Measurements of plaster models of jaws. Impressions and impression materials
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Diagnostic plaster casts of the jaws are made to create a replica of the patient's jaw. They are often also used to clarify the diagnosis. Using them, it is possible to obtain data on the peculiarities of the location of teeth, which is necessary for obtaining comfortable orthodontic structures and removable dentures. How are diagnostic jaw models made?
Making a plaster model of the jaw is an important stage in diagnostics and prosthetics. First, the doctor takes impressions using modern methods and materials. Later, with the help of plaster, it is possible to recreate plaster models of the jaws, which should closely replicate the main features of the patient’s real tissues.
After this, both plaster jaws are placed in an articulator, which simulates the movement of the jaws. You can easily buy a plaster model of the jaw at dental clinics. It will be possible to contact specialists if any diseases arise or the need to resort to prosthetics. Diagnostic jaw models must be of high quality. They necessarily provide information about the alveolar processes, tubercles, palate, frenulum and other soft tissue formations of the oral cavity. With the help of a high-quality plaster model of the jaws, many controversial situations that arise during examination and dental treatment can be clarified.
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In most cases analysis of occlusal relationships can be performed directly in the oral cavity, but if it is necessary to create extensive dentures or there are adaptive trajectories of movement of the lower jaw that prevent an adequate clinical examination, the analysis is carried out on plaster models in an articulator. For the production of single crowns and small bridges, the models do not need to be fixed in the articulator, provided that the doctor knows exactly what he is looking for.
Clinical assessment of occlusion
Should be paid attention to the following points:
Any symptoms of temporomandibular joint dysfunction: pain, muscle spasms, chronic toothache of unknown etiology.
The ease with which the patient makes conscious movements of the lower jaw in various directions.
Any occlusal failures and the possible impact of planned restorations on them.
Mobility of teeth during abduction of the lower jaw with closed dentition.
Presence, angle and smoothness of movement from the ZKP position to the FBK.
The type of lateral guidance and especially the degree of contact of the teeth to be replaced during lateral abduction of the mandible.
Availability of contact points on the non-working side.
The location and extent of abrasion facets on teeth to be dentures (or finished dentures) is the cause of abrasion.
The degree of occlusion stability and the impact of planned restorations on it.
Dentoalveolar elongation and inclination of teeth, especially those subject to prosthetics or their antagonists.
Clinical techniques for assessing occlusion
Articulation paper or foil to assess occlusion. Articulation paper of various colors and foil are used to mark occlusal contacts in various positions of the mandible. For example, FBK can be registered in one color, and ZKP in another. Articulating paper is quite difficult to use, the tops of the cusps often stain whether they are in contact or not, and on polished gold or glazed porcelain there is no staining at all. The degree of staining depends on the thickness of the paper, ideally it should be as thin as possible (teeth are very sensitive to the thickness of the material between them).
Wax for occlusion assessment. To record occlusion, thin plates of relatively soft wax with adhesive on one side are used. They are convenient, but quite expensive. As an alternative, 0.5 mm thick slabs of dark dental wax can be used. The advantages include the possibility of using a plate with impressions from the oral cavity when working with plaster models, as well as large sizes that allow you to obtain impressions of the entire dental arch. Contact areas in the mouth can be marked with a chemical pencil.
Silicone masses for assessing occlusion. Quick-curing silicone compounds can be used to record occlusal relationships. At first the material is very soft and does not offer any resistance when the teeth are closed, which can be a problem with more viscous materials such as wax if it is not softened properly. The feeling of resistance when biting can force the lower jaw into a different position. The silicone mass is flexible and at the same time dense enough to accurately record occlusal relationships and transfer them to plaster models.
Perforated areas in prints teeth indicate the presence of contacts in these places. All of the above determines the advantage of silicone mass over wax.
Plastic assessment strips. Plastic strips are used to identify contact points when teeth are closed in various leads. The thinnest of them, silver in color, has a thickness of 8 microns. The strip is placed between the teeth, and after they close, they try to remove it. Often this test is carried out simultaneously on 2 sides to determine the symmetry of the occlusion or in the area of the tooth covered by the crown and the adjacent tooth - this way you can make sure that the crown is in contact with the antagonist tooth, but is not overbite.
Sometimes special stripes It is permissible to replace them with matrices made of polyester film 40 microns thick; it is even more convenient to work with them, but they give a less accurate result.
