Somatic balance is a manifestation of posture corresponding to anatomy and function, especially coronal and cross-sectional balance has been a key indicator for joint surgeons to focus on and judge the success of surgery, however, trunk sagittal balance as an indicator for determining trunk balance involving the spine, pelvis, hip and knee, although it has begun to be emphasized and studied in spine surgery [1], it still has not gained enough attention in the field of joint surgery. The importance of the spine has been studied in spine surgery [1], but it has not received enough attention in the field of joint surgery. In patients with ankylosing spondylitis or a history of internal fixation of the spine for joint replacement, because of compensatory changes in the spine, trunk sagittal balance plays an important role in guiding and analyzing the treatment plan for hip and knee surgery in these patients. In this article, we review the literature and provide a concise review of anatomical parameters related to sagittal plane trunk balance of the spine, pelvis, and hip in the context of joint surgery. The ultimate goal of trunk balance is to keep the femoral head at the base of the pelvis in balance with minimal muscular effort, and in sagittal balance the pelvic incidence (PI), pelvic tilt (PT) and sacral slope (SS) are important for assessing the spine, pelvis and hip, SS), are important in assessing the morphology of the spine, pelvis, hip and knee joints and the center of gravity of the trunk. The pelvic incidence angle PI is the angle between the midpoint of the line connecting the centers of the femoral heads on both sides in the sagittal plane and the line connecting the midpoint of the upper sacral plate with the vertical line above the midpoint of the sacral plate. The pelvic tilt angle PT is the angle between the midpoint of the line connecting the torso vertical line in the sagittal plane with the center of the femoral head on both sides and the line connecting the midpoint of the upper sacral end plate, while the sacral tilt angle SS is the angle between the torso horizontal line in the sagittal plane and the angle above the sacral end plate [2].PI is mainly used to assess the morphology of the sacral pelvis by the relationship between the sacral plate and the femoral head, which is a morphological constant indicator for each body [3], while PT and There is a close relationship between PI, PT and SS, which has been shown to be PI = PT + SS, and this functional relationship between the three is similar in men and women [4]. The PI value determines the position of the sacral plate in relation to the femoral head [2]. Patients with higher PI values exhibit a horizontally oriented pelvis with a larger anterior-posterior diameter, when the femoral head is anterior to the midpoint of the sacral plate in the sagittal plane and has a better adaptation to sagittal imbalance, allowing a greater posterior tilt and increasing the posterior eccentric moment of the sacrum and femoral head in the sagittal plane, but the hip extension restrains the posterior tilt of the pelvis so that SS does not readily occur at 0°. In contrast, patients with lower PI values exhibit a very narrow pelvic ring in the anterior-posterior pelvic diameter, and the SS is more restricted in its ability to adapt, making SS more prone to 0°. SS is a directional reference indicator of the sacral plate in the horizontal line. PT increases when the pelvis is rotated posteriorly (posterior tilt) and decreases when the pelvis is rotated anteriorly (anterior tilt). Therefore, PT and SS are postural parameters that change inversely with postural changes. The ability of the pelvis to rotate along the femoral head (anterior or posterior tilt) around the bilateral hip-femoral axis may be one of the best regulatory mechanisms for sagittal plane balance. Based on the relationship between PI, PT and SS, PI can directly influence the possible values of PT and SS, thus regulating the maximum or minimum compensatory capacity for sagittal imbalance. Studies have indicated that the ideal non-pathological upper limit of PT should be less than 50% of the PI value . Similarly, the ideal value of SS should be greater than 50% of the PI value [5]. In patients with severe ankylosing spondylitis, whose PI values are low, as mentioned earlier hip extension does not limit the posterior tilt of the pelvis, thus a SS of 0 degrees is possible to achieve [6]. The pelvic morphology or position determines the sacro-pelvic parameters, and the anterior pelvic projection or Lewinnek plane (anterior pelvic plane) is the classical parameter used for joint prosthesis orientation in total hip replacement (THA), and the safe position from the orthopantomograph of the acetabular prosthesis is an anterior tilt angle of 5° to 25° and tilt angles of 30° to 50° [7]. Therefore, spinal fusion or ankylosis has a tremendous impact on the hip joint, and the difficulty of joint prosthesis adjustment in these patients further illustrates the importance of these sagittal plane anatomical parameters. Influence of sagittal