An employee was stocking shelves with product that was being removed from boxes. As the employee walked down the aisle, she attempted to step into a space between a step ladder and an open/empty box. The employee's foot hit the step ladder which in turn caused her foot to enter the empty box. The employee attempted to lift her foot out of the box and her forward momentum caused her to trip and fall to the ground. The employee sustained fractured vertebrae and was hospitalized.