The quickest way to stop the MBD from progressing is to take the chameleon to a vet and get calcium injections over a period of time until the blood calcium levels are high enough to give the chameleon a shot of calcitonin to draw the calcium rapidly back into the blood. Any vet should be able to do this even if s/he has to consult with another vet to get information about how much calcium, etc.
If you can't do that, the next best thing IMHO is to get some liquid calcium sandoz or calcium gluconate from a pharmacy and a needleless syringe. This type of calcium is supposed to be more readily absorbed.
Its important to correct things as soon as possible because MBD not only affects the bones but other systems in the chameleon...and the longer its let go the more permanent damage will likely occur. (The bone deformities will remain regardless.)
You also need to make the appropriate changes to your husbandry to bring the calcium, phos., D3 and vitamin A back into line...along with providing proper UVB (that does not pass through glass or plastic) and even gutloading/feeding your insects properly. Imbalances in the 4 minerals/vitamins I mentioned can push the chameleon towards MBD. So can constant overfeeding.
Here's some information I hope will help you with supplementing, etc.....
Appropriate cage temperatures aid in digestion and thus play a part indirectly in nutrient absorption.
Exposure to UVB from either direct sunlight or a proper UVB light allows the chameleon to produce D3 so that it can use the calcium in its system to make/keep the bones strong and be used in other systems in the chameleon as well. The UVB should not pass through glass or plastic no matter whether its from the sun or the UVB light. The most often recommended UVB light is the long linear fluorescent Repti-sun 5.0 tube light. Some of the compacts, spirals and tube lights have caused health issues, but so far there have been no bad reports against this one.
A wide variety of insects that have been well fed and gutloaded should be fed to it.
Since many of the feeder insects we use in captivity have a poor ratio of calcium to phosphorus in them, its important to dust the insects just before you feed them to the chameleon at most feedings with a phos.-free calcium powder to help make up for it. (I use Rep-cal phosphorus-free calcium).
If you also dust twice a month with a phos.-free calcium/D3 powder it will ensure that your chameleon gets some D3 without overdoing it. It leaves the chameleon to produce the rest of what it needs through its exposure to the UVB light. D3 from supplements can build up in the system but D3 produced from exposure to UVB shouldn't as long as the chameleon can move in and out of it. (I use Rep-cal phos.-free calcium/D3).
Dusting twice a month as well with a vitamin powder that contains a beta carotene (prOformed) source of vitamin A will ensure that the chameleon gets some vitamins without the danger of overdosing the vitamin A. PrEformed sources of vitamin A can build up in the system and may prevent the D3 from doing its job and push the chameleon towards MBD. However, there is controversy as to whether all/any chameleons can convert the beta carotene and so some people give some prEformed vitamin A once in a while. (I use herptivite which has beta carotene.)
Gutloading/feeding the insects well helps to provide what the chameleon needs. I gutload crickets, roaches, locusts, superworms, etc. with an assortment of greens (dandelions, kale, collards, endive, escarole, mustard greens, etc.) and veggies (carrots, squash, sweet potato, sweet red pepper, zucchini, etc.)
Calcium, phos., D3 and vitamin A are important players in bone health and other systems in the chameleon (muscles, etc.) and they need to be in balance. When trying to balance them, you need to look at the supplements, what you feed the insects and what you feed the chameleon.
Please note that various supplements have various amounts of D3 and vitamin A and so some can be given more often than others. The idea still is not to overdo the fat soluble vitamins like D3 and prEformed vitamin A.
Here are some good sites for you to read too...
http://chameleonnews.com/07FebWheelock.html
http://web.archive.org/web/200605020...Vitamin.A.html
http://web.archive.org/web/200406080...d.Calcium.html
http://www.uvguide.co.uk/
http://web.archive.org/web/200601140...ww.adcham.com/
If you can't access the sites above that have the word "archive" in you can do it through the WayBackMachine.