Movement of temporomandibular joint tissues during mastication and passive manipulation in miniature 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.
Section snippets
Animals
Twenty 8-month-old Hanford miniature pigs (Sus scrofa) were obtained from Charles River, Inc. (Wilmington, MA). Radio-opaque markers were surgically implanted as described below. Eight to ten weeks later, mandibular movements and deformation of the lateral capsule were recorded. Half the animals had received (5–6 weeks before the experiment) minor surgery to disrupt the condylar attachment of the disc, as reported by Liu and Herring (2000a), but because no significant movement differences were
Results
Of the 20 pigs, 10 produced both fluoroscopic and transducer data, four fluoroscopic only and six transducer only. Table 1 shows the sample size for each experiment. Missing or rejected fluoroscopic data were due to poor head position or marker visibility. Missing transducer data were due to malfunction.
Discussion
To understand the biomechanics of the TMJ, it is essential to know how the hard and soft tissues of the joint move and deform. In this study, we present what are, to the best of our knowledge, the first detailed data on these characteristics in pigs.
Acknowledgements
This work was supported by National Institute for Dental and Craniofacial Research awards DE08513 and DE11962. We thank Kathy Rafferty, Scott Pedersen, and Chris Marshall for their help with the experiments and analysis.
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