Ligaments of the Masticatory System
The ligaments of the masticatory system, as in all other freely movable joints, have three main functions: stabilization, guidance of movement, and limitation of movement. From a functional view, limitation of movement is the most important function (Mankin and Radin 1979, Osborn 1995). There are different interpretations concerning the number and nomenclature of the ligaments found within the masticatory system (Sato et al. 1995). Five or six ligaments have been described: lateral ligament, stylomandibular ligament, sphenomandibular ligament, discomalleolar (Pinto’s) ligament and Tanaka’s ligament. Sometimes the collateral attachment fibers between disk and condyle are included in the list as the lateral and medial collateral ligaments of the disk (Yung et al. 1990, Kaplan and Assael 1991, Okeson 1998), although from a functional viewpoint, this is not accurate.
The lateral ligament or temporomandibular ligament is made up of two parts: a deep, more horizontal part and a superficial, more vertically oriented part (Arstad 1954, Sicher and DuBrul 1975, Kurokawa 1986). The horizontal part limits retrusion (Hylander 1992) as well as laterotrusion (DuBrul 1980) and thereby protects the sensitive bilaminar zone from injury. The vertical part of the lateral ligament, on the other hand, limits jaw opening (Osborn 1989, Hesse and Hansson 1988). The superficial portions of the lateral ligament contain Golgi tendon organs (Thilander 1961). These nerve endings are very important for the neuromuscular monitoring of mandibular movements (Hannam and Sessle 1994, Sato et al. 1995). For this reason, anesthetizing the lateral portion of the joint permits a 10-15% increase in jaw opening (Posselt and Thilander 1961).
Lateral ligament
63 Situation with jaws closed
Unlike in formalin-fixed preparations, the lateral ligament (arrows) is usually clearly distinguishable in fresh preparations. The initial rotation during an opening movement is limited by the superficial part of the lateral ligament (von Hayek 1937, Burch and Lundeen 1971). Further opening of the jaws can occur only after protrusion has relieved tension on the ligament, following which the ligament is again stressed by renewed rotation (Osborn 1989).
64 Situation with jaws open
jaw opening is restricted by the length of the lateral ligament from its origin to its insertion. However, if the condyle can slip past the apex of the tubercle (eminence), the ligament (arrows) will no longer have this limiting effect. In addition, the lateral ligament will now impede retrusive and laterotrusive movements of the condyle (Posselt 1958. Brown 1975, Osborn 1989).
65 Function and structural adaptation of the ligaments
The chief function of the ligaments is to limit movement and thereby protect sensitive structures. In addition, they stabilize the joint and take over guidance functions (Rocabado and Iglarsh 1991). Depending upon the proportions of the types of collagen within the ligament and the direction of the functional overload on the joint, ligaments may become either stretched or shortened.
The stylomandibular ligament is a part of the deep fascia of the neck and runs from the styloid process to the posterior edge of the angle of the mandible. While part of the ligament inserts onto the mandible, its largest part radiates into the fascia of the medial pterygoid muscle (Sicher and DuBrul 1975). Although the stylomandibular ligament is relaxed during jaw opening, it restricts protrusive and mediotrusive movements (Burch 1970, Hesse and Hansson 1988). Even so, it should prevent excessive upward rotation of the mandible (Burch 1970), which sometimes causes problems in patients with a significantly reduced vertical dimension.
Stylomandibular ligament
66 Situation with jaws closed
Lateral view of a macroscopic anatomical preparation approximating the habitual condylar position. The ligament runs from the styloid process (1) to the posterior border of the angle of the jaw. In this mandibular position the ligament (arrows) is essentially free of tension. Chronic nonphysiological loading (Fig. 68) can lead to insertion tendinosis (Ernest syndrome; Brown 1996).
67 Situation during rotational jaw opening
Preparation shown in Figure 66 after the initial opening rotation. Rotational movement of the condyle against the articular protuberance causes a relaxation of the ligament (arrows). With further rotational opening, the angle of the jaw would swing farther posteriorly and allow even more slack in the ligament.
68 Situation during translation
Same preparation following anterior translation (= protrusion). Anterior translational movements in the temporomandibular joint always increase tension in the ligament (arrows). This helps to protect more sensitive structures (such as the superior stratum) from overextension during protrusion. Excessive closing rotation of edentulous jaws can likewise produce tension in the ligament.
The sphenomandibular ligament has its sole origin on the sphenoidal spine in only about one-third of patients (Burch 1966). In the majority of individuals it also inserts into the medial wall of the joint capsule, in the petrotympanic fissure or on the anterior ligament of malleus (Cameron 1915, Loughner et al. 1989, Schmolke 1994). By means of its insertion on the lingula of the mandible, the sphenomandibular ligament limits protrusive and mediotrusive movement (Langton and Eggleton 1992) as well as passive jaw opening (Hesse and Hansson 1988, Osborn 1989). The importance of the sphenomandibular ligament to the physiology of movement is negligible in comparison with the previously described ligaments (Williams et al. 1989), as is confirmed by the lack of related clinical symptoms.
The discomalleolar ligament (= Pinto’s ligament) was described by Pinto (1962) as a connection between the malleus and the medial wall of the joint capsule. However, a separate ligament can be demonstrated here in only 29% of temporomandibular joints (Loughner et al. 1989).
Tanaka’s ligament represents a cord-like reinforcement of the medial capsule wall, similar to the lateral ligament (Tanaka 1986.1988).
Sphenomandibular ligament
69 Situation at the habitual condylar position
Macroscopic anatomical preparation displaying the left sphenomandibular ligament from the medial. The ligament runs from the spine on the underside of the sphenoid bone (spina ossis sphenoidalis) to the lingula of the mandible (Rodriguez-Vazquez et al. 1992). With the jaws in this position the ligament (arrows) is essentially relaxed. Besides the ligament, a section of the lateral pterygoid muscle can be seen.
70 Situation during opening rotation
Same preparation after opening rotation. As long as the condlye is rotating against the articular protuberance without leaving the fossa, the ligament (arrows) becomes progressively more relaxed. Only after translation begins does the ligament re-acquire the same degree of tension it had when the jaws were closed.