Development of the Normal Occlusion.ppt - Development of the Normal Occlusion Mhmad karsh In a growing child many morphological changes take place in

Development of the Normal Occlusion.ppt - Development of...

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Unformatted text preview: Development of the Normal Occlusion Mhmad karsh In a growing child many morphological changes take place in size, shape and relationship of the dental arches. For purpose of ease in studying these developmental changes, of the dental arches and normal occlusion, Friel (1952) divided dental arch development into five stages as follows: 1. From birth until completion of the deciduous dentition (0 to 2 ½ years). 2. Deciduous dentition stage from completion of their eruption up to the eruption of the first permanent molars (2 ½ to 6 years). 3. 4. 5. Mixed dentition stages from the eruption of the first permanent molar to the final shedding of the deciduous teeth (6 to 12 years). The completion of the permanent dentition, by the eruption of permanent canines, premolars and permanent second molars (12 years and thereafter). Adult dentition stage. By the eruption of the third molars. Development of the Normal Occlusion: 1. 2. 3. 4. 5. Stage 1. Deciduous Dentition Stage. Mixed Dentition Stage. Early Permanent Dentition Stage. Adult Occlusion. 1. Stage 1: A. At Birth (Gum Pad): Description: In the mouth of the new born featus, gum pads cover the alveolar processes. These gum pads are firm and rounded as in an edentulous mouth. The maxillary gum pad is horseshoe shaped with a very shallow vault. The alveolar part is separated on its palatal side from the hard palate by a continuous horizontal groove known as dental or gingival groove. The lower gum pad is U shaped and again its alveolar part is limited lingually by a continuous horizontal groove. The alveolar part for both maxillary and mandibular gum pads are soon divided by transverse grooves into ten segments denoting the developing deciduous teeth (the mandibular are less defined than the maxillary). The groove distal to the canine segment continues on the buccal side of the gum pad and is called the lateral sulcus. Relationship Between Upper and Lower Gum Pads: The lateral sulcus of the maxillary pad is mesial to that of the mandibular when both are approximated. This refers to the backward position of the mandible in relation to the maxilla at that time. When the gum pads are approximated, there is rarely any contact in the anterior region, whereas in the back they touch at the first deciduous molar segment. This anterior vertical gap decreases by the growing alveolar processes and the erupting deciduous incisors. The upper gum pad is wider leading to overjet of the maxillary pad over the mandibular with a considerable overjet anteriorly. With the mandible in its physiological rest position these pads are apart "not in contact" with the tongue projection between them to lie behind the lower lip which forms the principal boundary to the front of the oral cavity. The upper lip appears short and flaccid at that age. Early in the first year of neonatal life the gum pads are not sufficiently wide to accommodate the developing incisors, which are crowded and rotated in their crypts. By the first year of life, gum pads grow rapidly especially in the lateral direction. This gives an increase in the arch width, which finally permits incisors to erupt in good alignment, helped by the molding effect of the lips and tongue. B. Eruption of the Deciduous Teeth: Early in the 4th month intrauterine life (I.U.L), calcification of the deciduous teeth begins. At birth 1/4 to 1/2 of the deciduous crowns have been calcified with the tips of the first permanent molar cusps. The first tooth to erupt is usually a mandibular central incisor at about 6 to 7 months followed by one of the maxillary centrals at about 8 months. At 9 to 10 months both maxillary and mandibular laterals erupt. The first deciduous molars erupt ahead of the canines at about 14 to 15 months then the canine at about 18 months. Finally second deciduous molars emerge at about 24 months. However, there are wide variations in both the eruption dates and the order of eruption. Racial and socioeconomic differences are likely. Matton (1955) has shown that heredity plays a greater role in eruption of primary teeth than environments do. 2. Deciduous Dentition Stage: This stage covers the period from completion of eruption of the deciduous dentition at about 2 ½ years till the emergence of a permanent tooth, which is usually to be either a first molar or a mandibular central incisor. There is less variability in occlusal relationship in the primary teeth than in the permanent dentition. Arch Shape: Most primary arches are ovoid in shape and display less variability in conformation than do the permanent arches. Spacing in Deciduous Teeth: Usually there is generalized spacing in the incisor region. These spaces allow for the difference between the size of the deciduous incisors and their permanent successors (the later being greater). These spacing contrary to the past popular opinion do not increase significantly after the primary dentition is completed. Although the spacing is likely to be generalized, somewhat wider spaces may be found mesial to the maxillary canines and distal to the mandibular canines and termed "Primate Space". The mandibular primate space is partially closed by the eruption of the permanent mandibular first molar, while the maxillary primate space is usually closed by the eruption of the permanent maxillary incisor. Changing Occlusion of Deciduous Teeth: Anterior Teeth: The deciduous incisors are vertical with minimal overbite and overjet however; at 3 years of age there may