Study of plaster models to assess occlusion
Plaster models outside the articulator are used to assess the stability of occlusion in the FBC and identify wear facets, which are often easier to detect on models than in the mouth. However, they are almost useless for determining contacts in various mandibular leads. It is important that the models are of good quality, with no air bubbles or pieces of plaster on the occlusal surface, and excess plaster on the posterior side must be carefully trimmed so that the models can be adequately matched. If you strictly follow the instructions, high-quality models can be obtained from alginate impressions, but it is advisable to use standard silicone or polyester impression materials for these purposes.
Studying plaster models in the articulator
If the data obtained during the assessment models in the hands of a doctor, it is not enough, it is doubtful that the necessary information can be obtained using a simple articulated articulator; it is advisable to use a semi-adjustable or fully adjustable articulator.
To register occlusion the following is required:
Data obtained using a facebow: the relationship of the maxillary teeth and the mandibular axis in 3 dimensions.
Sometimes registration of the PCP is not necessary, only registration of the FBK is sufficient, or models are easily compared into stable occlusion and registration of these positions is not needed at all.
Recording of protrusive movements of the lower jaw.
Recording of lateral movements of the mandible.
Semi-adjustable articulator has some functional limitations and allows only approximately to reproduce the movements of the lower jaw, but in most cases this is quite sufficient.
Plastering models into an occluder.
Formed competencies:
(PC-5 (1.5));
Purpose of the lesson: study the types, structure and characteristics of occluders, the technique of plastering models of jaws into an occluder, study the nature of the closure of jaws in an occluder.
Total lesson time: 200 minutes.
Lesson equipment: Study room, visual aids, computer class, computers, TV, tables, slides, computer programs, multimedia projector, video on the topic of the lesson.
Lesson plan:
| № | Stage name | Description of the stage | Pedagogical goal of the stage | Stage time |
| 1. | Organizational stage. | Check the students present, their appearance, discuss the lesson plan. | ||
| 2. | Test questions on the topic: 1. Occluders, their characteristics. 2. Comparison of plaster models according to the bite, gluing (fixation). 3. Plastering the model of the lower jaw into the occluder. 4. Plastering models of the upper jaw into the occluder 5. Studying the nature of the closure of the jaws (plaster models with teeth) in the occluder. Assessment of the state of occlusion. | Discuss questions that students had while preparing for the lesson. Control of background knowledge on issues. | ||
| 3. | Training stage. | Pedagogical story, demonstration, presentation of an algorithm for solving problems, instructions for completing tasks. | To teach students the technique of plastering jaw models into an occluder, as well as assessing the condition of the bite. | |
| 4. | Independent work | Development of methods for comparing plaster models by bite, gluing, plastering jaw models into an occluder. | Achieving the set goal of the lesson: study the types, structure and characteristics of occluders, the technique of plastering models of jaws into an occluder; study the nature of the closure of the jaws in the occluder. | 120 min. |
| 5. | Control of the final level of knowledge acquisition. | Tests, tasks, oral examination, test | Using an oral survey, determine the degree to which the goal has been achieved. | |
| 6. | The final stage. | Answers to students’ questions, assessment of the group’s work, assignment of assignments, notification of the topic of the next lesson, tasks for self-study for students | The teacher summarizes the content of the lesson |
Lesson No. 6
3rd semester
PROPAEDEUTICS
Structure of the orthopedic department,
Orthopedic office.
Equipment and tools
Used in clinical settings.
Formed competencies:
PC-1, PC-2, PC-5(1,5), PC-6(2), PC-7(1), PC-9(1)
the ability and willingness to implement the ethical and deontological aspects of medical practice in communication with colleagues, nurses and junior staff, adults and adolescents, their parents and relatives (PC-1);
the ability and willingness to identify the natural scientific essence of problems arising in the course of professional activity, to use the appropriate physical, chemical and mathematical apparatus to solve them (PC-2);
ability and willingness to conduct and interpret interviews and physical examinations, clinical examination, results of modern laboratory and instrumental studies, morphological analysis of biopsy, surgical and sectional material, write a medical record for an outpatient and inpatient patient(PC-5 (1.5));
the ability and willingness to conduct a pathophysiological analysis of clinical syndromes, to substantiate pathogenetically justified methods (principles) of diagnosis, treatment, rehabilitation and prevention among adults and adolescents, taking into account their age and sex groups (PC-6 (2));
ability and willingness to apply aseptic and antiseptic methods, use medical instruments, carry out sanitary treatment of medical and diagnostic premises of medical organizations, master patient care techniques (PC-7 (1));
ability and readiness to work with medical and technical equipment used in working with patients, own computer equipment, obtain information from various sources, work with information in global computer networks; apply the capabilities of modern information technologies to solve professional problems (PC-9 (1));
Purpose of the lesson: study the structure of the orthopedic department and dental laboratory, know the basic tools of an orthopedic doctor. Study the main components of dental units, drills and handpieces. Know the classification and main characteristics of impression trays.
