An employee was working with a catwalk, changing out a cable. After removing the skate cable, the employee and a co-worker were adjusting the skate to install the new cable. The co-worker positioned himself on the driller side of the catwalk; the injured employee was on the off driller side. As the employee made his way down the catwalk, he placed his right hand on the index pin of the catwalk trough to gain leverage. As the skate's momentum carried it forward, it pinched the employee's right thumb against the pin. The tip of the thumb was amputated.