The minimum set of records required for orthodontic treatment includes study models, clinical photographs, and panoramic and lateral cephalometric X-rays. However, in recent times, there has been a shift in thinking about the necessity of X-rays. For some children with malocclusion, X-rays may not provide significant additional information beyond what can be gathered from clinical examinations.

These may include additional X-rays such as occlusal views, posteroanterior (PA) cephalogram, tomograms of TMJ, 3D computed tomograms and biochemical studies related to bone metabolism, and technetium scan. The nature and severity of deformity and search for aetiology would decide the type of further investigations to be carried out.

The study models are first in the order of records to be obtained and of greatest importance in orthodontic diagnosis and treatment planning. Plaster study models are prepared from well-extended good quality alginate impressions. The digital models can be obtained through either intraoral scanning or 3D scanning of the impression(s). These replicas allow the teeth and associated dentoalveolar segments to be examined from all possible views, the buccal, lingual and occlusal, which may not be possible clinically.

Good study models will show dentition, dentoalveolar structures and well-defined sulci, frena and palate. An impression for a good study model should extend over all teeth and well into the sulci. When necessary, the orthodontic impression tray may be modified by the addition of wax to ensure full coverage of the dentition and extension to record vestibular depth. Beading wax is usually applied to the periphery of the orthodontic impression tray to extend the alginate deep into the sulci. Alginate impressions should be poured immediately or within 15 minutes of its recording to avoid distortions due to dimensional changes. Good and error-free alginate impressions showing surface details without distortion or voids are prerequisites for preparing a set of clean and accurate plaster models. After thorough washing, disinfection and gentle air dry, impressions are poured with orthodontic grade white stone plaster, trimmed to the specifications and finished for presentation and storage. Models are mounted on bases and so trimmed in the laboratory that when placed on their bases, patient’s occlusion is replicated.

Parts of a study model : An orthodontic study model consists of two parts ( Figs 19.1 and 19.2 ):

• 1.

  Anatomic region: The anatomic part of the study model consists of the actual impression of the dental tissues, arch and its surrounding structures.

• 2.

  Artistic region: The artistic part of the study model consists of a symmetrical plaster base that supports the anatomic portion and helps in analysing the occlusion and orientation of the study models.

Orthodontic study models.

(A–C) Well-trimmed dental study models in centric occlusion are a prerequisite and the most important orthodontic diagnostic record.

Parts of orthodontic study models and preparation guidelines.

(A and B) These figures serve as a guide for preparation of orthodontic study models.

The ratio of the anatomic portion to the artistic portion should be 3:1. Preferably, both the anatomic and the artistic parts should be poured in the same orthodontic grade dental stone.

Each set of study models should be trimmed to the most exacting specifications (American Board of Orthodontics [ABO]), smoothened with fine grade sandpaper, and polished to a high gloss finish, clearly inscribed with the patient’s particulars on the back of the model bases:

• •

  Name or initials

• •

  Unique hospital identity (UHID) number/registration number (ID)/barcode

• •

  Age/date of birth, sex

• •

  Date of impression

• •

  Stage of treatment, that is, pre-treatment/stage/post-treatment/follow-up

A maxillary study model should be symmetrical with the top of the model being parallel to the occlusal plane. The back of the model should be perpendicular to the midline of the palate as indicated by the orientation of the mid-palatal raphe. The plaster model, when placed on its back, should be such that the top of the cast or the model should be parallel to the occlusal plane of the teeth. The anatomical base of the maxillary model should be 13 mm thick. The total height of the cast should measure 35–40 mm from the occlusal surface to the top of the model. It is advisable to use mid-palatal raphe as a guide since the dental midlines are often not coincident with the skeletal midlines.

When placing the mandibular cast in occlusion with the maxillary cast, it is important to ensure that the bottom of the lower base is trimmed parallel to the upper base. The thickness of the base of the lower model should be equal to that of the maxillary model. The total height of both casts in occlusion should measure approximately 70–75 mm.

