfere with collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
cross-link formation showed similar eruption rates to control animals, provided occlusal forces were removed.
Timeline
Although tooth eruption occurs at different times for different people, a general eruption timeline exists. Typically, humans have 20 primary teeth and 32 permanent teeth. Tooth eruption has three stages. The first, known as primary dentitionDentition
Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age...
stage, occurs when only primary teeth are visible. Once the first permanent tooth erupts into the mouth, the teeth are in the mixed (or transitional) dentition. After the last primary tooth falls out of the mouth, the teeth are in the permanent dentition.
Primary teeth
Primary dentition starts on the arrival of the mandibular central incisorMandibular central incisor
The mandibular central incisor is the tooth located on the jaw, adjacent to the midline of the face. It is mesial from both mandibular lateral incisors. As with all incisors, its function includes shearing or cutting food during mastication, commonly known as chewing. There are no cusps on the...
s, typically from around six months, and lasts until the first permanent molars appear in the mouth, usually at six years. The primary teeth typically erupt in the following order: (1) central incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
, (2) lateral incisor, (3) first molar, (4) canine
Canine tooth
In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, fangs, or eye teeth, are relatively long, pointed teeth...
, and (5) second molar. As a general rule, four teeth erupt for every six months of life, mandibular teeth erupt before maxillary teeth, and teeth erupt sooner in females than males. During primary dentition, the tooth buds of permanent teeth develop below the primary teeth, close to the palate or tongue.
Mixed stage
Mixed dentition starts when the first permanent molar appears in the mouth, usually at five or six years, and lasts until the last primary tooth is lost, usually at ten, eleven, or twelve years. Permanent teeth in the maxilla erupt in a different order from permanent teeth on the mandible. Maxillary teeth erupt in the following order: (1) first molarMaxillary first molar
The maxillary first molar is the tooth located laterally from both the maxillary second premolars of the mouth but mesial from both maxillary second molars....
(2) central incisor
Maxillary central incisor
The maxillary central incisor is a human tooth in the front upper jaw, or maxilla, and is usually the most visible of all teeth in the mouth. It is located mesial to the maxillary lateral incisor. As with all incisors, their function is for shearing or cutting food during mastication . There are...
, (3) lateral incisor
Maxillary lateral incisor
The Maxillary lateral incisor is a tooth that is located distally from both maxillary central incisors of the mouth and mesially from both maxillary canines. As with all incisors, their function is for shearing or cutting food during mastication, commonly known as chewing. There are no cusps on...
, (4) first premolar
Maxillary first premolar
The maxillary first premolar is one of two teeth located in the upper jaw, laterally from both the maxillary canines of the mouth but mesial from both maxillary second premolars...
, (5) second premolar
Maxillary second premolar
The maxillary second premolar is one of two teeth located in the upper jaw, laterally from both the maxillary first premolars of the mouth but mesial from both maxillary first molars...
, (6) canine
Maxillary canine
The maxillary canine is the tooth located laterally from both maxillary lateral incisors of the mouth but mesial from both maxillary first premolars...
, (7) second molar
Maxillary second molar
The maxillary second molar is the tooth located distally from both the maxillary first molars of the mouth but mesial from both maxillary third molars. This is true only in permanent teeth. In deciduous teeth, the maxillary second molar is the last tooth in the mouth and does not have a third...
, and (8) third molar. Mandibular teeth erupt in the following order: (1) first molar
Mandibular first molar
The mandibular first molar or six-year molar is the tooth located distally from both the mandibular second premolars of the mouth but mesial from both mandibular second molars...
(2) central incisor
Mandibular central incisor
The mandibular central incisor is the tooth located on the jaw, adjacent to the midline of the face. It is mesial from both mandibular lateral incisors. As with all incisors, its function includes shearing or cutting food during mastication, commonly known as chewing. There are no cusps on the...
, (3) lateral incisor
Mandibular lateral incisor
The mandibular lateral incisor is the tooth located distally from both mandibular central incisors of the mouth and mesially from both mandibular canines. As with all incisors, their function is for shearing or cutting food during mastication, commonly known as chewing. There are no cusps on...
, (4) canine
Mandibular canine
The mandibular canine is the tooth located distally from both mandibular lateral incisors of the mouth but mesially from both mandibular first premolars...
, (5) first premolar
Mandibular first premolar
The mandibular first premolar is the tooth located laterally from both the mandibular canines of the mouth but mesial from both mandibular second premolars. The function of this premolar is similar to that of canines in regard to tearing being the principal action during mastication, commonly...
, (6) second premolar
Mandibular second premolar
The mandibular second premolar is the tooth located distally from both the mandibular first premolars of the mouth but mesial from both mandibular first molars. The function of this premolar is assist the mandibular first molar during mastication, commonly known as chewing. Mandibular second...
, (7) second molar
Mandibular second molar
The mandibular second molar is the tooth located distally from both the mandibular first molars of the mouth but mesial from both mandibular third molars. This is true only in permanent teeth...
, and (8) third molar. Since there are no premolars in the primary dentition, the primary molars are replaced by permanent premolars. If any primary teeth are lost before permanent teeth are ready to replace them, some posterior teeth may drift forward and cause space to be lost in the mouth. This may cause crowding and/or misplacement once the permanent teeth erupt, which is usually referred to as malocclusion
Malocclusion
A malocclusion is a misalignment of teeth or incorrect relation between the teeth of the two dental arches. The term was coined by Edward Angle, the "father of modern orthodontics", as a derivative of occlusion, which refers to the manner in which opposing teeth meet.-Presentation:Most people have...
. Orthodontics
Orthodontics
Orthodontics, orthodontia, or orthodonture is the first specialty of dentistry that is concerned with the study and treatment of malocclusions , which may be a result of tooth irregularity, disproportionate jaw relationships, or both...
may be required in such circumstances for an individual to achieve a straight set of teeth.
Permanent teeth
The permanent dentition begins when the last primary tooth is lost, usually at 11 to 12 years, and lasts for the rest of a person's life or until all of the teeth are lost (edentulismEdentulism
Edentulism is the condition of being toothless to at least some degree; it is the result of tooth loss. Loss of some teeth results in partial edentulism, while loss of all teeth results in complete edentulism....
). During this stage, third molars (also called "wisdom teeth") are frequently extracted
Extraction (dental)
A dental extraction is the removal of a tooth from the mouth. Extractions are performed for a wide variety of reasons, including tooth decay that has destroyed enough tooth structure to render the tooth non-restorable...
because of decay, pain or impactions. The main reasons for tooth loss
Tooth loss
Tooth loss is when one or more teeth come loose and fall out. Tooth loss is normal for deciduous teeth , when they are replaced by a person's adult teeth. Otherwise, losing teeth is undesirable and is the result of injury or disease, such as mouth trauma, tooth injury, tooth decay, and gum disease...
are decay or periodontal disease
Periodontal disease
Periodontitis is a set of inflammatory diseases affecting the periodontium, i.e., the tissues that surround and support the teeth. Periodontitis involves progressive loss of the alveolar bone around the teeth, and if left untreated, can lead to the loosening and subsequent loss of teeth...
.