Stress distribution under high-pull extraoral chin cup traction. A photoelastic study.

Birefringent materials were employed in the fabrication of a model of a human skull with distinct simulants for teeth, bone, and periodontal ligaments. The following observations were made subsequent to the application of force to this model with an extraoral high-pull chin cup: 1. Reciprocal stresses were observed in the posterior portion of the glenoid fossa and the contacting posterior part of the head of the condyle. 2. A tension coil spring representing the external pterygoid muscle applied reciprocal stresses at its insertions in the lateral pterygoid plate on the greater wing of the sphenoid and the anterior aspect of the head of the condyle. 3. Stresses were seen to radiate from the roots and apices of both the maxillary and mandibular posterior teeth. 4. A distinct concentration of stress was noted in the retromolar area of the mandible, away from the dentition, indicating a bending stress. 5. The posterior half of the sigmoid notch, below the anterior aspect of the head of the condyle, displayed a heavy band of stresses. 6. Selected sectioning of the mandible substantiated observations made from the intact hemimandible and also showed evidence of bending in the area corresponding to the head and neck of the condyle.