Movement of temporomandibular joint tissues during mastication and passive manipulation in miniature pigs

Abstract

Movement is an important aspect of the biomechanics of the temporomandibular joint (TMJ). To track the relative movements of TMJ components, radio-opaque markers were implanted in the left squamosal bone, mandible and retrodiscal tissue of miniature pigs. Medial–lateral (ML) and dorsal–ventral (DV) fluoroscopic records were made 8–10 weeks later during chewing and passive manipulation. Marker movements were digitized from the videotapes. During passive manipulation, the deformation of the lateral capsule was also measured with a differential variable-reluctance transducer. The results provide new details about porcine chewing pattern, which is distinguished by a regularly alternating chewing side. During masticatory opening, the mandible had a centre of rotation (CR) well inferior to the condyle and close to the angle. In contrast, the passive opening movement showed a higher CR location close to the condylar neck, indicating a different motion from masticatory opening. The retrodiscal tissue followed the movements of the mandibular condyle during both mastication and passive manipulation. The lateral capsule elongated during ipsilateral shifts and retrusion, implying a possible role in limiting such movements. These movement characteristics provide a useful reference for studies on the TMJ using pigs.

Introduction

Anatomically (Strom et al., 1986, Bermejo et al., 1993) and functionally (Herring, 1995), the oral apparatus of the pig is similar to that of man, making it a reasonable animal model for studies on the temporomandibular joint (TMJ). Previous studies (Liu and Herring, 2000a, Liu and Herring, 2000b) have documented functional loading in the pig TMJ, but to assess its biomechanics accurately, it is also necessary to understand the movements of the mandible and the joint’s soft tissues. Therefore, our aim now was to reveal such features in the pig during mastication and passive manipulations.

The TMJs of pigs and people appear to differ in the retrodiscal region. In man, the retrodiscal tissues are highly vascular and are believed to fill the glenoid fossa as it is vacated by the condyle during jaw opening (Rees, 1954), thus compensating for the volume (Rees, 1954) and pressure (Findlay, 1964) changes generated by condylar displacement. The mechanism is thought to be venous engorgement (Zenker, 1956, Osborn, 1985, Scapino, 1991). Recent histological evidence (Wilkinson and Crowley, 1994) supports this speculation with the finding that numerous venous channels in the retrodiscal tissues can be rapidly filled or evacuated from the adjacent venous pool. However, in the pig, the retrodiscal tissues are not particularly vascular but rather are fibrofatty (Herring, 1976). In addition, the postglenoid process, a prominent structure of the human TMJ that is considered to be a bony limit for posterior mandibular movement, is absent in pigs and replaced by the fibrofatty retrodiscal tissues (Strom et al., 1986, Herring, 1976). Because of these anatomical differences, it seems possible that the pig retrodiscal tissues might have unusual movement patterns during mastication and passive manipulation.

The lateral capsule is also of interest. In man, the central part of the lateral capsule is thickened to form the temporomandibular ligament, which arises from the articular tubercle of the zygomatic process of the temporal bone (immediately lateral to the articular eminence) and ends with two insertions, one to the posterolateral neck of the condyle (oblique band), and the other, with the disc, to the lateral pole of the condyle (horizontal band) (DuBrul, 1988, Ash and Ramfjord, 1995). Whether it is universal to have a distinct ligamentous structure on the surface of lateral capsule (Piette, 1993, Savalle, 1988), and whether there are always two distinct insertions (Sato et al., 1996), are a matter of debate. But there is agreement that the lateral capsule is important in ensuring the stability of the condyle and disc during movement (Schmolke, 1994). In addition, from a biomechanical perspective, the lateral capsule can bear tensile loads and will elongate when loaded under tension. The lateral capsule in the pig is thinner than that of man, but the attachments are the same. During mastication, the lateral capsule elongates during and immediately after the power stroke of mastication and then slowly shortens back to its original length (Liu and Herring, 2000b), but it is not clear how these distortions relate to passive movements.

The range of mandibular border movement in man may be a useful index of the maximum functional potential of the TMJ and the masticatory system. Reduced passive envelopes of motion or alterations of movement patterns are considered to be signs of temporomandibular disorders (Nielsen et al., 1990, Miller et al., 1999, Yoshida et al., 1998). Typical masticatory movements are much smaller than the range of border movement and coincide with it only at occlusal contacts. In the pig, the relation between chewing movements and the passive envelope of motion is unknown. To compare them, we here examined the range of active and passive motion and estimated the position of the centre of rotation. The location of that centre reflects the relative proportion of translation to rotation, and thus can aid in distinguishing the motion patterns of passive opening from those of chewing.

Various methods have been used to locate the position or path of the centre of rotation in man during jaw opening (Grant, 1973, Sperry et al., 1982, Chen, 1998). For the present study, we used the method of Weijs et al. (1989), who investigated the position of the centre of rotation in rabbits.