Clinical measurement of distally directed headgear loading

A method was developed to measure the distally directed load that is delivered to molar teeth by orthodontic headgear. Measurements on a three-dimensional model and on four patients showed that the intraoral load at the maxillary molar teeth was generally the same as or greater than the measured extraoral force. Higher loadings on the molar teeth were thought to be caused by lever arms present in the headgear assemblies. Both the power arm face-bow and the symmetrical face-bow were effective in delivering an unbalanced load when one arm was bent well outward. The unilateral effect was decreased as the extraoral force was increased. Bending of the face-bow by the stronger extraoral forces probably was responsible for the decreased unilateral effect. The effect of lever arms in the headgear assembly on the load delivered to the teeth warrants further study.