Dr. Dawson receives emails and letters from clinicians all over the world asking questions and looking for clarification on all matters of Functional Occlusion and challenges in dentistry.
From: A Dentist in Canada
Subject: Long Centric
Explanation of "Long Centric"
Assumption: Dawson centric relation is valid beyond a shadow of a doubt. My 37 years of successful clinical experience validates the truth of this assumption.
Normal swallowing initiates centric relation by placing the condyles in the most superior anterior position in the glenoid fossa. When CR encounters an interceptive incline it's almost always a premature incline. The lateral pterygoid is activated immediately to avoid the premature incline. Working concomitantly with the lateral pterygoids are the medial pterygoids (which are elevators, hence restricted mouth opening). Other elevators are also activated which will initiate inflammation at the insertion of these muscles (pain on palpation).
Erase the interfering premature contact and all the muscles immediately relax and allow the condyles to slide down the posterior incline of the temporal bone, moving the jaw slightly forward (long centric).Erase all prematurities and the patient will tell you that it feels like they have more teeth. In 20 minutes symptoms of MPD start to diminish.
Reply from: Dr. Dawson
RE: Long Centric
I think your understanding of long centric is correct. The fact that long centric results from relaxed elevator muscles is why we teach that long centric must be determined by gentle closure in order to avoid strong contraction of the elevator muscles which would pull the condyle's up into centric relation.
So remember: to determine long centric, it is a very gentle tap tap tap while the patient is sitting upright. Check the anterior teeth with thin articulating ribbon. Any marks on the lingual surface of an anterior tooth that are forward of CR indicates a need for slight clearance. This would rarely ever be more than 2/10 mm forward of CR.
Peter E Dawson DDS