2-1 Some of the components of the TMJ. A, articular eminence; C, condyle; D, articular disc; E, external auditory meatus; L, superior and inferior lateral pterygoid muscles; R, retrodiscal tissue (posterior attachment); S, thin superior wall of the glenoid fossa.
Fig 2-2 (a) In a dysfunctional joint with an internal derangement, the disc is displaced anterior to the condyle at the intercuspal position. (b) After initial rotational opening, the condyle is still posterior to the disc. (c) In translation of the mandible to maximum opening, the condyle recaptures the disc, clicking into position as it does.
Fig 2-3 (a) In a healthy joint, the condyle is in a superoanterior position in the fossa with the articular disc interposed when the teeth are in maximal intercuspation. (b) In the initial stage of opening, the condyle rotates in position, with the disc remaining stationary. (c) In maximum opening, the condyle translates forward, with the disc still interposed.
Fig 2-4 The mandible moves on a horizontal axis, as seen in a hinge axis opening.
Fig 2-5 Mandibular movement occurs around a vertical axis during a lateral excursion.
Fig 2-6 The mandible also rotates around a sagittal axis when one side drops down during a lateral excursion.
Mandibular Movement
Mandibular movement can be broken down into a series of motions that occur around three axes:
1 Horizontal axis (Fig 2-4): This movement, in the sagittal plane, happens when the mandible in centric relation makes a purely rotational opening and closing border movement around the transverse horizontal axis, which extends through both condyles.
2 Vertical axis (Fig 2-5): This movement occurs in the horizontal plane when the mandible moves into a lateral excursion. The center for this rotation is a vertical axis extending through the rotating or working-side condyle.
3 Sagittal axis (Fig 2-6): When the mandible moves to one side, the condyle on the side opposite the direction of movement travels forward. As it does, it encounters the eminence of the glenoid fossa and moves downward simultaneously. When viewed in the frontal plane, this produces a downward arc on the side opposite the direction of movement, rotating around an anteroposterior (sagittal) axis passing through the other condyle.
Various mandibular movements are composed of motions occurring concurrently around one or more of the axes. The up-and-down motion of the mandible is a combination of two movements. A purely hinge movement occurs as the result of the condyles rotating in the lower compartments of the TMJs within a 10- to 13-degree arc, which creates a 20- to 25-mm separation of the anterior teeth (see Fig 2-3b). This phenomenon was the basis for the terminal hinge axis theory in the early 1920s by McCollum.3 Kohno verified the presence of a transverse horizontal axis, which he termed the kinematic axis.13 There is also some gliding movement in the upper compartment of the joint if the mandible drops down farther (see Fig 2-3c). Then the axis of rotation shifts to the area of the mandibular foramen, as the condyles translate forward and downward while continuing to rotate.
When the mandible slides forward so that the maxillary and mandibular anterior teeth are in an end-to-end relationship, it is in a protrusive position. Ideally, the anterior segment of the mandible will travel a path guided by contacts between the anterior teeth, with complete disocclusion of the posterior teeth (Fig 2-7).
Mandibular movement to one side will place it in a working, or laterotrusive, relationship on that side and a nonworking, or mediotrusive, relationship on the opposite side; eg, if the mandible is moved to the left, the left side is the working side and the right side the nonworking side (Fig 2-8). In this type of movement, the condyle on the nonworking side will arc forward and medially (see A in Fig 2-8). Meanwhile, the condyle on the working side will shift laterally and usually slightly posteriorly (see B in Fig 2-8).
Fig 2-7 A protrusive movement occurs when the mandible moves forward.
Fig 2-8 When the mandible moves into a left lateral excursion, the right condyle (A) moves forward and inward, while the left condyle (B) will shift slightly in a lateroposterior direction. In this example, the left side is the working side (W), and the right side is the nonworking side (NW).
Fig 2-9 In the nonworking condyle (NW), the traditional Bennett angle (SCB) measures the angle from the sagittal plane to the endpoint of the movement of the condyle center. The Bennett angle used in articulators with an immediate lateral translation capability (S’PB) is measured from the sagittal plane after the immediate or early lateral translation (L) has occurred. The transverse horizontal axis (THA), or hinge axis of purely rotational movement, extends through both condyles. The working side condyle (W) slides laterally, or outward, in laterotrusion.
The bodily shift of the mandible in the direction of the working side was first described by Bennett.14 The angle formed in the horizontal plane between the pathway of the nonworking condyle, the mandibular lateral translation, and the sagittal plane is called the Bennett angle (Fig 2-9). The presence of an immediate or early lateral translation, or side shift, has been reported in 86% of the condyles studied.15 In addition to confirming the predominant presence of the early lateral translation, Lundeen and Wirth, using a mechanical apparatus, showed its median dimension to be approximately 1.0 mm with a maximum of 3.0 mm.16 Hobo and Mochizuki, using an electronic measuring device, found a lower mean value of 0.4 mm for the immediate lateral translation, with a high of 2.6 mm.17,18
Following the immediate lateral translation, there is a further gradual shifting of the mandible, or progressive lateral translation, which occurs at a rate proportional to the forward movement of the nonworking condyle.19 At one time, this was known as progressive side shift or Bennett side shift. Lundeen and Wirth found slight variation in the direction of the progressive lateral translation or Bennett angle, with a mean value of 7.5 degrees.16 Hobo and Mochizuki found a much greater variation, ranging from 1.5 to 36 degrees, with a mean value of 12.8 degrees.17,18
Determinants of mandibular movement
The two condyles and the contacting teeth are analogous to