The present results indicate that the 2 treatment strategies using splint therapy combined with OA, FP+OA and MRA+OA, are effective in reducing TMJ sounds (both clicking and crepitation) as long as they provide bilateral contact balance between the incisal edges of the lower incisors and the palatal surface of the upper incisors in the protrusive movement of the mandible. This finding is consistent with previous studies reporting that occlusion has an influence on the anterior region, resulting in TMJ disk placement.
18-20
Occlusal interferences that occur in protrusion have been found to be positively related to clicking, and occlusion is frequently cited as a major cause of TMD—with TMJ sounds being one of the TMD-related signs.
18,19,21 However, studies examining factors related to incisal overbite and overjet, posterior crossbite, and lateral excursions based on retruded contact position to maximum intercuspation suggest that, in patients with TMD, occlusal factors may be less important than previously believed.
22,23
Multilocular bone cyst, leading to surface irregularities in the posterior part of the left eminence of the temporal bone, has also been reported as a factor that may cause TMJ clicking.
24 In 2 studies, the Research Diagnostic Criteria for Temporomandibular Disorders were used to distinguish between anterior disk displacement with reduction and symptomatic hypermobility during protrusive mouth opening and closing; in both studies, the authors found it challenging to make such a distinction with any certainty.
25
The lateral pterygoid muscle is responsible for traction on the disk-condyle complex, thus displacing the disk and causing joint sounds,
5 which is related to the anatomy of the TMJ as reported in the present study. However, in many studies this muscle has not been palpated.
26 In a study investigating the relationship between TMJ disk displacement and osteoarthrosis, magnetic resonance images of the TMJ showed no influence of body length or mandibular height on the occurrence of disk displacement.
27 However, magnetic resonance images have shown that there is a relationship between muscle tension, disk anatomy, and presence of joint sounds.
26,28
For the management of TMJ sounds, studies have used stabilization splint therapy, often combined with OA and restorative treatment.
11 Other studies have interrelated TMJ sounds with occlusion during the protrusive movement of the mandible.
20,29
In the present study, the need for OA in the protrusive movement of the mandible can be explained by a force diagram schematically describing the occlusal forces applied to the teeth,
30 as shown in
Figure 3. The simultaneous bilateral contact of the lower incisors in protrusion with the palatal surface of the upper incisors generates equal forces (F) on both sides. Conversely, if unilateral rather than bilateral contacts occur between the lower and upper incisors, they will generate asymmetric force components (F1 and F2) that run perpendicular to the midline and do not cancel each other out, as demonstrated by the force diagram schematically shown in
Figure 4. The resultant force F2 starts at point O, which is the left-side TMJ of a patient with clicking, and ends at point N, coinciding with the right-side central incisor and being higher than the resultant force F1. Both forces (F1 and F2) run in the same direction as the lateral pterygoid muscles. Thus, based on the direction and intensity of force F2, we can conclude that the left lateral pterygoid muscle bundle has more freedom to displace the ipsilateral TMJ disk, because the mandibular hemiarch on this side is in contact only with the condyle on the articular eminence, with no support on the incisors. This lack of support can result in potential energy left over to the left lateral pterygoid muscle, facilitating disk traction and generating joint sounds on this side because, during the protrusive movement, friction occurs only on the condyle on the left TMJ eminence—there is no friction between the teeth on this side. Therefore, according to the force diagram, the possible cause of disk displacement in this case (
Figure 4) is the lack of contact between the left-side incisors. Also, friction between the condyle, the disk, and the articular eminence is reduced on this side, requiring less effort from the ipsilateral pterygoid muscle, which, with potential energy left over, can displace the disk from the condyle surface, thus causing joint sounds (clicking in this case).
For the management of severe TMJ clicking, injection of botulinum toxin A into the lateral pterygoid muscle was able to decrease the muscle action potential.
31 Studies have also demonstrated a tendency for the percentage of patients with clicking to decrease after orthognathic surgery, but improvements in crepitation were considered questionable.
32 Schiffman et al,
33 after the application of 4 treatment strategies (medical management, nonsurgical rehabilitation, arthroscopic surgery, and arthroplasty), found that nonsurgical treatment should be used for patients with TMJ closed lock because there was no significant difference in outcomes between the examined treatment strategies.
In the present study, we focused on tooth support in the anterior region, protrusive movement, and the need to correct the bilateral balance between the anterior teeth, which was achieved by OA in centric relation and lateral excursions, facilitating the action of the muscles involved in each mandibular movement. We also highlight the importance of the occlusal contact that occurs on the opposite side of the joint sound during the protrusive movement of the mandible. Based on the observation of occlusal contacts in protrusion, we were able to accurately identify in more than 94% of cases that the joint sound was either on the right or on the left side during protrusive movements.
Both treatment strategies prioritized the concept of mutually protected occlusion, in which all jaw and TMJ movements are reciprocally interrelated and must synchronize, which may be conveniently done by using the force diagram. Additional studies on this topic should be conducted to investigate other forms of cause and effect that are associated with joint sounds in different treatment modalities.