Influence of unilateral disc displacement on the stress response of the temporomandibular joint discs during opening and mastication

Abstract

The temporomandibular joint plays a crucial role in human mastication acting as a guide of jaw movements. During these movements, the joint is subjected to loads which cause stresses and deformations in its cartilaginous structures. A perfect balance between the two sides of the joint is essential to maintain the physiological stress level within the tissues. Therefore, it has been suggested that a derangement of the joint is a contributing factor in the development of mandibular asymmetry, especially if problems of the temporomandibular joint start in childhood or adolescence. To analyze the movement of the mandible and the stresses undergone by the discs, two finite element models of the human temporomandibular joint including the masticatory system were developed, one corresponding to a healthy joint and the other with a unilateral anterior disc displacement with their movement controlled by muscle activation. A fibre-reinforced porohyperelastic model was used to simulate the behaviour of the articular discs. The stress distribution was analyzed in both models during free opening and closing, and during the introduction of a resistant force between incisors or molars. It was found that a slight unilateral anterior disc displacement does not lead to mandibular asymmetry but to a slight decrease of the maximum gape. With the introduction of a restriction between incisors, the maximum stresses moved to the anterior band in contrast to what happened if the restriction was imposed between molars where maximum stresses were located more posteriorly. Finally, the presence of a unilateral displacement of the disc involved a strong change in the overall behaviour of the joint including also the healthy side, where the maximum stresses moved to the posterior part.

Fig. 1

Finite element model of the skull with the details of the two joints. The left joint was modified for the case of a unilateral anterior disc displacement.

Fig. 2

Connector elements defined to represent the masticatory muscles. Openers: digastric and lateral pterygoid; closers: temporalis, masseter and medial pterygoid.

Fig. 3

CASE 1: Magnitude of the displacement for the healthy and displaced joints.

Fig. 4

CASE 1: Maximum and minimum principal stresses in the right and left discs of the healthy and diseased joint at maximum open position. Results shown for 100% activation.

Fig. 5

CASE 2: Opening and closing movement of the mandible in the healthy joint. Results shown for 1% of activation of the closing muscles.

Fig. 6

Minimum principal stresses in the right disc for the three cases analyzed (without restriction, restriction between incisors and restriction between molars) at the middle of the second step (closing phase).

Fig. 7

Absolute maximum stresses in the posterior band (PB), intermediate zone (IZ) and anterior band (AB) without restriction (CASE 2), with a restriction between incisors (CASE 3) and between molars (CASE 4) (Positive values indicate tension, and negative values indicate compression).

Fig. 8

Maximum absolute stresses in the different parts of the disc for the healthy disc depending on the restriction; I: between incisors, L: between molars in the left side, R: between molars in the right side (positive values indicate tension, negative values indicate compression).

Fig. 9

Maximum absolute stresses in the different parts of the disc for the pathologic disc depending on the restriction; I: between incisors, L: between molars in the left side, R: between molars in the right side (positive values indicate tension, negative values indicate compression).