Wisdom teeth are the third molars of human beings. The teeth grow on the human jawbone, and the growth and development of the jawbone is completed at the age of about eighteen. On average, each person has 2-3 wisdom teeth, and up to one person can have 4 wisdom teeth. However, because the wisdom teeth in the jaw bone growth, development and eruption process is often affected by various factors and interference and can not be normal eruption, according to incomplete statistics of the maxillary wisdom teeth accounted for 8.5%, the mandible accounted for 52.4%. It can be seen that the number of obstructed wisdom teeth, the large number of people affected. So, what factors cause wisdom teeth obstruction? Let’s start with the analysis of the growth and development of the jaw bone.
First, the growth and development of the jawbone
The jaw bone is divided into the maxilla and mandible, the lower end of the maxilla and the upper end of the mandible is the alveolar ridge, the upper and lower alveolar ridge has 20 milk teeth first, and then there are 28 – 32 permanent teeth to replace the eruption. At the age of 6, the first molar erupts, commonly known as the “sixth age tooth”. After the upper and lower sixth teeth form an occlusal relationship, the dental relationship is also determined. The maxillary dental arch overlaps the mandibular dental arch. Although the growth and development of the maxilla and mandible are still going on at the same time, the growth and development of the mandible is controlled by the growth and development of the maxilla and the dental relationship.
1.The growth and development of the mandible
The mandible develops from the mandibular eminence of the first cheek arch, and a mandibular cartilage, also called Mechel cartilage, is first formed in the center of the mandibular eminence. By the 6th week of embryonic life, this cartilage is fully formed and a connective tissue aggregate appears lateral to the Mechel cartilage at the intersection of the incisal nerve and chin nerve, differentiating into osteoblasts, intra-membranous ossification, and the formation of the initial mandible. From there, the center of ossification gradually expands posteriorly below the inferior alveolar nerve and anteriorly along the inferior aspect of the incisal nerve to form bone tissue.
At the same time ossification also extends upward on both sides of the aforementioned nerve, gradually forming the medial and lateral bone plates of the mandibular body and the inferior alveolar and incisal nerve canals. The mandibular branch occurs from another ossification center, first a dense embryonic connective tissue appears posteriorly above the mandibular foramen, later intraperiosteal osteogenesis occurs and ossification forms the mandibular ascending branch and rostral process.
The condyle is posterior to the ossification center of the mandibular branch and is a conical cartilage. The condylar cartilage fuses with the ossification center of the mandibular body to form the mandible. The condylar cartilage undergoes endochondral ossification and forms the condylar head as it grows. Near birth, the fusion of the condyle with the mandibular body out i.e. the bone tissue at the angle of the mandible continues to form and muscle attachment occurs, resulting in enhanced union, while the growth of the condylar head increases the height of the condyle. However, most of the mandibular growth occurs after birth.
There are two main ways in which the mandible grows and develops after birth, with the exception of intracondylar osteogenesis at the condyle, where the increase in size of the mandible is mainly formed by intraperiosteal osteogenesis. This deposition of subperiosteal bone surface matrix is in turn related to the action of muscles, the growth of the condyle and the eruption of teeth, and thus determines the growth of the mandible, in accordance with the theory of functional growth.
(1) Vertical growth of the mandibular body: As the tooth germ develops in the mandible, the alveolar bone also develops; at the same time, new bone is constantly formed at the lower edge of the mandible, which increases the vertical height of the mandibular body.
(2) Growth of mandible in anterior-posterior direction: It mainly relies on bone formation at the posterior edge of the mandibular ascending branch and bone resorption at the anterior edge, so that the mandibular ascending branch moves back and the mandible lengthens. At the same time, bone formation is faster than bone resorption, and the width of the ascending branch increases. Insufficient growth of mandibular bone length is an important cause of wisdom tooth obstruction.
(3) Growth of the mandible in the internal and external direction: new bone is deposited on the outside of the bone plate and corresponding bone resorption on the inner surface of the bone plate, which increases the volume of the mandible while the bone plate is able to maintain a certain thickness.
(4) Growth of the mandibular condyle and rostral process: The condyle is the main growth center of the mandible and is the last of the maxillofacial bones to stop developing. During the fetal period, secondary cartilage appears on the surface of the formed ossified condyle and rostral process, and the cartilage is still thickening, while the side near the bone tissue is gradually ossifying, and due to the continuous proliferation and ossification of the condyle cartilage and rostral process cartilage, the mandibular ascending and rostral processes continue to grow. The condylar process is completely ossified and no longer grows until about 20-25 years old.
