A 68-year-old man from rural Maharashtra presents with progressive bowing of both legs over the past 3 years and recurrent bone pain. On examination, he has asymmetric limb deformity and warmth over the anterior tibiae. Serum alkaline phosphatase is markedly elevated at 420 IU/L (normal <100). Plain radiographs show characteristic 'blade of grass' appearance in the tibia with mixed lytic and sclerotic lesions. Which of the following complications is he at highest risk of developing in the next 5 years?