be what is called excessive overbite as the upper incisors almost cover the lower incisors when the teeth are brought into occlusion. This overbite is reduced by rapid attrition of the incisors. So at the age of 6 years, there may be an edge-toedge incisor relationship, which facilitates the early forward growth shift of the mandible. Posterior Teeth: The primary posterior teeth occlude so that the mesiolingual cusp of the maxillary molar occludes in the central fossa of the mandibular molar (i.e. the mesial cusp of the mandibular molar is ahead to the mesial cusp of the maxillary molar). The mandibular second primary molar is usually wide mesio-distally than the maxillary giving rise to a flush terminal plane of the distal surfaces of both maxillary and mandibular second primary molars. Flush Terminal Plane If this flush terminal plane remains, it will lead to an end to end relationship of the first permanent molars when they erupt distal to the second deciduous molars and arrive to occlusion. However, with natural occlusal wear of the cusps of the deciduous molars, the mandible is allowed to move forward by its forward component of growth, which exceeds that of the maxilla. Another contributing factor is the closure of the primate space distal to the mandibular canine. At such a case the distal surface of the mandibular primary second molar assumes a distinct mesial step terminally in relation to that of the maxillary allowing the first permanent molars to erupt in normal molar relationship (i.e. the mesiobuccal cusp of the maxillary permanent first molar occludes within the anterior buccal groove of the mandibular permanent first molar). In a case of absence of spacing in deciduous dentition with flush terminal plane, no space will be available for early mesial migration of the mandibular first permanent molar, thus the permanent first molars remain in an end to end relationship till the shedding of the mandibular deciduous second molars. 3. Mixed Dentition Stage: This covers the period from the eruption of the first permanent tooth till the shedding of the last deciduous tooth (from 6 - 12 years). Permanent dentition starts calcification after birth with the exception of cusps of the first permanent molars. So, post-natal environmental disorders and diseases might affect the calcification. It was found that the permanent teeth do not begin eruptive movements until their crowns are completed. They pass or pierce through the crest of the alveolar process when approximately two thirds of the root are formed and pierce the gingival margin and emerge to the oral cavity when early 3/4 of the roots are completed. The timetable of eruption of the permanent teeth may be as follows: 1. 2. First permanent molars: 6 ½ years. Lower permanent central incisors: 6 ½ years. 3. Upper permanent central incisors: 7 ½ years. 4. Lower permanent lateral incisors: 8 ½ years. 5. Upper permanent lateral incisors: 9 years. 6. Lower permanent canines: 10 years. 7. Upper first premolars: 10 ½ years. 8. Lower first premolars: 11 years. 9. Upper second premolars: 11 years. 10. Lower second premolars: 11 ½ years. 11. Upper permanent canines: 12 years. 12. Lower permanent second molars: 12 ½ years. 13. Upper permanent second molars: 12 ½ years. The above dates of eruption are averages and never the less few individuals are averages. The majority lies around these averages. Racial and socioeconomic differences are likely. The germs of all the permanent teeth are formed on the lingual side of their deciduous predecessors. The permanent incisors remain lingual to the deciduous throughout their development and if a deciduous incisor fails to shed the permanent one will normally erupt in a lingual position. It will tend to move to the line of the dental arch causing the deciduous to shed out or it may be locked in crossbite in the case of an upper incisor. The incisor teeth are often rotated before eruption to occupy less space, but at the time of their eruption they improve their position as lateral growth of the dental arches takes place. In a potentially crowded mouth this rotation may persist after eruption. The premolars migrate at an early stage from their lingual position so as to lie between the roots of the deciduous molars before they start calcification. The upper permanent first molar develops in the tuborosity of the maxilla with its occlusal surface facing somewhat distally. It moves downwards and forwards as growth takes place by bone apposition on the tuborosity until it erupts, its former position being occupied by the second molar and finally the third molar. Similarly the lower permanent first molar develops in the ascending ramus of the mandible with its occlusal surface tilted mesially. This tooth moves forwards with little or no vertical movements as growth takes place by bone remodeling at the angle between the anterior border of the ramus and the body of the mandible. The second and then the third molar replace it in the ascending ramus. Eruption of the Permanent Incisors: The deciduous incisors are much smaller than their permanent successors. Some or all of these growth mechanisms may accommodate the greater size of the permanent incisors in the dental arches: 1. The normal space present between deciduous incisors, which might increase but usually remain unchanged until they are shed. 2. 