Division of orthodontics



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KING SAUD UNIVERSITY

College of Dentistry

Department of Preventive Dental Sciences

DIVISION OF ORTHODONTICS

431 PDS


Introduction to Orthodontics
PRACTICAL MANUAL

Part I


Introduction to Orthodontics
PRACTICAL MANUAL

Part I

Contributors:
Dr. Eman AlKofide Dr. Huda AlKawari

Dr. Sahar AlBarakati Dr. Hana AlBalbeesi

Dr. Laila Baidas

Re-Edited By:

Dr. Eman Alkofide

2005-2006

Table of Contents



  1. Introduction




  1. Classification of Malocclusion

    1. Normal Occlusion

    2. Malocclusion

    3. Class I Malocclusion

    4. Class II Malocclusion

      1. Class II Div. 1

      2. Class II Div. 2

    5. Class III Malocclusion




  1. Diagnostic Aids:

    1. Study Models

    2. Radiographs;

a) Occlusal Films

b) Orthopantomographs

c) Hand and Wrist Radiographs

d) Cephalometrics



    1. Model Analysis;

  1. Arch Perimeter Analysis: Moyer's Analysis

  2. Arch Length Analysis: Nance Analysis

  3. Tooth Size Discrepancy: Bolton Analysis


IV. References

Introduction

Orthodontics is one of the oldest branches in Dental Science. Orthodontics (Ortho = Straight, Dontic = Teeth), is that branch of dental science concerned with genetic variation, development and growth of facial form. It is also concerned with the manner in which these factors affect the occlusion of the teeth and the function of associated organs. Therefore we are not only concerned with straightening of the teeth, but also of the growth, development, and function of the total orofacial complex.


During this course, the student will be familiarized with the term Orthodontics through a series of lectures and laboratory sessions. The lecture series of this course will deal with the abovementioned aspects of orthodontics in more detail. The laboratory session of this course will teach the student the technical part of Orthodontics.
The purpose of this manual is to introduce to the student the practical part of this course in a more simplified and understandable manner.
It is not considered a replacement of the required textbooks for the course, but as an adjunct to help the student during the laboratory session.


  1. Classification of Malocclusion

  1. Normal Occlusion

  2. Malocclusion

  3. Class I Malocclusion

  4. Class II Malocclusion

      1. Class II Div. 1

      2. Class II Div. 2

  5. Class III Malocclusion

  1. Classification of Malocclusion




    1. Normal Occlusion

Occlusion is considered to be normal when the dental arches are in correct alignment, with all the teeth in anatomically correct contact, and in physiologically optimal occlusion with the corresponding teeth in the opposite dental arch. The development of normal occlusion passes through several continuous stages from birth to the development of the permanent dentition. The deciduous dentition begins to appear at around the age of 6 months with the eruption of the lower central incisors. The deciduous teeth are usually complete by the age of 2½ year of age. At this stage there is often spacing between the teeth especially distal to the lower canines and mesial to the upper canines (primate spaces), with the distal surfaces of the second deciduous molars in line with each other (flush terminal plane).


At 6 years of age the first molars start to erupt, and the permanent incisors develop lingual to the roots of the deciduous incisors. At this time also, the ugly duckling state is evident.

As the child continues to grow, he/she passes through the transition period from early mixed dentition to late mixed dentition, to the permanent dentition. Within these periods, there lies a discrepancy between the mesiodistal widths of the deciduous molars and the premolars which creates spacing and is termed the “leeway space”. This develops to allow the lower permanent molars to move forward further than the upper molars and establish a class I molar relationship.




    1. Malocclusion

Malocclusion is defined as an irregularity of the teeth (The various types of malocclusion will be discussed briefly here; the detailed description of each will be elaborated in the next section).



