A simple answer to these condition is really difficult to provide. The most interesting fact is that the condition is located only on the toenails and the fingernails are not affected. This might indicate that something came into contact with the toenails and the same substance might have damaged them. Medical condition that affect the whole body such as : rheumatoid arthritis, diabetes, vascular disorders usually affect both the toenaiols and the fingernails.
Although you have already checked for fungal infection, bacterial infection might also worsen the condition of the nails if it lasts for long. Applying local antibiotic creams might help in this case. However, it is not very likely that you are dealing with bacterial infection because it usually does not stay only on the nail and spreads to the surrounding skin. Furthermore, some types of cosmetic products used for the toenails might be causing the condition if applied for longer periods of time.
In the end, the ridging needs more specific visual inspection by a dermatologist, because the ridging might indicate very different types of horizontal lines on the nails. For the possible types and their causes, you can seek help from this type of skin specialist.
Best,
Dr. Goce
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