Bite Registration

Much has been said about this topic recently and whilst I wouldn’t put this forward as the definitive text, I do submit this in the hope that a different view may help someone understand something that was previously obscure.

Why do a “bite registration”?

Bite registration is either taken in the ‘conformative’ (existing) or non-conformative (or reorganised).  The value of the latter is to allow the dentist to move outside the constraints of a worn dentition, improve the occlusal scheme, benefit appearance and/or in the case physiologic based occlusions; address numerous musculo-skeletal signs and symptoms, which often otherwise go undetected, i.e. headaches, neck and shoulder pain, tinnitus, etc.

Bite Registration Methods

Conformative or Intercuspal Position

Often referred to as “centric occlusion”, this is the position at which the teeth are in their highest enmeshment.  It is the position at which most dentistry is currently performed as is often seen as usually correct for that patient (and also that dentistry is currently seen by most practitioners and patients as primarily a repair service for individual teeth).  This view has been encouraged by Insurance Companies in the U.S. and the N.H.S. in the UK as a means of regulating fees and directing services.  (Neither is necessarily in the best interest of the patients).

Dr. Mike Wise’s book “Failure in the Restored Dentition” has an excellent chapter (12) on conformative techniques. Whilst there is nothing fundamentally wrong with the conformative approach an understanding of its limitation and other approaches and their rationales will help to prevent the “adoption of an existing problem”, - remember if the patient wasn’t aware of a problem which becomes apparent after you touch the tooth, you did it! 

The intercuspal position is often employed in orthodontic diagnosis.  Alas this takes no account of the relative position of the maxillae to the skull, or indeed the mandible to the maxillae, it’s just a tooth to tooth position.  Most Orthodontists would agree that muscles and soft tissues are of paramount importance in orthodontic diagnosis.

Reorganised or Non-conformative

Centric relation (CR) position.  “A maxillo-mandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective disks with the complex in the anterior-superior position against the slopes of the articular eminences.  This position is independent of tooth contact.  This position is clinically discernable when the mandible is directed superiorly and anteriorly and restricted to a purely rotary movement about a transverse horizontal axis.  This term is in transition to OBSOLESCENCE”.  Published in the Glossary of Prosthodontic Terms, J.P.D. – C.V. Mosby Co.  On its way to obsolescence, the confusion regarding the definition of CR becomes ever greater.  (The Glossary of Prosthodontic Terms, 6^th Ed. 1994).  Some clinicians maintain that it is not a position attainable by other clinicians, but despite this, it is a commonly adopted position as it is a commonly taught one.  Many schemes, appliances and instruments have developed around it.

The concern that this author has with it is three fold, first it is a border position.  As a position on the perimeter of Posselts envelope of movement it is not a position “through which” the mandible moves.  When you are examined by a Sports Medicine Specialist or Orthopaedic surgeon they will measure both your passive and active range of movements, but don’t base reconstruction around an extreme of motion. The second concern is that of “seating” of the condyles.  Many of those supporting the validity of CR feel that the TMJ’s are weight bearing, because they’re “loaded”.  This term indicates their ability to withstand stress with ideal strain resistance.  It is my belief that whilst they are not weight bearing joints (I feel the knee and hip are better examples), the TMJ’s are able to resist strain when “loaded” by the muscles of mastication.

The third concern is that of the centre of rotation, which has historically been stated to be in the head of the condyle.  Many feel that the first 10-20 mm of opening is in a rotation followed by a translational movement.  Recent research has confirmed the mathematical conclusions of Casey Guzary in that the centre of rotation appears to be further back in the neck, probably toward the top of the spine.

When the commonly seen picture of “clinical overclosure” is taken into consideration, the initial rotation around the condylar head will be seen to be pathological.  Translation is the physiological movement it would appear. This border movement (CR) may be attempted in both the upright and supine position, the latter using gravity to help hold the mandible drop distally.

Arbitrary Position

Whilst many would insist on evidence based practice, much of what we do is, by custom; arbitrary.  Even when we think we’re measuring, errors may arise.  Many clinicians feel they’ve developed a “feel” that works well in their hands.  It should be remembered that this is arbitrary, although it is not necessarily to be decried.  This includes the use of A.K. in the search for a position that suits the physiology of the body.

Gelb 4-7

Some positions revolve around the condyle/fossa position on radiographic interpretation.  Alas, we don’t know if the assumption that is made in this manipulation is in physiologic harmony with the other parts of the system (i.e. muscles) although it must be stated that in this area of Md-Mx relationships we don’t KNOW anything but we may BELIEVE things to be so.

Swallow Bite

We now move into the physiologic ‘dynamic’ area, utilising the bodies function to give “pointers” within the range outlined by Posselts Envelope of Motion. First described by Dr. Willie May it is based on working muscle lengths.  With the patient upright, two dice sized pieces of soft wax (and it should be soft) are placed on the teeth around the 1^st/2^nd molar area, the subject is asked to position their tongue to the roof of the mouth at the back, this “centres” the mandible.  The subject is asked to swallow allowing their “teeth” (if present) to sink into the wax and their mandible to move down and forward, until their face takes on a “relaxed” appearance.  Dr. Nick Mohindra has described this and its role in edentulous rehabilitation in the BDJ.  One major step forward with this technique, in the author’s opinion is the use of SS-19, which is the guide, proposed by the Canadian Dentist, Hank Shimbashi that between the CEJ of Mx/Md centrals should be the measurement of 19mm +/- 10% (17 – 21).  Although only a guide it has been the author’s experience that the 19mm guide has almost universal validity.

When the jaw position is determined, bite registration material may be introduced anterior and posterior to the wax blocks. This position may only be attempted with the subject upright so that gravity is uses as an ally in the vertical positioning of the mandible.

Relaxed Physiologic

The swallow bite can be modified further by relaxing the jaw musculature, or relaxing the jaw.  Musculature can be the basis of the registration at a set vertical (only one positional vector).  The Shimbashi SS-19, can once again be used to determine the mandibular position, although it must be remembered that the posterior position is still determined by the resting muscle length (not as previously thought, rotation in the head of the condyle).  Swiss researchers have shown this centre of rotation to be labile, but still not contradictory in premise to Casey Guzay’s quadrant theorem, based on engineering principles and based on the results of the construction of 125,000 dentures in 1952.  Consequently posterior support should be counted, and work on SDA (Shortened Dental Arches) despite being correct politically does not “hold water” physiologically.

The techniques usually adopted for muscle relaxation is ULF TENS (Ultra Low Frequency – Transcutaneous Electrical Neural Stimulation).

Physiologic using electronic instrumentation

The history of clinical medicine is in large measure the history of a search for diagnostic precision and therapeutic effectiveness – M.D. Altschule (1989). Whilst the “physiologic area” is gaining popularity its subjective nature must still be accepted.  The reason that many patients accept this and other subjective procedures is no doubt due not to its lack of importance as much as the patient’s “latitude” or as it’s been termed “Physiologic Adoptive Range”.  Many of the old school finds these non-dogmatic principles difficult to accept.

There are two companies that supply electronic instrumentation that will measure muscle activity and allow jaw tracking in order to achieve diagnostic and therapeutic Md. Positioning after muscle relaxation by ULF TENS.  One is the Myomonitor Company and the other is BioResearch, both based in the U.S.  This data now becomes objective and the next article will describe the benefits of this objectivity.