2.The growth and development of the maxilla
The maxilla is developed from the maxillary process, lateral nasal process and middle nasal process of the first gill arch, which form the maxilla of the posterior region, maxillary frontal process and premaxilla respectively. The maxilla develops through intraperiosteal ossification, with the center of ossification located at the infraorbital nerve emanating from the superior anterior dental nerve and at the anterior maxilla. From the center of ossification, the maxilla grows in all directions, forming the maxillary frontal process downward, the zygomatic process backward, the palatal process inward, the alveolar process downward, and the surface tissue of the maxilla forward.
The newborn maxilla is dense, short and wide, and later grows downward, forward and outward mainly due to surface proliferation of bone and interstitial proliferation of bone sutures, which increases the length, width and height of the maxilla. The lateral bone plate of the maxilla is thinner than the medial bone plate, and the bone of the maxillary tuberosity, where the maxillary wisdom teeth are located, is also thinner.
The maxillary sinus begins to form in the third trimester of embryo and remains a primitive structure at birth, with a diameter of about 5-10 mm. its development is mainly after birth, and the maxillary sinus development is basically completed at the age of 12-14 years, and later, as the maxillary sinus grows in the direction of the alveolar process, the maxillary sinus is very close to the root of the tooth, and the development is completely completed at the age of 18 years.
Second, the growth and development of wisdom teeth and eruption
1, the growth and development of wisdom teeth
The growth and development of the tooth is a continuous process, including the development of the tooth germ, tissue formation and eruption. The embryo of wisdom teeth starts to form in the mandibular branch at the age of 4-5 years, the hard tissue starts to form at the age of 7-10 years, and the eruption starts at the age of 17-21 years.
(1) Development of the tooth embryo
The dental embryo is formed by the proliferation of connective tissue cells at the end of the dental plate and is composed of three main parts.
Enamel-forming apparatus: originates from the oral ectoderm and forms enamel.
The papilla: originates in the ectodermal mesoderm and forms the pulp and dentin.
The dental capsule: originates in the ectodermal mesoderm and forms the dentine, periodontium and intrinsic alveolar bone. It is the ectodermal mesenchymal connective tissue that surrounds the enamel-forming apparatus and the surface of the dental papilla and plays an important role in the process of tooth eruption.
After the formation of the papillae, a corresponding permanent tooth germ is formed at the lower lingual end of the tooth germ, while the permanent molar germ is formed by the posterior free end of the tooth plate growing towards the distal center and keeping in line with the length of the maxillary and mandibular arches. At the beginning, there is not enough space in the jawbone to accommodate these dental embryos, so the upper and lower molars first develop with their conjunctival surfaces toward the distal-middle and proximal-middle directions, respectively, and then move to their normal positions due to the development of the jawbone and the growth of the tooth roots.
(2) Formation of dental tissues
The formation of dental hard tissues starts from the growth center, which is located at the cusp of the molar, and each growth center forms a growth lobe, and the fusion of the growth lobe forms a developmental sulcus.
(1) Formation of enamel and dentin
Dentin-forming cells first form a layer of dentin and recede toward the center of the pulp, followed by enamel-forming cells that secrete a layer of enamel and recede toward the periphery, and so on in a crosswise manner, depositing layers until they reach the thickness of the crown. Wisdom tooth crowns are fully formed at approximately 12-16 years of age. (See Figure 1)
② Formation of the tooth root
The root of the tooth begins to occur when the crown is about to complete its development. The root grows from the epithelial root sheath, which is surrounded by a papilla that differentiates into dentin-forming cells and forms the root dentin, within which the papilla forms the pulp. The epithelial root sheath continues to grow away from the crown and bends at an angle of 45 toward the pulp, forming a disk-like structure called the epithelial septum. The epithelial septum encloses a hole open to the pulp, which is the future apical foramen.
This is the way the roots develop before the formation of the bifurcation zone of multiple roots. When multiple roots are formed, first two or three lingual protrusions grow on the epithelial septum, which are connected to the contralateral protrusions, dividing the single foramen into two or three holes, forming two or three roots in the future, each with the same growth rate. During the development of the roots, the position of the epithelial compartment remains unchanged, and as the roots elongate, the germ moves toward the oral cavity and provides clearance for the continued growth of the roots.
(3) Formation of periodontal tissue
When the tooth root is formed, the cells in its surrounding capsule differentiate into adult osteoblasts, fibroblasts, and osteoblasts, forming dental bone, periodontium, and alveolar bone, respectively.
