In vitro strain measurements in the condylar process of the human mandible

Although there have been a number of experimental studies of temporomandibular joint loading, the precise relation between condylar load and condylar strain is incompletely understood. This in vitro study determined the magnitudes and directions of the principal components of strain on the four surfaces of the condylar process of human cadaver mandibles during loading with selected simulated muscle forces, with simultaneous measurement of occlusal and joint forces. Rosette strain gauges were placed on each of the four surfaces and the mandibles were loaded by a load cell to simulate the action of the masseter and medial pterygoid muscles. Force and strain values were measured at five different bite positions and nine different positions of the resultant muscle force. Forces and strain values were highest when the resultant muscle force was closest to the joint and the bite position was furthest from the joint. The ratios of bite force to joint force and the ratios of forces between the two joints conformed to theoretical predictions of many previous models, with the balancing-side joint being loaded more heavily than the working-side joint. At all gauge positions the maximum principal strain was tensile, the minimum principal strain was compressive, and the absolute strain values were correlated with the magnitude of the force on the condyle. However, under the chosen loading regimen, the ratio of compressive to tensile strain differed among the four surfaces. The highest levels of tensile strain occurred on the anterior and lateral surfaces and the highest compressive strain occurred on the posterior surface. It was not possible to detect significant changes in strain patterns due to bite position or muscle force position, that were independent of the force magnitudes.