The etiology of malocclusion is generally categorized into two causes: (1) Hereditary, such as jaw-teeth size discrepancy, and (2) Developmental, such as premature loss of teeth or habits (ex. Thumb sucking or tongue thrusting).
Malocclusion may be associated with one or more of the following:

  1. Malposition of the Teeth

This could be caused by:

  • Tipped teeth – the crown of a tooth is tipped or incorrectly positioned in comparison to the apex.

  • Displaced teeth – in this situation both the crown and the apex are displaced.

  • Rotated teeth – the tooth is rotated along its long axis.

  • Teeth in infra-occlusion – the tooth has not reached the occlusal level.

  • Teeth in supra-occlusion – the tooth has erupted pass the occlusal level.

  • Transposed teeth – two teeth have reversed their positions, for example a canine taking the place of first premolar.




  1. Malrelationship of the Dental Arches

This could occur in any of the three planes of space: antero-posterior, vertical, or transverse. The antero-posterior malrelationship is represented by the Angle Classification, which deals with the disproportion of the teeth in an antero-posterior plane. The vertical malrelationship is evident during the observation of overbite, while the transverse malrelationship is presented in cases with crossbites.

The most popular and world recognized classification of malocclusion is the one described by Edward Angle, which deals with the arch malrelationship in the antero-posterior position.



Angle’s Classification (Molar Classification):

This was the first useful orthodontic classification system that was developed in 1890, and it still used to our present date. Angle’s classification system was based on the upper first molars as being the "Key to Occlusion". According to Angle, the mesiobuccal cusp of the upper molar should occlude in the buccal grove of the lower molar. If this molar relationship exists, and the teeth were arranged on a smoothly curving line of occlusion, then normal occlusion would result.



Angle then described the three classes of malocclusion, based on the occlusal relationships of the first molars, which are as follows:


  • Class I Malocclusion;

The lower first permanent molar is within one-half cusp width of its correct relationship to the upper first permanent molar (i.e. the mesiobuccal cusp of the maxillary first permanent molar occludes with the mid-buccal groove of the lower first permanent molar.


Fig. 1
This is sometimes termed “neutro-occlusion”. There is a normal relationship of the molars, but the line of occlusion is incorrect due to crowded, rotated, spaced teeth, or others.

  • Class II Malocclusion;

The lower arch is at least one-half cusp width posterior to the correct relationship with the upper arch. This is also known as “disto-occlusion”. This type of malocclusion is further categorized into two divisions according to the relationship of the upper central incisors:
Class II Div. 1 - The upper central incisors are proclined or of average inclination with an increase in overjet (fig 2a).
Class II Div. 2 - The upper central incisors are retroclined. The overjet is usually average but can be decreased or a little increased. Sometimes the upper laterals are proclined (fig. 2b).


Fig. 2a Fig. 2b



  • Class III Malocclusion;

The lower arch is at least one-half cusp width too far forward in relation to the upper arch. This is also known as “mesio-occlusion” (fig. 3).


Fig. 3

In certain situations where early extraction of the first molars has occurred, the alternative to using the Angle’s classification of malocclusion is to use the position of the canine to determine which type of occlusion the patient has. Usually, in class I relationship, the position of the upper canine is between the embrasure of the lower cuspid and first bicuspid.


In class II cases, we have a mesial movement of the upper canine and a distal movement of the lower canine. In class III cases, the opposite is true. The upper canine is located more distal, with the lower canine migrating more mesial.

Other systems have been developed to further aid in classifying a malocclusion. They are also used when the first molars are absent. In these cases, an Incisor classification has been developed. Its benefit is also recognized during orthodontic treatment. Since one of the main objectives is to correct the incisor malrelationship during treatment, an understanding of incisor position is very important.


Incisor Classification:

This does not usually follow the buccal segment relationship. It can be divided into:



  • Class I - The lower incisor edges occlude with or lie immediately below the

cingulum plateau (middle part of the palatal surface of the upper central incisors) (fig 4a).

  • Class II - The lower incisor edges lie posterior to cingulum plateau of the upper incisors. The two divisions are:

Class II Div. 1 - The upper central incisors are proclined or of average inclination and there is an increased overjet (fig. 4b).

