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Original Article McHorris - Occlusal Adjustment via Selective Cutting of Natural Teeth 1 Occlusal Adjustment via Selective of Natural Teeth.* (Part 1) Cutting William H. McHorris A well-planned, precise occlusal adjustment of natural teeth has some distinct advantages over other forms of occlusal therapy. It should be emphasized, however, that an occlusal adjustment is an irreversible, "take away" procedure and has definite contraindications in some mouths. Before discussing types of treatment, it would be worthwhile to review the indications for any type of occlusal therapy. Although much controversy and confusion have surrounded the etiologic factors leading to occlusalrelated problems, dentistry has recognized that there are two etiologic factors that must be dealt with in treatment. The first is emotional or psychological stress; the second is existing occlusal interferences or maloccluding teeth. Their combined effect leads to periods of parafunctional activity. The parafunctional acts would include any use of the teeth other than the functional activities of chewing, swallowing, and speaking. Parafunctional activities result in much greater forces over a longer period of time than do functional activities. Such activities as bruxism and clenching can result in forces that exceed the gnathic system's ability to resist. Although the psychological stress factor must be recognized and dealt with, it is worrisome that dentistry is drifting toward increasing treatment of this factor. The intent of this article is to deal with the second etiological factor, occlusal interferences, and how to systematically eliminate them. The term occlusal interferences implies that the occlusion is interfering with something or that something is interPresented to the American Academy of Restorative Dentistry. Chicago . Illinois. February 1983. The Journal of Gnathology . Volume 16. Number 1. 1997 fering with the occlusion. In the stomatognathic system the term must be assumed to mean the obstruction of normal functional usage or the hampering of certain anatomical components from obtaining positions or postures that they were intended to be able to assume. The teeth are the interfering elements that must be dealt with. The postures or positions desired must first be defined and then a technique must be formulated to eliminate occlusal interferences that prevent the system from obtaining them. The objective of an occlusal adjustment, as with any form of occlusal therapy, is to correct or remove the occlusal interferences, or premature contacts, on the occluding parts of the teeth that prevent a centric relation closure of the mandible. It would require removal of the parts of posterior teeth that contact during working, nonworking, protrusive, and lateroprotrusive movements. During these movements, the only contacting surfaces should be related to the anterior teeth in general and to the mandibular anterior teeth in particular. So that a systematic, disciplined approach can be followed in treatment, the objectives should be listed. They are: 1. Centric relation occlusion of the posterior teeth 2. Proper "coupling" of the anterior teeth 3. An acceptable disclusive angle of the anterior teeth in harmony with the condylar movement patterns 4. Stability of the corrected occlusion 5. Resolution of the realted symptoms The options available to accomplish these objectives are: 37 2 McHorris - Occlusal Adjustment via Selective Cutting of Natural Teeth 1. Occlusal adjustment of the existing natural teeth 2. Use of removable orthopedic repositioning appliances 3. Orthodontics 4. Full or partial reconstruction of the dentition (fixed or removable denture) 5. Orthognathic surgical procedures 6. Any combination of the above The choice of treatment should always be based on the least offensive approach. The end result should be the resolution of the symptoms. Centric Relation Occlusion of the Posterior Teeth An occlusal correction should result in simultaneous contact of all posterior teeth in centric relation closure of the mandible. Centric relation occlusion is the most important determinant of mandibular stability. Consideration must be given to the anatomical components that participate in this important posture. Anatomical Considerations In healthy, well-functioning temporomandibular joints, the two mandibular condyles relate to the articular discs to form the condyle-disc assembly. It is important to recognize that these two components work as a unit. The temporomandibular joint is classified as a compound joint. By definition, a compound joint requires the presence of at least three bones, yet the temporomandibular joint has only two. The articulator disc functions as a "nonossified bone." Its superior and inferior surfaces are true articular facets. Therefore, the articular disc of the temporomandibular joint is not a meniscus at all. Considerable misunderstanding has resulted from the inaccurate use of this term. An anatomical meniscus (from the Greek meniskos, meaning crescent) is a crescent-shaped fibroartilaginous structure having one edge attached to the articular capsule and the other extended freely into the joint cavity.1 38 Original Article Thus a true meniscus does not separate the joint cavity or isolate the synovial fluid, but it functions passively to facilitate movement between the bony parts. Typical