Occlusion and orthodontics by almuzian

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Occlusion and orthodontics by almuzian

  1. 1. Occlusion and orthodontics by Almuzian The ‘ideal’ occlusion described by orthodontists today is derived from the work published by Angle (1900) and Andrews (1972, 1989). It is generally assumed (Andrews, 1976; Roth, 1976) that an ideal static occlusal relationship is compatible with an ideal functional occlusion, but this is not necessarily so (Tipton and Rinchuse, 1991). Ideal occlusion It refer to an occlusion which has ideal static (Andrews six keys) and functional occlusal relationship (mutually protected occlusion) Malocclusion  It is any deviation from the physiologically acceptable contact of opposing dentition. Occlusion  It is static contact between lower teeth with upper teeth. Static Occlusion  it is the relationship between the maxillary and mandibular teeth when the teeth are brought to maximum intercuspation. Functional Occlusion  It is the occlusal contacts of the maxillary and mandibular teeth during function (speech, mastication, and swallowing). Therefore, this type of occlusion should be free of interferences in the non-working side. Three types of occlusion can be aimed at to be the goal of functional occlusion after orthodontic treatment, which are:
  2. 2. 1. Group function occlusion 2. Canine guidance occlusion. 3. Anterior/ Incisal guidance occlusion. Intercuspal position (ICP) It is the occlusal position with the teeth in maximum intercuspation (Synonymous with centric occlusion). Working Side The side the mandible moves to-wards in lateral excursion. Non-Working side The side the mandible moves away from during lateral excursions. Lateral excursions Posterior teeth relationship is achieved by canine protected occlusion and group function occlusion. Terminal Hinge Axis (retruded/terminal arc of closure) It is terminology often used to describe the intercondylar axis in CR. The condyles rotate (hinge) on this axis during initial mandibular opening, approximately 25mm, before the translation phase of TMJ movement occurs down the articular eminences Retruded Contact Position (RCP) The occlusal position when the condyle in the retruded axis position. Few millimetre discrepancies between RCP and ICP are acceptable. Coincidence of ICP and RCP found in 22% of population only (Shefter and McFall, 1984)
  3. 3. Habitual closing movements The ICP of the successive closure is the result of a conditioned reflex, generated by a ‘memory’ in the neuromuscular system, known as an engram. In some individuals, the conditioned reflex makes manipulation of the condyles into the retruded axis position very difficult to achieve. If tooth contact is prevented by using an anterior jig or bite plane for a short period of time (approximately 10 minutes is usually adequate; Lucia, 1964) the proprioceptive feedback leading to reflex closure in ICP is broken. The mandible can then be more easily guided into the retruded axis position. (Short term deprogramming manoveour) Balanced occlusion During the entire lateral movement posterior teeth on both the working side and the non-working side are in contact. Present day thinking has completely dismissed this concept for restoring the natural dentition, although it is still useful in complete denture construction. Group Function During the entire lateral movements the buccal cusps of the posterior teeth on the working side are in contact. There are no tooth contacts on the non- working side. Anterior/Incisal guidance When the front teeth are placed together on their biting edges the posterior teeth should not touch. Canine protected Occlusion (canine guidance) During the lateral excursion, contact occurs only between the upper and lower canine on the working side. There is no contact between the teeth on the non-
  4. 4. working side. The canine tooth is the most appropriate tooth to guide the mandibular excursion. There are a number of reasons why this might be so: I. The canine has a good crown/root ratio, capable of tolerating high occlusal forces; II. The canine root has a greater surface area than adjacent teeth, providing greater proprioception; III. The shape of the palatal surface of the upper canine is concave and is suitable for guiding lateral movements. Andrews’s six keys of ideal occlusion: Andrews (1972) have described the ideal occlusion in adults based on his study of 120 casts of non-orthodontics norms; compared to 1150 post- treatment study casts that were presented at the American Association of Orthodontists. The six morphological characteristics of optimal occlusion was the basis for the development of straight wire appliance. It is worth mentioned that even these non orthodontic norm has a variable degree of SD (up to 15 degree) however the occlusion stil considered optimal. Andrews’s six keys are: 1. Molar relationship:  The distal surface of the distal marginal ridge of the upper first permenant molar contacts and occludes with the mesial surface of the mesial marginal ridge of the lower second molar.  The mesiobuccal cusp of the upper first permanent molar falls within the groove between the mesial and middle cusps of the lower first permanent molar.
  5. 5.  The mesiolingual cusp of the upper first molar seats in the central fossae of the lower first molar. 2. Crown Angulation (Tip) In normally occluded teeth the gingival portion of the long axis of each crown is distal to the occlusal portion of that axis. The degree of tip varies with each tooth type. 3. Crown Inclination (Torque) Crown inclination is the angle between a line that is perpendicular to the occlusal plane, and a line tangent to the middle of the labial or buccal clinical crown.  Anterior crowns: In upper incisors, the occlusal portion of the crown’s labial surface is labial to the gingival portion.  Upper posterior crowns: Lingual crown inclination is slightly more pronounced in the molars than in the cuspids and bicuspids.  Lower posterior crowns: Lingual inclination progressively increases from the cuspids through molars. 4. Rotations Teeth should be free of undesirable rotations. If rotated, a molar or bicuspid occupies more space than normally (a condition un-receptive of normal occlusion). A rotated incisor can occupy less space than normal. 5. Contact points (Spacing) In the absence of such abnormalities as genuine tooth-size discrepancies, contact points should be tight.
