An instrument knot is an excellent knot to use when one or both of the ends of the suture are short. In the diagrams below, adapted from the Ethicon guide, one "half" of the suture has been colored purple to make the instructions more clear -- a good way to practise is with a shoelace that you've colored to match this. It's a little confusing, but once you get the hang of it, it's like riding a bike and becomes almost reflex.
I strongly urge all practitioners to practise and perfect these techniques:
- 1. Hold the white end of the suture held between your left thumb and index finger. Form a loop by placing the needleholder on the side of strand away from you.
- 2. Grasp the purple half of the suture using the needleholder.
- 3. Complete the first half hitch by pulling the needleholder toward you and drawing the white half away from you. Let go of the purple half.
- 4. Pull the white half toward you with your left hand and loop it around the needleholder. Form a loop by placing the needleholder on the side of the strand toward you.
- 5. With the end of the suture held in the needleholder, pull it through the loop away from you.
- 6. The square knot is completed by horizontal tension applied with left hand moving toward you and the right hand away from you -- tension should be as horizontal as possible.
You can find video of the knot being tied at the UPENN surgery site and web searches should find you numerous other reference sites.