I have to admit that I'm in a similar boat as Rina in that I'm not really that surprised about this. Aika's been sidelined by this condition for a year, more or less. While I don't remember off the top of my head, I think that the initial prognosis/hopes were that she'd be back full-time after roughly 6 months (if even that). She wasn't my favorite member, but it's still hella lame that she's having to leave under these circumstances. Group graduations/departures suck enough on their own, the fact that in Aika's case, her injury/condition is such that it's not really giving her a choice and is basically forcing her to quit makes it that much worse.
I can't help but wonder though, everyone remember when she first joined the group, specifically around the time of Kanashimi Twilight, and how after watching the PV and "Making of" video for it, several of us had noticed and were fairly certain that we had never seen her wear anything other than flat shoes/boots (i.e. we had never seen her wear any type of footwear with a significant heel)? At the time, we had (or at least I had) just figured that since she was so young at the time, she may have simply not been used to/comfortable wearing them yet, and that in time she probably would be wearing them. However looking back at it all now, one can't help but wonder if this condition may have already been affecting her back then? If so, they (management, company higher-ups, TPTB, whatever) probably were hoping that as she grew up, her physical strength would increase and improve. Sadly, that's not what's happening.