Sciatica is caused by an irritation of one or both of the sciatic nerves. This is usually when a herniated disc puts pressure on the nerve root. Leg pain may make you think that sciatica is caused by leg problems, when it may not!!
Other causes of sciatica include spinal stenosis, spondylolisthesis, spinal tumors, trauma, muscle entrapment of the nerve and sciatic nerve tumor or injury.