I've proximal tibia fracture and its been 4 months now i'm trying to bend my knee wid cpm machine i've reached 80 but more than tat it pains a lot y?

Stiffness from cast: I assume you leg was immobilized during the healing of your tibia fracture. Any joint that is immobilized for any significant length of time (more than 7-10 days) would get stiff. Physical therapy working on motion after the cast or splint/brace is removed should allow knee to regain essentially full functional motion with time (6-12 weeks).