Orthodontic Study Models PDF

Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin Orthodontic study models They are accurate plaster reproductions of the teeth and their surroun...

Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin Orthodontic study models They are accurate plaster reproductions of the teeth and their surrounding soft tissues. The key to achieve good treatment results starts with proper diagnosis and treatment planning. Orthodontic records allow the orthodontist to carefully examine several parameters, such as dentition and jaw relationships, and make objective measurements for detailed evaluation and treatment planning. The study models are the 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. 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, frenum, and palate. An impression for a good study model should extend over all teeth and well into the sulci. Models are mounted on bases and so trimmed in the laboratory that when placed on their bases, the teeth are in the patient’s intercuspal relationship. Orthodontic study models consist of two parts (Fig 1). 1. Anatomic part. The anatomic part of the study model consists of the actual impression of the dental arch and its surrounding structures and is usually made in stone plaster. 2. Artistic part. The artistic part of the study model consists of a symmetrical plaster base that supports the anatomic portion and helps in analyzing the occlusion and orientation of the study models. The ratio of the anatomic portion to the artistic portion should be 2:1. Preferably, both the anatomic and the artistic parts should be poured in the same orthodontic grade stone plaster. (Fig 1) USES OF STUDY MODELS 1. Assess and record dental anatomy 2. Assess and record intercuspation 3. Assess and record arch form 4. Assess and record the curves of occlusion Page 1 of 11 Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin 5. Evaluate occlusion with the aid of articulators 6. Measure progress during treatment 7. Detect abnormality, e.g. localized enlargements, distortion of arch form, etc. 8. Calculate total space requirements/discrepancies 9. Provide records before, immediately, after and several years following treatment for the purpose of studying treatment procedures and stability. 1. Evaluation of study models The first step in assessing study models involves an assessment of the symmetry. Any gross asymmetry in occlusion or dentition would be obvious and should be recorded. Grids and occlusograms are placed over the occlusal arches of models to determine the exact location and amount of asymmetry. Thereafter, the characteristics of dentition, their number, mesiodistal width, rotations, crowding, ectopic positions, or the presence of supernumeraries are assessed. The findings of intraoral examination on dentition are reconfirmed in the models. This is followed by an assessment of the arch shape (whether tapering/V-shaped, oval, or u-shaped/square, Fig. 2), arch widths (intercanine, inter premolar, and intermolar), arch length, and arch perimeter. The arch form, arch widths, and arch length have considerable implications in orthodontic diagnosis since these govern the effective space available to accommodate dentition and govern 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 the extraction or otherwise. Palatal depth can be assessed, which may be increased in certain habits like mouth breathing and thumb sucking. In the case of cleft cases, the type of cleft, its extent, and any palatal fistula should be noted. Study models are also used to record the lateral and excursive paths of the mandible and are important when restorative dentistry is being planned, as the contours should accommodate the path of movement. The study models are mounted on an articulator in centric relation to evaluate CR-CO (centric relation - centric occlusion) discrepancies that may exist in a malocclusion. Page 2 of 11 Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin (Fig. 2) 2. Analysis of study models Various methods of model analysis have been described, and appropriate analysis must be selected for a given case. Different types of model analysis: I. Analyses used to assess the need for extraction 1. Analysis used for maxillary arch: Arch perimeter analysis 2. Analysis used in mandibular arch: Nance Carey's analysis II. Analyses used to assess the need for arch expansion Pont's analysis Linder Harth index Korkhaus analysis Ashley Howe analysis III. Analysis used to assess the tooth material excess Bolton's analysis IV. Analysis used in mixed dentition Mixed dentition analysis Armamentarium Required for Model Analysis The following things are needed to carry out model analysis: 1- Maxillary and mandibular study casts of the patient. 2- 0.012-inch soft round brass wire—to measure the arch length. 3- Bow divider—to measure the mesiodistal width of teeth and arch width. 