For the last 11 years my thumbnails have looked like this:
(image from http://www.dermnet.com/Habit-T
As you can see, it's a pretty gross condition to have. Your nails have a washboard effect to them and are extremely thin. For years I just hid them by covering them up with my shirt sleeves or ball up my fists.
My doctors didn't know why, I was told I was lacking this and missing that.... finally I was sick of not knowing and looked online for answers. It turns out that it was caused by my constant need to push back my cuticles after they had grown back from an unfortunate accident where I ripped both of my thumbnails completely off (more on that another time). The condition is called Habitual Tic Deformity and below is an exert from one of the medical journals I found. I am happy to report that since I started wrapping my thumbnails with band-aids I have broken the "tic". My nails are coming back like normal for the first time in 11 years!!!!! So if you know anyone suffering from this, inform them that there is a reason and that it can be cured.
The following text was borrowed from : http://www.stacommunications.c
Habitual tic deformity is a common self induced nail condition that results from chronic,habitual mechanical trauma to the cuticle. The most common scenario involves the thumbnail where a patient has repeatedly picked
at the cuticle and proximal nail fold with the index finger of that same hand as a compulsive habit. The nails of other fingers can also be involved but it is
less common. The typical nail plate finding of habit-tic deformity
is many parallel transverse grooves, sometimes with yellowish-brown discoloration, running down the middle of the affected nail. In addition, the cuticle may be pushed back with redness and swelling.
Patients must be encouraged to stop their compulsive habit-tic. One simple approach is to apply tape over the cuticle and proximal nail fold so as to
remind patients to stop their compulsive habit-tic and to lessen the habitual trauma experienced