Total lesson time: 150 minutes.
Lesson equipment: Study room, treatment room, functional diagnostics room, computer class, computers, TV, tables, slides, computer programs.
Lesson plan:
| № | Stage name | Description of the stage | Pedagogical goal of the stage | Stage time |
| 1. | Organizational stage. | Checking those present, reporting the topic of the lesson. | Check the students present, their appearance, discuss the lesson plan. | |
| 2. | Control of the initial level of knowledge. | Security questions |
On the first visit to the patient, impression mass is taken from the jaws up to the transitional fold so that the alveolar processes, apical bases and palatine vault, sublingual region, frenulum of the tongue and lips are clearly visible. Models are cast from plaster or super plaster. The base of the models can be shaped using special devices, rubber molds, or cut so that the corners of the base correspond to the line of the fangs, the bases are parallel to the chewing surfaces of the teeth. The models are marked with the patient's last name, first name, age and date of impression taking. Such models are called control or diagnostic models.
To study the size of the teeth, dentition, and apical bases of the jaws, it is advisable to use a meter or a special caliper, as well as various devices such as an orthocross, a symmetroscope, or an orthometer. The models are studied in three mutually perpendicular planes: sagittal, occlusal, tuberal (frontal) and the corresponding directions: sagittal, transversal and vertical.
Teeth measurements. Measuring the width, height and thickness of the crown of the tooth. The width of the crown part of the tooth is determined in the widest part of the tooth: at the level of the equator for all teeth, at the level of the cutting edge for the lower incisors. For the anterior group of teeth this is the medial-lateral size of the tooth, and for the lateral group it is the mesodistal one. However, in modern scientific literature, both domestic and foreign, the width of the coronal part of all teeth is referred to as its mesio-distal size.
The height of the crown part of permanent teeth is measured from the cutting edge of the tooth to its border with the mucous membrane: the front teeth - in the middle of the vestibular surface, the lateral teeth - in the middle of the buccal tubercle.
The thickness of the crown of a tooth is its mesiodistal size for incisors and canines and its mediolateral size for premolars and molars.
Measurements of the dentition are carried out in the transversal (transverse) and sagittal (longitudinal) directions. In the transversal direction, the width is studied, in the sagittal direction - the length of the dentition.
Transversal dimensions of the dentition. In children during the period of occlusion of primary teeth Z.I. Dolgopolova (1973) proposed measuring the width of the dentition on the upper and lower jaws between the central and lateral incisors, canines, first and second primary molars.
The measuring points for the central and lateral incisors and canines are located on the tops of the dental cusps, for the first and second primary molars - on the chewing surfaces in the front recess at the intersection of the longitudinal and transverse grooves.
During the period of occlusion of permanent teeth, the Pon technique is used to determine the transversal dimensions of the dentition, which is based on the relationship between the sum of the mesiodistal dimensions of the 4 upper incisors and the distance between the first premolars and the first molars on the upper and lower jaws. For this purpose, Pon proposed measurement points that, when the teeth of the upper and lower jaws are closed, coincide, and, therefore, the width of their dentition is the same.
In the area of the first premolars, the width of the dentition, according to Po-nu, is measured on the upper jaw between points in the middle of the intercuspal fissure, on the lower jaw between the distal contact points on the slope of the buccal cusps.
In the area of the first permanent molars, the width of the dentition is determined on the upper jaw between the points in the anterior recesses of the longitudinal fissure, on the lower jaw between the posterior buccal cusps.
During the period of changing teeth, instead of measuring points on the premolars, the distal dimples of the first primary molars in the upper jaw or their posterior buccal cusps in the lower jaw are used. In addition to the width of the dentition in the area of premolars and molars, it is advisable to study the width of the dentition in the area of the canines between the tops of their cutting edges.