4- Scale—to record the measurements obtained by the bow divider. 5- Pencil to mark on the brass wire. 6- Measured width using bow dividers is then transferred to the scale. Page 3 of 11 Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin (Fig. 3) Carey's Analysis Arch length—tooth material discrepancy is one of the important causative factors of malocclusion. Carey's analysis is aimed at determining the extent of the discrepancy. Carey's analysis is performed on the mandibular cast. If the same analysis is carried out on the maxillary arch, then it is called Arch Perimeter Analysis. Procedure Determination of Arch Length The arch length, anterior to the mandibular first molars, is measured using a 0.012-inch soft round brass wire. The wire is adapted to the model of the mandibular arch so that one end engages the first permanent lower molar near the marginal ridge. The wire is next passed over the buccal cusps of the premolars, then over the normal cuspal position of the cuspid, then over the anterior teeth at the incisal edges and finally around the same course on the opposite side, ending in the mesiobuccal line angle of the lower first permanent molar of the other side (Fig. 14.3). The wire is cut at this point and straightened, and the length is recorded. In the case of proclined anteriors, the brass wire is passed along the cingulum of anterior teeth. In the case of retroclined anteriors, the brass wire is passed labial to the anterior teeth. In the case of well-aligned anterior teeth, the wire is passed over the incisal edges of the anterior teeth. Page 4 of 11 Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin (Fig. 4) Determination of Arch Width/Total Tooth Material (TTM) Tooth material is determined by measuring the mesiodistal width of the teeth anterior to the first permanent molars (incisors, canines, and premolars) at the maximum contour using a bow divider (Fig 5). (Fig 5) Determination of the Discrepancy The discrepancy refers to the difference between the arch length and total tooth material. Inference Non-extraction case If the discrepancy is 2.5 mm or less, it indicates minimal tooth material excess, which can be managed by proximal stripping. Extraction of the second premolar If the discrepancy is 2.5-5 mm, the second premolar may need to be extracted. Extraction of the first premolar If the discrepancy is more than 5 mm, then extraction of the first premolar is advised. Arch Perimeter Analysis Arch perimeter analysis is similar to Carey's analysis, but it is performed on the maxillary cast. Page 5 of 11 Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin Pont's Analysis Pont reported his analysis in the year 1909 and proposed that the measurement of four maxillary incisors would automatically establish the width of the arch in the premolar and molar regions. Pont's analysis helps in the following: Determining whether the dental arch is narrow or wide. Determining the need for lateral arch expansion. Determining how much expansion is possible at the premolar and molar regions. Procedure Determination of Sum of Incisors (SI) The mesiodistal width of the four maxillary incisors is measured, and values are summed up. This is called the Sum of Incisors, i.e., the mesiodistal width of the following teeth: Right maxillary central incisor (11) Right maxillary lateral incisor (12) Left maxillary central incisor (21) Left maxillary lateral incisor (22) Sum of incisors (SI) = Mesiodistal width of (teeth 11 +12 + 21 + 22). *The FDI system of tooth notation is used. Determination of Measured Premolar Value (MPV) The width of the arch in the premolar region from the distal pit of one upper first premolar to the distal pit of the first premolar on the opposite side is measured (Fig. 6). It is called as Measured Premolar Value. (Fig 6) (Fig 7) Determination of Measured Molar Value (MM V) Page 6 of 11 Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin The width of the arch in the molar region from the mesial pit of one upper first molar to the mesial pit of the opposite side first molar is measured (Fig. 7). This value is called the Measured Molar Value. Determination of Calculated Premolar Value (CPV) The calculated premolar value or the expected arch width in the premolar region is determined by the formula, SI x 100 CPV = ---------------- 80 Determination of Calculated Molar Value (CMV) The calculated molar value or the expected arch width in the molar region is determined by the formula, SI x 100 CMV = ---------------- 64 Inference If the measured value is less than the calculated value, then expansion is indicated. How much expansion is needed in the premolar region is determined by the following formula: Amount of expansion = CPV-MPV needed in the premolar region How much expansion is needed in the molar region is determined by, Amount of expansion = CM -MMV needed in the m o la r region Bolton's Analysis Bolton’s analysis gives significance to