I think the “meat” may be gone too. You don’t want the added weight of fat anywhere else since she has arthritis. Just brush gently in that area when you groom so you don’t hurt her. My horse’s “meat” is gone from his wither area to shoulder so I use a soft brush there. You may also want to talk to your vet to see if there is a supplement or something you can feed her to bulk up that area. I also have been riding bareback to give his back a break from the saddle.