Seating the condyles for centric relation records

SEATING THE CONDYLES FOR CENTRIC RELATION RECORDS

H. W, FOUNTAIN, D.D.S. Kalamazoo,Mich. ENTRIC RELATION is not always easy to record even though the dentist knows

C

what is needed and the patient understands the problem and is attempting to cooperate.The proper seating of the condyles for equilibration or for making an interocclusalcentric relation record can be a frustrating experience. The fact that the patient can make rotary movementsof the mandible does not ensure that the condylesare seatedin the most posterior and superior positions in the fossae.Closure into a wax wafer which is softenedin the posterior region and firm in the anterior region is not adequatefor proper seating of the condyles for many patients. This article describes a procedure for seating the condyles in their centric positions either for visual examination of deflectiveocclusalcontacts or for mounting diagnosticcastson the articulator. PROCEDURE

The patient is taught to arc the mandible as near to hinge closure as possible. When the patient understands what is expected of him, a quarter segment of modeling compound* is doubled for thickness and placed on the lingual incisal aspectsof the upper anterior teeth. The dentist cradles the point of the patient’s chin with the thumb and forefinger of the right hand to detect a protrusive movement combinedwith the hinge closure. At no time should the mandible be pushed in a posterior direction. The mandible should be moved by the patient and not worked up and down by the dentist. The patient makesrepeatedclosureslittle by little into the modeling compound until the teeth that would contact first are about l/16 inch apart. The modeling compound is removed,cooled,and replaced between the teeth. The modeling compound should not extend distally to the cuspids. Then the patient repeatedly clenches the teeth on the modeling compound, which acts as a positive stop, without opening the mouth. The dentist places his finger tips over the massetermusclesand tells the patient to contract these muscles when he clenchesthem. If the muscles becometired or temporary discomfort develops, the patient may rest with the modeling compound between the teeth, resuming the clenching and relaxing after a brief pause. If the patient has had a centric occlusion in an eccentric relation, he will soon become aware of wanting to close the jaw posteriorly to the record made in the modeling compound, and another piece of modeling compound is softenedand the procedure repeated. *Kerr's Red Compound. 1n50

zk~t ‘2

SEATING

CONDYLES

FOR

CENTRIC

RELATION

RECORDS

1051

One or two changes of modeling compound may be needed if the existing occlusion is not close to the centric relation. A period of exercise ranging from 1 to 5 minutes permits vertical and posterior seating of the condyles not usually accomplished in a single attempt. LOCATING

DEFLECTIVE

CONTACTS

IN

NATURAL

TEETH

The clenching exercises are completed, and the modeling compound index is removed. If saliva has accumulated, the patient must not bring the teeth together when he expectorates. The dentist again places his thumb and forefinger on the chin, and the patient is asked to open and close the jaw in an arc of about H inch without permitting any teeth to contact. After the patient has made several smooth rotary movements, he is asked to close the jaw a little further each time until he feels the slightest “feather touch” between the upper and lower teeth. In most instances, the ligament system of the mandible has been stretched, and the muscles will not hold the condyle against the articulating eminence for more than a few seconds. Therefore, the modeling compound index is replaced from time to time, and the patient clenches the teeth tightly against it two or three times. This procedure will keep the condyles seated. Occasionally, patients with malocclusion and resulting neuromuscular confusion cannot cooperate in some of the simplest jaw movements. A few minutes of clenching the teeth on the modeling compound index helps to eliminate this neuromuscular confusion. This conditioning procedure should be performed before the hinge axis is located. It is easier to record the hinge location if the muscles and joints have received this exercise. MOUNTING

CASTS

ON

THE

ARTICULATOR

The modeling compound index made last is used as an anterior index for centric relation registration for mounting diagnostic casts or working casts or remounting finished restorations. A zinc oxide-eugenol cement* is mixed to a moderately firm consistency (2 Gm. of liquid to 5 Gm. of powder) and divided into two parts. The two parts of mixed cement are dabbed into cement powder to reduce the tackiness and are molded into two cylindric shapes about g inch in diameter and s/4 inch long. The modeling compound index is removed, and the pieces of soft cement are placed over the molar and bicuspid teeth. The modeling compound index is replaced, and the patient is asked to close the mouth firmly and hold the jaw in that position until the cement is hard. The thickness of the interocclusal record is determined by the thickness of the modeling compound index. The split-cast technique is used in mounting the upper cast on the articulator. It serves two purposes : ( 1) it permits an accurate check on the relation of the upper and lower casts as oriented by the first centric relation registration, and (2) it permits checking one centric relation registration against another. Three centric relation registrations should be made for each patient, and all should be iden*Ackerman.

FOUNTAIN

1052

tical. A new modeling compound index should be made for each centric relation registration. The fine detail in the zinc oxide-eugenol cement record is removed when seating casts of natural or prepared teeth or when transferring full coverage castings. The occlusal surfaces of the restoration and the modeling compound index are lubricated lightly. Similar modeling compound indices are made for the three eccentric positions when transfering jaw relations to an adjustable articulator. However, this procedure does not eliminate the need for remounting the completed restorations. Even though the occlusion appears to be perfect at the time of insertion, in most instances, a few days later, there will be deflective occlusal contacts in the second molar region. The adjustments are slight, and the need for more than one remounting has not been encountered. SUMMARY

The mandible is held in suspension by a muscle and ligament system. In many people, the skull-mandible ligament system has been stretched vertically as a result of occlusal disharmony, and it is not until the muscles of mastication have started to work with force that the joints approach their normal relationships. A method of positioning the condyles in their most posterior and superior positions has been described. 2901 SOUTH WESTNEDGE KALAMAZOO,

MICH.

AVE.