In a manner relating to or characteristic of arthritis; in a way affected by arthritis.
From 'arthritic' + '-ally' (in the manner of). The adverb form allows describing how something happens in relation to arthritis, following standard English adverb formation patterns.
Adding '-ally' to adjectives to create adverbs is such a reliable pattern that you can almost always guess how an adjective becomes an adverb—it's one of English's most predictable rules.
Complete word intelligence in one call. Free tier — 50 lookups/day.