The Centric Jaw relations
Definitions for centric relations :
" The Maxillomandibular relationship in which the condyles articulate with the thinnest awascular portion of their respective discs with the complex in the naterior - superior position against the slopes of the articular eminences . This position is independent of tooth contact This position is clinically discernible when the mandible is directed superior and anteriorly . It is restricted to the purely rotary movement about the transverse horizontal axis "
" The most retruded physiologic relation of the mandible to the maxilla to and from which the individual can make lateral movements . It is a condition that can exist at various degrees of jaw relation. It occurs around the terminal hinge axis "
" The most retruded relation of the mandible to the maxilla when the condyles are in the most posterior unstrained position in the glenoid fossa from which lateral movements can be made , at any given degree of jaw separation "
Importance of the Centric Jaw relations :
The centric relation position acts as a proprioceptive centre to guide the mandibular movements . The centric relation has some features : It is learnable , repeatable and recordable position which remains constant throughout life , It is a definite learned position from which the mandible can move to any eccentric position and return back involuntaily . It also acts as centre from which all movements can be made . If the mandible has to move from one eccentric position to another , it should go to the centric position before advancing to the target eccentirc position . Besides that , functional movements like chewing and swallowing are performed in this position , because it is the most unstrained position .
Various methods of retruding the mandible :
- Relaxing the patient . Making him feel comfortable
- The patient is requested to try to bring his upper jaw forward while occluding on the posterior teeth
- The patient should be instructed to touch the posterior border of the upper record base with his tongue
- The mandibular occlusla rim should be tapped gently with a finger . This would automatically make the patient to retrude his mandible .
- The temporalis and masseter are palpated to relax them .
There are various methods to record centric relations --
Physiological method : Tactile ( inter-occlusal check record method ) , Pressureless method , pressure method .
-------( The Tactile method ) : They are based on the proprioceptive impulses of the patient , Kinesthetic sense of mandibular movement , The visual acuity and sense of touch of the dentist . and no pressure is exerted on the interocclusal record . Procedure : two steps , First a tentative centric jaw relation is recorded using wax occlusal rims . In the second step , an inter-occlusal check record is made using the trial denture during try-in .
-------( The Pressureless method ) : Occlusal tims customized as usual and the patient is trained to close at centric relation position . Once the patient attains the centric relation position , the denture bases with occlusal rims are indexed / sealed in this position . The nick and notch method or the stapler pin method can be used to seal the occusal rims .
-------( Pressure method ) : Upper occlusal rim inserted first , the entire lower occlusal rim is softend in a bath of hot water and inserted carefully into the patient's mouth . Mandible is gently guided by you and firmly closed .
Functional method : Needlehouse method , Patterson method
-------( Needleshouse method ) : one of the most commonly used techniques , it involves the fabrication of occlusal rims made from impression compound , Four metal beads or styli are embedded into the premolar and moalrs areas of the maxillary occlusal rim and they are inserted into the patient's mouth and all the above mentioned factors affecting functional centric relation are considered , the patient moves his mandible and the metal beads will create markings on the lower occlusal rims . ( The posterior most point of the marking is the centric relation .
-------( Patterson method ) : Occlusal rims are made from modelling wax , and a trench is made along the length of the mandibular occlusal rim . ( A mixture of 1:1 Carborundum and dental plaster is loaded into the trench ) , and the patient is asked to perform mandibular movements . These movements will produce compensating curves on the plaster carborundum mix , as these movements are made the height of the plaster carborundum mix is also reduced .
Graphic method : Intraoral, extraoral
-------( Arrow point tracing ) : in 2 D , using a gothic arch tracer , it is usually recorded in the horizontal plane . Arrow-point tracing is done using an arrow point tracer . The general concept of this technique is thata pen-like pointer is attached to one occlusal rim and recording plate is placed on the other rim . When the mandible moves the pointer draws characteristic patterns on the recording plate . The pointer is known as the the " central-bearing point " and the recording plate is knwon as the central bearing plate .
-------( Pantograph ) : in 3 D , " A graphic record of mandibular movement in three planes as registered by the styli on the recording tables of a pantograph , tracings of mandibular movement recorded on paltes in the horizontal and sagittal planes "