**7.5 Loads at the TMJ**

Studies about whether the TMJ is subjected to load has been the subject of discussion for many years. Brehnan et al. in 1981 was able to corroborate in his studies carried out on monkeys that there is a load in the TMJ. It's accepted that mechanical loading is essential for growth [11]. During the natural function of the joint, a combination of compressive, tensile, and shear loads occur [5]. The efforts produced by the loads will generate a deformation which can be quantified by determining the change between the original length with the final length of a structure, this deformation is expressed as a percentage, there are two types of deformation: elastic

**Figure 2.** *Graph shows that the elastic limit and the maximun resistance.*

one in which eliminating the force the material recovers its original dimension, while plastic deformation is one in which the original dimension is not recovered. The elastic limit es the yield point beyond which permanent deformation occurs and the tissue does not return to its original shape. Ultimate strength is the stress a tissue can withstand, and breaking strength is the stress at which the tissue breaks (**Figure 2**) [20].

The value of the maximum resistance of the disc depends on the direction of the applied stress and the region where it is applied. For example, the ultimate strength of the intermediate zone of the disc is 37.4 MPa (1 MPa = 106N/m2) when a tensile stress is applied anteroposteriorly, while it is 1.6 MPa when the application of stress is medio-lateral [11].

During compressive loading the disk becomes smaller, during tensile loading, it is stretched in the direction of loading, during shear loading, one edge of the disk surface moves parallel to the adjacent surface (**Figure 3**) [16]. Therefore, an unloaded TMJ may show degenerative changes, which may lead to impaired masticatory function. However, an excessive load that exceeds the adaptive capacity can also lead to degradation of the joint structure [11]. If the surfaces of the condyle or fossa have significant bony irregularities, the distribution of force over an even smaller square area of the joint can make these ratios more diverse and destructive. Otherwise, an aging dysfunctional disc/capsule does not have the necessary viscoelastic properties to meet the functional demands of the TMJ [17].

Any surgical procedure must restore functional congruence between all four joint surfaces. Any intervention must limit the instability of the joint to eliminate the progressive influence of torque and shear at the lateral attachment of the disc/capsule to the mandibular condyle. Currently, no synthetic or biological material meets the viscoelastic properties disk/capsule Knowledge of biomechanics will guide the clinician in making decisions for the surgical treatment of TMJ.
