The numerous surgical techniques proposed for treating thumb basal joint arthritis (osteotomy of the first metacarpal, trapezial osteotomy, simple trapeziectomy, trapeziectomy with implant, total joint prosthesis, arthroplasty by interposition, etc.) necessitate an anatomically based radiological evaluation of the different pathological forms of thumb basal joint arthritis. Here, the author defines three parameters: narrowing of the trapeziometacarpal (TM) joint space: TMA; TM instability and subluxation: TM I; scaphotrapeziotrapezoid damage: STT. Four stages of TM osteoarthritic deterioration are defined: TMA0: no joint narrowing (painful and unstable joint); TMA1: narrowing 50%; TMA3: disappearance of the joint space, bone erosions. For TM instability/subluxation: TMI 0: reducible subluxation (unstable and painful TM); TMI 1: reducible subluxation but with imperfect reintegration; TMI 2: non-reducible subluxation 1/3. For STT damage, STT 0: radiograph is normal but anatomical damage is visible intraoperatively; STT 1: joint space narrowing <50%; STT 2: joint space is barely visible; STT 3: presence of erosion, sclerosis, irregularities. He outlines the shortcomings of the often-used Dell and Eaton-Littler classifications. A prospective study involving multiple cases having the same anatomical and radiological appearance that are assessed with sufficient follow-up is needed to standardize the modalities of surgical treatment.Copyright © 2020. Published by Elsevier Masson SAS.