Spinal movement is made possible by joints between the vertebrae that consist of two flat faces or "facets". If these degenerate, the two halves of the joint grate painfully against each other. Firstly, the disc herniates or wears down and secondly, this puts more stress on the facet joints behind it. These joints become inflamed and later wear out (osteoarthritis). The inflamed joints cause backache.
Later the arthritic joints become big and swollen (like an old lady’s arthritic finger joints) and these enlarged joints protrude into the spinal canal and cause narrowing of the spinal canal (spinal stenosis). This causes pinching of the nerves running down the buttocks and legs with nerve pain and weakness down the legs (sciatica).