meniscuses are those of the knee joint, where hinge action is the principal movement. In the temporomandibular joint, the articular disc not only divides the joint compartment into parts; it separates the joint into two distinct and quite different units, thus converting a simple joint into a compound joint. Both compartments, contain synovial fluid which lubricates the actions of the parts. The upper joint remains a freely movable sliding joint (arthrodial) while the lower functions as a pure hinge joint (ginglymoid). The synovial fluid also fulfills the metabolic requirements of the nonvascularized and noninnervated portions of the temporomandibular joints. The confinement of the synovial fluid to the articular surfaces requires encapsulation in a fibrous connective tissue bag which is attached at the circumferential peripheries of the articulating surfaces (Fig 1). This encapsulating structure is well vascularized and innervated and thus provides the blood and nerve supply to the joint. Its inner lining is the specialized synovial membrane which secretes the vital synovial fluid that provides nourishment to the joint. Furthermore, the temporomandibular articular disc is not a passive structure. It enters actively into joint function, being independently powered to make it capable of its own movements quite separate from those of the bony structures of the joint.2 The nonvascularized articular surfaces of the temporomandibular joint are composed of dense fibrous tissue rather than the hyaline cartilage found in all other synovial joints except those of the clavicle. This results in two unique advantages: 1. Since fibrous tissue is not as susceptible to the effects of aging as hyaline cartilage, these joints are less vulnerable to degenerative joint disease. 2. Since fibrous tissue is more capable of repair and regeneration than hyaline cartilage, these joints enjoy healing characteristics that have significance in treatment planning and prognosis.1 The Journal of Gnathology. Volume 16. Number 1. 1997 Original Article McHorris - Occlusal Adjustment via Selective Cutting of Natural Teeth 7 attached to the tympanic plate or posterior wall of the glenoid fossa (Fig 8). This retrodiscal or retroarticular attachment is also referred to as the bilaminar zone.5 The elastic connective tissue found in this attachment is primarily in the top layer of the two-layered (bilaminar) zone.6 The elastic properties of the retrodiscal attachment prevent excessive anterior and medial displacement of the articular disc . The attachment is thus the antagonist to the superior head of the lateral pterygoid muscle, which contracts during closure. This makes it a very active muscle during bruxing activities. Centric Relation The condyle-disc assembly is bodily seated against the fibrous-covered posterior slope of the articular eminence at the centric relation closed position (Fig 9). This relationship is a result of contractions of the closure muscles. The ability of the condyles to attain the seated posture, however, is dependent upon three important conditions. First, the inferior heads of both lateral pterygoid muscles, which are attached to the condyles, must be bilaterally relaxed. Second, there must not be an excessive amount of synovial fluid in the upper or lower compartments of the temporomandibular joint. An increased amount of synovial fluid in these compartments is not an uncommon finding in patients who chronically "brux" their teeth and who suffer from temporomandibular joint disorders. Third, and by far the most common deterrent, are the fittings of the occlud ing parts of the teeth. If the occlusal surfaces of the teeth, at maximum intercuspation, prevent the condyles from seating against the disc, the condyles must accommodate to an eccentric posture. This tooth-dictated, eccentric posture that the condyles are forced to assume is usually inferior and anterior to the desired seated, centric relation position. The posterior teeth should allow the condyles to assume their seated position against the articular disc. The inclines of posterior teeth should never have contacts that would force the condyle's seated closure position in a distal, inferior, or anterior direcThe Journal of Gnathology, Volume 16. Number 1. 1997 tion. Once the seated position is obtained, the posterior teeth must support and protect it. It is important to note that mandibular "rest position" of the condyles would be nearly the same as the centric relation occluded position, but it would not and does not result from muscular contraction (Fig 10). It is a posture in which the teeth are not occluded or touching and would require muscular rest or relaxation. By nature's law of "conservation of energy" the rest position of the condyles is not a seated position against the articular disc - the condyle-disc assembly is in a nonstress bearing or uncompressed relationship. Rest position would have to be considered the most therapeutic position for the mandible. Knowledge and understanding of these physiologic principles should be utilized in taking centric relation registrations for mounting diagnostic casts in an articulator. The principles should also be utilized in marking occlusal interferences that prevent the mandible from closing to a centric relation occluded position. Centric Registration Technique The technique preferred by this author when taking centric relation registrations is the one utilizing a "leaf gauge ." 