IA AAV-FIX expression within the joint protects from hemorrhage-induced synovitis. FIX ^−/− mice received AAV in left hind limb joint and normal saline in right hind limb joint under hemostatic coverage with IV FIX. Bilateral hind limb joints received puncture injury at 4 weeks after gene delivery; 2 weeks after injury, joints were collected. (A) Top panels: Representative pathologic changes of minimal synovitis at 2 weeks after puncture injury of knee receiving prior treatment with ssAAV5-CBA-hFIX (2.5 × 10⁹ vg). Left, original magnification ×40; right (area of detail from left figure, indicated by box), original magnification ×200. Bottom panels: Injured untreated knee demonstrates changes of hemarthropathy, including synovial proliferation narrowing the joint space, synovial thickening (black arrows), foci of hypervascularity (green arrows), and synovial overgrowth of the tibial articular surface. Left, original magnification ×40; right (area of detail from left figure, indicated by box), original magnification ×200. (B) Histopathologic grading of mice treated with AAV-hFIX IA delivery and comparison with pathology score of contralateral injured untreated knee (“No vector” control). High dose, 10¹⁰ particles/animal; low dose, 2.5 × 10⁹ particles/animal (n = 4 animals/group). *P < .05