Class II Div. 2 - The upper central incisors are retroclined, sometimes the upper laterals are proclined (fig 4c).

  • Class III - The lower incisor edges lie anterior to the cingulum plateau of the upper incisors. The overjet may be either reduced or reversed (fig. 4d).











a

b

c

d


Fig. 4

3. Class I Malocclusion:
This is the most common of all the malocclusions.

Dental Features:

  • Labial Segments; The lower incisor edge should occlude with or lie directly below the cingulum plateau of the upper incisors. Meaning that there should be a normal antero-posterior relationship between them (fig. 5).

  • Buccal Segments; The upper and lower molars are in “neutro-occlusion”. Because of the order of eruption, if there is a crowded dental arch, the last tooth within the arch to erupt will often be impacted or crowded out of the line of the dental arch. In some cases there may be an associated crossbite of one or two teeth, anterior teeth crowding, spacing, deep overbite or open bite, irrelevant of the canine and molar class I relationship.




Fig. 5

Skeletal Relationships:

  • Antero-posterior; the skeletal pattern is usually a class I, but it is possible to find a class I malocclusion in association with a class II or class III skeletal pattern.

  • Vertical and Transverse; They are usually within normal range.

  • Soft Tissue; The soft tissue form and activity are usually within normal range.


Growth:

There is a harmonious growth between the upper and lower jaw, which accounts for the skeletal and facial balance (fig. 6).






Fig. 6

Problems associated with Class I:

  • Crowding: This may appear in the labial or buccal segments due to a small or narrow arch, or in the premolar region due to early loss and drifting of teeth. It can be classified into mild, moderate, or severe.

There are two ways to measure crowding:

According to the broken contact: Mild = 1-2 broken contact

Moderate = 2-5 broken contact

Severe = more than 5 broken contact

According to measurement by mm’s: Mild = 1-4 lack of space

Moderate = 5-8mm lack of space

Severe = 8mm


  • Spacing: This could be localized or generalized. Localized, such as a midline diastema, could be caused by low frenal attachments, jaw-size discrepancy, or the presence of a mesiodens. Generalized spacing is usually due to a jaw-size to teeth-size discrepancy.

  • Deep Bite: Defined as the vertical overlap of the incisors (fig. 7). Normally the lower incisal edges contact the lingual surface of the upper incisors at or above the cingulum. It may cause traumatic occlusion and impingement of the palatal tissue.



Fig. 7

  • Open Bite: There is no vertical overlap of the incisors, and there is an evident vertical separation (fig. 8). This could be due to dental problems associated mainly with oral habits such as thumb sucking or mouth breathing, or skeletal problems such as arch deficiencies



Fig. 8

  • Cross Bite: It could be lingual or buccal, anterior or posterior, unilateral or bilateral, involving one tooth or a group of teeth (fig. 9 a-e). If it present anterior, this could be due to a pseudo-class III or a true class III. If it is posterior, it is usually due to a narrow upper arch.

Its causes vary from thumb sucking habits to dental problems such as teeth inclinations, to skeletal problems.



a. Normal Occlusion

b. Unilateral Buccal Cross Bite




c. Bilateral Buccal Cross Bite

d. Cusp relation tendency for crossbite [edge to edge] relationship



e. Scissors Bite [lingual cross bite]

(The upper buccal teeth are occluding buccally to the lower teeth)



Fig. 9

  • Localized Teeth Problems: Such as impacted or unerupted teeth. Most commonly observed in impacted cuspid cases.

4. Class II Malocclusion:
a) Class II Div. 1 Malocclusion
Dental Features:

  • Labial Segments; The lower incisor edge lies posterior to the cingulum plateau of the upper incisors. There is an increased overjet which may be due to proclined upper incisors, retroclined lower incisors, or a skeletal problem (fig. 10). Usually the overbite is increased and complete.

