Dental History

The Relation Between Mechanical and Anatomical Articulation

By RUDOLPH L. HANAU, M.E.

THE problems of denture construction with which dentists are daily confronted are so diversified that a simple standard or universal formula for their solution should be considered beyond reasonable expectations.

The basis of our conception of articulation is the articulation of natural teeth. Articulation is the change from one .occlusal position into another, while the occlusal surfaces maintain contact. The following main occlusions apply to the occlusal surfaces or to the maxillomandibular relation:

• Central occlusion,
• Protrusive occlusion,
• Right lateral occlusion,
• Left lateral occlusion,
• Intrusive occlusion.

I do not especially mention retrusive occlusion, which according to my interpretation is normally but an intrusive occlusion of the mandible to the posterior. In some dentures an actual retrusive movement is possible and used during the masticating strokes. This seems' to me an abnormal condition.

Central occlusion.

The occlusal surfaces are in central occlusion when the cusp of the upper interdigitates between the corresponding lower cusp and its distal neighbor, buccally; the upper and lower incisors are symmetrically arranged in two nearly concentric arcs between the cuspids and are in more or less intimate contact.

The mandible is in central occlusion when the condyle heads rest gently in their sockets and the teeth are in contact.

Protrusive occlusion.-'-The occlusal surfaces are in protrusion when the lower teeth are in contact with the upper, anterior to central occlusal relation.

The mandible is in protrusive occlusion when both condyle heads have made excursions forwardly and downwardly in their fossae.

Right lateral occlusion.-The occlusal surfaces are in right lateral occlusion when the lower teeth are in contact with the upper teeth in a position laterally to the right of central occlusion.

The mandible is in right lateral occlusion when the right condyle has made an excursion laterally and the left condyle an excursion forwardly, downwardly and somewhat inwardly~

Left lateral occlusion of the occlusal surfaces as well as of the mandible is a lateral image of the right lateral occlusion.

Intrusive occlusions of both the occlusal surfaces and the mandible are occlusions under applied forces which incur temporary displacement or compression of tissues. The effect of the unroundness of the condyle heads may be neglected within the limits of articulation.

Intrusive occlusion is not an independent occlusion; it is always associated with one of the main occlusions or an intermediate occlusion.

The above interpretation brings us to the realization that a variance of the magnitude of intrusion will associate a series of occlusal relations of the mandible with one occlusal relation of the masticatory surfaces, and vice versa. This applies to the anatomical masticatory apparatus involving resilient tissues, and not to the articular mechanism with its constrainedly guided parts and the dentures mounted rigidly therein by plaster of Paris. No intrusive .occlusion exists in the articulator. All other occlusions of the masticatory surfaces and of the jaw members are coincident.

Here we have the key to the situation, which, though complicated in itself, is easily mastered when understood.

We distinguish four kinds of articulation:

• (a) Anatomical articulation, which is articulation governed by at least three pairs of anatomical guiding elements; for example, by the two condyle heads in their fossil) and by two opposed guiding surfaces of natural teeth.
• (b) Semi-anatomical articulation, which consists of two anatomical guidances at the condylar joints and a third selected guide in the mouth, as is the guidance of artificial teeth upon natural teeth, or of opposed artificial teeth against each other.
• (c) Mechanical articulation, which is articulation produced in an particular set and adjusted to compensate for the difference between mechanism and anatomy.
• (d) Articulator articulation, which is articulation adapted to an arbitrarily chosen mechanism, irrespective of the anatomical requirements.

The search for a satisfactory solution of the problem of what constitutes articulation and of how to establish it in an auxiliary device known as an articulator has been very intense during the past few decades. Our dental literature gives ample proof thereof·.

Articulation itself is a purely physical function. It therefore should not surprise us that previous and present investigators accompanied the presentation of their conception of articulation with a mechanism which incorporates and illustrates their ideas' of the subject. I mention the names of Christensen, Walker, Snow, Gysi, Hall, Monson, Needles and Hanau.' Scores of names could be added. Snow, Gysi and their precursors have laid the foundation upon which I base my work. The conical theory of Dr. Hall, the spherical theory of Dr. Monson and the theory of the concentric spheres by Dr. Needles should never have been taken seriously, as they are neither scientific nor logical, in spite of their authors assertions.

The basic ideas of our present conception of articula:tion were very ably pre- sented to us by Snow's and Gysi's theoretical work and ingenious devices, illustrating their conception of the masticatory operation. The Gysi simplex device eliminates itself from consideration, as the principles incorporated therein are inconsistent with Dr. Gysi's valuable findings.

In the study of the work of each investigator, including the above named, it will be found that the endeavor was made to design an apparatus which reproduces mandibular movements. Some claimed an exact reproduction of such movements. As a matter of fact, some claimed that their mechanism represents that ideal movement which the Great Creator seemingly had failed to establish.

The natural masticatory apparatus consists of individual teeth rooted fairly firmly in the jawbones. The jaws themselves join in a sliding hinge, which is provided with two interposing' elements, the fibrocartilages. These permit intrusion of the condyle heads into their sockets. The shape of the condyle head, the shape of the cartilages and their resiliency are responsible for intrusion. The effect upon the relative position of mandible to maxilla, whatever the cause may be, is the same. We shall refer to the resiliency only in the following; yet keep in mind that the aforesaid contributary causes exist.

