For those wondering what the mechanisms underlying joint cracking are, Gregory N. Kawchuk and colleagues provided interesting insights within a study published in PLoS One in 2015. The authors used real-time magnetic resonance imaging to visualize the cracking of metacarpophalangeal joints, initiated through increasing mechanical long-axial traction. Therein, the authors were able to demonstrate that synovial joint cracking is not associated with the collapse of pre-existing bubbles (as presumed) but rather cavity inception. The observation is consistent with a process called tribonucleation whereas opposing surfaces resist separation force until they rapidly separate while creating gas cavities. This study might explain one of the questions osteopaths are facing most often in clinical practice: why do joint manipulations produce a cracking noise?
Kawchuk, G.N., Fryer, J., Jaremko, J.L., Zeng, H., Rowe, L., Thompson, R., 2015. Real-time visualization of joint cavitation. PLoS One, 10(4), e0119470.