Yeah, it is not super-rare, but just rare enough that most people don't know about it and that spell check still thinks it is a mistake. It falls under a blanket categorization of arthritic conditions referred to as spondyloarthritis (meaning arthritis of the spine, even though it involves much more than the spine). The most common condition that falls under the spondyloarthritis umbrella is psoriatic arthritis, which a lot more people are familiar with. As always, thanks for looking!