Hi Liz.
I have a Sports Medicine provider that bills multiple US on the same DOS also. Our provider is in a private practice and therefore owns the US equipment, so we do not bill with modifier 26. We do, however, bill using the RT or LT modifiers as well as the 59 or X series modifiers, depending on the carrier.
You did not say if the US of the knees are bilateral or if the finger joints are on one hand, therefore I am going to assume bilateral knees and right hand (for the sake of this example). As long as all the components for the complete US are documented for each joint, I would code as follows: 76881-RT, 76881-LT, 76881-59-RT, 76881-59-LT & 76881-59-RT (fingers, right hand). Using the X series modifiers it would be 76881-RT, 76881-LT, 76881-XS-RT, 76881-XS-LT & 76881-XS-RT.
Granted, the MUE is 2 and most carriers are going to deny the units greater than 2, and possibly all of them due to MUE, in which case you will need to appeal with report(s) / images.
I will be interested to see if there are any differing opinions.
Hope that helps a little.
Tina, CPC, CPCO