CR - Repeatable, But Not Necessarily Comfortable

This is a continuation of responding to the attack on LVI's occlusion philosophy by Dr. Dawson. He took exception to my comments in an article about CR repeatability and attacked my comments, "Even if a dentist is trained to do this, it (CR) is not necessarily a functioning physiologic position" and "The comfortable position of the mandible is determined by muscles, not joint anatomy." I would like to give you my reason for making these statements.

Neuromuscular vs. Centric Relation

The long-standing history of centric relation is confusing, since it has been changed numerous times over the years. It is obvious, as confirmed in the glossary of prosthodontic terms of definitions that there still is no consensus as to where is centric relation (1). That position ranges from the superior position of the fossa, the most anterior superior position of the fossa, or even to an exact position on a radiographic grid. The glossary of prosthodontic terms also clearly states that centric relation "is in TRANSITION TO OBSOLESCENCE." (The Glossary of Prosthodontic Terms, 1987). Today the glossary of prosthodontic terms list other definitions of centric relation of other strong advocates with still no consensus (1994).

Leaders of the mechanical occlusion philosophy have changed their definition over the past, trying to understand physiological and anatomical structures that are dynamic and living. Two crucial problems of the centric relation theory is: 1) the lack of strong supportive evidence to prove that centric relation is necessary when obtaining a bite for consistency and reproducibility, especially when there is presence of dysfunction of the masticatory system, pain and derangement of the temporomandibular joints (1) and (2) the lack of scientific proof that centric relation is physiologic.

Because of advances in scientific technology it is now possible to identify and find resting physiologic muscles by the use of electromyography (EMG) objectively rather than by educated guesswork. Computerized mandibular scanning (CMS) allows the serious student of occlusion to delineate between habitual mandibular positioning from a physiologic mandibular positioning. Mandibular movement patterns as well as muscle activity responses can now be observed to identify a physiologic rested starting position for maximum comfort.

A physiologic jaw trajectory has now been scientifically determined, eliminating the need to use the less accurate manual manipulation in establishing an occlusal position. The neuromuscular approach to dentistry is nothing more than adding to the knowledge base of the past using advanced technology of the twentieth century in a clinical setting.

The comfortable position of the mandible is determined by the muscles, not joint anatomy. Today, technology allows the clinician to measure the masticatory muscles and find the most comfortable position of the mandible ("Happy Muscles"). Manipulating techniques have proven invalid when using electromyography and jaw tracking instrumentation. Finding a manually induced position, even if repeatable, does not necessarily make it comfortable or physiologic!

With bilateral manipulation to CR, one can seat the joint and find a position where the joints can supposedly be loaded without any discomfort to the patient, and yet, have no definitive physiologic confirmation that this position results in synergy among teeth, muscles and TM joints. Hickman, et al, in a series of two controlled studies (1993, 1998) compared the efficiency of function within the stomatognathic system. He compared muscle efficiency and condylar position based on three techniques of occlusal positioning-bimanual manipulation, neuromuscular and leaf gauge. Study results concluded that the result of any therapeutic position should be an improvement in muscle function. With respect to muscle balance and activation, a neuromuscular condylar position proved to be the position capable of recruiting the greatest motor unit activity when compared with a bimanually manipulated (BM) position, a leaf gauge (LG) position, and a neuromuscular (NM) position."

The study further concluded, "Significant differences were found in the electromyographic (EMG) activation between the masseter and temporalis muscles for the leaf gauge (LG), manually manipulated (CR) and neuromuscular (NM) bite positions during maximal static clench..... the NM position displayed the highest degree of muscle activation upon function. Similarly, the ratio of the masseter/temporalis EMG activity during maximal clench was lower for the LG and CR positions and highest for the NM position. These data indicate that the NM position produced the greatest total muscle recruitment, with more masseter involvement during maximal clench, and enabled the subjects to generate greater clenching forces in the NM position as compared to the LG and CR positions."

Recently I took emg readings on a patient who had been restored by a CR advocate that teaches at a leading old school occlusal program. The patient was suffering from stuffy ears and some jaw pain. The emg readings in the CR position were elevated. I then found the NM position and took the emg readings in that position. They were incredibly low and comfortable. I also took some clench scores and found that masseter and temporalis activation during clench in CR was pathetic. When the patient clenched in the NM position, the muscle response was much six times higher and more efficient. Even though the patient is in a splint, he finds chewing efficiency so much better. Now, Dawson may call this anecdotal, but it's scientific validation of what we are doing, not anecdotal.

We just completed a study that compared muscle hypertonicity for four popular restoration positions. The existing bite (CO), Centric Relation (CR) as described by Dr. Peter Dawson (Bimandibular Manipulation), Swallow Bite (Willy May), and the Myocentric position as determined by computerized jaw tracking equipment (K6-I - Myotronics). All Swallow Bites were performed by me and all Centric Relation bites were performed by Dr. Mike Miyasaki, a respected clinician in the art of bimandibular manipulation. The Myocentric bites were obtained by Dr. Clayton Chan and myself.

Upon completion of obtaining the Myocentric bite, the subjects were asked to close into the recorded bites without clenching. The purpose was to record the muscle activity in the maximum intercuspation using EMG scans (K6-I - Myotronics) of the following muscles. Temporalis (both anterior and posterior division), Masseter, and Digastric. The readings would record the hypertonicity of the muscles to move the mandible in the position of maximum intercuspation. Knowing these results would indicate which position was the most comfortable for the patient.

Results

Highest Level of Hypertonicity (least comfortable position)
15 - CR
2 - CR/CO
1- CO

Lowest Hypertonicity (most comfortable position)
13 - M
4 - S/M
1 - S

CO = Centric Occlusion
CR = Centric Relation
CR/CO = Not a significant difference between CR and CO
M = Myocentric
S = Swallow Bite
S/M = Not a significant difference between S and M

The conclusion would support the theory that the manually induced centric relation position is not the most comfortable position with which to restore the human occlusion. It would appear from the results obtained that the Myocentric position or a physiological position will create more neuromuscular harmony.

As I mentioned in the last issue, find out for yourself about the validity of neuromuscular dentistry and the profound effects it can have on your practice. Don't let the concerns of those with vested interests in competing programs influence your decision. It's an incredible world which has the potential of making dentistry much more than just being a tooth doctor.

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