[Criterias for the selection of neckstraps in therapy with extraoral forces]

1. Considerable differences could be measured in the force-deflexion-diagram whilst using different neckstraps. These differences could be explained by the different material qualities. 2. Force-deflexion-diagramm achieved in vitro could not be transfered directly to the therapy of patients. 3. Extreme asymmetrical force distributions at the outer bows will arise during the therapy with extraoral forces, because of the friction between skin, textile neckstrap and adjustable traction bands. These asymmetrical force distributions will arise particularly during functional movements of the head. Turnings and noddings lead to extreme values, by reaching the maximum asymptotically. 4. The use of a force snap-away neckpad also often leads to asymmetrical force distributions at the outer bows. 5. Asymmetries occur less often whilst using the low-cervical-pull according to Sander, due to the reduced friction between the two plastic parts of this headgear system. Head movements nearly always lead to an equalization of the forces at the outer bow. Extreme forces last less than one second. The mean value of the force at the outer bow, ascertained over a longer period, is only a little bit less than the force of 500 g adjusted at the beginning. 6. The mean value of the forces measured over a longer period while wearing an adjustable traction band is nearly 50% less than the force adjusted at the beginning. 7. The adjustment of the force at the outer bows is done while the patient is sitting in an upright position. However the patient is normally very seldom in such a position so that, as a rule, the forces at the outer bows are less than the adjusted force. 8. The problem of lesser forces at the outer bows can be compensated by a longer wearing period and by a higher initial force adjustment. 9. The adjustment of the force at the outer bow with the help of a spring-balance leads to inaccuracy and, as rule, the real force application to the molar is significantly less. 10. Only the wearing of the low-cervical-pull according to Sander leads to reproducible force values during head turning and noddings. Also the mean force value corresponds to the initial applied force. 11. While using an adjustable traction band it is advisable to change this often during the treatment. Otherwise the function of the treatment with the extraoral force will be reduced because of the loss of elasticity of the traction band.