Metabolic bone disease (MBD) is a weakening and deformation of the skeleton caused by a lack of calcium or vitamin D3 — usually from missing UVB or unsupplemented food — leading to soft, bendy bones and a rubbery jaw.
It's one of the most common and preventable reptile illnesses. Prevent it with correct UVB, a proper basking temperature (needed to use the calcium), and calcium/D3 supplementation. Early MBD can be halted; advanced cases cause permanent deformity — a vet's call.