Bone surface strains and internal bony pressures at the jaw joint of the miniature pig during masticatory muscle contraction

The long-standing debate on whether the jaw joint is loaded is due in part to the complexity of the factors involved, including a number of different muscles, each with a potentially unique role. This study sought to elucidat e how two major jaw muscles, the masseter and the lateral pterygoid, influe nce jaw-joint loading. Twenty five 10-month-old miniature pigs were divided into two groups, controls and pigs with the lateral capsular ligament of t he jaw joint stripped surgically; this was expected to affect loading by de stabilizing the joint. Rosette strain gauges were bonded to: (1) the latera l surface of the squamosal bone (equivalent to the squamosal portion of the temporal bone in humans) at the level of the articular eminence; (2) the l ateral surface of the condylar neck, and (3) the lateral surface of the man dibular corpus below the molar region. Semiconductor pressure transducers w ere placed underneath the surfaces of the anterior slope of the condyle and the posterior slope of the articular eminence through drilled holes. Strai ns and internal bony pressures were recorded during stimulated tetanic cont ractions of the masseter or lateral pterygoid muscles. Masseter contraction , either alone or with the contralateral muscle, caused net tension in the squamosal bone and net compression in the condylar neck. The orientations w ere approximately vertical to the occlusal plane. Masseter contraction elev ated both the condylar and eminence pressures from their resting values, Th e strains caused by lateral pterygoid contractions were much smaller than f or the masseter with the exception of the condylar location. Ipsilateral la teral pterygoid contraction decreased both the condylar and eminence pressu res from their resting values, perhaps because condylar movement altered th e contact between the joint surfaces. Surgical disruption enhanced both pre ssure changes and bone strains under either muscle contraction but their ov erall patterns were not altered. In conclusion, both strains and pressures in the jaw joint varied according to specific muscle activity. (C) 2000 Els evier Science Ltd. All rights reserved.