Hip dysplasia is defined as an abnormal relationship between the femoral head and the acetabulum, and is generally referred to as dislocation, subluxation, and some other manifestations of hip instability. Numerous studies have shown that ultrasound has unique advantages for early diagnosis of hip dysplasia, but there are still controversial issues. In this article, we will review the images, image analysis and problems of ultrasound diagnosis of hip dysplasia in infants. Hip dysplasia is an abnormal relationship between the femoral head and the acetabulum, which generally refers to dislocation of the femoral head, subluxation and some other manifestations of hip instability. Early detection of hip dysplasia has become a hot spot in clinical research because it often leaves serious sequelae for the children if it is not detected at an early stage and treated actively. Ultrasound can detect hip abnormalities earlier than physical examination and X-ray. Because it is non-invasive and has the ability to distinguish between bone and soft tissue, ultrasound is preferred by foreign countries for the examination of infant hip development [1]. I. Ultrasound anatomy of the joint On ultrasound images, the normal cartilaginous femoral head in infants up to 6 months of age is spherical, with a diameter of about 1.2-2.1 cm, which is very low in echoes, and has a pointy internal hyperechoic distribution, probably due to vascularization. Some of the greater trochanters share a common cartilaginous origin with the femoral head and are ultrasonographically continuous with the femoral head and have the same echogenic properties. Ultrasound distinguishes very small ossification centers of the femoral head earlier than X-ray. Enlargement of the ossification center can obscure the display of the acetabulum. The acetabulum is composed of three bones connected by a Y-shaped cartilage. The vertical portion of the cartilage is a valuable landmark for ultrasound because it can identify the true acetabulum and provide the approximate extent of the acetabular center. In the transverse sweep obtained by placing the probe on the lateral thigh, the acetabulum is shown as a V-shaped structure with acoustic shadows caused by the pubic bone and sciatic bone. The sciatic component is seen to be larger than the pubic component through the vertical portion of the Y-shaped cartilage, but the femoral head is roughly equidivided by a continuous line through the acoustic beam in the central portion of the acetabulum. A full view of the vertical Y-shaped cartilage component is seen in the coronal plane. Occasionally a small amount of fluid is visible within the normal hip joint. The muscles surrounding the joint are also well visualized. On real-time ultrasound, the femoral head can be seen rotating within a fixed acetabulum. II. Types of sonography Depending on the method of examination, it can be categorized into real-time ultrasound and dynamic ultrasound.Graf et al. used real-time ultrasound to observe the development of the hip joint. Graf et al. used real-time ultrasound to observe the development of the hip joint, which was mainly examined when the infant was in hip flexion 35, internal rotation 10, and lateral recumbency, emphasizing the morphological observation of the hip joint. It can not only observe the relationship between the femoral head and acetabulum from different angles or dynamically, and distinguish the femoral head, acetabulum and cartilage tissues, but also has the characteristics of non-invasive and non-radiation. Real-time ultrasound probes are divided into line array and sector type. Only line array probes can be used to observe the morphology of the hip joint, and different frequencies can be used to obtain high-resolution images depending on the age of the infant, thus allowing serial quantification of the development and maturation of the hip joint and semi-quantitative evaluation of its stability. The method emphasizes the viewing angle to ensure that the femoral head and acetabulum are viewed through the bony markers of the hip.Graf draws three lines near the acetabulum: the first line (baseline) connects the bony acetabular rim (bony convexity) to the point at which the capsule attaches to the periosteum, the second (inclined line) connects the bony convexity to the glenoid rim of the acetabulum, and the third (acetabular apex line) connects the inner lower rim of the acetabulum to the bony convexity. Two angles are then measured. α angle is the angle between the acetabular apex line and the baseline, which measures the convexity of the bony acetabulum. a small α angle indicates a shallow bony acetabulum. β angle is the angle between the baseline and the obliquity line, which indicates the extent to which the cartilaginous acetabular rim provides additional coverage of the femoral head. a large β angle indicates lateral displacement of the femoral head. Based on these two angles, the hip is classified into five types. type I is normal, with an α angle greater than 60 and a β angle less than 55. type II has delayed ossification, with an α angle between 44 and 60, and a β angle between 55 and 77. type II is classified into two categories, type IIa is within three months and has an α angle less than 50, and type IIb is greater than three months. type III has an α angle less than 43, and a β angle greater than 77, and is further classified into IIIa and IIIb. type IIIa has an α angle less than 43, and a β angle greater than 77, and is further classified into IIIa and IIIb. Type IIIa is characterized by a hypoechoic