An experimental study of pressure-volume dynamics of casting materials

Casting materials are commonly used in a trauma and post-operative setting in orthopaedic practice. Swelling after trauma or surgery is universal, hence, the importance of understanding the pressure-volume dynamics of various materials commonly used for casting. This study attempts to define the pressure response of casts made from three commonly used materials to increasing volume, using a cylindrical model cast. Plaster of Paris (PoP), rigid fibreglass and semi-rigid non-fibreglass (Softcast) were chosen for comparison. Softcast had the best compliance and rate dependency characteristics, accommodating significantly more volume of fluid compared to plaster of Paris or Rigid fibreglass material. The latter two had similar compliance. All three materials demonstrated stress-relaxation which is of advantage in reducing peak pressures for a given volume change. This study shows that the casting materials behave in a viscoelastic manner, which allows them to accommodate more volume change than would otherwise be possible. The use of semi-rigid material may be safer than other materials as far as response to swelling (volume expansion) is concerned.