71 Situation during translation
Macroscopic anatomical preparation after anterior translation (= protrusion). In synergy with the stylomandibular ligament, tension in this ligament is increased as translation progresses. The stylomandibular and sphenomandibular ligaments together restrain protrusive and mediotrusive movements. If a pain-producing lesion in one of these ligaments is suspected, passive movements must be used to test the ligaments.
Arterial Supply and Sensory Innervation of the Temporomandibular Joint
The arterial blood supply of the temporomandibular joint is provided primarily by the maxillary artery and the superficial temporal artery (Boyer et al. 1964). Both of these arteries are also the principle supply for the muscles of mastication. Apart from the network of arteries surrounding it, the condyle is also supplied from the inferior alveolar artery through the bone marrow (Okeson 1998). The venous drainage is through the superficial temporal vein, the maxillary plexus, and the pterygoid plexus.
The sensory innervation of the joint capsule and its receptors has already been addressed briefly on page 27. The temporomandibular joint is innervated predominantly by the auriculotemporal, masseter, and temporal nerves (Klineberg et al. 1970, Harris and Griffin 1975). Proprioception occurs through four types of receptors (Thilander 1961, Clark and Wyke 1974, Zimny 1988): Ruffini mechanoreceptors(type I), pacinian corpuscles (type II), Golgi tendon organs (type III), and free nerve endings (type IV). These receptors are located in the joint capsule, the lateral ligament, and in the bilaminar zone and its genu vasculosum. The anteromedial portion of the capsule contains relatively few pain receptors, of type IV (Thilander 1961).
72 Arterial supply
Diagram of the arterial blood supply of a left temporomandibular joint (modified after Voy and Fuchs 1980). The condyle is supplied with blood from all four sides. In addition, there are anastomoses with the inferior alveolar artery within the marrow spaces. Because of the abundant blood supply, avascular necrosis is rarely found in the condyle (Hatcher et al. 1997). Compression of the anterior vessels by anterior disk displacement (Schell-has et al. 1992) will not interfere with the condyle’s blood supply.
73 Sensory innervation of a left temporomandibular joint
The afferent nerve fibers arise from the mandibular branch of the trigeminal nerve and exhibit four types of nerve endings. In rats, fret nerve endings (type IV), which are potential pain receptors, have beer found in the capsule, lateral ligament, bilaminar zone, and in the pars anterior and pars posterior o the disk (Ichikawa et al. 1990, Kido et al. 1991). This has not been verified for human disk structures however.
74 Innervation in the capsule region
Schematic diagram of the different areas of innervation (modified from Ishibashi 1974, Schwarzer 1993) Activation of the type-IV receptor in the capsule increases the activity of sympathetic efferent fiber (Roberts and Elardo 1985). Because of the sympathetic innervation o the intrafusal muscle fibers (Grass et al. 1993), a secondary rise ii muscle tone is brought about by activation of the afferent fibers a the muscle spindles and the efferent α-motoneurons (Schwarze 1993).
Sympathetic Innervation of the Temporomandibular Joint
The sympathetic innervation of the temporomandibular joint comes from the superior cervical ganglion (Biaggi 1982, Widenfalk and Wiberg 1990). Neurons with the neuropeptides CGRP (calcitonin gene-related peptide) and SP (substance P), that are associated with the sensory nervous system, are found chiefly in the anterior part of the joint capsule (Kido et al. 1993). Sympathetic fibers containing neuropeptide A (NA), Y (NPY), or VIP (vasoactive intestinal peptide) are more numerous in the posterior part of the joint. The ratio of sympathetic to sensory nerve fibers is approximately 3:1 in the temporomandibular joint Schwarzer 1993). Sympathetic neurons serve primarily for monitoring the vasomotor status. This monitoring allows optimal adjustment of the blood volume in the genu vasculosum during excursive and incursive condylar movements. There is evidence that, in addition to the vasomotor effect, the sympathetic nervous system also plays a role in pain perception (Roberts 1986, Jähnig 1990, McLachlan et al. 1993). Both NA and SP effect the release of prostaglandins, which heighten the sensitivity of pain receptors (Levine et al. 1986, Lotz et al. 1987).
75 Effects of the sympathetic nervous system on the temporomandibular joint
Certain neuropeptides can increase the sensitivity of nociceptors and thereby directly influence pain perception. Special importance has been attributed to the neuropeptides SP and CGRP in the production of synovial cells (Shimizu et al. 1996). Bone remodeling processes are likewise guided by neuropeptides. A nonphysiological increase of pressure within the genu vasculosum caused by a sympathetic or hormonal malfunction during incursive condylar movement results in an anteriorly directed force on the articular disk (Ward et al. 1990. Graber 1991) and this can contribute to an anterior disk displacement. (Revised drawing from Schwarzer 1983.)
76 Afferent paths of the trigeminal nerve and conections of neurons in the brain stem
Schematic representation of the interrelationships between afferent fibers of the trigeminal nerve and the so-called nociceptive specific (NS) neurons and/or the wide dynamic range (WDR) neurons (Dubner and Bennett 1983, Sessle 1987a. b) in the region of the cervical spinal column. The specific connections (A-D) in the sensory trigeminal nucleus in the brain stem can result in identical perceptions in the cortex regardless of where the pain was first perceived.
Muscles of Mastication
Anatomically the muscles of mastication can be divided into simple and complex muscles (Hannam 1994, 1997). The lateral pterygoid and the digastric muscles are counted among the simple muscles. These muscles work through a favorable lever arm relative to the joint and so do not have to produce a great deal of force to bring about functional mandibular movements. The parallel muscle fibers in these muscles have their sarcomeres arranged in series, and these are responsible for the adequate muscle contraction. During contraction, the diameter of each muscle increases and is at its greatest near the midpoint of the muscle.
In contrast, the complex muscles include the temporal, masseter, and medial pterygoid muscles with their many aponeuroses and varying sizes. During function the aponeuroses can shift and become deformed (Langenbach et al. 1994). The muscle fibers in this group run obliquely and increase their angle to one another during contractions. A complex muscle can produce a force of approximately 30-40 N per cm2 of cross-section (Korioth et al. 1992, Weijs and van Spronsen 1992). The orientation of the muscle fibers and their facultative activation during various mandibular movements is one of the reasons that muscle symptoms can be reproducibly provoked by loading in one certain direction but not in others. Although there are recurrent principles in muscle architecture (Hannam and McMillan 1994), variations in the areas of muscle attachment and differences in intramuscular structure have an effect on craniofacial development (Eschler 1969, Lam et al. 1991, Tonndorf 1993, Holtgrave and Müller 1993, Goto et al. 1995).
The motor units in the muscles of mastication are small and seldom extend beyond the septal boundaries (Tonndorf and Hannam 1997). The “red” muscle fibers (with higher myoglobin content) contract slowly. They maintain postural positions and are slow to become fatigued. The “white” fibers (with lower myoglobin content) have fewer mitochondria and can contract more rapidly, but they fatigue sooner because of their predominantly anaerobic metabolism. The muscles of mastication are composed of varying mixtures of fibers of types I, IIA, IIB, IIC, and