Artificial full dentures are associated with four resilient elements interposed between the parts which comprise the restored masticatory apparatus. At each of the condylar joints and below each plate we have an interposition of resilient tissues, through which coactive forces are transmitted during the execution of physical mastication. ,

We shall first dwell upon the conditions pertaining to full dentures, for they present all the features found in natural normal dentures, traumatic dentitions, partial dentures and edentulous masticating apparatus. Lastly we shall consider the latter four by elimination of elements.

The masticatory apparatus, equipped with a full denture, is comparable to a power and working machine. The power is generated by coordination of a complex arrangement of simultaneously and alternately operating muscles arid ligaments. The peculiarities of the elements comprising the power mechanism are important and interesting for many reasons, yet their consideration is not necessary for our analysis, provided we are satisfied with our conception of their effect upon the working mechanism.

Let me illustrate by an example. The lathe is a working machine, invariably hitched to, relayed with, or combined with a power mechanism. A description or an analysis of the lathe mechanism may, without detriment to succinctness, start with the framework and wind up with the spindle, rotated by motive power. Hence we may apply the logic: the working mechanism of the restored masticatory apparatus is made up of the following coacting main elements:

• The upper denture.
• The tissues oetween the upper denture and the maxilla.
• The maxilla, consisting of an assembly of rigidly joined bones.
• The fibrocartilages interposed in separated places between maxilla and andible.
• The mandible, a single bone.
• The tissues between mandible and lower denture.
• The lower denture.

That part of the working cycle of mastication which is' of prime importance to prosthodontists obtains the moment the chain of elements becomes closed either by direct contact of the upper and lower dentures or by an interposed bolus of food. Of further importance is the relation of the elements for an open chain: during relaxation, speech and apprehension. In either case forces act between the elements and, of course, through them. The magnitude of these forces ranges from the negative to the positive, or we may say forces act which tend to separate two elements or hold them together.

All elements except the food itself will withstand the applied forces without immediate destruction, and the anatomical elements are securely linked without danger of their sudden collapse or dismemberment. The effective operation of restored' dentures is largely dependent upon the permanency of retention of the substituted elements to the complemented anatomical elements.

Numerous difficulties in denture construction may be attributed to the failures of complying with the two principal and reciprocal requirements: (1) establishment of occlusions and (2) establishment of retention.

Occlusions and impressions are not a fifty-fifty proposition. The share belong- ing to each factor cannot be expressed in mathematical terms. It varies in each case and is indeterminable in all cases-two good reasons why we should endeavor to obtain perfection for both.

Perfect occlusions contribute more effectively to permanent stability than perfect impression work, but do not infer that perfect impressions are to be looked upon indifferently. It is well known that the best impression work will not maintain stability when associated with defective occlusions, and it is also conceded that the establishment of balanced occlusions will not only be of assistance in maintaining stability, but is also a stimulant to physiological changes which im- prove adaptation and assist in maintaining stability. On the other hand, it is evident from clinical experience that the best impression does not maintain stability when associated with faulty, unbalanced occlusions. The many cases which require rebuilding on account of defective occlusions amply justify this contention.

Natural articulation in its entirety is not suitable for reproduction in artificial dentures. The latter require balanced occlusions through the entire stroke of applied articulation. Balanced occlusions infer a contact relation of the masticatory surfaces which will counteract displacement of the plates by masticatory forces. All extreme and intermediate occlusions, except central occlusions, require at least three-point contact, at points widely distributed over the masticatory surfaces. Two or more contact points at the posterior ends of the arch and one (or more) in the anterior region of the denture constitute a favorable distribution.

I am using the customary expressions three-point contact and multiple-point contact; in fact, each point spoken of represents a fragmental surface of contact or closely spaced independent surface fragments of adjacent cusp inclines. I frequently spoke of contact areas and intimacy of contact in these areas, explained in Dental Engineering.

Balanced central occlusion is not an evenly intimate contact relation along the entire masticatory surfaces. A perfectly satisfactory central occlusion for dentures under applied muscular forces has a greater intimacy of contact on the left and the right side about the first molars; to the posterior and anterior a gradual decrease of intimacy of contact occurs, and it winds up in total loss of contact at the anterior or the posterior or at both regions. This means that spaces between various regions of the opposed masticatory surfaces exist for different applied forces.

One may compare such arrangement with a frequently met type of rocking chair, one having a base below the rocker. One purpose of such a rocking chair design is evident. The occupant's weight, which acts at the center line upon the base, is favorably transmitted to the carpet. The weight is evenly distributed over a greater area and, in consequence, exerts less per unit area. When the occupant's center of gravity shifts then his weight is transmitted to another part of the base. In consequence, the back part of the chair will transmit a greater share of the applied weight to the carpet. The weight is less favorably distributed; it becomes somewhat greater per unit area at the back.

A similar condition, that of distributing the pressure, is produced by a bolus of food between two dentures backed by resilient tissues, and also when the dentures are in contact under muscular tension.

A rocking-chair fit of the masticatory surfaces (pardon such unscientific term) serves a double purpose. It secures favorable pressure distribution in central occlusions under loads and it prevents interference of* the masticatory surfaces which is invited by dentures slightly lifted by relieved resilient tissues. In lateral and protrusive occlusions, when initiating articulation of the surfaces, forces may or may not act. In either case the masticatory surface formation as described presents advantages.