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1 Mandibular growth rotations Mandibular growth rotations Prepared by: Prepared by: Dr. Mohammed Alruby Dr. Mohammed Alruby M Mandibular andibular growth rotations growth rotations Dr. Mohammed Alruby Dr. Mohammed Alruby
2 Definition: Mandibular growth rotation (MGR): defined by Bjork and coworker as: the complex system of rotational mandibular movements resulting from condylar growth and functional remodeling that take place during active growth periods and may proceeds after puberty N: B: Mandible consists of core and functional process: 1-Alveolar process: serving in function of mastication 2-Muscular process: serving as muscular attachment 3-Condylar process serving to articulate the bone with the rest of skull Types of mandibular growth rotation MGR I- According to direction: Bjork: 1-Forward and upward (counterclockwise) rotation: This type occurs in most individuals during pre-pubertal and post-pubertal growth, can divided into: a-Forward rotation with fulcrum located at the incisal edge of lower incisors, occurs mainly due to vertical condylar growth b-Forward rotation with fulcrum located at the premolar area, occurs mainly due to vertical condylar growth and lack of incisal stop ------ deep bite 2-Backward and downward (clockwise) rotation: The mandible rotates backward and downward with the fulcrum located near the condyles. This type is expected in certain pathological condition such as tonsillitis, adenoids, nasal obstruction or abnormal habits such as tongue thrust, and mouth breathing II- According to origin: Skiller: 1-Matrix rotation: The mandible rotates as a whole by the influence of overlying soft tissue matrix, with center of rotation located at the condyle. This type represents 1/3 of total MGR 2-Intra-matrix rotation: Usually caused by condylar growth and functional remodeling, the direction of this type is usually determined from the shape of lower border of mandible. This type represents 2/3 of total MGR 3-Total rotation: It is the sum of the previous two types III- Mandibular auto-rotation: In which the mandible rotates as a whole without change in its morphology ------ clockwise rotation N: B: == Mandibular growth rotation is an important phenomenon in human facial growth, however the changes in vertical height of dentition, the inclination of occlusal plane and subsequent mandibular rotation during growth are poorly established == the mandibular growth rotation is associated with occlusal plane rotation, which caused by greater height increase in the posterior than anterior segment IV- Other types of rotations: 1-Internal rotation: Rotation of mandibular core relative to cranial base Around condyle ------- matrix rotation M Mandibular andibular growth rotations growth rotations Dr. Mohammed Alruby Dr. Mohammed Alruby
3 Within body -----------intra-matrix rotation 2-External rotation: Rotation of mandibular plane relative to the core of mandible and it is produced by surface changes (Bjork 1969, Bjork and Skiller 1972, Mills 1978, Huston et al 1985) V- Other types: 1-Forward rotation: a-Type I forward rotation: Center at joint that give rise to deep bite Underdevelopment of anterior facial height The cause may be occlusal imbalance due to loss of teeth or powerful muscular, may occur at any age b-Type II forward rotation: = Center located at incisal edges of lower anterior teeth due to the combination of marked development of posterior facial height and normal increase in the anterior height = posterior part of mandible rotates away from maxilla increasing the height of ramus = mandibular symphysis swing forward to make the chin more prominent c-Type III forward rotation: Center of rotation displaced backward to the level of premolars Anterior facial height become underdeveloped Posterior facial height increase Chin more prominent than type II Displace the path of eruption of all teeth in mesial direction Mandibular posterior teeth more upright in relation to maxillary posterior teeth. 2-Backward rotation: a-Type I backward rotation: Center at joint, this case when bite raised by orthodontic means as, change inter-cuspal or bite raising appliance Increase anterior facial height ------- open bite Occurs in case of flattening of cranial base or incomplete development of middle cranial fossa as in oxycephaly. b-Type II backward rotation: Center of rotation situated at the most distal occluding molars The lower incisors become retroclined, this induce crowding in lower anterior teeth Premolars and molars are inclined forward in relation to the maxillary teeth because of their proximity to the center of rotation N: B: In short face: the rotation also progressively upright the incisors displacing them lingually toward crowding In long face: anterior open bite will develop. The rotation of the jaws caries the incisors forward creating dental protrusion Mechanisms of mandibular rotation: 1-Displacement type of rotation: Mandible rotate upward and downward at the condyle pivot Mandible rotate upward and forward to meet a short mid face or closed basi-cranial flexure, and it rotate M Mandibular andibular growth rotations growth rotations Dr. Mohammed Alruby Dr. Mohammed Alruby
