Bondage restraints, specifically wrist cuffs or ankle cuffs come in many materials and styles.
Many serious bondage practitioners avoid using metal cuffs, or use them only symbolically. If you do use them, make sure they aren't too tight (the wrist should move easily inside them), avoid putting any kind of tension or pressure on them once on (don't pull someone by them) and avoid putting weight on them (such as lying face-up with hands cuffed behind back). These metal bondage items, altho popular in film and tv, have real practical drawbacks: they can be painful and damaging to nerves and bones, and like all metal bondage items should be used with great care
There are many metal cuff afficionados, who practise the safety at the same time as aesthetics, knowing the dangers and and watching for signs of damage. Personally I find leather, lined or rubber/neoprene to be more confortable and easier to use. Well except for rope and other wrap, but thats a whole other section
With all bondage one has to pay attention to the wearer, for blood flow restriction such as, pain, numbness .. it can happen very quickly and be dangerous. check fingers/toes for temperature and flexibility. Damaging your partner should not happen ....through ignorance or accident...Be aware and be safe.