Note: Overjet is defined as the horizontal overlap of the incisors. Normally the upper incisors are 2-3mm ahead of the lower incisors.

  • Buccal Segments; The upper and lower first molars are in “disto-occlusion”, meaning that the mesiobuccal cusp of the upper first molar is anterior to the mid-buccal groove of the lower first molar.




Fig. 10

Skeletal Relationships:

  • Antero-posterior; there is usually a class II skeletal pattern. In severe malocclusion cases, with poor skeletal relationships, orthodontic treatment alone is compromised. In other cases, the inclination of the lower teeth will compensate for the skeletal pattern and thus the overjet will be less than expected.

  • Vertical; The anterior skeletal face height is usually average, although it may be high. A high angle or “dolichofacial” pattern is usually associated with an unfavorable facial profile with little chin prominence, hence leading to a compromised orthodontic treatment result.


Soft Tissue:

The lips are frequently incompetent, which leads to the uncontrolled proclination of the upper incisors. Sometimes a lip seal will be maintained, but frequently there is a tongue-to-lower lip seal with the lower lip lying behind the upper incisors.


Growth:

Patients with a class II div. 1 pattern exhibit more vertical growth. Unlike patients with a class II div. 2 pattern who exhibit more mandibular horizontal growth. A typical class II div. 1 case presents with a “dolichofacial” pattern or “Long Face Syndrome”, and has less favorable growth direction of the mandible than the “brachyfacial” patient or “Square Jaw Patient in class II div. 2 cases.




b) Class II Div. 2 Malocclusions
Dental Features:

  • Labial Segments; The upper central incisors are retroclined, while the upper laterals may be proclined or retroclined (fig. 11). When the upper laterals are proclined, they are usually mesially inclined and mesiolabially rotated. The lower anterior segment is frequently retroclined, which may lead to crowding of the lower incisor area. This increases the interincisal angle and hence has an effect on the amount of overbite. The overjet is usually not a problem here. There is an increase in the lower curve of Spee and the patient may appear with a gummy smile due to retroclination of the incisors.




  • Buccal Segments; Here the lower arch is at least one-half cusp width post normal to the upper arch, and there may be crowding due to early loss of the deciduous molars with a forward drift of the lower first molars.





Fig. 11

Skeletal Relationships:

  • Antero-posterior; the profile is usually well balanced, with the chin in a good position with the rest of the face. In some cases, the skeletal discrepancy is severe. This may be due to an increase in the length of the anterior cranial base, leading to a more distal positioning of the glenoid fossa and hence the mandible.

  • Vertical; The lower facial height is reduced or average. The Frankfort mandibular plane angle is often low. The lower anterior facial height may contribute to the depth of the overbite.

  • Transverse; In rare cases we may find a “scissors bite”, with the upper buccal teeth occluding outside the lowers.

  • Mandibular Positions and Paths of Closure; Usually, the path of closure is a simple hinge movement and the habitual position of the mandible is the rest position. But in severe cases, the mandible is habitually postured downwards and forward. With true posterior displacements of the mandible, and where there has been a loss of posterior teeth, patients will complain of pain in the early adult life, leading to TMJ problems.


Soft Tissue:

The lip line here is usually high, with the lower lip covering more than the occlusal half of the upper incisors. There may be an accentuated labiomental fold, and an increased nasiolabial angle with flattening of the upper lip profile.


Growth:

These patients exhibit a closing growth rotation, which contributes in part to the reduced facial height and the deep overbite. Treatment in these cases is difficult.


Oral Health:

In cases with severe overbite, direct trauma (traumatic bite) to the gingival mucosa may occur. This is due to the lower incisors occluding with the palatal mucosa and the upper incisors occluding with the labial mucosa. In these cases proper oral hygiene is a must and treatment of the traumatic occlusion is indicated.



