This is an interesting and challenging problem to try to answer, particularly in a way that is succinct and understandable at the same time. I am not one of your "wonderful coders," but an Orthopedic Surgeon who has seen both of these disorders alone and together many times in the past. I think the first/best way to approach this is to get the Principle/Primary Diagnosis and the Secondary Diagnosis properly prioritized (cart and horse), then from there the procedures done. What was the main problem for which any procedure was done? These could be CTS with some Flexor Tenosynovitis, or Flexor Tenosynovitis with associated, resulting, or secondary CTS. In my experience, when there is florid hypertrophic proliferative tenosynovitis, such as RA (which is the most common), I considered the CTS to be the resulting from/secondary to the Synovitis, which extends from the distal forearm flexor tendons across/through the wrist (Carpal Tunnel section) then out into the palm of the hand. Since the Synovitis within the Carpal Tunnel adds "volume" and increases "pressure" in/on/to the normal contents of the Carpal Tunnel (tendons and Median Nerve), the result is the CTS/Median Nerve Compression, since the nerve is the structure that is affected by/vulnerable to the increased volume and pressure (i.e. Compressive Neuropathy). Also, in my experience, Carpal Tunnel Syndrome alone is rarely associated with florid proliferative synovitis. There often is some synovial scarring/thickening and adhesions of the tenosynovium around the nerve and flexor tendons, i.e. "sticking them together," but not really an "inflammatory" synovitis.
So getting to the procedures, when a Carpal Tunnel Release is performed open (as opposed to some of the "Closed/Endoscopic" or "Limited Open" techniques), where the nerve and tendons are exposed and visualized throughout the length of the Carpal Tunnel incision and into the palm of the hand, then the Median Nerve and tendons that are incorporated in the scarred/thickened and adherent tenosynovium all need to be freed from this thickened/scarred synovium. As such, then a "Limited Tenosynovectomy" was done to free all of them from each other and remove this tenosynovium, along with the transection/release of the Transverse Carpal Ligament (the essential aspect of the Carpal Tunnel Release procedure). More often than not, I would then "incorporate" (bundle) the "Limited Tenosynovectomy" into my CTR Code, 64721, and not "charge" additionally or separately for it. If it were a really bad scarring/adhesion situation that added a significant amount of time and work to the procedure, I would probably list it (25115) as an Additional Procedure (51). Since I did remove the scarred/adherent synovial tissue from the nerve and tendons, it is technically a "Tenosynovectomy."
If however the principle/primary problem was the florid hypertrophic proliferative flexor tenosynovitis with secondary CTS, then the main procedure is the 25115: Flexor Tenosynovectomy of the wrist. But, the synovitis as noted above is not really confined to the wrist, but usually extends all the way into the palm. So in order to expose and access the full length of the disease process, the incision has to go from the distal forearm out into the palm, which requires that the Transverse Carpal Ligament be incised/transected/released resulting in an "incidental" but "intentional" Carpal Tunnel Release, which is the treatment of the Carpal Tunnel Syndrome part of the problem. That would explain why 64721 is "bundled" into the 25115 code.
I realize this is a wordy and lengthy discussion, but these are not rare or unusual conditions to occur, but it takes this much to try to sort it all out. For what it is worth, always send the Operative Report.
I hope this helps you some.
Respectfully submitted, Alan Pechacek, M.D.