The so-called less common diseases of the hip joint are relatively common diseases of the hip that are often treated by orthopedic surgeons and have more clinical practice, such as osteonecrosis of the femoral head, osteoarthritis secondary to hip dysplasia, ankylosing spondylitis and rheumatoid arthritis involving the hip. This issue focuses on several related papers, reviews and tutorials, with the aim of disseminating knowledge and experience in the treatment of the less common diseases of the hip, so that orthopaedic surgeons can be more alert to these diseases, in order to diagnose them earlier, more accurately and treat them more appropriately. Osteonecrosis of the femoral head (ONFH) is a common and difficult to treat disease in orthopedics in China. Orthopedic surgeons have rich experience and correct judgment on the definition, diagnosis points, staging and treatment of this disease, but it is undeniable that in daily medical work, it is not uncommon to misdiagnose other similar diseases of the hip, such as acetabular dysplasia secondary to osteoarthrosis, ankylosing spondylitis involving the hip, as ONFH, and even some experts and monographs engaged in research and clinical work on osteonecrosis also show the above confusion. In this issue, we published a paper on the definition, diagnostic criteria and differential diagnosis of ONFH, in the hope of reducing misunderstandings in this area and preventing the expanded diagnosis and overtreatment of ONFH. After imaging was used for the diagnosis of hip diseases, the diagnosis and treatment of hip diseases have been revolutionized. MRI can enable early and accurate diagnosis of many hip diseases, making up for the shortcomings of ordinary X-rays, CT scans, etc., and has been widely used in orthopedics in recent years. However, there is always a duality of things. The MR images of many hip diseases are very similar, which makes differential diagnosis difficult. Bone marrow edema of the hip is one of the more difficult changes to differentiate. Many hip pains, such as ONFH, bone marrow edema syndromes (BMES), inflammation, trauma, and tumors, often show bone marrow edema on MR examination, and the differential diagnosis of bone marrow edema is a knowledge that orthopedic surgeons should have. In this issue, we have published a review, tutorials and papers that describe the key points of differential diagnosis of bone marrow edema from different perspectives. It is important to emphasize that the differentiation from MR imaging features alone can still be difficult, but close clinical integration, including detailed patient history, signs and other imaging, such as nuclear bone scan, CT and radiographs, will make the differentiation easy and accurate. The primary differentiation of bone marrow edema is to identify whether the edema is reversible or irreversible. Reversible bone marrow edema is defined as BMES, including transient osteoporosis of the hip (TOH), regional migration osteoporosis (RMO), reflex sympathetic dystrophy (RSD), and other forms of osteoporosis. sympathetic dystrophy (RSD) and femoral head contusion and inflammation. Although the etiology of BMES remains unclear, its common feature is self-limiting. In contrast, bone marrow edema in ONFH is irreversible. The concept of bone marrow edema as an early change in ONFH has been incorrectly considered, but this concept has been proven to be completely wrong [3,4]. bone marrow edema in ONFH never occurs before the appearance of band hyposignal, and bone marrow edema in ONFH is always accompanied by severe pain in the hip and limited joint motion, especially internal rotation of the hip. If not effectively treated, the majority of patients subsequently develop femoral head collapse. In addition to irreversible bone marrow edema that occurs with ONFH, there are also subchondral incomplete fractures and osteochondral injuries. MR imaging with the application of various procedures can identify whether the bone marrow edema is reversible or irreversible. Reversible bone marrow edema has low signal on T1WI, high signal on T2WI lipid suppression or STIR, and lack of subchondral changes on T2WI and control-enhanced T1WI, whereas irreversible bone marrow edema has a low signal band in the subchondral bone and a double line sign on T2WI in some patients. In general, reversible myelomeningocele can completely dissipate in 6 to 12 months after drug treatment, while irreversible myelomeningocele inevitably progresses to femoral head collapse in a significant proportion of patients despite effective surgical treatment. Femoroacetabular impingement and the resulting acetabular glenoid labral injury are receiving increasing attention from orthopedic and sports medicine physicians. This type of disease was initially recognized and reported in the 1950s and 1960s, but it was not named FAI until this century after a series of studies on the anatomy, pathology, biomechanics, diagnosis, and treatment of FAI by Ganz and his orthopedic team in Bern, Switzerland. the use of radiographs in various projection orientations, CT, and especially arthrography-assisted magnetic resonance imaging (MRA) were applied. The diagnosis of FAI, especially acetabular labral injury, became accurate and specific. The discovery has also led to a hidden anatomical cause for primary hip osteoarthritis, the etiology of which has been poorly understood for many years. A large number of FAI cases have been diagnosed and treated in Western countries, but not many cases have been reported in China, and the disease is not rare, but the lack of alertness and knowledge of orthopedic surgeons has led to many FAI patients being missed and misdiagnosed. Other less common conditions of the hip include adjacent joint cysts, osteoid osteoma, and pigmented villous nodular synovitis, all of which should be considered in the identification of hip pain and dysfunction, and the related paper is also published in this issue. Total hip arthroplasty is undoubtedly one of the most successful surgeries of the 20th century and occupies an important place in the treatment of hip disorders. However, while THA is widely performed, other hip procedures should not be neglected, such as hip osteotomy (periacetabular, proximal femoral), joint fusion, head and neck resection arthroplasty (e.g. Girdlestone arthroplasty) and hip arthroscopy. The orthopaedic surgeon should learn and master a variety of techniques in order to obtain reasonable treatment options for patients with different hip diseases, ages and conditions.