5. Class III Malocclusion:
Dental Features:

  • Labial Segments; A class III incisor relationship exists when the lower incisor edge is lying anterior to the cingulum plateau of the upper incisors. The lower incisors may lie anterior to the upper so that there is a reverse overjet (fig. 12). The upper incisors are often crowded and they are usually proclined. The lower incisors are usually spaced and frequently retroclined. This inclination compensates the extent of the underlying sagittal arch malrelationship.

  • Buccal Segments; The lower arch is at least one-half cusp width too far forward relative to the upper arch. Usually the upper arch is crowded with canines buccally excluded, while the lower arch is well aligned. It is not uncommon to observe a crossbite in the buccal segments because of a narrow maxilla, which may be unilateral or bilateral. A unilateral crossbite is usually associated with lateral displacement of the mandible to obtain maximal intercuspation.


Fig. 12

Skeletal Relationships:

  • Antero-posterior; there is a combination of factors which leads to this malrelationship. The mandible is usually large, with a short retrognathic maxilla. The patient will appear with a concave profile. There is a more forward position of the glenoid fossa on the skull base so that the mandible is more anteriorly positioned than usual, with a short anterior cranial base.

  • Vertical; The Frankfort mandibular plane angle is usually high, with an associated reduced overbite or anterior open bite. The intermaxillary height is an important factor to consider.

  • Transverse; In most cases the maxillary base is narrow with a wide mandibular base. This transverse discrepancy is compensated for by a buccal inclination of the upper teeth and a lingual inclination of the lower teeth. It is common to see crossbites in the buccal segments due to a large mandible and a narrow maxilla.

  • Mandibular Positions and Paths of Closure; In patients with a mild class III malocclusion, the incisors meet edge to edge in centric relations. But in order for the mandible to obtain a position of maximal occlusion, there is a forward displacement of the mandible which accentuates the skeletal discrepancy. When there is a unilateral crossbite with the teeth in occlusion, there will usually be an associated lateral displacement of the mandible on closure.

In cases of skeletal disharmony, a more pronounced anterior displacement of the mandible occurs. It will be more difficult if not impossible for the mandible to retrude to obtain maximal occlusion. In fact, the only way the lower arch can meet with the upper arch in maximum occlusion is through the forward displacement of the mandible.
Soft Tissue:

In cases where the lips are frequently incompetent, the anterior intermaxillary height is large. These cases usually present with an anterior open bite with an adaptive swallowing behavior, where the tongue comes forward into the gap between the incisors.


Growth:

Here any growth is unfavorable, since the mandible may grow more prognathic (fig. 13). When the height of the intermaxillary space is normal or reduced, growth may worsen the reverse overjet and the horizontal profile of the face. When the height of the intermaxillary space is increased with growth, the tendency to a skeletal anterior open bite may become greater.




Fig. 13

Class III Molar and Jaw Relationship


Oral Health:

Mandibular displacements due to occlusal disharmonies eventually may be associated with muscle pain. Also when there is a premature contact in the incisor region there may be gingival recession around one or more lower incisors. And in cases with anterior open bite, periodontal changes can be expected around the non-functional teeth.





  1. Diagnostic Aids:

  1. Study Models

  2. Radiographs;

      1. Occlusal Films

      2. Orthopantomographs

      3. Hand and Wrist Radiographs

      4. Cephalometrics

  3. Model Analysis;

  4. Arch Perimeter Analysis: Moyer's Analysis

  5. Arch Length Analysis: Nance Analysis

  6. Tooth Size Discrepancy: Bolton Analysis

  1. Diagnostic Aids



1. Study Models
In order to properly diagnose an Orthodontic case, several steps are required. These steps

allows us to gather information pertinent to developing diagnosis, and hence, treatment planning of the case.

One aspect of orthodontic diagnosis that is part of the common knowledge of the profession, and yet has not been described often in the literature, is the fabrication of study models (fig. 14).