the tooth size. According to Bolton, there exists a ratio between the mesiodistal width of maxillary and mandibular teeth. Malocclusion occurs when there is a disparity between the mesiodistal dimensions of maxillary and mandibular teeth. Bolton's analysis helps in determining the disproportion in size between maxillary and mandibular teeth. Procedure Sum of Mandibular 12 Teeth The mesiodistal width of all the teeth mesial to the mandibular second permanent molars is measured and summed up. Sum of Maxillary 12 Teeth The mesiodistal width of all the teeth mesial to the maxillary second permanent molars is measured and summed up. Sum of Mandibular 6 Teeth (Anteriors) Page 7 of 11 Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin The mesiodistal width of all the teeth mesial to the mandibular first permanent premolars, i.e. mandibular anteriors, is measured and summed up. The sum of Maxillary 6 Teeth (Anteriors) The mesiodistal width of all the teeth mesial to the maxillary first permanent premolars, i.e. maxillary anteriors, is measured and summed up. Determination of Overall Ratio According to Bolton's study, the sum of the mesiodistal width of the mandibular teeth anterior to the second permanent molars should be 91.3% of the mesiodistal width of maxillary teeth anterior to the second permanent molar. The overall ratio is determined by the formula, Sum of mandibular 12 Overall Ratio = --------------------------------- x 100 Sum of maxillary 12 Inference If the ratio is less than 91.3%, then it indicates maxillary tooth material excess. The amount of maxillary tooth excess is determined by the formula, Sum of mandibular 12 Sum of maxillary 12 = ---------------------------------- x 100 91.3 If the ratio is more than 91.3%, then it indicates mandibular tooth material excess. The amount of mandibular excess is determined by the formula, Sum of maxillary 12 Sum of mandibular 12 = ------------------------------- x 91.3 100 Determination of Anterior Ratio According to Bolton's study, the sum of mesiodistal widths of mandibular anteriors should be 77.2% of the mesiodistal width of maxillary anteriors. The anterior ratio is obtained by the formula, Sum of mandibular 6 Anterior Ratio = -----------------------------------x 100 Sum of maxillary 6 Inference If the ratio is less than 77.2%, then it indicates maxillary tooth material excess. The amount of maxillary excess is determined by the formula, Sum of mandibular 6 Sum of maxillary 6 = ------------------------------- x 100 77.2 If the ratio is more than 77.2%, then it indicates mandibular tooth material excess. The amount of mandibular excess is determined by the formula, Page 8 of 11 Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin Sum of maxillary 6 Sum of mandibular 6 = ---------------------------- x 77.2 100 Mixed Dentition Analysis 1-Radiographic Method This technique makes use of radiographs, as well as a study cast model, to determine the width of the unerupted teeth. Radiographic measurement of an unerupted tooth may not be precise because of distortion and thus is unreliable. However, it is possible to predict the width of the unerupted tooth by comparing the measurement of an already erupted permanent tooth on the cast as well as on the radiograph. The formula is, XI x Y 2 Y l= ----------------- X2 Where, Y1 = Width of an unerupted tooth whose measurement is to be determined. Y2 = Width of the unerupted tooth on the radiograph. XI = Width of a tooth that has erupted on the cast. X2 = Width of a tooth that has erupted measured on the radiograph. 2-Non-radiographic method Moyer's analysis Moyer’s analysis employs prediction tables. The lower incisor mesiodistal widths are measured and added to predict the sizes of unerupted teeth from the table. Step-by-step procedure for the use of probability tables Measure and obtain the mesial-distal widths of the four permanent mandibular incisors and find that value in the horizontal row of the appropriate table (sex-wise). Reading downward in the appropriate vertical column, obtain the values for the expected width of the cuspids and premolars corresponding to the level of probability. Moyer used a 75% of probability Mandibular incisors are used for the prediction of both the mandibular and maxillary cuspid and bicuspid widths. The amount of space available for the eruption of permanent canines and premolars is determined by measuring the distance between the distal surface of the lateral incisor and the mesial surfaces of the first molar. The predicted value is compared with the available arch length to determine the discrepancy. If the predicted value is greater than the available arch length, crowding of the teeth can be expected. Whereas if the predicted value is less than the available arch length, it will result in the spacing of the teeth. Page 9 of 11 Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin Tanaka and Johnston’s method