Jaw relation (or the maxillomandibular relationship) refers to the position of the mandible (lower jaw) relative to the two maxillae (upper jaw) and can be described as a tooth-to-tooth relationship between maxillary and mandibular teeth or as a bone-to-bone relationship between the maxillae and mandible. Several terms describing different tooth and jaw relations are discussed in this section.

A. MAXIMAL INTERCUSPAL POSITION

Maximal intercuspal position (MIP) or maximal intercuspation is a tooth-to-tooth relationship that is not dependent on where the jaw muscles or joint anatomy would like to position the mandible. It is the tightest or best fit between maxillary and mandibular posterior teeth and can be demonstrated on handheld casts of the upper and lower arches without looking into the mouth (Figs. 9-21B and 9-22B).

B. CENTRIC RELATION

Centric relation (CR) or centric jaw relation is the relationship of the mandible to the maxillae where healthy muscles and joint anatomy can comfortably guide the mandible if there were no teeth. It is an important reproducible and repeatable relationship of the mandible to the maxillae because it is normally the relationship people return to each time after they chew and swallow, and it is the relationship that dentists use when they mount diagnostic casts of the mouth prior to major restorative procedures. This jaw relationship is not affected by the presence (or absence) of teeth, so it does not change due to tooth malocclusion. It includes the range of positions of the mandible during a hinge-like opening and closing without moving bodily forward and without teeth touching, or just until the first two teeth initially just touch but do not yet begin to close more tightly into MIP.^18–20,J

It is a relatively rare but ideal occurrence when CR coincides with the MIP. This occurs when the mandible closes in its CR position and there is simultaneous even contact of teeth in maximal intercuspation when the teeth first touch.^8,19–22 This type of ideal occlusal relationship results in a harmony between the guidance afforded by jaw muscles, the position of condyles against the discs and fossae, and the maximum fitting together of the teeth. This condition does not occur in most people unless they have just had a well-executed occlusal reshaping (equilibration) where small amounts of interfering occlusal enamel were removed by the dentist to equalize occlusal stress,²² have a well-made removable denture, or have had a complete dental arch rehabilitation replacing or reorienting all occlusal surfaces (described later in this chapter).

When centric jaw relation does not coincide with the MIP, a prematurity or deflective occlusal contact exists. Most people have deflective malocclusion to some degree. Premature or deflective occlusal contacts refer to the teeth that are the first to contact as the mandible closes into its most retruded position in CR. These deflective occlusal contacts result in a mandibular deviation away from its relaxed, CR closure in order for the teeth reach MIP. The direction of the deviation of the mandible is usually forward (about 1 to 2 mm) and upward, with or without simultaneous lateral movement.^{18,20,21,23,24} This is illustrated in Figure 9-21 where premature contacts deflected the mandible forward and to the left as the teeth move from CR into MIP (from A to B) and in Figure 9-22 where the mandible was deflected forward 2 mm (from A to B). In Figure 9-22, compare the short vertical pencil lines on two pairs of opposing maxillary and mandibular teeth that line up when the teeth are in their MIP but reveal how distally the mandible was positioned in its centric jaw relation. A prematurity is most obvious on the skull in Figure 9-23 where the supraerupted maxillary third molar occluded before any other teeth when the mandible closed in its CR. This deflective occlusal contact forced the mandible to move considerably forward and superiorly in order to reach MIP.

Edentulous people (with no teeth) who wear complete dentures or false teeth are provided with denture teeth where CR coincides with MIP because they can learn to pull the mandible back and close into a stable and repeatable position of CR during jaw closure. This enables the tight occlusion of denture teeth to coincide with the repeatable centric jaw position, so the dentures will remain tightly secured against the mucosa and not rock loose when functioning.

An articulator is a mechanical device that holds casts of the two arches, permitting a close duplication of the patient’s opening and closing centric jaw relations. Notice the fit of the ball of the lower (mandibular) part fitting into a concavity on the upper (maxillary) part (Fig. 9-24A and B). This design simulates the heads of the condyles fitting into their articular fossae. It is easier to study tooth relationships with the patient’s dental arches (dental stone casts) on the articulator in your hands, rather than with your hands in the patient’s mouth. What better way is there to determine whether or not the maxillary and mandibular lingual cusps fit together tightly or properly in the maximal intercuspal relationship?

LEARNING EXERCISE

With your jaw muscles relaxed, open your mouth so that your teeth are slightly apart, and close very slowly in a hinge motion without sliding the jaw forward until the first teeth initially touch gently. The relationship of your jaws prior to your first gentle tooth contacts is your centric jaw relation. The relation of this pure hinge opening is a most important one to record when making extensive dental restorations for a patient. If your mandible is deflected (hits and slides) forward as you continue to close your teeth together into their MIP where they fit together most tightly, you are experiencing deflective or premature occlusal contacts, and you are among the majority of people whose CR does not coincide with the MIP. The mandible will almost always slide forward from CR into MIP, either straight forward or to one side. More than likely, your own deflective tooth contacts will not be as severe as that shown in Figure 9-22. Can you determine in which direction your premature tooth contacts deflect your mandible? Compare the location of your first (premature) tooth contact in CR with those in Table 9-1. Less than 1% of this group had MIP coincide with CR, yet most were asymptomatic.

TABLE 9-1

Deflective Centric Relation Tooth Contact Data from 811 Dental Hygienists

Location of First Centric Relation Tooth Contact	Number of Hygienists	Percent
Premolars one side	232	28.6
Premolars both sides	90	11.1
Molars one side	328	40.5
Molars both sides	113	13.9
Molar one side; premolar one side	38	4.7
Canine	4	0.5
MIP = centric jaw relation (no prematurity)	6a	0.7
TYPE AND PLACE OF DEFLECTIVE CONTACT	PERCENT
Premolars only	39.7
Molars only	54.4
Unilateral prematurity	69.2
Bilateral prematurity: same tooth	25.8
Bilateral prematurity; premolar–molar	4.7

^aThree of the six recently had an equilibration by their dentists.

Research conducted by Dr. Woelfel at the Ohio State University, 1974–1986.

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