They usually use a tendon from your wrist called the Palmaris longus. . It's not really useful but not everyone has one. If they don't have one they'll use a portion of the semitendinousus hamstring tendon. They aren't as strong as a ligament but they weave it together in a figure 8.
Believe it or not, when that tendon dies because it has been removed, it comes back with physical properties more of a ligament and not a tendon as it was. Still not exactly a ligament but closer.