this is from AAOS and may be of some help
Basal joint arthroplasty
Anatomy: The basal joint of the thumb is also known as the carpometacarpal (CMC) joint. Located at the base of the thumb, the basal joint usually moves quite freely to help position the thumb.
Coding: Arthritis of the basal joint is often treated with an arthroplasty. Years ago, the anchovy procedure, as it was called, was represented by 25447 (Arthroplasty, interposition, intercarpal or carpometacarpal joints), and this code is still used for the treatment of basal joint arthritis. If the tendon graft is harvested from a separate incision, 20924 (tendon graft, from a distance [eg, palmaris, toe extensor, plantaris]) can also be reported. The GSD includes the following procedures under code 25447: arthrotomy/synovectomy of wrist or intercarpal joints; excision of osteophytes, bone fragments and joint debridement; partial or total excision of trapezium or trapezoid; capsular release, repair and/or reconstruction; and internal fixation of implant. The harvesting of tendon graft through separate skin or fascial incision is excluded from code 25447.
A variation of this procedure adds a sling or suspension aspect. The first metacarpal is suspended to the second metacarpal to inhibit the proximal migration of the first metacarpal that often occurs after the excisional arthroplasty of the first CMC joint. Typically, one half (or all) of the flexor carpi radialis (FCR) is used to create the new intercarpal ligament between the first and second metacarpals.
The transfer of the FCR to the base of the first metacarpal is not a part of the basic CMC arthroplasty and must be coded separately. Use either 26480 (transfer or transplant of tendon, carpometacarpal area or dorsum of hand; without free graft, each tendon) or 25310 (tendon transplantation or transfer, flexor or extensor, forearm and/or wrist, single; each tendon, as appropriate). Modifier 51 would be appended to the secondary procedure in either the 25447 + 26480 or 25310 code pairs, according to the January 2005 CPT Assistant.