In orthopedic dentistry, the term "occlusion" is used. By it is understood the closing of teeth. There are 4 main occlusions and many intermediate ones. The first include central, front and 2 side.
Central occlusion is distinguished by the maximum contact of the surfaces of the opposed teeth being closed. It is considered the initial and final stage of articulation, since the first stage begins with the exit of the lower jaw from the state of central occlusion, and the last ends with bringing it to its original state.
The articulation in dentistry is the whole complex of movements (chewing and non-chewing) performed by the lower jaw, possible options for occlusion.
One type of articulation is central occlusion. With it, the muscle fibers that raise the lower jaw are maximally and uniformly strained on both sides.
Signs of a correct bite
They are used to determine central occlusion (or central jaw ratio ). The correct bite in dentistry is called orthognathic. It is determined by the following criteria:
- On the upper jaw, each tooth is located opposite (antagonizes) the same name and behind the lower one. Each lower, in turn, antagonizes with the same upper tooth, standing in front. Exceptions are the central incisors, as well as the last teeth located on the upper jaw. They are located opposite only the lower teeth of the same name.
- The central incisors of the lower and upper jaws are separated by one median line.
- The front lower teeth overlap by approximately 1/3 of the height with the upper front teeth.
- The medial (lying inward, closer to the midline) vestibular tubercle on the upper first molar (third tooth from the end) is located in the transverse groove of the first lower molar.
It is worth saying that these signs can be detected only in the intact (intact, non-pathological) bite.
Specificity of application of criteria
As practice shows, most people lose the first molars in the first place, the relative position of which determines the content of the fourth symptom.
If we talk about the third criterion, then, as a rule, it is not applied in determining the central ratio of the jaws .
The most reliable in clinical terms are the first two signs. The essence of central occlusion is the maximum contact of the surfaces of the teeth located opposite each other, regardless of their number. Accordingly, with an intact bite or such a number of teeth that would be sufficient to determine the central ratio of the jaws , you can use the signs characteristic of their ethnic or even pathological position. The fact is that the latter also differs, albeit with a distorted, but characteristic mutual arrangement of the jaws.
If, due to secondary (acquired) adentia (partial / total loss of teeth), the number of signs decreases, the determination of the central ratio of the jaws can be carried out with a thorough study of the facets (flat surfaces) of the last pair of oppositely located (antagonizing) teeth. With their complete absence, the state of central occlusion is determined by indirect signs.
Central jaw ratio: definition
In the presence of opposing teeth, the central ratio is easy to determine. Difficulties arise when the patient does not have them.
In the second case, the specialist needs to establish the most advantageous functionally central jaw ratio. The position is determined in three planes mutually perpendicular to each other: horizontal, frontal and sagittal (longitudinal). However, the doctor does not have the necessary guidelines.
Of course, with the complication of the task, the probability of medical errors in determining the central ratio of the jaws increases.
Incorrect vertical sizing: consequences
The interalveolar height (the distance between the jaws) is determined in the frontal plane. A correct understanding of this process will eliminate errors in determining the central ratio of the jaws . Each incorrect movement provokes certain morphological and functional disorders with characteristic symptoms.
For example, with an increase in vertical size (interalvolar height), there is a knock on the teeth during meals, and in some cases when talking. In addition, patients talk about fatigue of chewing muscles.
The decrease in interalveolar height causes even more negative consequences.
So, with a decrease in the distance between the parts fixed by the prosthesis, the vertical size of the lower third of the face decreases. In this case, the upper lip becomes shorter, the nasolabial folds become deeper, the corners of the mouth drop. As a result, a person’s face acquires senile features. Often you can observe maceration of the skin in the corners of the mouth (pathological swelling that occurs with prolonged contact with water).
It is also worth saying that a decrease in vertical size leads to a decrease in the functionality of the prosthesis. This fact has been proven by chewing tests.
Together with a decrease in the distance between the jaws, the oral cavity also decreases. This, in turn, entails constraint in the movements of the tongue, speech disorders. Accordingly, in this case, patients can talk about the rapid fatigue of the chewing muscles.
