**7.2 Rotational movement**

Rotation occurs when the condyles rotate around a fixed point or axis during mandibular opening and closing. Rotational motion can occur in three reference planes: horizontal, vertical, and sagittal. Each of them occurs around a point called the axis [11].


vertical axis in the opposite condyle, which remains in said axis. This type of movement does not occur normally [16].

• Sagittal axis of rotation: Occurs when one of the condyles moves inferiorly while the other remains in the position of the terminal axis. This movement occurs in conjunction with other movements. Mathematical studies indicate that in this plane there is the same contact and muscle activity from one side to the other, so there are no alterations in dental occlusion that result in a joint without load [11, 16].

The amount of condylar rotation does not differ between men and women. A finding that contrasts with the greater maximum interincisal opening of men compared to women due to differences in jaw length. In fact, with the same degree of rotation, the greater the length of the mandible, the greater the opening of the mouth. Consequently, the degree of interincisal opening cannot be considered as a measure of joint mobility or laxity, unless corrected for mandibular size [8].

### **7.3 Translational motion**

Translation can be defined as a movement in which every point of the object t simultaneously has the same speed and direction. In the masticatory system, it occurs when the mandible protrudes. During normal movements, rotation and translation occur simultaneously, as the mandible rotates in one or more axes, each of the axes is changing orientation in space [16].

The total movement of the mandible does not consist only of rotation and translation. Side-to-side or eccentric bodily movement of the mandible and rotation and translation of the joints indicate that the mandible acts as a free-moving or floating; structure. Controlled by pairs of complementary and opposing functional muscle groups that gradually exert impulse force with numerous force vectors, the threedimensional movement of the mandible with a dual-operation joint system is unlike any other orthopedic system in the body [17].

Classical records analyzed mandibular movements in terms of their geometry, using mechanical systems. Posselt designed an instrument called a gnatho-tensiometer, which could record border movements in all three planes, obtaining the Posselt diagram. Currently, technology has made it possible to improve position tracking techniques and thus be able to analyze mandibular kinematics with high spatial and temporal resolution (**Figure 1**) [18].

#### **7.4 Temporomandibular joint and its relationship with occlusion**

Movement is not only guided by the shape of the bones, muscles, and ligaments, but also by the occlusion of the teeth [1]. The Glossary of Prosthodontic Terms defines occlusion as the static relationship between the chewing surfaces of the maxillary and mandibular teeth. Dental contact has to be studied from a functional perspective and a more adequate definition of occlusion would be the biological and dynamic relationship of the components of the masticatory system that determines dental relationships [19].

Occlusion comprises a wide range of topics, the biomechanics of occlusal contact between two teeth with different cusp inclinations form a complex system [16]. From a clinical point of view, TMJ changes including intracapsular exudate and joint tissue

#### **Figure 1.** *Posselt diagram.*

loss can result in occlusal changes such as anterior or posterior open bites. It is important to mention that a particular occlusal scheme is not a determinant of disease. There is no evidence to suggest that one scheme predominates over another. Group functions compared to canine guides cause less condylar displacement, this displacement is small and has no clinical significance [19].

The range of vertical movement is dictated by anterior determinants such as overbite and posterior determinants such as TMJ condylar guidance. From a biomechanical point of view, anterior versus posterior determinants have a greater influence on tooth contact due to their proximity to the teeth. On the other hand, the condylar guide will influence when the molars are in contact or close to contact during mandibular movements [19].
