I dust with a phosphorous-free calcium powder at most feedings. Most of the feeder insects we use have a poor ratio of calcium to phos. and this helps to make up for it.
I dust twice a month with a vitamin powder that contains a beta carotene source of vitamin A. PrOformed (beta carotene) sources of vitamin A won't build up in the system. However, there is controversy as to whether some/any chameleons can convert beta carotene or not so some people give a little prEformed vitamin A once in a while to ensure that they get some. PrEformed vitamin A can build up in the system and excess can prevent the
D3 from doing its job and lead to MBD.
If your chameleon only gets UVB from linear/tube lights and gets no direct sunlight, then you might want to dust twice a month with a phos.-free calcium/D3 powder. D3 from SUPPLEMENTS can build up in the system and cause problems so don't overdo it.
UVB from lights or sunlight should not pass through glass or plastic.
Appropriate basking temperatures assist in proper digestion.
Its also important to gutload your insects and provide them with a nutritious diet.
Phos., calcium, D3 and vitamin A are all players in bone health and its important that they are in balance. Look at what you feed the chameleon, what you feed to the insects and the supplements you use when trying to attain a balance.