1. The cause of cyst formation lies in the major overloading of certain regions caused by the unevenness of the articular surface produced by the arthritic process.

2. Photoelasticity investigations disclose an overburdening of the bone beneath the surface of the joint, which in turn markedly resembles the shapes of actual cysts.

3. The results of this investigation fit organically into the logical sequence of processes in accordance with our theory.

4. This theory is further borne out by the successful formation of cyst phenomena in healthy joints by overloading (Trias 1961).

5. Our theory has no immediate practical bearing on the treatment of arthritic cysts. However, it uncovers certain interesting aspects of the behaviour of bone tissues under mechanical overloading above the limits of biological, though below those of mechanical bearing capacity. Conclusions arising therefrom might in many respects be useful in research into the physiology of bone.