The sagittal dimensions of the dentition in children are determined at the age of 3 to 6-7 years (during the period of occlusion of baby teeth).
The length of the anterior segment of the dentition is measured from the middle of the distance between the mesial corners of the central incisors from their vestibular surface along the sagittal plane to the point of intersection with the line connecting the distal surfaces of the crowns of primary canines, and the total sagittal length of the dentition - to the point of intersection with the line connecting the distal surfaces second primary molars.
The longitudinal length of the dentition is also measured, which is normally equal to the sum of the mesiodistal dimensions of 12 teeth.
The symmetry of the dentition and the displacement of the lateral teeth are examined by comparing the sizes of the right and left halves of the dentition and determining the unilateral mesicentral incisors and Pon's points.
The mesial displacement of the lateral teeth on plaster models of the jaws can be determined by comparing the distances from the interincisal papilla to the apexes of the canines or Pont's points on the first premolars and first molars on the right and left. On the side of the expected mesial mixing of the lateral teeth, this distance will be less compared to the opposite side and the norm.
The position of the posterior teeth can also be assessed relative to the "O" point, located at the intersection of the midpalatal suture and the tangent to the distal surfaces of the first permanent molars. The distance from this point to Pon’s measuring points on the first premolars (line b) and first molars (line a), as well as the distance along the median palatal suture from point “O” to the top of the interincisal papilla. The distance from point “O” to the measuring points on the right and left must be equal.
It is necessary to examine segments of the dentition and the palatine vault.
The values of the parameters of the palatal vault (length, height, width and angle of the palate) are determined using the following method:
The length of the palatal vault - from the top of the interincisal papilla (lateral approximal surfaces of the central incisors) along the median palatal suture to the line connecting the distal surfaces of the first permanent molars;
the depth of the palatal vault - according to the size of the perpendicular from the deepest point on the drawn contour of the palate to the line connecting the tops of the interdental papillae between the second premolars and the first molars;
the width of the palatal vault - along the line connecting the tops of the interdental papillae between the second premolars and the first molars;
angle of the sky (angle "a") - according to the method of Persin and Erokhin, based on certain provisions when constructing it. The reference plane is the plane parallel to the tuberal plane, which passes through the Pon measuring points in the area of the first premolars. At the point of its intersection with the sagittal plane on the median palatal suture - point 1 - an angle is constructed, the components of which are a line parallel to the base of the symmetrograph plane and a line to the apex of the interincisal papilla - point 2.
The palate height index is determined on plaster models of the jaws and calculated using the formula: 100.
Sky Height Index = Sky Height
Dental width
Apical base measurements.
The width of the apical base of the upper jaw is determined on a plaster model along a straight line between the deepest points in the f-ssae canina area (in the recess between the tips of the canines and first premolars), and on the model of the lower jaw - between the same teeth, departing from the level of the gingival margin by 8 mm 13.23).
The length of the apical base is measured on the upper jaw from point A (the intersection of the median palatal suture with the line connecting the central incisors in the neck area with the palatal surface) along the median palatal suture to the line connecting the distal surfaces of the first permanent molars; on the lower jaw - from point B (the anterior surface of the cutting edges of the central incisors) along the perpendicular to the intersection with the line connecting the distal surfaces of the first permanent molars.
Study of the shape of the dentition.
The upper and lower dental arches during the period of occlusion of primary teeth are a semicircle; during the period of occlusion of permanent teeth, the upper dental arch has the shape of a semi-ellipse, the lower - a parabola. The shape of the dentition can be assessed using graphic methods, using various devices or geometric constructions - symmetroscopy, photosymmetroscopy, symmetrography, parallelography, Howley-Gerber-Gerbst diagram.
Symmetroscopy. Using this method, the location of the teeth is studied in the transversal and sagittal directions. Ortho-cross (orthodontic cross) is used for express diagnostics. It is a transparent plate on which a cross with millimeter divisions or a millimeter grid with divisions of 1-2 mm is applied. The plate is placed on a plaster model of the upper jaw, orienting the cross along the median palatal suture, and then the location of the teeth is studied in relation to the midline and transverse lines 13.24).
Photosymmetroscopy is a method of symmetroscopy of diagnostic models of jaws with their subsequent photographing in a certain mode. A photograph of jaw models with a millimeter grid projected onto it is subsequently studied and measurements are taken.
