People who have had injuries often avoid the standing forward bend because of fears that it will re-injure them. In fact the contrary is often true - guarding or protecting the back can lead it to become less and less mobile, and increase the chances of further injury.
Indeed, the standing forward bend is actually a good movement to practice, because it is the type of movement that we often perform in real life when stooping to pick things up for example.
The key to this exercise it to do it slowly and carefully. As with all stretches, this is not a competitive thing - just because someone else may be able to touch their toes does not mean you should try to do so.
Performing this movement is simple - slowly bend over as if you are trying to touch your toes, relax and hold it for a few seconds, then slowly stand back up. Don't bounce or push, simply move slowly up and down.
Variations of this include crossing your feet one in front of the other, or doing it with legs shoulder width apart - a standard yoga stretch. Don't worry if you don't get anywhere near your knees, simply doing this stretch will help you to maintain a healthy back.