4 Downward and backward to accommodate vertically long mid face and / or open basi-cranial flexure 2-Remodeling type of rotation: Remodeling occurs at the angle between corpus and ramus (but not at gonial angle) The ramus become more upright. This occurs due to the resorptive and deposit growth processes occurs at the ramus – corpus junction Prediction of mandibular growth rotation Prediction of amount and direction of MGR in the individual pt is very important consideration in orthodontic treatment planning. As this enable clinician to decide when extrusive or intrusive mechanics should be used and when environmental factors to be modified. Bjork introduce a simple method for predicting MGR from single cephalogram, he determines 7 morphologic criteria in the mandible developed as a result of bone remodeling in particular type criteria forward Backward Straight or slop backward Straight Notch at angle Slop backward and weak chin Acute Acute acute Increase Inclination of condylar head Curvature of mandibular canal Shape of lower border Inclination of symphysis Slop forward curved Curved downward Slop forward and strong chin obtuse obtuse obtuse Decrease Inter-incisal angle Inter-molar angle Inter-premolar angle Lower anterior facial height Clinical consideration of MGR in treatment planning 1-Forward MGR: a-Forward rotators should be treated as early as possible, treatment should be directed to correct the incisor relationship and provides proper incisal stop to support the occlusion during active growth, however the correction of over jet into the adult value is not advisable at this early age, as this would have an adverse effect on the future mandibular incisor alignment. In such instance a maxillary anterior bite plate is recommended to stabilize the anterior occlusion b-Posterior extrusive mechanics such as cervical headgear and inter-maxillary elastics should be considered c-Functional appliance that affect extrusion of posterior teeth is valuable in treatment as well as in retention. d-In the presence of moderate crowding, non-extraction approach such as distalization is preferable. As extraction tend to close the bite and forward closure is very difficult of steep cusps distalization tend to open the bite, thus aid in correcting the case e-Non growing pt often require orthognathic surgery f-Owing to the hyper active elevators, so high tendency to relapse which require especial consideration in retention M Mandibular andibular growth rotations growth rotations Dr. Mohammed Alruby Dr. Mohammed Alruby
5 g-Natural anchorage is very good h-Maxillary bite plate is mandatory, and retention in lower arch should be extended until growth is completed 2-Backward MGR: a-Special consideration should be given to control the vertical dimension: -Avoid any extrusive mechanics, select mechanics that help intrusion on posterior teeth as: high pull head gear, posterior bite blocks, open face activator -In the presence of crowding, extraction therapy is preferable as it tend to close the bite b-Anchorage requirement is very high because of higher tendency to anchorage loss c-Non growing pt often require orthognathic surgery d-Habit control is the key for success e-Owing to hypoactive elevator and defect on muscle activity of orofacial musculature, higher degree of relapse should be expected f-Retention should be done with part time high pull headgear or functional appliances until growth is completed. Retention in lower arch should be fixed for unlimited time Two approaches were recommended for early treatment: First: 1-If case require extraction, the U and L 1st premolars should be extracted as it just emerge to the level of gum 2-High pull chin cup exerting light force (16 ounce/side) should worn at least 12 hours / day. Anterior open bite is completely closed before the insertion of fixed appliances 3-Full appliance is inserted after the remaining teeth erupt Second: 1-High pull headgear is used to intrude maxillary posterior teeth 2-After 1st molars intruded about 2 –3mm, the remaining deciduous teeth is removed to allow the mandible to close In both approaches: Habit braking appliance should be used together with fixed appliance Retention should be done by using functional appliances such as: Frankel, Mandibular bite block to control the habit Hazards of MGR 1-Forward MGR: a-Contribution to skeletal dysplasia: = The potential risk for the forward growth rotation during the active growth periods is the development of skeletal deep bite. = This is largely influenced by incisor relationship: for example: if there is proper incisor relation and adequate incisal stop, there will be a great chance deep bite will not develop. = Bjork reported that, under ideal circumstances, the fulcrum for anterior MGR will located at incisor, the incisal stop will act as safety valve against deepening the bite = for any reason there is a lack of incisal stop, the pt develops skeletal deep bite, because the fulcrum of rotation will move further back to the premolar area = growth related skeletal deep bite is often developing early and continuous to increase throughout the active growth M Mandibular andibular growth rotations growth rotations Dr. Mohammed Alruby Dr. Mohammed Alruby
6 b-Contribution to dental malposition: Bjork suggested a role for closing MGR in late lower incisor crowding, he stated that MGR increase the mesial component of force and causes packing of lower incisor with resultant retroclination and crowding, this view was supported by Richardson who concluded that, ((the lower jaw grows forward more than the upper jaw, if the lower incisors were not free to move forward because of restraining of upper incisors, then it is more likely to become retroclined and consequent crowded)) 2-Backward MGR: a-Contribution to skeletal dysplasia: Backward rotation may result in: -Increase LAFH -------- skeletal open bite -More posterior position of chin -Worsen class II pt with associated functional problems such as: tongue thrust, mouth breathing, weak elevator muscles These functional problem often complicate treatment and contribute higher percentage of failure b-Contribution to dental malposition: Sakuda anf Belfast suggested that, opening rotation of the mandible causes retroclination of lower incisors while the posterior teeth are not guided distally which lead to the development of crowding Thanks: M Mandibular andibular growth rotations growth rotations Dr. Mohammed Alruby Dr. Mohammed Alruby
