A fused joint may impair the ability to use a limb effeciently if at all, but should not necessarily cause constant pain, especially when you mention that there is no favoring of the leg. This leads me to believe that the licking of the leg or paw is not the result of yoru little dog's deformity, but instead the result of another reason altogether.
Dogs will obssessively lick for a couple of different reasons. Probably the most common reason for excessive licking, epecially when confined to the feet, is skin allergies. Interestingly, patients afflicted with skin allergies often choose to obssessively lick one or two paws rather than all four.
Another reason dogs will obssessvively lick an area is because of obssessive/compulsive disorder, where like people affilicted with the same disease will obssessively repeat certain behaviors.
I would consider managing your dog as a skin allergy case first and foremost, which can be accoplished through the use of topical anti-itch sprays, oral anti-histamines, and/or cortisteroid treament. For more imformation on skin allergies here is an article I wrote on the disease:
If focusing on skin allergies does nto help your little dog, then you can try managing OCD by treating with antidepressants, such as prozac or amitriptylline (both have well established use in veteirnary medicine).
Feel free to discuss my recommendations with your vet.