Fifth Stage Orthodontics Study Models PDF

Study models By: Dr. Hussain Haitham Study models They are accurate plaster reproductions of the teeth and their surrounding soft tissues. Study models are essential diagnostic records, which help to study the occlusion and dentition from all three dimensions. Ideal requirements of orthodontic stu...

Study models By: Dr. Hussain Haitham Study models They are accurate plaster reproductions of the teeth and their surrounding soft tissues. Study models are essential diagnostic records, which help to study the occlusion and dentition from all three dimensions. Ideal requirements of orthodontic study models 1. Models should accurately reproduce the teeth and their surrounding soft tissues. 2. Models are to be trimmed so that they are symmetrical and pleasing to the eye and so that an asymmetrical arch form can be readily recognized. 01 Study models 3. Models are to be trimmed in such a way that the dental occlusion shows by setting the models on their backs. 4. Models are to be trimmed such that they replicate the measurements and angles proposed for trimming them. 5. Models are to have clean, smooth, bubble-free surfaces with sharp angles where the cuts meet. 6. The finished models should have a glossy marproof finish. 02 Why we make study models? (Benefits) 1. They are invaluable in planning treatment, as they are the only three dimensional records of the patient's dentition. 2. Occlusion can be visualized from the lingual aspect. 3. They provide a permanent record of the intermaxillary relationships and the occlusion at the start of therapy; this is necessary for medicolegal considerations. 4. They are a visual aid for the dentist as he/she monitors changes taking place during tooth movement 03 Why we make study models? (Benefits) 5. Help motivate the patient, as the patient can visualize the treatment progress. 6. They are needed for comparison at the end of treatment and act as a reference for post-treatment changes. 7. They serve as a reminder for the parent and the patient of the condition present at the start of treatment. 8. In case the patient has to be transferred to another clinician, study models are an important record. 04 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 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 record before, immediately, after and several years following treatment for the purpose of studying treatment procedures and stability. 05 Parts of the study models The study models can be divided into two parts for the purpose of description: The anatomic portion The anatomic portion is that part which is the actual impression of the dental arch and its surrounding soft tissue structures. This is the part, which must be preserved when trimming the model. 06 Parts of the study models The artistic portion The artistic portion is the stone base supporting the anatomic portion. This portion is trimmed in a manner, which depicts, in a general way, the dental arch form and is pleasing to the eye. 07 Study model fabrication and trimming Preliminary procedures in the fabrication of study models are: 1. Remove any excess flash or obviously excessive bulk on the periphery of the models 2. Remove any nodules that may be present on the occluding surfaces of the teeth 3. Remove any extensions in the posterior areas that prevent occluding of the models 4. Using the wax bite, occlude the models. 08 Model analysis 1. Pont’s analysis In 1909 Pont presented to the profession a system whereby the mere measurement of 4 maxillary incisors automatically established the width of the arch in the premolar and molar region. It gives an approximate indication of the degree of narrowness of the dental arches in a case of malocclusion and also the amount of lateral expansion required for the arch to be of sufficient size to accommodate the teeth in perfect alignment 09 Model analysis Where, Sum of Incisors (SI): The greatest width of the incisors is measured with calipers recorded on a line (In mm). Measured Premolar Value (MPV): The distance between the upper right first premolar and upper left first premolar (i.e. the distal end of the occlusal groove). Measured Molar Value (MMV): The distance between the upper right first molar and upper left first molar (i.e. the mesial pits on the occlusal surface). 10 Model analysis Calculated premolar value (CPV): The expected arch width in the premolar region is calculated by the formula: SI x 100/ 80 Calculated molar value (CMV): The expected arch width in the molar region is calculated by the formula: SI x 100/ 64 The difference between the measured and calculated values determines the need for expansion. If measured value is less, expansion is required. 11 Drawback of Pont's Analysis 1. Maxillary laterals are the teeth most commonly missing from the oral cavity. 2. Maxillary laterals may undergo morphogenetic alteration like 'peg' shaped lateral. 3. This analysis is derived solely from the casts of the French population. 4. It does not take skeletal mal-relationships into consideration. 5. Pont's index does not account for the relationship of the teeth to the supporting bone, or the difficulties in increasing the mandibular dimensions. 12 Drawback of Pont's Analysis 6. It may be useful to know the desired maxillary dimension for a case, but it is more difficult to achieve the corresponding mandibular dimensions that are necessary to maintain a balanced occlusal relationship It should always be remembered that the patient's original mandibular and maxillary arch form should be considered as the ultimate guide for arch width rather than the values arrived at by using the Pont’s index. 