Cortisone injections are helpful but only temporarily. I'm not so sure it will help decrease the dose of prednisone you may need for PMR. I had severe degenerative arthritis in both knees and that pain I could distinguish from PMR pain. It is true that prednisone will help pain caused by degenerative and inflammatory arthritis but taking more prednisone for that purpose is not a good idea.
That is a huge problem when taking prednisone because it helps many types of pain. It is very easy to take more prednisone for pain that isn't caused by PMR. When the pain goes away, it is easy to justify the higher dose of prednisone by calling it PMR pain.
I have had both knees replaced and that did help me decrease my prednisone dose. I think it may prove that I was taking more prednisone for the wrong reason. I also have arthritis on my lower back. After awhile, it becomes difficult to distinguish PMR pain from arthritis pain. My rheumatologist frequently told me and a "localized" cortisone injection is more preferable then taking "systemic" prednisone.