Fig. 14

The following section describes in detail the process of constructing study models as advised by the American Board of Orthodontics:



1. Impressions:

Standard aluminum trays should be used to obtain accurate impressions of the dentition and associated soft and hard tissue structures. The edges of the trays usually are lined with a border of wax that prevents the edge from impinging on the soft tissue. Care should be taken to ensure that the trays are neither too wide nor too narrow, so that minimal soft tissue distortion occurs. The areas of tissue attachment, particularly in the area of the labial frenum and in areas of soft tissue attachment adjacent to the upper first premolars, should be reproduced in the impression. Obtaining a proper impression of the hard and soft tissue of the dentoalveolar region is critical for the proper fabrication of diagnostic casts.

After the impression has been made, it should be checked thoroughly. The impression should appear smooth with no major voids, and the borders of the impression should be rolled with good extension into the vestibular areas. The impression also should extend posteriorly in the palatal area and lingually in the mandibular region. Lastly, the impression should be checked for the presence of any large air bubbles, especially on the occlusal surfaces of the teeth. After making the impression, it should be disinfected to prevent possible contamination of the laboratory area.
2. Wax Bite Registration

Clinicians use a wide variety of substances to record the orientation of the upper and lower dental arches. Normally, the bite registration is taken in centric occlusion, a tooth-guided position. In instances in which there is a substantial difference between centric occlusion and centric relation, an additional bite registration should be taken in centric relation as well. The clinician then must decide whether the study models are to be trimmed in centric occlusion or centric relation. In instances in which a centric relation registration is desired, it is often useful to mount the articulated model on an appropriate articulator, utilizing a facebow transfer.

For routine procedures, one or two thicknesses of yellow bite registration wax are used. The horseshoe-shaped wafers of wax are softened first in warm water and then placed on the maxillary dental arch. The patient then closes his or her mouth so that the lower teeth bite into the softened wax. The patient should be instructed to bite through the wax, to avoid producing study models that “rock” or are unstable when trimmed. Using finger pressure, the clinician then will press the wax against the teeth to achieve a three-dimensional registration of the bite. Some clinicians advocate keeping the labial surfaces of the upper and lower anterior teeth free of wax. The incisal edges can be registered in the wax, but the midlines are still visible so that the lateral orientation of the wax bite (using the midlines) can be determined.

After the impression and wax bite have been taken, they are wrapped in moistened paper toweling and placed in sealable plastic bag.


3. Disinfecting The Impression

A thorough rinsing of impressions reduces the number of microorganisms on the surface of the impressions, removing plaque and secretions. The impressions should be immersed in Biocide and then allowed to stand for approximately 10 minutes. A 1:100 solution of chlorine bleach and water also can be used as disinfecting solution. Impressions then are rinsed in lukewarm water and rebagged until the time they are poured.


4. Pouring The Impression

The first step in pouring the impressions is to fill in the area occupied by the tongue in the mandibular impression. This can be accomplished by first placing a thumb or piece of moistened paper towel or tissue in the tongue space. One scoop of alginate is mixed to normal consistency and placed in the area normally occupied by the tongue. As the alginate begins to harden, smoothen the alginate with finger pressure. After the initial set is completed, the impression is put aside until a final set is completed. After that time, the impression should be checked for accuracy, making sure that the alginate addition does not obstruct the anatomical structures in the lingual region of the mandibular impression.

The impressions are poured using white orthodontic stone or plaster. The impression should have been rinsed previously; not only to eliminate the residue of the disinfectant but also to eliminate traces of saliva that otherwise might affect the integrity of the finished surface of the stone. The stone is mixed in a vacuum mixer to eliminate bubbles that otherwise would be trapped in the stone. The stone is poured in the tooth portion of the impression first, using a vibrator and a waxing instrument or spatula. Additional stone is added with the spatula to complete the anatomical portion of the impression.

