BACKGROUND: Trapezial excision with ligament reconstruction and trapezial exci sion with ligament reconstruction combined with tendon interposi tion have proven to be highly ef fective techniques for treating primary osteoarthritis of the thumb carpometacarpal joint. To determine whether tendon inter position and proximal migration of the thumb metacarpal affect the objective and subjective out comes, we compared the long-term outcomes of these two pro cedures performed in similar pa tient groups. METHODS: Forty-three patients (fifty-two thumbs) were randomized to un dergo either trapezial excision with ligament reconstruction or the same procedure combined with tendon interposition. Fif teen patients treated with liga ment reconstruction (group I) and sixteen patients treated with the same procedure with concomi tant tendon interposition (group II) were evaluated after a mean follow-up period of 48.2 months. The outcomes were assessed with the Buck-Gramcko score, with the total score calculated on the basis of the objective and subjective results. The ability to perform activities requiring use of the thumb and to return to work was analyzed as well. Radio graphs were evaluated to deter mine the amount of proximal metacarpal migration at rest and under stress. RESULTS: Postoperatively, the mean total Buck-Gramcko score was rated as excellent in group I and as good in group II (p = 0.036). Group I had significantly better mean scores for palmar and ra dial abduction, cosmetic appear ance, and willingness to undergo the surgery again under similar circumstances (p < 0.05). The mean scores for tip-pinch strength and the mean subjec tive scores for pain, strength, daily function, dexterity, and overall satisfaction did not dif fer significantly between the groups. Both groups had satis factory results with regard to their performance of activities of daily living and their ability to re turn to work. With the numbers available, the amount of proxi mal metacarpal migration, at rest and under stress, did not differ significantly between the groups. CONCLUSIONS: Tendon interposition does not affect the outcome after the ligament reconstruction for the treatment of osteoarthritis of the thumb carpometacarpal joint. Furthermore, proximal migration of the thumb metac arpal does not appear to influ ence the functional outcome.