anatomical parameters of the pelvis on acetabular abduction angle in hip replacement The interdependence of acetabular tilt and SS in the sagittal plane can be clearly seen in the lateral view [8]. In the standing position, SS values are higher while the acetabular cup tilt is smaller. In contrast, in the sitting position, the SS value decreases and the cup tilt increases, while the acetabular prosthesis in THA shows a vertical, i.e. increased, abduction angle in the sitting position compared to the standing position in the lateral views. The mean coronal abduction angle of the acetabular cup in the standing position ranged from 49 to 52°, while in the sitting position it ranged from 57 to 64°. Meanwhile, the abduction angle in the sagittal plane ranges from 36 to 47° in the standing position and from 51 to 58° in the seated position [9, 10]. Changes in these parameters in acetabular tilt can guide the direction of the “anterior opening” of the acetabular prosthesis and reduce the impingement of the acetabular and femoral head prostheses, which has implications for the stability of the hip replacement and early and late prosthetic wear. However, abnormal body posture can disrupt this distribution, for example, excessive posterior pelvic tilt is usually a compensator for anterior trunk tilt, and this adaptive postural change can cause the acetabulum to be more vertical than normal in the frontal view, i.e., increased abduction, and the hip to be in a hyperextended position in the standing position. This posterior extension of the hip will result in posterior impingement of the joint. A typical example is when hip replacement is performed in patients with ankylosing spondylitis, because the compensatory posterior tilt of the pelvis increases, and if the prosthesis is placed in a standard position with reference to the bony markers of the pelvis, resulting in a large abduction and excessive “anterior opening” of the acetabulum, postoperative posterior impingement and anterior dislocation of the hip prosthesis occurs when the patient is in a standing position The excessive anterior tilting of the pelvis in the sitting position causes anterior impingement [11-15], which is the cause of pain and accelerated lining wear after hip arthroplasty in patients with spinal ankylosis. Influence of pelvic sagittal plane anatomical parameters on the anterior tilt angle of hip replacement acetabular prosthesis The anatomical anterior tilt angle is the angle between the pelvic orthostatic axis and the acetabular axis when the transverse reference plane projected to the middle of the pelvic sagittal plane is perpendicular to the longitudinal axis [16].CT is considered a direct method of measuring the anatomical anterior tilt angle from a transverse section, but this machine based arbitrary adjustment of the measured anterior tilt angle is not suitable. Therefore, it is particularly important to assess the true THA acetabular anteversion angle (functional acetabular anteversion angle) in cases of trunk instability, especially when the lumbosacral joint is stiff or in an abnormal position [17-19]. Functional acetabular anteversion is measured by the forward opening of the acetabular cup and varies with PT, with a corresponding increase in anteversion with increasing PT. The functional anteversion angle calculated at the CT level, especially at the level of the upper sacral plate CT, can be seen as a reflection of the SS measured on lateral slices. In the standing position, the functional anteversion angle is shown to be smaller in cross section than in the sitting position, while in the sitting position the acetabular cup is shown to open completely forward, thus avoiding acetabular impingement at the femoral neck, which is beneficial for hip flexion. In the supine position, although the PT is greater than in the standing position and the anterior spinal convexity increases, with the extension of the lower limbs, the anterior tilt remains smaller than in the standing position. Some patients with ankylosing spondylitis who have complete stiffness of the lumbosacral joints have less change in the angle of anterior tilt in these adaptations in the standing and sitting positions, resulting in being in a repetitive impingement state. These patients experience a reduction in spinal pronation and compensatory posterior pelvic tilt, while the acetabulum is always in excessive anterior tilt so as not to interfere with hip flexion, but this causes posterior impingement of the acetabulum, progressive loss of hip extension in the standing position, and accelerated wear of the lining [20]. Summary The acetabular abduction angle and functional anteversion angle are positively correlated with PT and negatively correlated with SS. The influence of acetabular sagittal parameters on the placement of acetabular abduction angle and anteversion angle should be considered during hip replacement in patients with spinal fusion or ankylosis to enable optimal placement of the prosthesis position in such patients and to reduce postoperative prosthesis impingement syndrome and liner wear.