Estimation of an Optimal Boot Cast Height: An Experimental Study

Background: Application of a lower leg cast is a very common method for treating injuries involving the foot and ankle, such as fractures and sprains. This study goal was to determine the optimal cast height for providing sufficient immobilization for various kinds of lower leg injuries.

Methods: This study was designed to prospectively assess the effect of cast height on ankle range of motion (ROM). Ten healthy individuals (nine men and one woman) were included. A standard plaster of paris cast was applied to the level of the knee and sequentially shortened. Sagittal ankle ROM was measured by a portable fluoroscopy machine.

Results: The mean sagittal ankle ROM was minimal with the cast at maximum length (13.8 degrees, range to 24) and maximal without the cast (60 degrees, range 45 to 74). Shortening of the cast halfway between the ankle and the knee resulted in an increase in the mean sagittal ROM of only 10 degrees (from 13.8 degrees to 23.3 degrees). Further shortening of the cast resulted in a dramatic increase in sagittal ROM (to 60 degrees).

Conclusion: Lower leg casts should be applied to a maximum length in order to achieve maximal immobilization of the ankle. Short lower leg casts, i.e., up to one-half the length between the ankle and the knee, provide reasonable immobilization of the ankle with only a 10-degree increase in sagittal ROM.