The angle between the lateral surfaces of the maxillary cast should be 65 degrees to the posterior surface, whereas it should be 55 degrees for the mandibular cast. Posterior corners of the cast are perpendicular to a line formed between the intersection point of the posterior and lateral surfaces and the intersection point of the lateral and frontal surfaces of the cast on the opposite side. The length of the corner segments should be 13–15 mm. The models should be polished and trimmed with exacting symmetry and specifications. ABO allows a 5-degree tolerance in base angles.

In day-to-day practice, dental models can be mounted on preformed plastic bases. Occlusion for dental plaster models is determined by placing the separated, properly trimmed study casts on a flat surface and then bringing them together into maximum intercuspation.

The pre-treatment study models are prepared to record the status of malocclusion at the time of reporting for first consult.

• I.

  Pre-treatment study models are replicas of occlusion and dentition that objectively record malocclusion traits such as overjet, overbite, canine relations and buccal segment relationships.

• II.

  These models allow for a direct assessment of malocclusion and facilitate measurements of dental arches, tooth sizes and the space required to correct malocclusion. Study models are valuable in total space analysis and essential in this diagnostic procedure.

• III.

  They help to assess and record dental anatomy and intercuspation of teeth and detect abnormalities such as localised enlargement or distortion of arch form.

• IV.

  Unlike clinical examination, study models allow viewing of the occlusion from the lingual aspect.

• V.

  These models also enable objective evaluation of dental midlines. For determining the skeletal midline, a facial examination is required.

• VI.

  Study models are an excellent tool for explaining the treatment plan to the patient and their parents, assessing the treatment progress and increasing motivation.

• VII.

  They are a 3D presentation aid that enables patients to visualise their mouth structure and identify issues more efficiently. Unlike just looking at a picture, patients can touch, handle and see their problems in different views.

• VIII.

  Study models also help patients understand the expected outcomes of the prescribed treatment more clearly. The use of study models in, case presentation, allows a ‘co-diagnosis’ approach. We can also explain, using a model of perfect teeth, the steps that should be undertaken to rectify the malocclusion problems.

Orthodontic study models are created to record the progress of the treatment. These records are called ‘stage models’ during treatment and ‘post-treatment models’ upon completion of treatment.

• I.

  Progress study models help record the sequential progression from pre-treatment through mid-treatment to completion of therapy and during followup.

• II.

  The pre-treatment, post-treatment and follow-up models are used to evaluate the outcomes of the treatment, and the follow-up models are also used to record relapse. Many research studies have used these study models as data sets.

• III.

  A set of study models through various stages of treatment and follow-up are excellent teaching aid.

The first step in assessing study models involves a thorough visualisation and an assessment of the symmetry. Any gross asymmetry in occlusion or dentition would be apparent and should be recorded. A transparent grid or occlusogram is used to quickly identify the position and severity of asymmetry in the dental arch.

Multiple dentition features are evaluated in a specific order, including but not limited to, shape and anatomy, number, mesiodistal width, rotations, crowding, ectopic positions and extra or missing teeth.

This is followed by an assessment of the arch form (V/narrow/square) ( Fig. 19.3 ), arch widths (inter-canine, inter-premolar and inter-molar), arch length and arch perimeter. The arch form, widths and length have considerable implications in orthodontic diagnosis, for these govern the effective space available to accommodate dentition, the stability of the treatment outcome and aesthetics to a great extent. These considerations, in association with anteroposterior movements of the dentition, will determine the requirements for extraction, if any, or a non-extraction treatment plan.

Arch forms.

(A) V shaped, (B) Parabolic and (C) wide/square.

Korkhaus 3D bow divider instrument is helpful in making measurements on the dental cast.

In cases of cleft palate, the type of cleft, its extent and any palatal fistula should be noted down.

When restorative dentistry is being planned, study models are used to record the lateral and excursive paths of the mandible. The models are mounted on an articulator in centric relation for evaluating centric relation-centric occlusion (CR-CO) discrepancies that may be present in a malocclusion.

