Friday, July 29, 2011

A Note On Mandibular movements and definition of terms.... with 3D Animated Mandibular Movement Videos

The movements that the mandible can make and the  names  of  the  important  positions  within  this range  of  movements  are  following  videos
                                                                 Sagital Plane           

                                                          3D Illustration
Terminal hinge axis (THA) and  the retruded arc of movement (or closure)
This  is  an  axis  which  passes  through  both condyles and about which the mandible rotates in its  most  retruded  (comfortable)  position  of  the condyles    the  retruded  arc  of  closure.  This is a clinically reproducible movement and recording it is often useful in making crowns and bridges and essential in making complete dentures. The THA can  be  measured  in  individual  patients  but  it  is sufficient for most applications to use an average THA  such  as  that  recorded  by  an  ear-bow.

Intercuspal position (ICP)
This  is  the  position  of  maximum  contact  and maximum  intercuspation  between  the  teeth.  It  is therefore  the  most  cranial  position  that  the mandible  can  reach.  The  term  ‘centric  occlusion’ has  been  used  to  describe  this  position,  but  this is confused with ‘centric relation’ (see below) and may also imply centricity of the condyles in their fossae,  centricity  of  the  midline  of  the  mandible with  the  midline  of  the  face,  or  centricity  of  the cusps  within  the  fossae  of  the  opposing  teeth, none of which may be the case. The term ‘centric occlusion’ is  therefore  better  not  used.

Retruded contact position (RCP)
This  is  the  most  retruded  position  of  the mandible with the teeth together. It is a clinically reproducible  position  in  the  normal  conscious patient.  Patients  with  conditioned  patterns  of muscle activity may not be able to manipulate the jaw into it, even with assistance by the dentist. In less than 10% of the dentate population the RCP coincides with the ICP. In the remainder the RCP is up to 2 mm or more posterior to the ICP. The term ‘centric relation’ has been used to describe this position, but it has the same disadvantages as the term ‘centric occlusion’ and will not be used. ‘Centric  occlusion’  (CO)  and  ‘centric  relation’ (CR)  are  terms  sometimes  used  in  complete denture  construction  where  they  mean  different  things  to  ICP  and  RCP.

Mandibular movements
Those  patients  who  have  a  discrepancy  between the  RCP  and  ICP  usually  close  straight  into  the ICP from the postural or rest position when the movement is made subconsciously. Patients sitting in  dental  chairs  making  voluntary,  conscious movements  when  asked  to  do  so  by  the  dentist often  make  bizarre  movements  rather  than closing  into  the  ICP  directly. These aberrant movements and contacts are the result of patients trying  too  hard  to  help  and  not  understanding what is required. Students have been heard to ask non-dental  or  medical  patients  to  bite  on  their ‘posterior’  teeth!
However, contact  does  occur  in  the  range between the ICP and RCP during empty swallowing (particularly nocturnal swallowing), during the mastication of a tough bolus and during parafunctional  activity.  Thus the  mandible  can  slide  from the ICP in four main directions with the teeth in contact,  or  in  an  infinite  number  of  directions  at angles  between  these  main  pathways.  The four excursions  are:
• retrusive
• protrusive
• left  lateral
• right  lateral.

Retrusive movements
Movements between the ICP and RCP are usually guided  by  a  limited  number  of  opposing  pairs  of cusps of posterior teeth. The  angle  of  the  slide  between  RCP  and  ICP,  its length  and  the  individual  pairs  of  teeth  that produce it are important and should be examined. Of even greater importance is any unevenness of the movement  producing  bulges  or  lumps  in  the path  of  movement.  These disturbances  to  the smooth movement of the mandible are one form of  occlusal  interference.

Protrusive excursion
In  forward  movement  of  the  mandible  with  the teeth together. It is usually the incisor teeth that guide the movement. This will not be the case in anterior open bites or in Class III incisor relationships. The  angle  and  length  of  movements  will  be determined  by  the  incisor  relationship  so  that,  for example, in a Class II Division II incisor relationship with an increased overbite and reduced overjet, the movement of the mandible has to be almost vertically  downwards  before  it  can  move  forwards. Anterior  guidance  is  important  when  making anterior  crowns  or  bridges.  Sometimes,  when  the teeth are a normal shape, it is helpful to reproduce the  patient’s  existing  guidance  as  accurately  as possible;  on  other  occasions,  for  example,  with worn teeth, it is unnecessary or undesirable to do so,  and  in  fact  the  purpose  of  the  treatment  may be  partly  to  alter  the  incisor  guidance.

Left and right lateral excursions
In lateral excursions the side that the mandible is moving  to  is  known  as  the  working  side  and  the opposite  side  the  non-working  side.  The  term ‘balancing side’ has been used to refer to the non-working  side,  but  since  it  implies  a  balanced occlusion,  balancing  or  stabilizing  a  complete denture  base,  it  should  not  be  used  in  reference to  natural  teeth.
The  contacts  on  the  working  side  are  either between  the  canine  teeth  only  (canine-guided occlusion)  or  between  groups  of teeth on the working side (group function).  Occasionally,  individual  pairs  of posterior teeth will guide the occlusion in lateral excursion,  but  this  is  not  regarded  as  ideal.  The canine-guided  occlusion  is  considered  to  be protective  of  the  posterior  teeth  which  disclude in  lateral  guidance.

Canine guidance 

Group function

Contact  on  the  non-working  side  in  lateral excursions  should  not  normally  occur.  It  does sometimes  occur  after  extractions  and  over-eruption  and  occasionally  following  orthodontic treatment,  particularly  when  this  treatment  has been  carried  out  with  removable  appliances  that have  allowed  the  posterior  teeth  to  tilt. Contact  may  also  occur  in  cases  of posterior  crossbite  where  the  lower  teeth  are placed  buccally  to  the  upper  teeth.


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