Predicting the outcome of twin block functional appliance treatment: a prospective study.

A prospective study was undertaken to investigate the relationship between various measured pre-treatment parameters and the reduction in overjet achieved when using a twin block functional appliance. Forty-three subjects were fitted with a twin block functional appliance, and a number of pre-treatment clinical and radiographic morphological features were recorded. The functional appliance wear was monitored for 6 months and any individual who did not co-operate with wear was excluded from the subsequent analysis. Multiple regression analysis with stepwise inclusion was used to relate the percentage reduction in overjet achieved by functional appliance wear to any of the pre-treatment parameters. The data from 22 individuals was included in the final analysis. The overbite and SNB angle were the most strongly related variables to percentage reduction in overjet. These were then used to construct a predictive equation for the expected percentage reduction in overjet: Percentage reduction in overjet in 6 months = 132 + 4.9x1 - 1.4x2, where x1 = overbite and x2 = SNB. The pre-treatment overbite was, in isolation, the most influential feature in predicting the percentage of overjet reduction.