2.Emergence of wisdom teeth
Tooth eruption is a complex process in which the crown is formed to move toward the symphyseal surface and cross the bone crypt and oral mucosa to reach the functional position.
(1) Eruption mechanism
Tooth eruption is a complex process involving multiple factors, and the mechanism of eruption is not very certain.
① Resorption of alveolar bone
During tooth eruption, there is extensive bone remodeling in the alveolar crypt, osteoclast-regulated bone resorption in the symphysis, and bone resorption in the basal area around the developing tooth root. Alveolar bone resorption widens the guide canal on the symphyseal side of the permanent tooth embryo to accommodate the passage of the crown, a process that is important in tooth eruption, and in the case of unerupted teeth buried in the alveolar bone, the key to eruption is the formation of an eruptive channel by bone resorption. In addition, the formation of new bone in the apical part of the developing tooth is also one of the driving forces of tooth eruption.
②The role of the dental capsule
During eruption, bone resorption in the symphysis forms the eruption channel, bone formation in the root bifurcation area, root growth and bone deposition at the base move the erupting tooth to the eruption channel. The capsules in the symphyseal region are connected to the lamina propria of the oral mucosa and form a guide canal, where the mononuclear cells increase during eruption and receive signals from the enamel-forming epithelium, which then migrate to the surface of the bone traps to transform into osteoclasts and absorb the alveolar bone to form the eruptive channel as the tooth moves crown-side.
The root then grows to fill the passively generated space, the apical capsule tissue provides the necessary space for root formation and guides the root to grow in the normal direction, the deposition of alveolar bone at the base is also one of the driving forces for tooth eruption; around the root, the capsule tissue forms the periodontium, in which the majority of fibroblasts contract to drive the periodontium fibers to contract and pull the tooth to move in the eruption direction, this change is the result of the tooth entering the mouth This change begins to occur after the tooth enters the mouth. In addition, the dental capsule can also secrete regulatory factors that interact with cytokines secreted by the stellate reticular layer to regulate the eruption of the tooth.
It has been suggested that the permanent dental embryo is in a hydrostatic pressure equilibrium system in the alveolar bone, and the equilibrium is disrupted when the milk tooth is subjected to occlusal or orthodontic forces, and the dental capsule surrounding the permanent dental embryo transforms the hydrostatic pressure into biochemical signals from the intra- and extracellular matrix, resulting in a series of morphological, cytological and biochemical changes that regulate tooth eruption.
(2) The process of eruption can be divided into three phases as follows.
① Pre-eruption
The main change is the movement of the tooth embryo within the alveolar bone during the formation of the tooth root. This movement is carried out in two ways: one is the overall movement of the tooth embryo, and the other is the change of the center of the tooth embryo by keeping one part of the tooth embryo fixed while the other part continues to grow. The former refers to the movement of the tooth germ as a whole in the direction of bone formation due to bone tissue resorption and its formation in the opposite direction during jaw bone development; the latter refers to the formation of the tooth root when the epithelial compartment is in a fixed position and the crown gradually moves in the direction of the oral mucosa and the height of the alveolar process increases.
②Emergence period
The tooth enters the mouth through the eruption channel to the occlusal contact, starting from the root formation 2/3. Wisdom tooth eruption usually occurs at the age of 17-21 years.
③Late eruption period
Also called the functional eruption period, when the tooth erupts until the occlusion is established, the surrounding periodontal and bone tissues are remodeled, the apical dentine and dentin are deposited, and the root tip is fully formed, generally after 2-3 years, and the wisdom tooth root is fully formed at about 18-25 years of age. The continuous wear of the symphyseal surface is also compensated by a slight movement of the tooth in the syncline, in addition to a slight lateral movement of the tooth.
Third, the relationship between jawbone, wisdom tooth development and wisdom tooth obstruction
1. Congenital factors
(1) Degeneration of jaw bone development
With the continuous refinement of food, human chewing function decreases, and the growth and development of jaw bone also has obvious degeneration, mainly manifested by the reduction of the length, width and height of the mandibular branch and jaw arch, of which the width of the mandibular branch and the alveolar recession are the most obvious. In addition, the mandibular angle becomes smaller with age, the resorption of the anterior edge of the mandibular branch is reduced, and the lack of mandibular length growth is also the cause of mandibular wisdom teeth obstruction.
