A. ENVELOPE OF MOTION

A helpful method for analyzing a patient’s mandibular movement is to obtain an outline of their jaw movement called an envelope of motion. This is done by attaching a marking device (stylus) to the mandibular teeth that can trace on paper the movements of the mandible as viewed from the front (a frontal view) or from the side (a sagittal view). Figure 9-33 shows examples of these two tracings, both reproducing the outer border movements during maximal movement of the mandible. When facing a person, the frontal envelope is the outline formed (traced) by a marker located between the mandibular central incisors while the mandible moves maximally in all directions. Beginning with teeth in their most intercuspal position (MIP), the mandible (with teeth lightly touching) moves the maximum distance to the right; then, in its maximal right position, depresses to its most open position; from there closes in its most left position until teeth lightly touch; and finally returns (with teeth lightly touching) to the MIP.

Now, analyze an actual tracing of a frontal envelope in Figure 9-33A in order to appreciate what it reveals. Begin in the MIP at the top and follow clockwise. The mandible with the teeth in light contact first slides laterally to the patient’s left (our right) as far as possible. The outline reveals the amount of canine overlap resulting in the mandible initially moving down as it moves to the side until the canines are end to end and then moves upward as the canines move laterally beyond their end-to-end alignment. Next, the jaw opens downward in its most left lateral position until open about 30 mm and then begins veering toward the center to a maximum opening of 51 mm. From this point, the jaw moves to the patient’s right (our left) as far as possible as it begins to close. Finally, from the closed maximum right side position, the teeth slide into MIP as the jaw slowly moves back and upward (due to the canine overlap) into the starting point (MIP).

The sagittal envelope can be visualized, when viewing a person from the side, as an outline formed (traced) by a dot located between the mandibular central incisors while the mandible begins in the centric relation (CR) position, just before the teeth move forward into the MIP. Next, with teeth lightly together, the mandible moves into its most anterior (protruded) position, then to its most open position, and from there, the mandible closes in its most posterior position into CR until teeth lightly touch. Finally, the mandible returns (with teeth lightly touching) to the MIP. To analyze the tracing of a sagittal envelope of motion in Figure 9-33B, begin at the centric relation or CR. Due to a slight deflective (premature) contact, the mandible is directed forward and slightly upward into the MIP. With the teeth held together lightly as the mandible continues to protrude maximally, the initial downward movement of the mandible is due to incisal overlap (normal overbite) where the lingual surfaces of maxillary incisors guide the mandible downward as it goes forward, followed by an upward and forward movement as mandibular incisors move beyond the edge-to-edge position into the most protruded position. With the mandible protruded, it moves down to the maximum opening of 51 mm. From this point, the jaw closes while firmly retruded, which develops the curved translation portion of closure, followed by the straighter hinge-opening boundary (with rotary motion only), and finally back to the starting point (MIP). We can tell from this envelope of motion that upon opening, this person can rotate his retruded mandible open 30 mm at the incisors with a hinge movement before it begins to translate forward.

Now, study Figure 9-34 to compare the uppermost portions of the frontal envelopes of motion of three subjects in order to visualize differences in the superior portion related to the amount of canine guidance (overlap). Subject A has the smallest and narrowest range of movement for his mandible (32 mm vertically, 21 mm sideways). No lowering of the mandible on either side of the MIP indicates that he did not have canine protection (i.e., there are no deeply overlapping canines) to lower the mandible and disocclude the posterior teeth. Subject B can open his mandible 53 mm and move it laterally 31 mm. He has a canine-protected occlusion as indicated by the steep portion where the mandible drops on either side of the MIP. Subject C has a medium-sized envelope of motion with canine protection on his right side (left side of envelope C) and group function occlusion with shallow canine rise on his left. This patient preferred to chew mostly on his left side where his envelope is lopsided.

To appreciate the amount of mandibular movement during chewing relative to the entire envelope of motion, once again, analyze Figure 9-33A. Focus on the smaller pattern of lines with arrows (enclosed within the larger frontal envelope of motion) of that person chewing peanuts on the right and left sides. The lines traced during the opening stroke (denoted by opening arrows pointing downward) are somewhat straight, whereas the closing stroke lines are considerably convex (or bulge) as the mandible moves toward the working side to obtain working side tooth contacts. The chewing cycles occupy only 25 mm of the maximum 51 mm opening range for this man. The chewing strokes on peanuts in a lateral direction utilize only 12 mm of the total side-to-side range of mandibular movement.

