Which burn stage involves the underlying muscle or bone?

In burn classification, stage 4 burns involve the most severe damage, extending through the skin and subcutaneous tissues to affect underlying structures, including muscle and bone. This type of burn can result from prolonged exposure to heat, flames, chemical reactions, or electrical sources, leading to extensive tissue destruction. The characteristic presentation of a stage 4 burn includes not only the depth of the injury but also a lack of sensation in the affected area, as nerve endings may be destroyed.

This contrasts with other stages: stage 1 burns only affect the outer layer of the skin, stage 2 penetrates into the dermis, and stage 3 burns extend through the dermis but may not reach the underlying muscle or bone. Stage 4 is thus uniquely defined by its involvement with deeper tissues, including muscle and bone, marking it as a critical medical emergency that typically requires surgical intervention and extensive rehabilitation.