Background:Rhizarthrosis, or osteoarthritis (OA) of the first carpometacarpal joint, is currently considered a common pathology, associated with aging,affecting approximately 7% of men and 15% of women over the age of 50.Despite its high prevalence, only a small number of patients seek medical attention.Adequate conservative treatment can alleviate symptoms and even reduce the need for surgery in up to 70% of cases. The possibilities of conservative treatment include intra-articular (IA) injection of corticosteroids and hyaluronic acid (HA). The efficacy of IA HA has been proven especially in knee but the data in rhizarthrosis are scarce.Most HAs had been registered as medical devices worldwide and the EU Medical Devices Regulation (MDR 745/2017) requires a continuous post-market follow-up to ensure the safety and performance of these products. In compliance with the MDR, this work aimed to evaluate the efficacy and the impact on quality of life, of a marketed HA, in a cohort of patients with rhizarthrosis under real conditions in clinical practice. Methods:Observational, post-marketing, retrospective, follow-up study. Between January 2020/June 2022, patients were treated in the Rheumatology Dpt. of Hospital General Universitario de Elche, Spain,with a single injection of HA (Adant, Meiji Pharma Spain) and followed for 6 months. Pre/post Visual Analogue Scale (VAS) for pain, and functional questions (key, grip and button) were used for efficacy assessment. Patients data were pseudonymized and included in a database for further analysis. The chi2 test was used, for qualitative variables, and the T or Mann Whitney tests for quantitative ones. The pre/post comparison of the VAS was made with the t test for paired samples. The study was approved by the Ethics Committee of the hospital. Results:Twenty patients with a mean age of 61 years, 80% women, were studied. An 80% had bilateral rhizarthrosis, 70% had Kellgren-Lawrence gradeIII-IV (moderate-severe) and 65% had other chronic medical condition (e.g., hand OA, osteoporosis, diabetes, etc.).The 80% of the patients had received 2 previous treatments with HA injections. The volume administered varied from 1 to 2 ml (55%-45%). The mean absolute change from baseline in VAS pain score over 6 months was -5.95, a reduction of 77% (p<0.001). A 35% of the patients had an improvement 80% and the others between 70 and 79%. Regarding functional capacities, the 80% of the patientsachieved complete recovery.No significant statistical correlations were observed between baseline characteristics, the number of prior injections, or the volume administered, and the degree of improvement in pain or functional outcomes.All patients were satisfied with the treatment. There were no adverse events recorded. Conclusion:This study suggests that viscosupplementation using Adant is an effective and well tolerated therapeutic option in managing pain and improving function of rhizarthrosis with an excellent safety profile.