Anatomically, it is said that the knuckles consist of the metacarpophalangeal[1] (MCP) and interphalangeal (IP) joints of the finger.

However, the ordinal terms are used inconsistently and may refer to any of the knuckles.

[3][4] The physical mechanism behind the popping or cracking sound heard when cracking joints such as knuckles was elucidated in 2015 by cine MRI to be caused by tribonucleation as a gas bubble forms in the synovial fluid that bathes the joint.

[5] Despite this evidence, many still believe it to be caused by synovial fluid filling the vacuum left by the joint's displacement.

[6][7] In 2009, Donald L Unger was awarded the Ig Nobel for Medicine[8] for his informal experiment with knuckle cracking.