Korkhaus palatal index

Korkhaus considered palatal vault depth in relation to posterior arch width to objectively calculate the palatal index. Palatal height is measured on a mid-sagittal plane in the region of upper first molars at the level of the occlusal plane. The height is defined as the perpendicular distance from the connecting line between midpoints of fissures of both upper first molars to the palate ( Fig. 19.4 ). The following formula is used:

Palatal height index = Palatal height × 100 Posterior arch width % Average index value = 42 %

Korkhaus cast measurement instrument.

Korkhaus device is designed to simultaneously measure arch width, arch depth and palatal depth.

High palatal depth values are indicative of narrow maxilla, and smaller values suggest a shallow palate.

Total tooth material

To determine the amount of space required, the mesiodistal width of each tooth mesial to the first molar should be measured and then added up. The measurement of each tooth’s mesiodistal width should be taken as close to the contact point as possible, using either a sliding or digital vernier calliper.

If the arch perimeter is more than the TTM, as determined by the sum of the mesiodistal widths of teeth mesial to the first molars, spacing between the teeth can be expected. Conversely, if the TTM or the space required is greater than the amount of space available, there is arch length/perimeter deficiency and crowding would occur ( Figs 19.6 and 19.7 ).

Measuring the arch length. (A) The arch length is measured from mesial of the first molar to another side of the first molar with the help of soft brass wire. The wire is contoured as a curve following the contact points of teeth in a smooth curve looking at basal skeletal arches in the buccal region and at the base of the maxilla, which is the post-treatment position of the anterior teeth. (B) Alternatively, the arch length can be measured from mesial of the first molar to another side of the first molar in segments as shown in the figure.

Measuring the tooth size.

The vernier calliper is used for the measurement of the mesiodistal width of the individual tooth. The total tooth material is the sum of mesiodistal widths of teeth mesial to the first molars.

Orthodontic movement of lingually inclined incisors in a labial direction increases the perimeter, whereas the lingual movement of the incisors decreases the arch perimeter. According to Tweed, inclining the lower incisors one degree labially increases the arch perimeter by 0.8 mm and vice versa. The inclination of the lower incisors is measured by the angle formed by the long axis of the mandibular incisor with the mandibular plane on the lateral cephalogram. The anteroposterior tooth position can be determined by measuring the perpendicular distance of the incisal tip of the mandibular incisor with the nasion-point B line.

The curve of Spee’s depth is measured by determining the greatest depth of the curve on both sides of the mandibular arch in the premolar region. This is done by placing a rigid plastic square sheet on the occlusal surfaces of a study model, in contact with the permanent molars and mandibular incisors. A Boley gauge is used to measure the deepest point between the plastic sheet and the buccal cusps.

The depths of the curve on the right and left sides are summed up, divided by two and then an additional 0.5 mm is added to provide the total space required for levelling the curve of Spee ( Fig. 19.8 ).

Curve of Spee is the depth at the lowest tooth on a plane from the incisor to the erupted last molar.

Flattening the curve of Spee can be achieved either by supra-eruption of premolars, the intrusion of mandibular incisors or a combination of both. Incisor intrusion requires additional space in the bony bases; otherwise, proclination will result.

The discrepancy between the arch length and the tooth material is a common cause of malocclusion. Carey’s analysis helps in determining the extent of this discrepancy.

Arch length is determined by contouring a piece of brass wire touching the mesial surface of the first molars, passed over the buccal cusp of premolars and along the incisors from one side to the other such that it is aligned along the dentoalveolar segment where the teeth should be ideally positioned; that is, if the anterior teeth are well aligned and not protrusive, the wire passes over the incisal edge of anterior. In case the incisors are proclined, the wire is passed along the cingulum of the anterior teeth. In contrast, if the anterior teeth are retroclined, the wire in the anterior segment passes labial to the teeth. The tooth material is determined by sum of the mesiodistal width of the teeth anterior to the first molars (i.e. second premolars to second premolars). According to Carey’s:

• •

  If the difference between the arch length and the tooth material is between 0 and 2.5 mm, it indicates minimum tooth material excess and interproximal reduction can be considered to reduce the tooth material, or it can be treated with a non-extraction approach.

