Does a Good Cast Index Always Reflect a Good Cast?

The cast index is the most commonly used measurement of cast quality in pediatric forearm fractures, with low values representing a good quality cast and higher values suggesting poor technique. However, in some instances the cast index may not accurately reflect cast quality. The purpose of this study was to critically evaluate the quality of casts deemed "good" or "bad" based on the cast index against other measurements in order to identify these examples.

Pediatric patients casted for a distal or midshaft forearm fracture were identified from a prospectively gathered database. There were 121 casts on 97 forearms. Measurements were made on forearm or wrist x-rays to calculate the cast index, gap index, and the anatomic dimensions of the forearm at the level of the fracture. Fisher's exact test was used to assess the likelihood of a good cast index (< 0.75) to also have an acceptable gap index (< 0.15). Pearson's correlation compared the anatomic measurements of the forearm and the cast index.

There were 65 casts with a good cast index, and 58% also had an acceptable gap index (< 0.15). Among the 56 casts with a bad cast index, 20% had an acceptable gap index (p < 0.001). The anatomic measurements of the forearm and the cast index were strongly correlated (r = 0.96).

Many casts with a good cast index have an unacceptable gap index. Forearm measurements and cast index are highly correlated, suggesting that a well-molded cast may have a higher cast index due to the shape of the forearm. While the cast index is a useful tool, "good" or "bad" casts are not uniformly captured by this measurement.