'Cracking joints'. A bioengineering study of cavitation in the metacarpophalangeal joint.

The cracking of joints is a common phenomenon which interests patients and clinicians alike. Its exact nature has remained in doubt, as evidenced by a recent comment in the British Medical Journal (1969). At the knee and hip, tendons over bony prominences can cause 'clicking' which may sometimes be audible. The tensor fascia lata is particularly apt to do this over the greater trochanter. The majority of cracks do not appear to arise from this mechanism. The only detailed study of cracking in the metacarpophalangeal joint, as far as the authors are awNare, is that of Roston and Wheeler Haines (1947). Before this work bubbles had been observed in joints by Fick (1911), Dittmar (1933), and Nordheim (1938), who were interested in them as a means of obtaining radiographs of fibro-cartilage in the knee without using a contrasting medium. In the present communication it will be shown that this bubble is not the cause but the effect of the crack, and that fluid 'cavitation' is responsible for the cracking noise. In addition, suggestions will be made to explain why some joints cannot be cracked and why, having been cracked, about 20 minutes must elapse before a joint can be cracked again.