• •

  If the difference between the arch length and the tooth material is between 2.5 and 5 mm, it indicates moderate anchorage and the need to extract second premolars.

• •

  If the difference between the arch length and the tooth material is more than 5 mm, it indicates the need to extract first premolars.

Bolton considered that widths of maxillary and mandibular teeth have a pre-determined unique proportion which is a feature of normal occlusion relationship. An alteration in this balance would lead to unsatisfactory occlusion, exhibited as improper intercuspation, overjet or spacing.

Bolton’s analysis is carried out by measuring the mesiodistal widths of permanent teeth (except second and third molars) and then comparing the summed widths of the maxillary to the mandibular anterior teeth, and the total width of the upper and lower teeth with a standard ratio ( Table 19.1 ). The table shows standard corresponding mesiodistal width values of maxillary and mandibular anterior as well as overall Bolton’s ratio.

The following formula derives the anterior ratio (AR):

Bolton’s analysis helps determine size disproportions between maxillary and mandibular teeth. An excess of maxillary tooth material would lead to an overjet, whereas a mandibular excess would lead to crowding or a negative overjet.

The following formula derives the overall ratio (OR):

An anterior ratio of less than 77.2% indicates maxillary anterior tooth material excess, whereas an anterior ratio of more than 77.2% indicates mandibular anterior tooth material excess. An overall ratio of less than 91.3% indicates maxillary tooth material excess, whereas an overall ratio of more than 91.3% indicates mandibular tooth material excess. Bolton’s ratio has implications in clinical practice during finishing and detailing of the occlusion.

Royal London Space Analysis (RLSA) takes into account all possible factors that can influence the use of space or make the space available. The analysis is done in two steps ( Fig. 19.9 ). First step involves calculation of need for space and second step involves effect of treatment procedures including growth. The effects of favourable and unfavourable growth are also considered. ^, The values obtained are charted as + or −.

Royal London space analysis. The residue should be zero in both arches. It may be necessary to adjust the treatment objectives to achieve this, but these must remain attainable and not simply manipulated in order to achieve the zero residue.

Source: Based on Kirschen RH, O’higgins EA, Lee RT. The Royal London Space Planning: an integration of space analysis and treatment planning: Part I: Assessing the space required to meet treatment objectives. Am J Orthod Dentofacial Orthop. 2000 Oct;118(4):448–55. doi: 10.1067/mod.2000.109031. PMID: 11029742.

First step takes into consideration crowding and spacing, occlusal curves, arch width, anteroposterior position of labial segments, mesiodistal angulation and incisor inclination. The second step of the analysis involves effect of treatment procedures, such as extractions, tooth-size modifications, distal or mesial molar movements, as well as natural growth. However, any space planning should be regarded only as a useful guide, for several factors which includes growth, biological response and patient compliance are beyond the control of the clinician. RLSA is a reliable space analysis; however, its impact on decision making process in the treatment is questionable. The analysis is not a robust model for treatment planning.

When analysing the space required for mixed dentition, the sizes of unerupted permanent teeth can be obtained using dental radiographs. Traditionally, the mesiodistal widths of unerupted teeth are measured using intraoral periapical (IOPA) radiographs, as shown in Figs 19.10 and 19.11 . However, measurements of tooth size on radiographs are not always accurate due to the inherent distortion of the radiographic image. To compensate for this distortion, a calculation method was devised based on the principle that if we can measure an object that is visible on both the radiograph and a dental model, we can determine the correct mesiodistal width of the crown of the unerupted tooth. The amount of distortion can be calculated using the following formula:

(A) The orthopantomogram (OPG) is taken to evaluate the number, position and eruption status of the successive teeth in the dental arch. The red arrow indicates the measurement of the mesiodistal width of a second premolar. (B) An intraoral periapical (IOPA) radiograph of a child in mixed dentition. The mesiodistal widths of canines and premolars are directly measured on radiograph taken by long cone method.