3. Increase in the inter-canine width. In average terms, there is 1 - 2 mm. increase in inter-canine width during the time the deciduous incisors are in the mouth. During the eruption of the permanent incisors and especially the permanent canine, there is about 3 - 5 mm. increase in the upper arch and much less in the lower (the increase in mandibular intercanine width stops earlier than that of the maxilla). The fact that the permanent incisors are more proclined than their predecessors giving rise to a wider arch and more space is available to accommodate larger permanent incisors. Inter-canine width If the deciduous incisors are in contact and not spaced, this should be regarded as essentially a crowded condition. However, as the correlation in size of deciduous teeth and permanent teeth is not very high, large deciduous teeth that can be replaced by relatively small permanent successors may cause this lack of spacing between deciduous teeth. On average un-spaced deciduous teeth tend to be associated with slightly more growth in intercanine width. If the deciduous teeth are crowded themselves, this is indicative of crowding of permanent teeth. The permanent incisors are frequently crowded when they first erupt, but this crowding may be improved as eruption takes place. Eruption of the Permanent Canines and Premolars: In an average individual the combined mesiodistal width of the mandibular deciduous canine, first molar and second molar are in average 1.7 mm. greater than that of the mandibular permanent canine, first premolar and second premolars. This difference in size is called the leeway space. However, this Leeway space is lesser in the maxillary arch as the combined mesio-distal width of the maxillary deciduous canine and molars is on average greater 0.9 mm. than that of the maxillary permanent canine and premolars. Thus there is a Leeway space of 3.4 mm. in the mandible bilaterally while it is only 1.8 mm. in the maxilla bilaterally in the transition from mixed dentition to the full permanent dentition. Some of this Leeway space is taken by the distal movement of the larger permanent canine falling into the primate space, while other part is utilized by the late mesial shift of the mandibular molars in order to establish a normal molar relationship. The maxillary permanent canine develops distant from the dental arch and close to the floor of the orbit. At 9 to 10 years as it moves downward and forward towards occlusion, it comes to lie against the apices of the erupted permanent lateral and central incisors causing mesial pressure on their roots. The centrals clinically respond to that pressure by central diastima and distal crown flaring while the laterals show labial tipping. Broadbent called this clinical picture "UGLY DUCKLING PATERN OR STAGE". This temporarily clinical picture improves as the permanent maxillary canine continues to erupt and exert mesial wedging action on the crowns of the lateral and central incisors. Since the maxillary canine erupts after its neighbors, it is particularly liable to be the victim of crowding. 4. Early Permanent Dentition Stage: After the eruption of the permanent premolars, canines and second molars at about 12 to 13 years, now all teeth are in good intercuspation, normal molar relationship but some proclination of the incisors and fullness of the lips may be observed. This may be attributed to the larger tongue size relative to the surrounding skeleton of the jaws. As the jaws catch up to the tongue in growth, they bring the apices of the incisors forward giving rise to more upright position of the incisors. Finally this will lead to a decrease in the lip convexity. As the mandible continues to grow forward more than the maxilla, the mandibular incisors are locked lingually to the upper, they tip lingually and the mandibular arch perimeter decreases more. This may lead to the so-called "Late Teen Crowding" of the lower incisors especially seen in boys. As the mandible continuous to grow downward and forward, the incisors tend towards an edge to edge occlusion. Adult Occlusion: After the eruption of the third molars very little changes take place in the dental arch size and relations. Dimensional Changes in the Dental Arches: Dental arch dimensions may be enumerated as length, width and perimeter or circumference. 5. A. Arch Width: The width is a term which may be given to the transverse linear measurements between the right and left canines and called the intercanine width or between molars and called inter-molar width or bi-molar width. The maxillary inter-canine width increases from 3 to 14 years by about 5 mm., less increase in width is observed in the intermolar width for both arches. Inter-canine width Inter-molar width B. Arch length and Arch Perimeter: Arch length is usually defined as the length of a perpendicular line drawn from the mesial contact point of the upper central incisors to a line joining the distal contact points of the second deciduous molars or second premolars. The arch perimeter is measured from the mesial surface of the first permanent molar around the arch over the contact points between premolars or deciduous molars and over the incisal edges of the incisors to the mesial surface of the first permanent molar of the opposite side. Arch Length Arch Perimeter In the maxilla there is a slight increase in the arch length and the arch perimeter (which may reflect the same thing) between 6 and 10 years as the permanent incisors erupt while the deciduous molars are still present. This is followed by a decrease as the later teeth are replaced by the smaller permanent premolars. In the mandible there is no increase in the arch length or perimeter but a noticeable decrease after 10 years. This decrease in mandibular arch perimeter may be attributed to: 1. Late mesial shift of the mandibular molar as the Leeway space is utilized. 2. The inter-proximal wear of the teeth. 3. The mesial drift tendency of the teeth throughout the life. 4. The lingual tipping of the mandibular incisors as the mandible grows forward. Thank You ...
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