Praise be to Allah.
It is permissible to remove defects and deformities that appear on the face or body, by means of natural or chemical treatments, or lasers, so long as that does not result in any harm that is greater or equal (to the defect being dealt with).
That includes treating freckles, acne and dark spots, even if it is by using some kinds of permissible food, because of the general meaning of the evidence that it is permissible to use medical treatment, and because of the report 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).
Ash-Shawkaani (may Allah have mercy on him) said: With regard to the words “except when there is a disease (that would justify doing that)”, what appears to be the case is that the prohibition mentioned only applies if it is done for the purpose of beautification, not to treat a disease or problem, for in that case it is not prohibited.
End quote from Nayl al-Awtaar (6/229)
Ibn al-Jawzi (may Allah have mercy on him) said: With regard to medical treatment that removes freckles and beautifies the face for the husband, I do not see anything wrong with that. Narrated by as-Safaareeni in Ghadhaa’ al-Albaab (1/432).
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: Recently there have appeared medicines that make a dark-skinned woman more fair. Can she use them? Or is using such medicines prohibited on the grounds that they change the creation of Allah?
He replied: Yes, that is prohibited if it permanently changes the colour of the skin, because that 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. Hence we must understand the difference between what is done for the purpose of beautification and what is done to remove a defect, because 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.
End quote from Fataawa Noor ‘ala ad-Darb.
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.
It says in al-Mawsoo‘ah al-Fiqhiyyah (31/47): The fuqaha’ are of the view that the ‘awrah of a woman in front of another woman is the same as the ‘awrah of a man in front of another man, i.e., the area between the navel and the knee. Hence it is permissible for a woman to look at all of another woman’s body, except the area between the navel and the knee. That is because usually there is no provocation of desire. But that is haraam when there is desire and the fear of fitnah. End quote.
But if there is a need to uncover more than that, for the purpose of medical treatment, there is nothing wrong with that, as has been explained previously in the answers to questions no. 5693 and 95891
And Allah knows best.