Why do people with leprosy get deformed bones?

Weak bones. Leprosy is an infection like tuberculosis and can deform bones in several ways. It can invade bone and replace marrow with granulation tissue and cysts weakening it so that norma force on it causes the bone bends over time from. It can damage the nerves and blood supply which affect bone formation. It can cause osteoporosis. Generally bone deformity is only seen in advanced cases.
Numbness side effect. Leprosy leads to numbness. This can lead to repetitive injury to an area with fracture formation. All happening without pain. Also happens in diabetics for the same reason.