DCNA, A Functional Cranial Analysis of Centric Relation
We previously have used the method of functional cranial analysis to good effect in the study of cranial growth. A fundamental attribute of this method is its operationalism, that is, it is predicated on an analysis of function.
A review of a variety of current clinical thought on the nature of centric relation as well as of the techniques advocated for the determination of the mandibular position, leaves the impression that many clinical workers regard centric relation, at least in adults, as real, functional, fixed and reproducible.
However desirable such a position may be, particularly with reference to any given technique of clinical therapy, it is not correct. On the basis of biomechanics of the TMJ, centric relation is never a functional position nor is it habitual or common.
Further, current knowledge of the mode of growth and responsiveness of skeletal tissues to dynamically fluctuating functional environments suggests that the TMJ is significantly adaptive to alterations in oral function occurring in real time.
Moss Melvin L.
1975, 02, Vol.19, No. 3 pp 431-442
Accordingly, possible, and biomechanically permissible, mandibular positions are not immutably fixed.
To the extent that any therapy is based upon a correct comprehension of the relevant biological principles, it is more likely to be successful.
If an articular surface is perfectly hemispherical, its axis of rotation will be fixed. If the articular surface is ovoid, as is the case in mandibular condyles, the axis will constantly shift producing an evolute, that is, constantly shifting, axis of rotation.
The first action of muscles, which produce motion in at a diarthrosis, is to produce compression of the articular surfaces, following which motion occurs. During function, there is always a compressive force exerted upon the functioning joint surface; the TMJ is “load-bearing”.
Once the muscles producing the motion at a joint contract and compress the reciprocal functioning joint surfaces, the envelope of motion occurring at` the joint is determined entirely by the specific morphology of these same joint surfaces.
Moss Melvin L.
1975, 03, Vol.19, No. 3 pp 431-442
Gross anatomical, comparative, histological and analytical biomechanical studies correlatively establish that` the almost cartilage-free mandibular fossa is a non-functioning joint surface. Normally, no compression ever exists between the condylar head and this fossa.
By any functional definition, utilizing the commonly accepted principle of biomechanics, the centric relation is not a functional position. Inn fact it mat be doubted if this “relation” is any other than iatrogenic.
At no time, under any normal conditions, will an isolated (“hinge”) rotation occur in the lower joint compartment without a simultaneous rotation in the upper joint compartment about the tubercular axis.
By the process of repetitive and circular citation, coupled with an apparent inability to integrate more recent studies, the field remains troubled.
What appears to be a “translation” of the condylar head during protrusive and retrusive positioning of the mandible, is in fact, a motion of rotation at the upper compartment with the axis of movement passing transversely through a given articular tubercle.
Moss Melvin L.
1975, 04, Vol.19, No. 3 pp 431-442
From this work two critical points easily emerge. First, that all motions at this joint are rotations: there is no “translation” or slide with this joint during function; and
Second, at not time does a “hinge-axis” rotation occur at the condylar head (lower compartment) without a simultaneous rotation at a mechanical axis passing through the tubercular eminence (upper compartment).
The ability to place the condylar head on the anterior slope of the tubercle is found normally in a majority of people (70% in one study), and so the concept of clinical subluxation being equivalent to pathology is to be seriously questioned.
Normal motion of the TMJ is produced by the action of any combination of the muscles of mastication, the muscles of facial expression and the muscles of the suprahyoid compartment. When these muscles contract voluntarily, they bring the functioning joint surfaces into compressive articulation.
Hence, as any normal mandibular function begins, by definition, then the condylar heads are not, and cannot be, in centric relation.
Moss Melvin L.
1975, 05, Vol.19, No. 3 pp 431-442
However, there is no scientifically satisfactory demonstration by any worker that the centric relation is either position usually (or habitually) assumed by a normal individual or, more importantly, that it is ever a functioning position; indeed, it is neither.
To the extent that a dentist believes that this relation is static, constantly reproducible and unchanging, even in the adult, he bases his therapy on poor grounds. If he further adds to this a belief in the isolated “hinge-axis” rotation, he is totally incorrect.
There is every reason to believe that the TMJ is significantly responsive to a dynamically altering environment and capable of significant morphological (and hence biomechanical) adaptation
Based on an operational view of the head as a region where certain functions are carried out, the method then describes for each function: (a) all of the tissues, organs and functioning spaces (oral, nasal, pharyngeal) necessary to completely carry out the given function as well as
(b) all of the skeletal tissues (bone, cartilage, tendon) required biomechanically to support and/or protect the former; (a) is called the functional matrix and (b) the skeletal unit.
Moss Melvin L.
1975, 06, Vol.19, No. 3 pp 431-442
The very maintenance in being of all skeletal units are always, and without exception, secondary, compensatory and mechanically obligatory responses to temporally (and functionally) prior demands of the specifically related matrices.
In clinical circles often there is a misunderstanding of the processes by which the form and growth of skeletal tissues are organs (bones) are regulated. Certainly some continue to believe that the intrinsic (genetic) factors play, if not a sole, at least a predominant, role.
Available data from many fields and from many workers make it more reasonable to suggest that extrinsic (environmental) factors primarily regulate skeletal unit growth. In this view, for example, the mandibular teeth would be a functional matrix and their supporting alveolar bone its skeletal unit. (Eruption)
Let it be clearly understood that this position does not deny the role of intrinsic factors in development. The selective repression and de-repression of one or another genetically regulated cellular metabolic pathway must occur for active skeletal tissue growth to occur. In a phrase, the intrinsic factors in skeletal tissue cells provide only the possibility of an appropriate response to extrinsic factors.
Moss Melvin L.
1975, 07, Vol.19, No. 3 pp 431-442
Centric Relation –Despite its theoretical clinical utility, the search for an immutable condylar position, defined as a centric relation, within the TMJ unfortunately is an ephemeral undertaking. This is so for three reasons.
1. In biomechanical terms, the centric relation is a nonfunctional position... This is so at any age and in any normal condition.
2. Over relatively long periods of time, the morphology of all functional surfaces of the TMJ is capable of significant adaptive alterations. These are normal compensatory responses of skeletal units to the prior (progressive - rgp) alterations of functional matrices.
3. In much shorter time periods, the dynamically fluctuant state of the neuromuscular apparatus makes it reasonably certain that intra-individual variation in condylar positions can exist.
Any clinical procedure that demands the location of a given individuals “centric relation” position within the tolerances of a millimeter or two must ignore the considerations given above. If we add to that the possibility that an isolated “hinge-axis” rotation of the condyle is being sought clinically at that “centric relation” position, we are even further removed from sound biology.