How do you manage dark elbows and knees?

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I have a problem with my bony prominence such as my knees and elbows, They look darker than other areas of my body. I have been told that calamansi or lemon juice help whiten this part of my skin. I used it for up to one week and because I didn't see any good result I just gave up and stop using it.I also use whitening toner, cream and lotion but it was still ineffective.It was so embarrassing every time I wear shorts or skirt it was very disappointing. I have fair skin except form this part may be I just get it from frequent kneeling when I have to reach something lower. How about you how do you manage dark spots like this? Can you give me some tips on how to improve this part of my body?