The space available in the arch of a case of mixed dentition can be measured with a calliper.

The space required is the sum of the mesiodistal widths of the unerupted first premolar and second premolar. The combined mesiodistal width of deciduous canine and two molars is greater than combined mesiodistal width of permanent canine and two premolars. The difference is called leeway space.

X1 = Width of unerupted tooth whose width is to be determined

X2 = Width of the unerupted tooth on the radiograph

Y1 = Width of erupted tooth as measured on the cast

Y2 = Width of the erupted tooth as measured on a radiograph

This method is relatively accurate and practical and does not require the use of prediction tables. It can be used in both maxillary and mandibular arches. However, the accuracy of this method is dependent on the quality of the radiographic image. Therefore, periapical films are preferred over panoramic films.

Researchers have calculated correlation among the size of anterior and posterior teeth. By measuring the size of an erupted anterior tooth, it is possible to predict the size of the unerupted canine/premolar from the prediction table. The main advantage of non-radiographic prediction methods is that they can be performed by measuring the erupted mandibular incisor(s) without the need of additional measurements from radiographs. However, these measurements are less accurate and have a larger standard error as compared with the correctly adjusted radiographic methods.

i. Moyer’s analysis

This mixed dentition analysis utilises Moyer’s prediction tables ( Table 19.2 ). Prediction is based on the premise that there is a reasonably good correlation between the size of erupted permanent incisors and the unerupted canines and premolars. A person with large teeth in one part of the mouth will have large teeth elsewhere also, presumably having been controlled by the same genetic mechanism.

TABLE 19.2

Moyer’s analysis probability tables for predicting the sizes of unerupted cuspids and bicuspids

Based on the concept of Moyers RE. Handbook of orthodontics for students and general practitioners . Chicago: Year Book of Medical Publishers; 1958:369–72.

Mandibular bicuspids and cuspids
21|12	19.5	20.0	20.5	21.0	21.5	22.0	22.5	23.0	23.5	24.0	24.5	25.0	25.5
Males 75%	20.3	20.5	20.8	21.0	21.3	21.5	21.8	22.0	22.3	22.5	22.8	23.0	23.3
Females 75%	19.6	19.8	20.1	20.3	20.6	20.8	21.1	21.3	21.6	21.9	22.1	22.4	22.7
Maxillary bicuspids and cuspids
21|12	19.5	20.0	20.5	21.0	21.5	22.0	22.5	23.0	23.5	24.0	24.5	25.0	25.5
Males 75%	20.3	20.5	20.8	21.0	21.3	21.5	21.8	22.0	22.3	22.5	22.8	23.0	23.3
Females 75%	20.4	20.5	20.6	20.8	20.9	21.0	21.2	21.3	21.5	21.6	21.8	21.9	22.1

Advantages

• •

  It can be done with equal reliability by the beginner and an expert clinician.

• •

  It is not time-consuming and does not require any special equipment.

• •

  It can be done on the mouth as well as on the cast.

• •

  Mandibular incisors are used for the prediction of both the mandibular and maxillary cuspid and bicuspid widths.

Step by step procedure for the use of Moyer’s probability tables:

• •

  Measure and obtain the mesiodistal widths of the four permanent mandibular incisors and find that value in the horizontal row of the appropriate table (sex-wise).

• •

  Read downwards in the appropriate vertical column to obtain the values for the expected width of the cuspids and premolars corresponding to the level of probability.

• •

  Moyer used 75% of probability rather than the mean of 50% since the values distribute generally towards crowding and spacing. Crowding is a much more serious clinical problem and 75% predictive values thus protect the clinician on the safe side.

To determine the space required for the eruption of permanent canines and premolars, the distance between the distal surface of the lateral incisor and the mesial surfaces of the first molar is measured. The predicted value is then compared with the arch length to determine any discrepancy. If the predicted value is greater than the available arch length, it could result in crowding of the teeth. Conversely, if the predicted value is lesser than the available arch length, it may cause spacing between the teeth.

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