In this case, they use a symmetrograph, on which the studied diagnostic model of the jaw is oriented and then fixed relative to perpendicularly located measuring scales. It is advisable to use a parallelograph, which allows sagittal, transversal and angular measurements. A conditional reference point is found on the jaw model. As such a point, the authors use the point of intersection of the sagittal and transversal planes with the mesial surface of the first permanent molars. In diagnosis, diagrams are used to determine the sum of the mesiodistal dimensions of the three upper teeth. To determine the shape of the dentition, the model is placed on the drawing so that its midline, running along the palatal suture, coincides with the diameter AM, and the sides of the equilateral triangle FEG pass between the canines and premolars. Then, with a finely sharpened pencil, outline the outline of the dentition and compare the existing shape with the curve of the diagram.
To obtain a plaster model, it is necessary to collect a cast, accurately place its parts in a spoon, and then glue them together and to the spoon with molten wax.
The collection of the cast begins no earlier than 30-40 minutes after removing it from the oral cavity, so that the moisture on the surface of the cast can evaporate.
Before placing parts of the cast in a tray, it is necessary to very carefully clean their surface adjacent to the tray, as well as the inner surface of the tray, from small particles of plaster that interfere with the accurate preparation of the cast.
The largest parts of the cast are placed first, and then the small ones. All parts of the cast must be accurately placed in the tray so that there is no gap anywhere between the tray and the outer surface of the cast. There should be no gaps on the inner surface of the cast, between its parts. The outer edges of the assembled impression are glued to the impression tray with hot wax. Pouring wax within the prosthetic field is not allowed; the slightest inaccuracy made during gluing the cast leads to distortion of the model.
The technique for obtaining a plaster model involves filling a cast or impression with liquid plaster, which is why this process is called model casting.
To make it easier to separate the cast from the model, it must be coated with an insulating substance. For these purposes, a number of substances are used that are applied to the surface of the impression. For this purpose, soap alcohol, kerosene with stearine and a number of other substances have been proposed. However, practice has shown that any insulating substance leaves a layer on the cast, resulting in an inaccurate model. Therefore, it is better to immerse the glued cast in cold water for 6-8 minutes; it fills all the pores, so that the model plaster does not adhere to the cast plaster.
For greater strength of the model, the plaster with which the cast is poured should have the consistency of sour cream.
The cast begins to be filled with small portions of plaster, and it is poured first onto the most convex part of the cast. The impression is shaken constantly to remove air bubbles. This is repeated until the entire cast is filled with plaster.
When the entire cast is filled, a mound is made from the remains of the plaster, which is placed on the cast; the latter is turned down and, together with the mound, pressed against a smooth object (glass, metal plate, etc.); The result is models with a wide base-stand, convenient for work. Thus, the model consists of two parts:
- 1) the working part corresponding to the prosthetic field, i.e. the location of the future prosthesis,
- 2) a stand that serves to stabilize the model.
Please note that the height of the stand should be at least 2-2.5 cm; This is of particular importance in the case of a deep sky, since the thinning of the model in this place can lead to the dream being pressed through during pressing under pressure from the press.
After the plaster has hardened, the edges of the model are trimmed with a spatula (Fig. 14).
Separating the cast from the plaster model. The cast is separated from the plaster model 8-10 minutes after casting, that is, when the plaster of the model begins to generate heat. This is the most favorable moment for separating parts of the cast from the model. The cast is removed very carefully to avoid damage to the model. First of all, you should free the teeth, guided by the dental formula, which indicates where and which teeth are located. To separate, use a dental spatula, inserting it shallowly along the fracture line of the cast, and using a lever-like movement, separate parts of the cast from the model. When all the teeth are freed, they tap the cast with a horn or metal hammer until a specific hollow sound of emptiness appears, meaning that a gap has formed between the cast and the model; After this, the model is completely separated from the cast. If, when separating the cast from the model, a tooth breaks off, which has retained the clear contours of the fracture line, you can glue it to the model using a special liquid glue (a solution of celluloid in acetone). Gluing with cement is not recommended due to the fact that it prevents the exact fit of the tooth to the model.
In case of more serious damage to the model, for example, separation of part of the alveolar process, fracture of the model, scratches in the area of the prosthetic field, etc., the impression should be retaken.
Separation of impression material from the model. When casting a model from an impression, no insulating substance is required so that the impression can be easily separated from the plaster model. After the plaster has hardened, the impression with the model is immersed in hot water for several minutes; The impression mass softens and is easily separated from the model.
