Knuckle

The knuckles are the joints of the fingers. The word is cognate to similar words in other Germanic languages, such as the Dutch "knokkel" (knuckle) or German "Knöchel" (ankle), i.e., Knöchlein, the diminutive of the German word for bone (Knochen). Anatomically, it is said that the knuckles consist of the metacarpophalangeal[1] (MCP) and interphalangeal (IP) joints of the finger. The knuckles at the base of the fingers may be referred to as the 1st[1] or major knuckles while the knuckles at the midfinger are known as the 2nd[2] and 3rd, or minor, knuckles. However, the ordinal terms are used inconsistently, and can be found referring to any of the knuckles.[3][4]

Knuckle
Anatomical terminology

Cracking

The physical mechanism behind the popping or cracking sound heard when cracking joints such as knuckles has recently been elucidated 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]

  • Clenched fist with knuckles (encircled in red)
  • Motorcycle racing gloves with titanium knuckle protectors

References
  1. Utah Mountain Biking; Thumb Sprain; First as metacarpo.
  2. Second knuckle as thumb's interphalangeal, "anchor knuckle" as metacarpophalangeal
  3. First knuckle as thumb's interphalangeal
  4. First knuckle as distal interphalangeal
  5. Kawchuk GN, Fryer J, Jaremko JL, Zeng H, Rowe L, Thompson R (2015) Real-Time Visualization of Joint Cavitation. PLoS ONE 10(4): e0119470. https://doi.org/10.1371/journal.pone.0119470
  6. Johns Hopkins Arthritis Center, Knuckle Cracking and Arthritis
  7. Brodeur R. (1995). "The audible release associated with joint manipulation". J Manipulative Physiol Ther. 18 (3): 155–64. PMID 7790795.

 This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Knuckle". Encyclopædia Britannica. 15 (11th ed.). Cambridge University Press. p. 883.

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