Volumetric Considerations for Valving Long-Arm Casts: The Utility of the Cast Spacer

Following valving, casts were secured via 3 separate techniques: overwrap with a 3-in elastic wrap (Econo Wrap; Vitality Medical), application of two 10-mm and 15-mm cast spacers (CastWedge; DM Systems) (Figure 2). After securement, cast pressures were recorded, and circumference measurements were performed at the 2 previously identified points. The cast padding was then cut at the valve site and secured via the 3 listed techniques. Cast pressure and circumference measurements were performed at set time points (Figure 3). Changes in cast pressure were recorded in terms of the amount of change from the initial cast placement to account for differences in the size of volunteers’ forearms. Volumetric calculations were performed only for the spacer subgroups owing to the added material in the elastic wrap group. Estimated cast volume was calculated using the equation for volume of a frustum (Figure 4).

We used a 2-cast type (univalve and bivalve) by 4 securement subgroups (initial, elastic wrap, 10-mm spacer, and 15-mm spacer) design, with cast type serving as a between-subject measure and securement serving as a within-subject variable. An a priori power analysis showed that a minimum sample size of 15 subjects per condition should provide sufficient power of .80 and alpha set at .05, for a total of 30 casts. Statistical analyses were performed using IBM SPSS Statistics software version 21 (IBM). Experimental groups were analyzed using mixed-design analysis of variance (ANOVA). Post hoc comparisons between valving groups and cast securement were performed using Scheffe’s test to control for type II errors. Change in cast volume between the initial cast and cast spacers groups was compared using paired Student’s t tests. Statistical significance was predetermined as P < .05.

RESULTS

A summary of collected data for cast pressure and volume is detailed in Table 1, subdividing the variables on the basis of cast type and type of securement. Recorded pressures of the different subgroups are depicted in Figures 5 and 6 according to type of securement (initial, elastic wrap, 10-mm spacer, or 15-mm spacer). Results of the mixed-design ANOVA demonstrated significant differences between the initial cast pressure and univalve and bivalve groups (P < .05). There was a main effect for bivalve having lower pressure overall (F [1, 1)] = 3321.51, P < .001). There was also a main effect indicating that pressure was different for each type of securement (elastic wrap, 10-mm spacer, 15-mm spacer) (F [1, 28] = 538.54, P <. 01). Post hoc testing confirmed pressure decreased significantly, in descending order from elastic wrap, to 10-mm spacers, to 15-mm spacers (P < .05).

Table 1. Cumulative Data for Two Casting groups at Each Timepoint

Continue to: Table 2...