After the pouring of the anatomical portion of the impression is completed, the remaining stone is poured into a large base former, again using the vibrator. The impression tray is turned upside down and pushed into the stone in the base former. Care should be taken to verify that the occlusal surface of the impression remains parallel to the bottom surface of the base former. Also, the impression tray should not be pushed into the plaster in the base former. If the impression tray becomes trapped in the plaster, difficulties will be encountered in removing the tray, and the vertical thickness of the study model may be reduced.

The impression tray is removed from the poured stone after it is hardened. Ordinarily, a wait of 30-60 minutes after the onset of the mix is adequate to make sure that the orthodontic stone is set. Care should be taken in removing the impression from the set stone so that the teeth (particularly the upper and lower incisors) are not fractured during tray removal.
5. Trimming The Study Cast

The trimming needs to be done slowly and carefully. As a first step in the procedure, a laboratory knife is used to remove any large or small chunks of plaster that interfere with the occlusion of the cast. Such interferences include bubbles on the occlusal surfaces as well as lateral extensions in the posterior regions, particularly behind the last erupted molar.




  • Rough Trimming the Maxillary Model

The maxillary model is trimmed symmetrically with the top of the model trimmed parallel to the occlusal plane (Fig. 15). The back of the model is trimmed perpendicular to the midline of the palate as indicated by the orientation of the midline palatal raphe. Rough trimming of the stone bases first can be accomplished free-hand, using the platform on the model trimmer as a guide.

After the model is rough trimmed, the model is placed on its back so that the top of the cast can be trimmed. The teeth rest against the attachment of the model trimmer that slides into the groove on the trimming platform. The top surface of the model is trimmed so that it is parallel to the occlusal plane of the teeth (fig. 15).

The anatomical base of the maxillary model should be about 1.5 cm thick (about 13 mm) (fig. 16). If the maxillary base has been poured to an inadequate thickness, or if the base has been trimmed excessively, the finished study models will look uneven.

The total height of each cast should measure 3.5-4.0 cm from the occlusal surface to the top of the model.




Fig. 15



  • Trimming the Back of the Models

Once the top of the cast has been trimmed flat, the cast is placed with the top of the cast against the trimming platform. The cast then is oriented so that the palatal raphe is perpendicular to the wheel of the model trimmer. It is advisable to use the palatal raphe as a guide since the dental midlines often are not coincident with the skeletal midlines.

The cast is trimmed so that there is about 5 mm of stone distal to the most posterior tooth. In instances of severe Class II malocclusion, additional space should be allotted in the posterior region until the final occlusion is determined.




  • Establishing the Interarch Relationship

The upper and lower casts are placed together, and the operator checks for any interference that might prevent a proper occlusion from being established. The wax bite registration is placed on the maxillary cast, and the mandibular cast is occluded into the wax indentations.

The models then are placed on the trimming table with the casts in occlusion and the maxillary model on the bottom, with the backs of the casts facing the trimming wheel. The casts are held firmly together, and the back surfaces of the models are trimmed. At this point, only the mandibular cast touches the trimming wheel. The casts should be held gently but firmly together as the casts are pushed into the coarse grinding wheel. Trimming continues until both the upper and lower casts are touching the trimmer.

After it has been determined that the backs of the casts have been trimmed in a parallel fashion, the models are removed from the trimmer, and the backs of the casts placed on a flat surface. At this point, the models should lie flush on the surface with the wax bite in place. If this is not the case, the models should be placed back in the trimmer with the wax bite in place and retrimmed until both surfaces are flush against the trimmer.

The wax bite is removed and the casts occluded in a hand-held fashion. Once again, the models should be checked for the appropriate bite orientation by placing the backs of the models on a flat surface. If the backs of the models are not flush, they should be retrimmed without the wax bite in place. If there is any uncertainty concerning the accuracy of the wax bite, a new bite should be taken.




