The bones of the temporomandibular joint
The mandibular condyle
The adult mandibular condyle is roughly elliptical in shape with the largest diameter being mediolateral (Figure 2.8). There is considerable individual variation in both condylar size and angulation to the various planes, and there are often differences between the right and left sides in an individual. The mediolateral dimension varies between 13 and 25 mm and the anteroposterior dimension varies between 6 and 16 mm. The mediolateral angulation to the transverse plane is between 15° and 33° and from 0° to 48° in the horizontal plane. These dimensions vary not only between individuals but between the right and left sides in an individual.
Figure 2.8 The mandibular condyle.
(M. Ziad Al‐Ani, Robin J.M. Gray.)
The temporal bone
The articular surface of the temporal bone consists, posteriorly, of the concavity of the glenoid fossa and, anteriorly, of the convexity of the articular eminence; it extends from the anterior margin of the squamotympanic fissure to the margin of the articular eminence. The roof of this fossa is very thin, indicating that this part is not a load‐bearing area. Anteriorly, however, the articular eminence is thicker and this area, together with the disc, may be the area that bears most of the load during function (Figure 2.9).
Figure 2.9 The articular part of the temporal bone.
(M. Ziad Al‐Ani, Robin J.M. Gray.)
Innervation of the TMJ
This arises from the mandibular division of the trigeminal nerve. The auriculotemporal nerve innervates most of the TMJ mainly anterolaterally and small branches of the masseteric and deep temporal nerves supply the posterior aspect.
Vascular supply to the TMJ
Vascular supply to the TMJ is from the external carotid artery via the internal maxillary artery and the superficial temporal artery.
Mandibular (jaw/masticatory) muscles
The jaw muscles form another component of the articulatory system.
The muscles commonly symptomatic in temporomandibular disorders that are accessible for clinical examination are the masseter, temporalis, lateral pterygoid, and digastric muscles. Other muscles involved but not comprising part of the routine clinical examination are the medial pterygoid, mylohyoid, suprahyoid, infrahyoid, and cervical muscles.
Masseter muscle
This muscle originates from the anterior two‐thirds of the zygomatic arch and extends obliquely downwards to its insertion over the lateral surface of the angle of the mandible (Figure 2.10).
Figure 2.10 The masseter muscle.
(Figure courtesy of Dr Paul Rea and Caroline Morris, University of Glasgow.)
Function
There are two portions to this muscle: the superficial and the deep. The superficial masseter is one of the primary elevator muscles of the mandible during jaw closure, but, as some of its fibres are angled anteriorly, it also assists in protrusion of the mandible. The deep portion is one of the main elevators of the mandible but, as some of its fibres run posteriorly, it is active in retrusion of the mandible.
Parafunction
This muscle is active during clenching of the teeth and is frequently found to be tender at its origin and less frequently at its insertion.
Examination
The muscle is examined bimanually with one finger inside and one finger outside the mouth to palpate the origin and insertion of this muscle. It is usually tender where it inserts into bone (Chapter 3).
The temporalis muscle
This is a large, broad, fan‐shaped muscle that has its origin in the temporal fossa between the superior and inferior temporal lines, which run across the parietal bone, temporal bone, and greater wing of the sphenoid, extending forwards to the temporal surface of the frontal bone. The fibres of this muscle run in various directions and converge into a tendinous insertion which runs under the zygomatic arch and inserts into the coronoid process and anterior border of the ascending ramus of the mandible (Figure 2.11).
Figure 2.11 The temporalis muscle.
(Figure courtesy of Dr Paul Rea and Caroline Morris, University of Glasgow.)
Function
The anterior fibres of this muscle, which form its major bulk, are mainly vertical and elevators of the mandible. Progressing posteriorly along the middle and posterior parts of the muscle, the fibres become increasingly oblique and the posterior fibres are almost horizontal. The anterior fibres are elevators of the mandible. The posterior fibres retrude the mandible.
Parafunction
In parafunction, this muscle becomes symptomatic and painful, usually in the anterior part of the temple, in patients who perform the parafunctional activity of bruxism or tooth grinding.
Examination
This muscle is accessible to digital palpation only over its origin. It is usually the anterior vertical fibres that are tender to digital palpation, although the posterior horizontal fibres can on occasion also be found to be tender. The insertion of this muscle is not accessible for digital palpation (Chapter 3).