(2) Abrasive gaps
In primitive humans, the height of the teeth and the width of the proximal and distal middle are constantly worn away, so that the teeth behind can be tilted to the proximal middle in turn, and due to the wear between the neighboring teeth, the wisdom teeth have sufficient space to erupt and avoid ambiguous obstruction. Modern humans because of the reduction in tooth wear, wisdom teeth eruption time and postponed to 18-25 years old, the growth of the jawbone has nearly stagnated, wisdom teeth eruption by resistance to increase.
(3) Heredity and mixed descent
Heredity and pedigree mixing is also one of the reasons. There are three types of hereditary traits: one is mixed, the other is mixed, and the third is mutually exclusive. The mixed type is a mixture of traits from both parents, and the offspring form new traits that are different from any parent. The hybrid type is a mixture of offspring expressing the shapes of both parents. Mutually exclusive type refers to the offspring showing only the genetic traits of one parent alone.
(4) Pathological factors
Some systemic diseases or local lesions affect the development of the jaw length, resulting in tooth misalignment or obstruction, such as congenital anomalous jaw syndrome, cartilage hypoplasia, and endocrine disorders, such as hypothyroidism or hypopituitarism, can also affect the growth of the jaws, delayed tooth eruption, and wisdom teeth obstruction. Trauma to the jaw bone or infection or temporomandibular joint ankylosis and other diseases can disrupt the growth of the jaw bone and lead to dental obstruction or malocclusion.
2.Acquired factors
(1) Changes in the angle of inclination of the crown of the wisdom tooth and the root morphology of the tooth.
As mentioned earlier, the mandibular wisdom tooth is in the mandibular branch of the development of the beginning of its tooth surface and the mandibular plane is a certain angle that is the anterior inclination angle, to obtain normal eruption, must undergo the upright process, the smaller the anterior inclination angle is conducive to eruption. The change in the anterior inclination angle may be caused by the fact that the growth rate of the proximal and distal portions of the crown of the wisdom tooth and the proximal and distal roots are not exactly the same. If the growth of the proximal part of the crown and the proximal root precedes or is greater than the growth of the distal middle of the crown and the distal root, it may cause the wisdom tooth to erupt upright, and the proximal root, which is curved to the distal middle, can often be seen on the x-ray and is slightly longer than the distal root; if the growth of the distal part of the crown and the distal root precedes or is greater than the growth of the distal middle of the crown and the distal root, it may cause the wisdom tooth to tilt to the proximal middle and appear as a proximal block, which can often be seen on the x-ray. The distal mesial root can be seen on the x-ray, which is curved to the proximal mesial and is slightly longer than the proximal mesial root.
(2) Distance between the distal middle of the second molar and the mandibular ascending branch
The horizontal distance between the high point of the distal mesial profile of the crown of the second molar and the mandibular ascending branch should be greater than or equal to the distance between the proximal and distal mesial crowns of the third molar in order for the wisdom tooth to have the possibility of eruption. (See Figure 3 and Figure 4)
(3) Anterior extension occlusion
The right lower wisdom tooth is obstructed by two factors: one is the angle of inclination of the crown; the other is the small gap between the proximal mesial margin of the wisdom tooth and the distal mesial gap of the second molar.
(4) Clearance between the distal mid of the second molar and the proximal mid of the wisdom tooth crown margin
Even if the occlusal surface of the crown of the third molar, the angle of inclination is less than 35 degrees or the angle that can erupt, even if the gap between the high point of the distal mesial profile of the crown of the second molar and the proximal mesial of the mandibular ascending branch can reach the eruption of the third wisdom tooth, if the proximal mesial margin of the third molar is very close to the distal mesial crown, neck or root of the second molar or has been in contact or can enter, then the wisdom tooth cannot reach the purpose of eruption, therefore, the distal mesial crown of the second molar, the cervical root Therefore, the horizontal distance between the level of the distal mesial crown and the proximal mesial incisive margin of the third molar crown is also a factor in whether the wisdom tooth erupts.
(5) Occlusal relationship of the first molars and wisdom teeth obstruction
The occlusal relationship between the maxillary and mandibular dentition is related to the eruption of wisdom teeth, and we rarely see the phenomenon of wisdom teeth obstruction in patients with anticlinal cooperation, while patients with microconglomerative deformity not only often have obstruction of the third molar, but also often have crowding of the anterior teeth. The clinical finding is that mandibular obstructive wisdom teeth are associated with the jaw relationship between the upper and lower sixth teeth and the coverage of the maxillary dentition over the mandibular dentition.