A proposed in vitro model for investigating the mechanisms of 'joint cracking': a short report of preliminary techniques and observations.

Joint "cracking" is common but not a clearly understood audible phenomenon. In this brief report we propose an in-vitro model to potentially assist in revealing a mechanism for, and therefore source of, this phenomenon. Using a suction cup under tension and de-nucleated fluid to simulate synovial fluid, an audible release with intra-articular cavity formation was elicited. This was followed by a refractory period during which no audible crack could be elicited until the observed cavity had slowly reabsorbed back into the joint fluid. Conversely, if regular fluid containing pre-existing nuclei was used, a cavity formation occurred but with neither an audible release nor subsequent refractory period. With this simple in-vitro model, we were able to reproduce the characteristic audible release, cavity formation and related refractory period typically observed in related experiments in human joints. This simple in-vitro model may be of use in helping to discern both the timing and precise nature of other yet to be discerned mechanisms related to joint cracking.