This is a randomized double-blinded randomized comparison between injection of platelet rich plasma (PRP) and placebo for thumb basale osteoarthritis.

The aim of this study is to evaluate if high-concentration PRP injection decreases pain and disability in patients with thumb base osteoarthritis in the short-term and compare the effect to placebo.90 patients will be inlcuded (45+45). The primary outcome is pain on load (numerical rating scale) after 6 months. Patient-rated outcome measures, pinch grip and key pinch, range of motion of the CMC.1 joint will be assessed before treatment and at 3, 6 and months after the injection. Participants and assessors will be blinded.

Inclusion Criteria:

• Osteoarthritis in the thumb base
• Radiological Eaton Littler class 1-3.
• Clinical signs of thumb base osteoarthritis )pain at palpation of the CMC-1 joint and pain during provocation/grinding test).

Exclusion Criteria:

• Rheumatoid arthritis
• Ongoing infection in the hand or wrist
• History of gout or pseudogout in the hand
• Inability to co-operate with the follow-up protocol (language difficulties, severe psychiatric disorder, cognitive impairment, drug addiction).
• Chronic pain syndrome / centralized pain.
• Intra-articular injection in the affected joint within 6 months.
• Eaton Littler class 4 (symptomatic STT joint osteoarthritis).

blind fold on participants. No infomration about given treatment to outcome assessors.