I read with interest the case report entitled “Severe Class II Division 1 malocclusion in an adolescent patient, treated with a novel sagittal-guidance Twin-block appliance” (Li P, Feng J, Shen G, Zhao N. Am J Orthod Dentofacial Orthop 2016;150:153-66).

In the Discussion section, the authors pointed out that Clark originally introduced a 45° inclination on the blocks in his 1982 and 1988 articles. The original Twin-block traction technique relied on extraoral traction and intermaxillary elastics from the Concorde face-bow to enhance the inclined plane effect of the 45° incline on the blocks.

A year later in 1989, Trenouth introduced the concept of forward reflex posturing using a 70° incline on the blocks to create an artificial forward bite of accommodation. This meant that the extraoral traction and intermaxillary elastics on the original Clark appliance could be dispensed with, because they were no longer necessary for the appliance to function efficiently.

Most of the time during the day is spent with the mandible in the rest position, with tooth contact only occurring during swallowing, speech, and mastication. A 70° block incline maintains the mandible in a forward position even when it is open in the rest position. This is because the hinge axis of opening is approximately 70° to the occlusal plane as demonstrated by Posselt on mandibular movement. This allows 24-hour forward posturing when the masticatory system is at rest as well as in function.

Thus, the Twin-block is more a postural appliance than a functional one. Such forward posturing has been shown to lead to distraction of the mandibular condyle from the glenoid fossa. There is a clear mechanism to explain the resulting skeletal response that is well documented in the literature. This forms the basis of the tropic premise postulated by Mew that a jaw that is constantly postured forward will grow forward.