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 elucidate how two major jaw muscles, the masseter and the lateral pterygoid, influence jaw-joint loading. Twenty-five 10-month-old miniature pigs were divided into two groups, controls and pigs with the lateral capsular ligament of the jaw joint stripped surgically; this was expected to affect loading by destabilizing the joint. Rosette strain gauges were bonded to: (1) the lateral 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 lateral surface of the condylar neck; and (3) the lateral surface of the mandibular corpus below the molar region. Semiconductor pressure transducers were placed underneath the surfaces of the anterior slope of the condyle and the posterior slope of the articular eminence through drilled holes. Strains and internal bony pressures were recorded during stimulated tetanic contractions 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 were approximately vertical to the occlusal plane. Masseter contraction elevated both the condylar and eminence pressures from their resting values. The strains caused by lateral pterygoid contractions were much smaller than for the masseter with the exception of the condylar location. Ipsilateral lateral pterygoid contraction decreased both the condylar and eminence pressures from their resting values, perhaps because condylar movement altered the contact between the joint surfaces. Surgical disruption enhanced both pressure changes and bone strains under either muscle contraction but their overall patterns were not altered. In conclusion, both strains and pressures in the jaw joint varied according to specific muscle activity.