Crepitus is a meical term to describe the grating, crackling or popping sounds and sensations experienced under the skin and joints. The sound can be created when two rough surfaces in the human body come into contact - for example, in osteoarthritis when the cartilage around joints has eroded away and the joint ends grind against one another, or when the fracture surfaces of two broken bones rub together. In soft tissues, crepitus can be produced when gas is introduced into an area where it normally isn't present. The term can also be used when describing the sounds produced by lung conditions such as interstitial lung disease—these are also referred to as "rales". Crepitus is often loud enough to be heard by the human ear, although a stethoscope may be needed to detect instances caused by respiratory diseases. In times of poor surgical practice, post-surgical complications involved anaerobic infection by Clostridium perfringens strains, which can cause gas gangrene in tissues, also giving rise to crepitus. Subcutaneous crepitus is a crackling sound resulting from subcutaneous emphysema, or air trapped in the subcutaneous tissues.