THE CAST INDEX: A SIMPLE RADIOLOGICAL PREDICTOR OF PLASTER CAST FAILURE IN PAEDIATRIC DISTAL FOREARM FRACTURES

Abstract

Introduction: We aimed to assess a simple radiological method of predicting redisplacement of paediatric forearm fractures. The Cast Index (CI) is the ratio of sagittal to coronal width from the inside edges of the cast at the fracture site. A CI of > 0.7 was used as the standard in predicting fracture redisplacement. The cast index has previously been validated in an experimental study.

Methods: Case records and radiographs of 1001 children who underwent a manipulation under general anaesthesia for a displaced fracture of the distal forearm were studied. Redisplacement was defined as more than 15 degrees of angulation and/or more than 80 percent of translational displacement on check radiographs at 2 weeks. Angulation (in degrees) and translation displacement (in percentage) were measured on the initial and check radiographs. The CI was measured on postoperative radiographs.

Results: Fracture redisplacement was seen in 107 cases at 2 week follow up. Of the 752 patients (75%) with a CI of less than 0.7 the displacement rate was 5.58%. Of the 249 patients (25%) with a CI greater than 0.7 the redisplacement rate was 26%. The CI was significantly higher in the redisplacement group. No statistically significant difference was seen for age, sex or ethnicity. Nor were statistical differences noted in initial angular deformity, initial displacement and seniority of the surgeon. Good intra and inter observer reproducibility was observed. There was no statistical difference in patients with a cast index between 0.7 and 0.8.

Conclusion: The cast index is a simple and reliable radiographic measurement to predict the redisplacement of forearm fractures in children. Previous studies have used a CI of > 0.7 as the predictor of redisplacement although this study suggests a plaster with a CI of < 0.81 is acceptable. A high cast index is associated with redisplacement of fractures and should therefore be considered when moulding casts in distal forearm fractures.