Answer: According to the 2005 Medicare Physician Fee Schedule, you can charge Medicare for an assistant surgeon when you report either 29824 (Arthroscopy, shoulder, surgical; distal claviculectomy including distal articular surface [Mumford procedure]) or 29826 (... decompression of subacromial space with partial acromioplasty, with or without coracoacromial release).
The carrier may have denied your claim based on the modifier that you reported. Because you submitted the charge to a Medicare payer, you should have appended modifier AS (Assistant at surgery service). If you used modifier 80 (Assistant surgeon) instead, a Medicare carrier would deny the claim.
If you submitted the claim to Medicare with modifier AS and the payer still denied the charge, you should appeal with a letter from your surgeon explaining the work that the PA performed, a copy of your operative report, and a copy of the appropriate page of the Fee Schedule that shows that an assistant is allowed with both of the shoulder procedures that your surgeon and PA performed.