13 2. Linder Harth index Linder Harth proposed an analysis, which is very similar to Pont's analysis. However he made a variation in the formula to determine the calculated premolar and molar value. The calculated premolar value is determined using the formula: SI x 100/ 85 The calculated molar value is determined using the formula: SI x 100/ 64 Where, SI = Sum of mesiodistal width of incisors 14 3. Korkhaus analysis This analysis makes use of the Linder Harth's formula to determine the ideal arch width in the premolar and molar region. An additional measurement is made from the midpoint of the inter-premolar line to a point in between the two maxillary incisors. According to Korkhaus, for a given width of upper incisors a specific value of the distance between the midpoint of interpremolar line to the point between the two maxillary incisors should exist. 15 3. Korkhaus analysis In case of proclined upper anteriors, an increase in this measurement is seen while a decrease in this value denotes retroclined upper anteriors. For the values noted the mandibular value (LL) should be equal to the maxillary value (Lu) in millimeters minus 2 mm. 16 4. Ashley Howe's analysis Ashley Howe considered tooth crowding to be due to deficiency in arch width rather than arch length. He found a relationship between the total width of the mesiodistal diameters of teeth anterior to the second permanent molars and the width of the dental arch in the first premolar region. Total tooth material (TTM) Refers to the sum of the mesiodistal width of the teeth from first molar to first molar (inclusive of the first molars), taken on casts of the dental arches, measured with dividers or a Boley’s gauge. 17 Basal arch length (BAL) In the maxilla the median line measurement from Downs A point is projected perpendicularly to the occlusal plane, then to the median point on a line connecting the distal surface of first molars. In the mandibular arch the measurement is made from Downs B point to a mark on the lingual surface of the cast in the same manner as in the maxilla. Premolar diameter (PMD) is the arch width measured at the top of the buccal cusp of the first premolar. 18 Premolar basal arch width (PMBAW) Is obtained by measuring the diameter of the apical base from canine fossa on one side to the canine fossa on the other side, about 8mm below the chest of interdental papilla below the canine and first premolar with the lower end of the Boley’s gauge. To determine whether the apical bases of the patient could accommodate the patients' teeth, the following measurements have to be obtained: 19 Percentage of premolar diameter to tooth material= (PMD x 100)/ TTM Percentage of premolar basal arch width to tooth= (PMBAW x 100)/ TTM Percentage of basal arch length to tooth material= (BAL x 100)/ TTM Comparison between PMBAW and PMD gives an idea of the need and the amount of expansion required and PMBAW% gives an indication towards an extraction or non-extraction treatment plan. 20 5. Wayne A. Bolton analysis Bolton pointed out that the extraction of one tooth or several teeth should be done according to the ratio of tooth material between the maxillary and mandibular arch, to get ideal interdigitation, overjet, overbite and alignment of teeth. Bolton's analysis helps to determine the disproportion between the size of the maxillary and mandibular teeth. To attain an optimum inter-arch dental relationship, the maxillary tooth material should approximate desirable ratios, as compared to the mandibular tooth material. 21 5. Wayne A. Bolton analysis The sum of the mesiodistal diameter of the 12 maxillary teeth (sum of maxillary 12) and the sum of the mesiodistal diameter of the 12 mandibular teeth (sum of mandibular 12) including the first molars are determined. In the same manner, the sum of 6 maxillary anterior teeth (sum of maxillary 6) and the sum of 6 mandibular anterior teeth from canine to canine (sum of mandibular 6) is determined. 22 Overall ratio The sum of the mesiodistal widths of the 12 mandibular teeth should be 91.3 percent the mesiodistal widths of the 12 maxillary teeth, according to Bolton. This ratio is calculated using the following formula: Overall ratio =sum of mand. 12 x 100 ÷ sum of max. 12 If the overall ratio is greater than 91.3 percent, then the mandibular tooth material is excessive. 23 Anterior ratio The sum of the mesiodistal diameter of the 6 mandibular anterior teeth should be 77.2 percent the mesiodistal widths of the 6 maxillary anterior teeth. This ratio can be found out using the formula: Anterior ratio= sum of mand. 6 x 100 ÷ sum of max. 6 If the anterior ratio is greater than 77.2 percent, then the mandibular anterior tooth material is excessive. 24 Drawbacks of the Analysis 1. This study was done on a specific population and the ratios obtained need not be applicable to other population groups. 2. Bolton analysis doesn't take into account the sexual dimorphism in the maxillary canine widths. Bolton advocated the reduction of tooth material in the anterior region if the anterior ratio shows an excess of tooth material. He prefers to do proximal stripping on the upper arch if the upper anterior tooth material is excess and extraction of a lower incisor, if necessary, to reduce tooth material in the lower arch. 25 6. Carey's analysis The arch length- tooth material discrepancy is the main cause for most malocclusions. This discrepancy can be calculated with the help of Carey's analysis. This analysis is usually done in the lower arch. The same analysis when carried out in the upper arch is called as arch perimeter analysis. 