7 The gauge consists of 15 to 20 leaves of acetate, vinyl, or some other plasticlike material held together by rivets or brass brads (Fig 11). The gauge is placed between the anterior teeth, and the patient is asked to close in a retruded position. The interocclusal space is observed and leaves are added or subtracted to secure the vertical relation desired to obtain the registration (Fig 12). A dependable plasticized wax in two separate strips is softened in hot water and then placed on the maxillary posterior teeth, one strip on each side of the arch. The leaf gauge is replaced between the anterior teeth, the mandible is retruded, and the patient is asked to apply moderate biting pressure on the leaf gauge until the wax is hardened or set (Fig 13). Only cusp tip indexing of the wax is preferred. If the indentions in the wax are too deep or too shallow, leaves can be added or subtracted 43 Original Article McHorris - Occlusal Adjustment via Selective Cutting of Natural Teeth 9 and the technique repeated to obtain the desired registration. The leaf gauge between the anterior teeth aids the patient in retruding the mandible while the biting force of the closure or elevator muscles will seat the condyle-disc assembly against the posterior slope of the articular eminence. This eliminates the operator error often encountered in attempts to manipulate a patient's mandible to centric relation. Since the patient is applying all of the pressures, the seated position of the condyledisc assembly is not likely to exceed the physiologic limits of the system. In the case of end-to-end or Class Ill relationships of anterior teeth, or in the case of the absence of anterior teeth, a downward or "resistance-to-closure force" is applied against the patient's chin at the midline as closure into the wax is occurring. Centric Relation Occlusion (Objective 1) The centric relation occluded position can be maintained through the proper distribution of the forces exerted by the interocclusal contacts. The interocclusal contacts between opposing posterior teeth should occur on the occluding surfaces in such a way that they will provide mesiodistal and buccolingual stability of the teeth and mandible. Closure stoppers* and equalizers, when equal and opposite, assure mesiodistal stability (Fig 14). A, B, and C contacts, when equal and opposite, assure bucculingual stability (Fig 15).8 all the forces in eccentric bruxing attempts. They will be the only teeth contacting in any movement of the mandible away from the centric relation occluded position. If there is proper coupling, anterior contact will take place within one-half thousandth of an inch (.0005) or within the first degree of rotation of the mandible away from the centric relation occluded position. This resultant "dis-occlusion" of the posterior teeth is called disclusion. 10 Disclusion can exist only through the efforts of the anterior teeth working in harmony with the condyles if posterior eccentric interferences are to be prevented. Natural teeth resemble gothic pyramids because they are rounded or spheroidal in all three dimensions. Two rounded surfaces meet at a point and, if either surface is moved along a tangent to the radius of the curvature, the two surfaces will immediately separate or disclude. This is the mechanical principle utilized in obtaining disclusion of posterior teeth (Fig 16 and 17). In the gnathic system, the angle of movement can be determined through mandibular recordings or approximated by carefully obtained lateral checkbites. Thus articulators can become a mechanical likeness of patient's existing maxillomandibular relationship and allow diagnosis and treatment of their occlusal relationships. 0 Proper "Coupling" of the Anterior Teeth (Objective 2) Equal and opposite distribution of forces on the posterior teeth will stop the hinge closure of the mandible without deflection and prevent hard contact to the more sensitive, more vulnerable anterior teeth. The "together," almost touching, relationship of the anterior teeth to each other is known as "anterior coupling." 9 This coupling relationship of the anterior teeth will enable them to quickly assume • Closure stoppers are any interocclusal contact found on the distal inclines of maxillary teeth and the mesial inclines of mandibular teeth. •• Equalizers are any interocclusal contact found on the mesial inclines of maxillary teeth and the distal inclines of mandibular teeth. The Journal of Gnathology, Volume 16. Number 1. 1997 Acceptable Disclusive Angle of the Anterior Teeth (Objective 3) The angle of thrust of the mandibular anterior teeth against their maxillary opponents can be controlled so as not to be too restrictive. The less restrictive, the less friction, and therefore the less tooth movement and wear. The dictating factor is the angle of the articular eminentia. These vertically inclined slopes dictate the condyles' angle of thrust or disclusive movement. This is particularly true when the condyle-disc assembly is held in forceful contact against the eminence during periods of bruxism. This angle can be approximated through instrumentation to serve as a guide to approximate what the anterior disclusive angle should be. It is the likeness of the condylar disclusive angles and the anterior disclusive angles that facilitates harmonious, effortless mandibular movement. This harmony 45