  6. 6. 6. Curve of Spee: A flat occlusal plane should be a treatment goal. Measured from the most occlusally prominent-cusp of the lower second molar to the lower central incisor, no curve was deeper than 1.5mm in the non-orthodontics norms. Roth mentioned that for a functional occlusion we need (Roth 6 keys of functional occlusion) 1. ICP coincide with RCP 2. Cuspal guidance or Canine guidance (the decision to provide cuspal or canine guidance depend on the FMPA. In high angle case, group function is the aim and opposite in low angle case. Di Pietro 1977). 3. Incisor guidance 4. Upper teeth overlap lower 5. Biting along the LA of the teeth The six identified characteristics were deemed to be realistic treatment objectives for more than 90% of the patients accepted by North American orthodontists and for white people only Another problem lies in the dynamics of wire bending effects. For example, as you place torque in the anterior part of the archwire you negate tip by a ratio of four-to-one (wagon Wheel effect)  Bennett and McLaughlin Seventh Key:
  7. 7. A seventh key was added by Bennett and McLaughlin (1993) which is correct tooth size; i.e. no tooth-size discrepancies. Importance of functional tooth contacts It has frequently been proposed that the following problems can result from certain patterns of functional tooth contact. 1. Mandibular dysfunction, egermark-erikson et al (1983)show no evidence 2. Bruxism, Egermark-Erikson et al (1983)show no evidence 3. Periodontal disease, there is very little work to support it except the work by Ericsson, Thilander and Lindhe 1978) on beagle dogs with experimental periodontitis which done on animal and can not rely on it. 4. Instability of tooth position, Roth claims that but there is no evidence. Types of articulators in orthodontics 1. Hinge 'articulators' should be considered a cast holding instrument rather than a true articulator. does not permit lateral movements. 2. Plane Line and Average Value articulators permit both vertical and horizontal movements but do not address finer movements of the TMJs. The horizontal movements are based on average angles which are usually 30 (for condylar guidance, 15( for incisal guidance and an intercondylar distance of 110mm) 3. Semi-adjustable articulators can accept a facebow (see below) and can be adjusted in the vertical, sagittal and horizontal planes to approximate lateral and protrusive mandibular movements.  ARCON - ARticulator CONdyle. The condylar ball is on the lower arm and the articular fossa is on the upper arm (e.g. Denar).
  8. 8.  Non-ARCON - The condyles are on the upper arm (e.g. Dentatus). 4. Fully-adjustable articulators aim to provide more adjustments and a greater degree of accuracy. They require pantographic tracings, which attempt to record border movements of the mandible, and the articulator is adjusted to simulate these movements. These instruments are expensive, time-consuming and rely on excellent laboratory support to ensure the degree of accuracy is maintained. The use of articulator in orthodontics by Clark 2001 There are few indications for the use of articulator-mounted casts in orthodontics. Their use is advocated in the following circumstances: 1. Where a significant discrepancy (>2 mm) exists between RCP and ICP. 2. Multiple missing teeth. 3. Orthognathic cases. 4. Although the evidence for occlusal parameters in the aetiology of TMD is equivocal, the articulator-mounting of study models pre-orthodontic treatment and pre-debond in individuals with TMD is recommended. This will enable the clinician to ensure that occlusal interferences are eliminated prior to debond and a record retained of the funcional occlusal relationships for medico-legal purposes.
  9. 9. Facebows  A facebow is an instrument which records and allows the transfer of the relationship of the terminal hinge intercondylar axis and the maxillary dentition from the patient to the articulator.  The procedure permits a more accurate simulation of lateral and protrusive mandibular movements  The facebow method is specific to the articulator system. For the more commonly used Denar system, the posterior reference point is the external auditory meatus (in the region of the inter-condylar axis) whilst the anterior reference point is 42mm vertically above the incisal edge of the right lateral incisor Inter-occlusal Record (IR) 1. ICP  Injectable silicones (Jet-Bite, BluMouse)  Conventional wax-bites. 2. CR  This position, also known as 'pre-centric', is recorded to assess the first contact for the re-organised approach for restorative dentistry  To obtain the pre-centric record a jig is made to hold the teeth apart and stabilise the jaws whilst the IR material hardens. How to achieve dynamic occlusion? A. Di Pietro, 1977, investigated the frequency of naturally occurring patterns of occlusion.
  10. 10.  He found canine guidance truly exists in only 35% of normal occlusions, and these always have low Frankfort/Mandibular plane angles and flat occlusal planes.  Group function is more frequent, and becomes progressively more posterior as the cant of the occlusal plane increases. B. The decision whether to aim for canine guidance or group function should thus be based on evaluating the probability of finishing a case to a low angle, flat occlusal plane dento-facial pattern.  In forward growth rotation, canine guidance is the aim,  a posterior rotation will favour group function. C. Correct bracket height, giving good levelling of the arches with adequately stiff finishing wires will enable smooth intercuspal contacts to be achieved. D. Consistent arch forms, with particular attention to final co-ordination will also ensure that the arches fit together on closure. E. The clinician must develop the ability to assess centric relation correctly and treat to that position, eliminating any mandibular shift that may be present initially.

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