Thank you for getting back to me.Your chameleon has symptoms of Metabolic Bone Disease (MBD). Unfortunately, the pet store sold you a UVB light that is nowhere near the potency a chameleon needs. You need one with an output of 10.0, rather than 5.0. The 5.0 lights produce almost no UVB at all after 3 months. The 10.0 are good for 6 months. Swollen and broken legs are one of the most common symptoms.Chameleons are particularly susceptible to MBD and even with a fresh UVB light, it's difficult for them to produce enough vitamin D without access to natural sunlight. It can take months to years for MBD to progress to the point where it causes symptoms, and once it does, it's very difficult to treat. MBD can result from too little vitamin D, lack of calcium, too much phosphorus, or an imbalance of these and other nutrients. The most common cause, however, is insufficient UVB light.You can read more about it at the following site:http://www.anapsid.org/mbd.html
The first thing to do is get a new UVB light. I recommend the Reptisun 10.0 in the straight tube style. Second, if your climate is mild enough to take him outdoors, carry him out for a supervised sunbath every day. Of course, if it's cold, you won't be able to do that. You should also make an appointment with a reptile vet. The vet can make a certain diagnosis of the problem, which of course is impossible with only an online description. Then, if MBD is present, the damage will have to be assessed, followed by proper treatment. If it turns out MBD is not the problem, the vet will be able to determine what is wrong (infection is another possibility), and treat that. This link will take you to a directory of reptile vets:
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