On the sagittal view (Fig. 9-33B), note that the chewing stroke begins at the MIP and that the opening stroke is more posterior than the closing stroke. The opening stroke is only 7 mm anterior to the hinge-opening boundary, whereas the closing stroke is 10 mm anterior to that boundary. However, as the jaws are closed to crush the food bolus, the mandible is slightly more retruded than the MIP. Crushing of the food bolus (a rounded mass of food) occurs at this MIP.

B. ACCURATE RECORDING OF THE CENTRIC RELATION JAW POSITION

The process of obtaining an accurate centric relation jaw registration or occlusal record is seen in Figure 9-35. First, a leaf wafer^19,24,38 is selected and deformed in the mouth as the patient bites into it. Then, an anterior deprogrammer^{18,22,24,38–42} is inserted at an upward angle between the incisors as the patient arcs the mandible open and then closes (hinge type or rotational opening) until the incisors engage the leaf gauge of sufficient thickness so all other teeth separate slightly (Fig. 9-35C). Anterior deprogramming^{18,22,24,38–46} is the process of getting the TMJ into a relaxed or comfortable neuromuscular position (centric relation) by interrupting or negating the proprioceptors surrounding the teeth in the periodontal ligaments. These proprioceptors would otherwise automatically or subconsciously direct the mandible into the habitual or acquired intercuspal position. Anterior deprogramming is usually accomplished in 10 to 15 minutes by interposing something between the anterior teeth^{20,38,40,41,44,45} (such as a leaf gauge, Lucia jig, or sliding guide) while the patient retrudes the mandible and squeezes slightly on the centered anterior fulcrum (see Fig. 9-35C). In this manner, the mandible is “tripodized” (stabilized by two condyles and the leaf gauge) by the patient’s nerves and muscles, and the patient is momentarily unable to aim the jaw into the acquired or habitual occlusion because no signals can be sent to the brain from the proprioceptors in the separated teeth.³⁷ Otherwise, the teeth could cause the mandible to deflect forward from the centric relation position. The posterior teeth must remain separated several minutes for deprogramming to occur.^20,42 Once it has occurred, the patient will feel as if the posterior teeth occlude (contact) improperly or in a strange way (with deflective contacts). In some instances, the deprogramming will not occur until the patient has worn an occlusal device for several weeks and has maintained a stable and comfortable mandibular position for at least 1 week.⁴²

Next, the leaf gauge is inserted in the wafer, and an impression material is thinly spread over tooth indentations in the leaf wafer (Fig. 9-35D). Then, the entire assembly is carried to the mouth (Fig. 9-35E), and the patient closes firmly, as previously, onto the leaf gauge until the recording medium sets. The imprints of the upper and lower teeth in this centric relation registration (Fig. 9-35F) are used to relate casts of each arch mounted on the articulator (Fig. 9-35G). This diagnostic mounting procedure should always be accomplished prior to attempting any type of tooth equilibration in the mouth.^18,19,22 These dental stone casts, mounted in their relaxed centric relation position, can be used to determine the extent of tooth reshaping required in order to decide the best treatment. The dental stone teeth with interfering or premature contacts can be reshaped (reduced) in order to predict the amount of tooth reduction that will be required during the equilibration. If the amount of tooth structure that must be removed during the equilibration would likely expose dentin or pulp, then orthodontics or surgical procedures must be considered.

Another device used for anterior deprogramming of the mandible and for recording centric jaw relation is the sliding guiding inclined gauge or sliding guide^39–43 (Fig. 9-36). It comes in three maximum thicknesses, and the thickness used is dependent on the severity of the malocclusion (Fig. 9-36A). The thickness gradually increases from tip to handle, and the curvature of the sliding guide is critical, so that it can be placed in the mouth between overlapping incisors at a relatively steep angle relative to the plane of occlusal without injuring the tissue of the roof of the mouth (Fig. 9-36B). The exact thickness between the incisors is read on the millimeter scale (Fig. 9-36C). Minimal incisal separation is the goal for deprogramming and jaw position registration, just so long as no posterior teeth touch, thus avoiding proprioceptive impulses. This is particularly important for the centric jaw relation registration to minimize errors between the articulator and the patient. The sliding guide is made of a nonbrittle autoclavable plastic and works well with a custom bite deformed Woelfel leaf wafer for centric relation jaw registrations (Fig. 9-36D and E).^39–42

C. LONG CENTRIC ARTICULATION

Long centric articulation or the intercuspal contact area is actually a range of mandibular movement where a person can smoothly (without interferences) move the mandible from centric relation directly forward in a horizontal plane to the position of maximum intercuspation. There is no upward or lateral component. This range of movement is often the goal during an equilibration to provide the patient with a long centric relationship by relieving all deflective or premature tooth contacts that had previously caused the mandible to deviate either sideways or upward from centric relation to the MIP. The patient with a long centric articulation will have a small anteroposterior range (0.5 to 2.0 mm) of uniform posterior tooth contact occurring at the same vertical dimension of occlusion.

