We present an objective method for predicting the redisplacement of paediatric forearm and wrist fractures. Novel radiographic measurements were defined and their value assessed for clinical decision making. In Phase I of the study we defined the cast index and padding index and correlated these measurements with the incidence of fracture redisplacement. Phase II assessed these indices for their value in clinical decision making. Cast Index (a/b) is the ratio of cast width in lateral view (a) and the width of the cast in AP view (b). Padding Index (x/y) isthe ratio of padding thickness in the plane of maximum deformity correction (x) and the greatest interosseous distance (y) in AP view. The sum of cast index and padding index was defined as the Canterbury Index. In Phase I, 142 children's radiographs were analysed and a statistically significant difference was identified between redisplacement and initial complete off-ending of the bones, cast index > 0.8 and padding index of > 0.3. There was no significant association with age, fracture location, seniority of surgeon or angulation. In Phase II, radiographs of 5 randomly selected cases were presented to 40 surgeons (20 consultants & 20 registrars). Following an eyeball assessment they were asked to measure the cast index and padding index (after instruction). With eyeballing the consultants predicted 33% and registrars 25% of the cases that redisplaced. After learning to measure the indices the accuracy increased to 72% for consultants and 81% for registrars (p<0.001). We conclude that the cast index, padding index and Canterbury Index are validated tools to assess plaster cast quality and can be used to predict redisplacement of paediatric forearm fractures after manipulation. They can easily be taught to orthopaedic surgeons and are more accurate than eyeballing radiographs in the clinical setting. Redisplacement can be predicted if cast index > 0.8, padding index > 0.3 and Canterbury Index > 1.1