26 6. Carey's analysis Procedure: Determination of arch length: The arch length is measured anterior to the first permanent molar using a soft brass wire. The wire is placed touching the mesial aspect of lower first permanent molar, then passed along the buccal cusps of premolars, incisal edges of the anteriors and finally continued the same way up to the mesial of the first molar of the contralateral side. 27 6. Carey's analysis Determination of arch length: The brass wire should be passed along the cingulum of anterior teeth if anteriors are proclined and along the labial surface if anteriors are retroclined. The mesiodistal width of teeth anterior to the first molars are measured and summed up as the total tooth material. The difference between the arch length and the actual measured tooth material gives the discrepancy. 28 6. Carey's analysis Interpretation: The amount of discrepancy between arch length and tooth material is calculated. The followings could be implied: If the arch length discrepancy is: 0 to 2.5 mm; proximal stripping can be carried out to reduce the minimal tooth material excess. 2.5 to 5 mm; extraction of 2nd premolar is indicated Greater than 5 mm; extraction of first premolar is usually required. 29 Mixed dentition analysis The purpose of a mixed dentition analysis is to evaluate the amount of space available in the arch for succeeding permanent teeth and necessary occlusal adjustments. Many methods of mixed dentitions analysis have been suggested; however, all fall into two strategic categories: 1. Those in which the sizes of the unerupted cuspids and premolars are estimated from measurements of the radiographic image. 2. Those in which the sizes of the cuspids and premolars are derived from knowledge of the sizes of permanent teeth already erupted in the mouth. 30 Mixed dentition analysis 1. Moyer's mixed dentition analysis The correlation between the sizes of the mandibular incisors and the combined sizes of cuspids and bicuspids in either arch is high enough to predict the amount of space required for the unerupted teeth during space management procedures. The mandibular incisors have been chosen for measuring, since they erupt into the mouth early in the mixed dentition. 31 Mixed dentition analysis 1. Moyer's mixed dentition analysis The maxillary incisors are not used in any of the predictive procedures, since they show too much variability in size, and their correlations with other groups of teeth are of lower predictive value. Procedure in the mandibular arch Measure the greatest mesiodistal width of each of the four mandibular incisors. Determine the amount of space needed for alignment of the incisors. 32 Mixed dentition analysis Procedure in the mandibular arch Compute the amount of space available after incisor alignment. Predict the size of the combined widths of the mandibular cuspid and bicuspids. Procedure in the maxillary arch. The procedure is similar to that for the lower arch, with two exceptions: A different probability chart is used for predicting the upper cuspid and bicuspid sum. 33 Mixed dentition analysis Procedure in the maxillary arch. Allowance must be made for overjet correction when measuring the space to be occupied by the aligned incisors. Remember that the width of the lower incisors is used to predict upper cuspid and bicuspid widths. 2. Tanaka and Johnson analysis The prediction of the size of unerupted canines and premolars in contemporary orthodontic population can be done with the Tanaka and Johnson analysis. They found Moyer's prediction table to be equally appropriate for contemporary population. 34 Mixed dentition analysis 2. Tanaka and Johnson analysis However they have simplified Moyers 75 percent level of the prediction table into a formula: 𝑈𝑝𝑝𝑒𝑟 𝑐𝑎𝑛𝑖𝑛𝑒 & 𝑝𝑟𝑒𝑚𝑜𝑙𝑎𝑟𝑠 𝑤𝑖𝑑𝑡ℎ𝑠 = 𝑤𝑖𝑑𝑡ℎ 𝑜𝑓 𝑓𝑜𝑢𝑟 𝑙𝑜𝑤𝑒𝑟 𝑖𝑛𝑐𝑖𝑠𝑜𝑟𝑠 + 11𝑚𝑚 2 𝐿𝑜𝑤𝑒𝑟 𝑐𝑎𝑛𝑖𝑛𝑒 & 𝑝𝑟𝑒𝑚𝑜𝑙𝑎𝑟𝑠 𝑤𝑖𝑑𝑡ℎ𝑠 = 𝑤𝑖𝑑𝑡ℎ 𝑜𝑓 𝑓𝑜𝑢𝑟 𝑙𝑜𝑤𝑒𝑟 𝑖𝑛𝑐𝑖𝑠𝑜𝑟𝑠 + 10.5𝑚 2 35 Mixed dentition analysis 2. Tanaka and Johnson analysis However they have simplified Moyers 75 percent level of the prediction table into a formula: Predicted width of maxillary canine and premolar Estimated width of maxillary canine and premolar in one quadrant= (Mesiodistal width of four lower incisors/ 2) +11 Predicted width of mandibular canine and premolar Estimated width of canine and premolar in one quadrant= (Mesiodistal width of four lower incisors/2) +10.5 35 Kesling DIAGNOSTIC SET-UP Kesling, HD introduced the diagnostic set-up which is made from an extra set of trimmed study models. The diagnostic helps the clinician in treatment planning as it simulates various tooth movements, which are to be carried out in the patient. The individual teeth along with their alveolar process are sectioned off from the model using a saw and replaced back in the desired final position. 36 Kesllng DIAGNOSTIC SET-UP Uses of Diagnostic Set-up 1. Aids in treatment planning as it helps to visualize tooth size-arch length discrepancies and determine whether extraction is required or not. 2. The effect of extraction and tooth movement following it, on occlusion can be visualized. 3. It also acts as a motivational tool as the improvements in tooth positions can be shown to the patient. 37