Emp was prying open 2 flanges to install valve. Utilizing a 3/4 ton come-along & a one inch nylon strap to open flanges. One inch strap broke, causing flanges to slam together with left thumb located in pinch point. One inch nylon strap was not large enough to withstand tension, which resulted in strap breaking. Negative x-ray on 8/22. Nail repositioning done on 8/24/12.