The author comes into contact with many patients with different spinal disorders in his daily work, among which lumbar disc herniation and lumbar spondylolisthesis are the most common. These two diseases are distinct, and the average age of patients with disc herniation is relatively small compared to that of lumbar spondylolisthesis. So is age a determining factor? Why do patients with lumbar spondylolisthesis not have herniated discs when they are younger? To figure this out, we have to understand the sagittal balance of the spine, which is called the sagittal plane, meaning the side. Humans are the only animals that walk upright. In order to adapt to upright walking, the human spine developed four physiological curves from the sagittal plane with anterior convexity of the cervical vertebrae, posterior convexity of the thoracic vertebrae, anterior convexity of the lumbar vertebrae and posterior convexity of the sacral vertebrae. The importance of sagittal balance of the spine in maintaining an upright posture can be seen when the human body is standing and the back muscles maintain proper tension to keep the entire trunk balanced and not leaning forward or backward. In the process of describing and studying the sagittal plane of the spine, spine experts have developed different sagittal parameters of the spine, the most important of which is the pelvic incidence PI (pelvic incidence), as shown in the figure below, the line between the center of the femoral head or hip axis and the center of the upper endplate of sacrum 1 reflects the thickness and direction of the pelvis, and its angle with the vertical line of the upper endplate of sacrum 1 is the pelvic incidence PI, which is an anatomical parameter used to describe pelvic morphology independent of posture and is constant after skeletal development. The other two parameters are postural correlates: pelvic tilt angle (PT) and sacral tilt angle (SS), which describe the spatial orientation of the pelvis and are influenced by the individual’s position and posture. These three parameters have the following mathematical relationship: PI=PT+SS. Moreover, the anterior convexity of the lumbar spine LL is approximately equal to PI+9, which means that the larger the pelvic incidence angle is, the more the lumbar spine is bent forward, and the physiological curve of such people may look better, for example, the pelvic incidence angle of European and American people is generally larger than that of Chinese people, but the more the lumbar spine is bent, the greater the shear force between the vertebrae, and the vertebrae are prone to The more curved the lumbar spine is, the greater the shear force between the vertebrae, and the more likely it is for the vertebrae to slip out of place. This biomechanical analysis is supported by a number of clinical studies at home and abroad. At present, several studies at home and abroad have shown that the PI of the pelvic incidence angle is smaller in patients with lumbar disc herniation, while the PI is larger in patients with lumbar degenerative slippage than in controls, and the PI often exceeds 60° in severe slippage (3-4 degrees), and overweight and large sacral tilt angle are also risk factors for anterior slippage of L4, which cannot be limited by a sagittal position of the articular eminence joint. Sagittal balance of the spine also interacts with other spinal disorders. Scoliosis is often associated with sagittal deformity, and sagittal force line changes have been reported in lumbar scoliosis >15°. Degenerative scoliosis often has reduced LL and vertebral rotation with poor compensatory capacity, resulting in a tendency for anterior trunk displacement and forward lean during standing and walking, and is an important factor in pain and dysfunction, which often outweighs scoliosis. Posterior convexity deformities such as congenital deformities, Sheuermann’s disease, ankylosing spondylitis, medically induced flat back, tuberculosis, tumors, and trauma result in a direct impact on the sagittal force line, with compensatory capacity depending on a variety of factors such as age, site and degree of deformity. Congenital deformities are adequately compensated, but can be lost with age. Ankylosing spondylitis posterior convexity deformity PI increases and loss of compensatory capacity after spondylopelvic fusion. Multiple vertebral fractures due to osteoporosis and osteogenesis imperfecta increase TK and decrease LL, even with sagittal imbalance. Therefore, for people with a relatively small pelvic incidence angle, it is important to pay attention to the posture of daily life and work to avoid excessive stress on the intervertebral discs, while for people with a relatively large pelvic incidence angle, exercising good lumbar back muscles and avoiding obesity may be an effective way to prevent lumbar spondylolisthesis.