Tanaka–Johnston analysis is a mixed dentition analysis. It predicts the widths of unerupted canines and premolars based on the sum of the widths of lower incisors. This method is very convenient to use in orthodontic practice. No radiographs or reference tables are involved. Method The estimated width of the mandibular canine and premolars in one quadrant is determined by adding 10.5 mm to the measured value of half of the mesiodistal width of four mandibular incisors. The estimated width of the maxillary canine and premolars in one quadrant is determined by adding 11 mm to the measured value. Tanaka and Johnston combined the sexes in their study. This is a reasonably accurate method of mixed dentition space analysis and does not require either radiographs or reference tables. 3. E-models or digital models E Models are three-dimensional digital models that help to eliminate the need for traditional plaster models in orthodontic practice. There are essentially two different technologies to produce digital models: 1. 3D scanner-based, which can work either on models or directly on mouth. 2. 3D CT-based uses alginate/rubber base impressions of the dental casts. Methods of making E models 3D Scanners. In order to make E models, the orthodontist/ laboratory uses 3-D images that are recorded through a laser scanner. This laser scanner uses a flashing white light, much like a video camera, which digitally maps the teeth or plaster models into precise, high- resolution, three-dimensional electronic records. These pictures of teeth are sent to the computer to build a complete model of the patient’s teeth and are stored on the computer’s hard disk, which can be assessed like any other digital picture. The models are viewed as 3D images on the screen and are fairly accurate in dimension. The E models can be transported as files from one orthodontist to another and to the patients or can be put on the web. 3D CT. In this method, the alginate impressions of a patient are transported in a special tray to an industrial scanner, which, like a CT scan of the skull, scans the impressions and uses highly sophisticated software to construct a 3D image of the dentition/oral structures. The artistic portion of the models is then superimposed for the purposes of presentation and aesthetics. These models can be viewed from all possible views on the computer screen. Page 10 of 11 Fourth stage Orthodontics 03/12/2024 lecture 2 2024-2025 Orthodontic Diagnosis Dr. Anwar Ahmad Amin Advantages of E models Digital models deliver significant benefits that leverage the power of digital technology to advance the practice of orthodontics. E models eliminate the production, storage, and archiving costs of plaster models, freeing up space and allowing online digital file storage with 24-hour access. They eliminate the geographic barriers of plaster models and paper files for multiple site practices and empower dental professionals with access via the Internet from any location. Digital models allow easy and seamless communication between orthodontists, general dentists, oral and maxillofacial surgeons, and patients and, therefore, facilitate comprehensive interdisciplinary treatment planning. Most of the digital model viewing software includes analytical tools to facilitate quick and easy measurements of teeth and arch length. Since E models can be measured directly on screen, this is time-saving and, therefore, enhances the efficiency in orthodontic diagnosis, treatment planning, and patient education processes. Digital set-up When using digital models, treatment planning occurs on the computer, where the orthodontist is able to map out the entire treatment sequence precisely. There are also options available to design custom arch-wires that work more efficiently and thus require fewer wire changes and adjustments throughout orthodontic care, and thus reducing treatment time. These commercial options are now available through several services in the USA and other countries. Disadvantages of E models Digital models have some inherent disadvantages: Digital models are technique-sensitive as the whole procedure of making the E models depends upon the proper scanning of the teeth and adjacent hard and soft tissues. There should be no vibration or shaking of the hand while scanning, and there should be no patient movement. Although this may not be applicable to 3D CT-based methods, time lost in transporting impressions and resultant distortion could be a contributor to inaccuracy if not taken care of. A personal touch or feel is absent with the use of digital models since a digital image can only be seen on a computer or printed on paper. There are no actual study models. Conventional study models made from plaster are better appreciated as these can be seen and felt by the patient. With digital models, there are chances of data loss as all the information is stored on the computer. Page 11 of 11