I don't know, but i have them, too, and they progressively worsen, it seems.  doctor has looked and doesn't know (imagine that?!).  with mine, the ridges are deep and horizontal.  the cuticles begin the process it seems with disintegrating, so to speak little by little, to where there is no more cuticle at the "moon" part of the nailbed.  then, the ridges begin as small dents in the middle of the nail, and progressively more dents become obvious and they deepen.    it don't look good, if ya know what i mean, but the main thing is what does it mean???  anybody else ever have it?