Errors in determining the central ratio of the jaws lead to a change in the position of the mandibular head in the articular fossa. The head moves deeper, and the thick posterior layer of the articular disc puts pressure on the neurovascular bundle. In this area, patients often begin to experience pain.
Incorrect determination of the interalveolar height affects the design of prostheses. In case of its overstatement, the products become massive. When the height is underestimated, the dentures are low with short teeth.
Determination of the central ratio of toothless jaws
The process includes:
- Preparation of bite rollers.
- Determination of the vertical distance between the jaws.
- Determination of the central position of the lower jaw.
- Drawing lines on rollers.
- Bonding models.
Let's consider some stages separately.
Roller preparation
During this phase:
- Borders of wax patterns are being specified.
- The vestibular surface and thickness of the upper roller are formed.
- The height of the upper roller is determined.
- A prosthetic plane is formed. It acts as a guideline for the proper placement of staged glass.
Clarification of the boundaries consists in eliminating interference with the fixation of the roller on the prosthetic bed. It helps prevent deformation of the upper lip. The technician checks all the boundaries of the template, releasing from it the frenum of the tongue, lips, cheeks, pterygo-maxillary and lateral folds of the mucosa.
A number of circumstances affect the formation of the thickness of the bite of the upper roller and the vestibular surface.
Atrophy after tooth loss manifests itself in different areas in different ways. On the lower jaw, for example, the bone first decreases from the lingual surface and the top of the crest. On the upper jaw, on the contrary, the bone begins to disappear from the apex and vestibular surface.
At the same time, the alveolar arch narrows, the conditions for staging teeth significantly worsen. In the anterior section, a lowering of the upper lip is noted, as a result of which the face acquires senile features.
The height of the upper roller is determined taking into account the following factors. The cutting edges of the upper central incisors with closed jaws coincide with the line of contact of the lips. When talking, they protrude about 1-2 mm from under the lip. A person looks several years older if the edges of the incisors are not visible with a smile.
The template is inserted into the mouth, and the patient is asked to close his lips. A line is drawn on the roller along which the height is set. If the edge of the platen is below the line of contact, it is shortened, if it is higher, it is increased with a strip of wax. Then the height of the roller is checked with a half-open mouth. Its edge should protrude 1-2 mm from under the upper lip.
Having determined the height of the roller, the specialist brings the occlusal surface in accordance with the pupil line. Two rulers are used for this. One is installed on the pupil line, the other on the occlusal plane of the roller. If they are parallel, then all actions were performed correctly.
Lateral departments
As a result of measuring a large number of skulls, it was revealed that the occlusal surface of the posterior teeth is parallel to the Camper horizontal. This is the line of contact of the lower edge of the auditory (external) passage and the nasal spine.
On the face, the horizontal runs along the nasal line, which connects the base of the wing with the middle of the tragus.
Two rulers are also used to check parallelism.
Adjustment of the lower and upper rollers
When fitting, it is important to achieve complete closure of the elements in the anteroposterior and transverse (transverse) directions and the location of the buccal regions in the same plane.
Corrections that may be necessary are carried out only on the lower roller. For well-fitted elements, the surfaces are in close contact over the entire length. When the jaws are closed, they fit both in the lateral and in the anterior regions.
First you need to check the contact in the anteroposterior direction. In case of non-simultaneous closure, a shift of the roller can be noted. All identified deficiencies are eliminated by building or removing wax in the corresponding sections of the roller.
Cross direction
When determining the central ratio of the jaws in the complete absence of teeth in the patient, it is rather difficult to identify violations of the contact of the occlusal areas of the ridges in the transverse direction.
When closing the mouth, they first fit on the right, and then on the left. In some cases, this violation is invisible visually . This is due to the fact that with closed rollers there is no clearance between them. This situation, in turn, is caused by the fact that the patterns are sagging on the one hand. Accordingly, a gap is formed between the mucosa of the alveolar ridge and the rollers, which is not visible to the specialist.