hyaline cartilage rim in the hip joint, while type IIIb is characterized by degenerated hyaline cartilage with stronger echoes than the femoral head. type IV is characterized by a complete dislocation of the hip joint. The main disadvantage of real-time ultrasound is that the results are influenced by subjective factors, but its diagnostic criteria are easier to establish. Dynamic ultrasonography is performed when the hip joint is in motion and under stress, and the method is based on the Ortolani and Barlow test.Haroke et al. evaluated hip development by ultrasonography during postural position of the Ortolani and Barlow test, which emphasizes the position and stability of the femoral head. This multidirectional dynamic examination evaluates the position and stability of the femoral head and hip development. Under compression, the position of the normal femoral head is stable, whereas mild dislocation of the femoral head can occur in newborns. this should disappear after 4 weeks, but in infants with hip dysplasia strong echoes are obtained due to the persistence of the dislocation of the femoral head, with fibrofatty tissue filling the space between the femoral head and the acetabulum. Grissom and Harcke performed longitudinal and transverse views of the hip through the lateral approach. Their positions included supine and lateral positions. The presence of subluxation of the femoral head under the labrum should be noted in hip flexion of 90. It should be noted that there can be a physiologic 4-6 cm subluxation during the first week of life in newborns. Dynamic ultrasound is more subjective than real-time ultrasound, but it is based on the Ortolani and Barlow test, and its clinical use has demonstrated its effectiveness in the early detection of hip abnormalities in infants. Based on the fact that these two methods have their own advantages and disadvantages, Harcke, Grat, and Clarke et al. proposed the use of a combination of real-time and dynamic ultrasound with evaluation criteria that include hip morphology and stability. The examination must include coronal views at rest and longitudinal views under compression. In addition bony landmarks and some angles can be measured depending on the circumstances. The detection rate is improved. Ultrasonography in infants Ultrasonography observes the development of the femoral head and acetabulum and uses these anatomical structures as the markers for evaluation of development. 2-8 months old infants should pay attention to the development of the ossification center of the femoral head, which can be obviously changed in a short time, and as the ossification center grows the lower rim of the acetabulum becomes fuzzy, and ultrasonography can’t distinguish the different key anatomical structures of the hip joint when approaching the age of 1 year. However, since there can be a physiologic 4-6 cm subluxation in the first week of life, the first ultrasound examination should be performed within the first 4-6 weeks of life. One of the current controversies is whether to perform ultrasound on all or on newborns with risk factors. Although ultrasound is noninvasive, it is not a gratuitous examination [2]. Some authors have suggested that ultrasound should be performed on newborns with risk factors, but the question is whether this would increase the incidence of late diagnosis; Clarke et al. compared the two in newborns with risk factors and concluded that the latter would not increase the incidence of late diagnosis; Boeree et al. examined 26,952 high-risk infants using a combination of clinical examination and ultrasound, and the rate of late diagnosis was 0.02%, with a rate of 0.3%, and a rate of 0.3%, with a rate of 0.4%. The rate of overdiagnosis was 0.02%, which is similar to the rate of overdiagnosis from previous censuses. The second point of controversy is whether the sensitivity of ultrasound leads to overtreatment [3]. It has been shown that not all infants with abnormalities detected by ultrasound in the first 4-6 weeks of life require treatment. It is now recognized that in infants older than 4 weeks, if sonographic abnormalities persist, the physician should decide as soon as possible whether to continue observation or begin treatment. Most scholars now believe that mild abnormalities can continue to be closely monitored, but ultrasound should be performed every 4 weeks [4]. Ultrasound monitoring of treatment Children younger than 3 months of age with mild hip instability should receive ultrasound every 1-2 months to clarify hip development and take timely treatment. For children who have been braced, they should be examined every 3-4 weeks for those younger than 3 months and every 6 weeks for those older than 3 months. For type IIb and III hips should be examined every 3 months within one week of age. Infants treated with Pavlik stent should be followed up with ultrasound, which has its own unique advantages in monitoring the efficacy of stenting such as improvement in stability of the hip and position of the femoral head after stenting [5]. V. Conclusion Ultrasonography is an effective method for early detection of hip dysplasia. Real-time ultrasonography emphasizes morphological features. Dynamic ultrasound is performed during hip motion and stress, which is based on the Ortolani and Barlow test, and also evaluates hip development. There is controversy about the scope of ultrasound and whether it can lead to overtreatment, but in general ultrasound is an effective, rapid and safe method and allows effective monitoring of treatment outcomes [6].