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Why can deep burns cause contractures around joints?

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Cross-section diagram of human skin showing burn depth by degree, with arrows indicating that first-degree burns affect only the epidermis, second-degree burns reach the dermis, third-degree burns extend into subcutaneous tissue, and fourth-degree burns penetrate to muscle. — burn This diagram shows how burn injuries are classified by severity. First-degree burns damage the outer skin, known as the epidermis. First-degree burns are the least severe form of burn injury. Second-degree burns damage the outer skin and some of the underlying skin, known as the dermis, which also includes some connective tissue. Third-degree burns damage all the layers of skin, including subcutaneous tissue. Fourth-degree burns, which are the most severe, destroy every layer of skin and also damage muscle, tendons, and bones.

Deep burns can cause contractures around joints because of the nature of the scar tissue that forms during the healing process.12

When tissue damage is substantial, the body repairs the area by producing fibrous connective tissue composed primarily of collagen. Unlike healthy skin, this scar tissue is relatively inelastic and lacks the elastic fibers that normally allow for flexibility. As the scar matures, the densely packed collagen fibers can cause the tissue to shrink and tighten.12

When this process occurs over a joint, the resulting lack of elasticity restricts the range of motion, effectively "locking" or pulling the joint into a fixed position. These contractures are a common, debilitating aftereffect of extensive burns, often requiring physical therapy or corrective plastic surgery to restore mobility.312