LEARNING EXERCISE

Evaluate your own jaw mobility or mandibular movement capability.

It should be an educational and interesting experience for you to complete this simple exercise in order to increase your awareness of your own jaw movements. It will take about 20 minutes to do this exercise.

Obtain a clean, plastic millimeter ruler cut off even with the zero mark. Make the following measurements as described while observing your tooth relationship in a mirror.

1. Horizontal overlap (H) of incisors and canines

Using a mirror, measure the horizontal overlap (H in Fig. 9-37) in the following three locations while holding your teeth together in MIP (or on handheld tooth models with teeth tightly closed).

1.a. ___ mm = horizontal overlap between the labial surfaces of central incisors at the midline

1.b. ___ mm = horizontal overlap between labial surfaces of left canines

1.c. ___ mm = horizontal overlap between labial surfaces of right canines

2. Protrusive overlap (P) = ___ mm

Measure the protrusive overlap (P in Fig. 9-37) with your lower jaw moved forward as far as possible (like a bulldog), between the labial surface of the maxillary central incisors and the labial surface of the mandibular incisors.

3. Horizontal overlap of canines during lateral excursions (right and left sides)

Measure the horizontal distance between the facial surfaces of the upper and lower canines during maximum movements in lateral excursions (arrow on Fig. 9-38A), first with the lower jaw moved to the left as far as possible. This is just like the measurement P in Figure 9-37, only between the facial surfaces of the maxillary and mandibular left canines.

3.a. ___ mm = left horizontal overlap of canines

Next, measure the horizontal distance between the facial surfaces of the upper and lower canines during a maximum lateral movement of the jaw to the right side as far as possible (red arrow in Fig. 9-38B).

3.b. ___ mm = right horizontal overlap of canines

4. Vertical overlap of central incisors (V) = ___ mm

Measure the vertical overlap (V in Fig. 9-39) at the midline between the incisal edges of your central incisors while holding your back teeth tightly closed or on your tooth models in MIP.

5. Opening movements (hinge opening and total opening)

The hinge opening is the distance between incisal edges at the maximum hinge-only opening. Practice opening your jaw slowly as far as possible with a hinge movement in centric relation. Hinge opening is usually only half or less than half of the maximal opening (first portion of O in Fig. 9-39, and represented in Fig. 9-20B as the limit of the hinge opening). If you open properly, there should not be any crepitation because the articular discs and condyles are fixed posteriorly.

5.a. ___ mm = hinge opening

Now, open as widely as you can and measure the maximum opening (O in Fig. 9-39) between the incisal edges (usually you can fit four fingers between your incisors). If you noticed a noise near one or both ears when you opened widely, it is probably caused by a disharmony between the movement of the jaw and the movement of the disc that fits between the jaw condyle and the skull on either side. It is usually not a serious problem, and many people experience crepitation for a while during their lifetime.

5.b. ___ mm = maximum opening (O in Fig. 9-39)

6. Calculate maximum movements

6.a. Add measurements 4 plus 5b to obtain total incisor opening = ___ mm.

6.b. Add 4 and 5a to obtain maximum hinge opening at incisors = ___ mm.

6.c. Add measurements 1b and 3a to obtain maximum left lateral movement = ___ mm.

6.d. Add measurements 1c and 3b to obtain maximum right lateral movement = ___ mm.

6.e. Add measurements 1a and 2 to obtain maximum protrusion = ___ mm.

6.f. Add totals 6c and 6d to obtain total lateral movement (from right to left) = ___ mm.

Are you surprised that you can move your mandible farther from side to side than you can move it directly forward? Usually, your jaw can move about twice as far sideways (laterally) as it can protrude or move directly forward. Compare the results of your own jaw movement capability with that of 796 dental hygienists and 318 dental students in Table 9-3.

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