Hello! This is a good question because there can be several side effects of having metal pins inside your body – but only if the pins have been inside your body for a very long period of time.
1) If the metal pins have been used to hold two pieces of bone together (like in a fracture) it is possible that the bone grows around the metal pins and the pins become embedded in the bone. But this would take a long time to happen.
2) It is also possible that the body reacts to the metal causing inflammation around the pins which can cause infection and a lot of pain.
3) Finally, it is possible that tiny fragments of the metal may erode (break off) and enter the bloodstream and then get lodged somewhere else in the body – this can also cause infection and pain.
The main thing to remember is that doctors have very specialised knowledge about this and would not put someone at risk – technology and medicine have advanced so much over the past few decades and this procedure is probably safer than ever before.
There shouldn’t be any obvious side-effects. Most implants these days are made from metal alloys (mostly titanium) that are extremely unreactive and hard-wearing. I believe surgeons advise that things like artifical hips are replaced every 15-20 years, but I think that is mainly precautionary and is not strictly necessary