The influence of maxillary incisor inclination on arch length

This ex vivo study was designed to investigate Andrews' hypothesis that there is a space implication when incisors are torqued correctly. A working model was constructed to allow acrylic typodont incisors of varying known values of inclination to be substituted into the model. The arch lengths of the various 'set-ups' were measured using a reflex microscope linked to a PC. In order to quantify the space requirement of clinical relevance for adequate incisor torque, the method was repeated by substituting replicas of patients' 'natural' incisors. For both acrylic and natural incisors it was found that, as the inclination of the teeth increased, there was an increase in all arch lengths, this being greater for the natural incisors. This larger increase for the natural incisors was related not only to their increased size, but was also dependent on the morphology of the incisor. Those incisors which were parallel-sided showed the greatest increase in arch length, whereas the incisors that were relatively triangular in shape showed the smallest increase. When the inclination of an 'average' set of 21/12 is increased by 5 degrees, an increase in the arch length of approximately 1 mm may be expected.