To detect it, a cold spatula is inserted between the elements. If the rollers have a snug fit and lie on the same ridge, the tool cannot be inserted effortlessly.
Determination of interalveolar height: general information
It consists in finding the distance between the processes of the jaws, the most convenient for the work of muscles and joints, ensuring better fixation and work of the prosthesis. When determining the central ratio of the jaws with complete loss of teeth according to the indicator of interalveolar height, the contours of the face are restored. Thus, the aesthetic part of the prosthetics issue is also solved.
Finding the interalveolar height, in fact, acts as a stage in determining the vertical component of the
central ratio of the jaws. The determination of distance is currently carried out in two ways: anatomical and anthropometric. Let's consider them in more detail.
Anthropometric method
When applying it, the following landmarks are used:
- AC line is divided by point B in the middle and extreme relation;
- the ac line is divided in the same way by point b, and the ac or ab line by d;
- Frankfurt horizontal - Fe;
- nasal line - cl e.
The anthropometric method for determining the central ratio of the jaws is based on information about the proportionality of individual areas of the face.
The 19th-century German philosopher and poet Adolf Zeising, in his works, developed the law of proportionality of division. He found several points through which the human body is divided according to the principle of the "golden section". Their finding is associated with rather complex mathematical constructions and calculations. Facilitates the solution of the problem using the Göringer compass. This tool automatically determines the desired section point.
The technique for determining central occlusion and jaw ratio is as follows. The patient should be asked to open his mouth wide. The extreme leg of the Goeringer compass is superimposed on the tip of the nose, and the second on the chin tubercle. The distance between them will be divided by the middle leg in the middle and extreme positions. A larger indicator corresponds to the distance between the points with adjacent rollers or teeth.
There is another technique for determining the central jaw ratio - according to Wordsworth-White. It is based on the equality of the distances from the center of the pupils to the line of contact of the lips and from the base of the nasal septum to the lower point of the chin.
Alternative
It is worth noting that the above methods for determining the central ratio of the jaws can be used with the classic profile of the face. As practice shows, they do not give accurate results, therefore, they are used with some restrictions. The anatomical and functional method for determining and fixing the central ratio of the jaws is considered optimal.
The technique of the anatomical and functional method
The patient is involved in a short conversation, which is not associated with prosthetics. Upon completion, the lower jaw is brought to rest; lips close normally usually freely. In this position, the specialist measures the distance between the marks on the chin and the base of the nasal septum.
Templates with rollers are introduced into the mouth. The patient is asked to close them. The interalveolar height is determined with the central position of the lower jaw. When processing rollers, the mouth repeatedly closes and opens. Typically, the patient sets the lower jaw in a central position.
After introducing the rollers, the specialist again measures the distance - the occlusal height - between the points indicated above. It should be less than the height at rest, by 2-3 mm.
If the height of the lower third of the face when closing the rollers and at rest turned out to be equal, then the interalveolar distance is increased. If the occlusal height is lower than the resting height by more than 3 mm, the height of the lower roller should be increased.
After the measurements, the specialist pays attention to the tissue near the mouth gap. If the interalveolar height is correct, the normal lines of the lower third of the face are restored. At a lower rate, the corners of the mouth will lower, the nasolabial folds will become more pronounced, and the upper lip will become shorter. When identifying such signs, it is necessary to take measurements again.
In the case of an increase in the interalveolar height, the closure of the lips is accompanied by a certain tension, the nasolabial folds are smoothed, and the upper lip becomes longer. In such a situation, the following test is very revealing. When the fingertip touches the closing line, the lips open instantly, which is uncharacteristic for the situation when they fit freely.
Conversation Test
It is considered the second addition to the anatomical technique.
After identifying the interalveolar height, the specialist asks the patient to pronounce individual syllables or letters (f, p, o, m, e, etc.). The doctor at the same time monitors the level of separation of the rollers. If the interalveolar height is normal, it is about 5-6 mm. If the distance exceeds 6 mm, a reduction in height may be necessary. If it is less than 5 mm, then, accordingly, the height can be increased.