The concept of “ankylosing spondylitis” was introduced in about 1900, when the clinical diagnosis could only be made based on the fact that the patient’s spine was already ankylosed. In the 1930s, the clinical use of radiology led to the discovery that the onset of ankylosing spondylitis often began in the sacroiliac joint. Therefore, sacroiliac arthritis became an important basis for the diagnosis of ankylosing spondylitis. The diagnostic classification criteria for ankylosing spondylitis have evolved through the Rome criteria in 1961, the New York criteria in 1967, and the revised New York criteria in 1984. Although the revised New York criteria are still widely used in clinical practice, radiological changes in the sacroiliac joint at grade II or above are a necessary condition, while radiographs can only detect structural damage complicated by inflammation, not inflammation itself. This results in a delay of 5-10 years in the diagnosis of AS, which obviously does not meet the requirements of early diagnosis and early treatment. In 1974, the term “spondyloarthropathy (SpA)” was proposed to generalize the spectrum of clinical diseases with common features such as AS, psoriatic arthritis, Wright’s syndrome/reactive arthritis, enteropathic arthritis, and leukoarthrosis. Neither the diagnosis of SpA is based on radiological alterations of the sacroiliac joint as a necessary condition. the classification of SpA by the ESSG criteria is mainly based on inflammatory low back pain and asymmetric peripheral oligoarthritis, since SpA diagnosed by this criterion includes two groups of patients: those with radiological damage of the sacroiliac joint and those who have not yet developed radiological damage of the sacroiliac joint. Therefore, a new concept of “medial SpA” was developed. It is more appropriate to refer to those patients with SpA with inflammatory low back pain as “medial SpA” rather than AS, because AS implies that “ankylosis” has already occurred and does not apply to patients who are still in the early stages, or even to the minority of patients with SpA who are mild and do not have radiological damage throughout their lives. A small number of patients with SpA have mild disease and do not experience radiological damage throughout their lives. The current majority view is that non-radiographic mid-axis SpA and AS are two stages of one disease, i.e., the radiographic manifestations of sacroiliac arthritis will appear sooner or later as time progresses. Similar clinical features exist for both subtypes, particularly in relation to disease activity and comparable treatment response rates. However, non-radiographic mid-axis SpA has more heterogeneity compared to AS due to the self-limiting nature of the disease itself and its slow progression. The concept of non-radiological mid-axis SpA was developed more on the basis of the need to classify certain patients rather than the need to diagnose a particular disease.