When considering the role of injections, pain felt in the spine itself needs to be differentiated from referred limb pain in the arm, (brachalgia), or leg, (sciatica).
Pain in the spine may be generated by two principle mechanisms; directly as a result of the mechanical clanging of spinal structures together or inflammation of these structures. It may be generated from the discs, facet joints, ligaments and the associated muscles around the spine which may go into spasm. The mechanism whereby problems in these tissues cause pain is complex though often relates to inflammation – this is what causes the pain, heat, redness and swelling you see develop around a cut or sprained ankle. Spinal pain can be felt centrally over the spine i.e. in the back or back of the neck. In some people, it is felt to one side, perhaps in the shoulder/scapular or in the sacroiliac area, buttock, groin or upper thigh.
Limb pain, either in the leg or arm, arises when pressure or inflammation affects the spinal nerve roots that pass to the legs or arms from the spine. Spinal nerve roots can become irritated due to direct pressure from a disc prolapse or narrow spinal canal or by being affected by inflammation in the joints or discs. Worn joints can swell like an arthritic joint and weakened discs may prolapse (herniate/slip/bulge). Similarly, worn discs and joints are susceptible to inflammation and therefore there is often a combination of both mechanical pressure and inflammation at play. This irritation of the nerve root is felt primarily by its covering – the nerve root sheath. So even though the pathology is in the spine, the pain is perceived in the brain as coming from the area of the arm/leg supplied by that nerve – this is called referred pain.
So pain felt in the spine itself comes from the discs, joints, ligaments and muscles of the spine. By contrast limb pain comes from the nerve roots in the neck or lumbar spine (back) when they are irritated or pressed on by these spinal structures.
However, the pain from discs, facet joints, ligaments and muscles may also be felt in the arm or leg. This is because the body does not always localise where pain is coming from very accurately. So, pain in the arm and the leg may be due to irritation of the spinal nerve roots or from inflammation of other spinal structures. Referred pain from irritation of the spinal nerve roots is usually sharp or burning and felt further down the arm or leg. Pain from other spinal structures is usually a duller pain or ache and felt mainly in the buttock / thigh or shoulder / upper arm (though it can travel further down the limb).