It is permissible to remove defects or deformities that may appear anywhere on the body. Both the natural as well as chemical procedures are permissible as long as the procedure does not cause any other greater harm or defect that is equally difficult to deal with.
The defects or deformities also include acne, freckles, dark spots etc. It is permissible to get them treated using some foods or medical treatment. However, there are certain things that are clearly forbidden. Narrated by Abu Dawood (4170) from Ibn ‘Abbaas (may Allah be pleased with him), who said: The woman who does hair extensions and the woman for whom that is done, the woman who plucks eyebrows and the woman for whom that is done, the woman who does tattoos and the woman for whom that is done when there is no disease (that would justify doing that) are cursed. This hadith was classed as saheeh by al-Albaani in Saheeh Abi Dawood.
Ahmad (3945) narrated from Ibn Mas‘ood (may Allah be pleased with him) that he said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) forbid the woman who plucks eyebrows, the woman who files teeth (for the purpose of beautification), the woman who does hair extensions and the woman who does tattoos, except when there is a disease (that would justify doing that).
Some people may have a great number of freckles and if a woman wants to get them medically removed to look.more beautiful for her spouse then there is nothing wrong with that.
The beautification should not take a person to the point of changing the creation of Allah. Nowadays many options are available for men and women both by which they can permanently change the skin color. These methods tend to change the dark-skinned person to become more fair.
Doing this is prohibited as it is akin to tattooing and the Prophet (blessings and peace of Allah be upon him) cursed the woman who does tattoos and the woman for whom that is done. But if it is done to remove a defect, such as if there is a disfiguring black mole on the skin, so the individual uses something to remove it, there is nothing wrong with that.
Therefore we must understand that the difference lies in the intention and purpose of what for the treatment is being undertaken.
The Prophet (blessings and peace of Allah be upon him) gave permission to the Sahaabi whose nose had been cut off to wear a nose made of gold, so as to remove the defect that resulted from his nose having been cut off. But he cursed the woman who files teeth for the purpose of beautification, and the one for whom that is done, which refers to creating gaps between the teeth and the like. But if we assume that there is unevenness in the teeth, as some of them stick out and others do not, in a manner that is disfiguring, then there is no problem with doing procedures to straighten them.
It is permissible for a woman to uncover her knees and lower legs for the purpose of medical treatment, whilst being careful to cover that which she does not need to uncover.
And Allah knows best.
May Allah heal all.