'Ely's cysts' are a feature of:

In severe degenerative joint disease (DJD), the glenoid fossa may appear grossly enlarged because of erosion of the posterior slope of the articular eminence. This erosion may allow the condylar  head  to  move  forward  and  superiorly  into  an  abnormal anterior  position  that  may  result  in  an  anterior  open  bite.  The condyle may also be markedly diminished in size and altered in shape  because  of  severe  erosions.  In  some  cases,  small,  round, radiolucent areas with irregular margins surrounded by a varying area of increased density are visible deep to the articulating surfaces. These lesions are called Ely cysts or subchondral bone cysts, but they are not true cysts; they are areas of degeneration that contain fibrous tissue, granulation tissue, and osteoid. Oral radiology ; White and Pharaoh 7th edition page no: 509