Fig. 16



  • Rough Trimming the Mandibular Model

With the models in occlusion and the wax bite in place, the models are placed on the trimming table with the lower base against the trimming wheel. Using the perpendicular attachment of the trimming table against the top surface of the upper cast, the bottom of the lower cast is trimmed parallel to the top of the upper cast. The cast is trimmed so that the base of the lower model is equal in thickness to that of the upper model (Figs. 15 and 16). The total height of both casts in occlusion should be about 7-7.5 cm.
6. Final Trimming

  • Maxillary Model

The precise angulations of the study models are determined using an angulator that can be screwed into the trimming table. The angulator allows the operator to set the correct angle for each surface. By placing the back of the cast against the flat surface of this device, an angle is formed between the surface of the cast and the trimming wheel surface that allows for the correct angulations to be determined. The screw on the angulator should be tightened firmly to prevent slipping of the device that could result in trimming errors.

First, the angulator is set at 70 (Fig. 17). The cast is placed in the appropriate position and the first side trimmed until the deepest extent of the vestibule is reached. The thickness of the initial trim should be roughly the thickness of a standard wooden pencil. It is best to be cautious at this point, because it is, of course, possible to retrim any surface. The opposite side also is trimmed at 70.

The angulator then is set to 25 and the front of the maxillary cast trimmed so that both sides meet anteriorly. The tip of the cast should approximate the midline as determined by the palatal raphe. The anterior borders of the maxillary cast are equal in length.


Fig. 17

The last portions of the maxillary cast to be trimmed are the back edges. These edges are trimmed perpendicular to a line drawn from the intersection of the lateral and posterior borders of the cast and the intersection point of the lateral and frontal surfaces of the cast on the opposite side (Fig. 17). The length of the corner segments should be 13-15 mm. Care should be taken to avoid trimming this area too quickly, or excess stone may be removed.





  • Mandibular Model

The angulator is set at 65 when initially trimming the lower model (Fig. 18). Each side is trimmed to the depth of the vestibule, again using the width of a standard wooden pencil as an initial guide.

The next step in trimming is to establish the posterior angles of the mandibular cast. As with the maxillary model, the posterior edges of the mandibular model is trimmed perpendicular to a line bisecting the angle formed by the lateral aspect of the cast and the posterior aspect of the cast (fig. 18). The length of this posterior surface should be 13-15 mm (fig. 19).

The anterior part of the cast is not angled but rather is rounded. The determination of the curvature is accomplished free-hand through gentle movement of the cast in a smooth arcing fashion. The anterior curvature is trimmed to the depth of the vestibule in most instances. In instances of dentoalveolar protrusion, care must be taken to avoid damaging the teeth during the trimming process.
7. Finishing Procedures


  • Filling Voids

The casts are inspected carefully and any remaining bubbles removed with a cleoid-discoid instrument or any waxing instrument. Particular attention is paid to the gingival margin as well as to other soft tissue areas. Any airholes or voids are filled with stone and the surface smoothed carefully, using either a finger or small brush to add plaster to the model.

All voids are filled, regardless of whether they are on the anatomical or artistic portions of the model.




Fig. 18


  • Finishing

The edges of the dental cast are smoothed slightly with a laboratory knife so that they are smooth and even. If there are obvious asymmetries in the extension of the vestibule that are due to impression technique rather than anatomical variation, these areas may be modified using a plaster knife or a rotary instrument.

The fine polishing of the artistic portions of the dental cast is initiated using a piece of fine-grained sand paper. The edges of the study models are smoothed under warm water. Also, the flat surfaces of the model are smoothed in areas of voids that previously have been filled with plaster.

The edges of the models should not be rounded. The finished models should have sharp angles but should be generally smooth in appearance. The models are set aside in an area to dry and allowed to dry for at least 24 hours.


  • Polishing the Casts

The casts are placed in a soaping solution for one hour. The casts then are removed from the soap bath to be rinsed under warm water and allowed to dry for approximately 20 minutes. Using a soft rag, the bases are buffed until the casts are smooth and shiny. The casts are labeled in the appropriate manner, noting the name and age of the patient, as well as the date of the impression.



Fig. 19

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