Fig. 1.

The OA and PsA overlap model and variation of the enthesis with pathology

Normal entheses are usually observed in individuals <40 years of age. Thickened entheses are often seen with age with no arthritis, but similar appearances are also evident in early OA or in asymptomatic joints of OA. Classical inflamed entheses are seen in most PsA joints, where there can be diffuse bone oedema on MRI, while classical degenerative entheses are typical of advanced OA joints often accompanied by osteophytes and cartilage loss with joint space narrowing. However, there is a subset of joints where there are overlapping features that can cause a diagnostic challenge, often with some degree of inflammation or degenerative changes that can be accepted as either OA or PsA.