慢性下腰痛成人发病率15%至45%，而下腰椎退行性疾病是其发生的结构性因素。骨盆入射角（PI角），是骨盆形态的量化，成人后保持恒定。大量文献报道PI角对下腰椎退行性疾病的发生具有显著影响。但尚无PI角对下腰椎三关节复合体的三维运动和形态学影响的生物力学研究。我们前期研究了PI角与椎小关节退变的关系。我们推测：PI角增加会导致下腰椎三关节复合体的矢状位活动度的增加和椎小关节面发生矢状位化。为此，本研究将根据三维模型上的测量PI角，将受试对象分5组。测量各组下腰骶椎的椎间隙高度和椎小关节冠矢状面成角；完成7个功能性体位下双C型臂正交X线摄像，经图像和系统校对后，导入工程软件，在虚拟环境内完成3D-2D图像配准，再现下腰椎三关节复合体的在体三维运动。精准分析PI角大小对生理载荷下的下腰椎三关节复合体的生物力学影响，为深入研究PI角奠定理论基础，为预测下腰椎退行性疾病的发生发展提供科学依据。

The incidence of chronic low back pain is about 15 to 45 %, and degenerative diseases at the lower lumbar spine is the structural factor. Pelvic Incidence （PI） angle is the quantification of the pelvis anatomical shape, but remains constant since skeletal maturity. Many literatures reported that PI angle has significant effect on the occurrence of various lumbar degenerative diseases. However, there is no biomechanical studies about impacting of PI angle on the three-dimensional movement and morphology at low lumbar spine until now. We evaluated the relationship between PI angle and facet joint degeneration before. We hypothesized that the increase of PI angle resulted in increase of sagittal activity for the lower lumbar three-joint complex and sagittal change for the facet joint articular surface. Therefore our current study will measure the PI angles on the three-dimensional (3D) models and then the enrolled subjects were divided into five groups according to the PI angle. The height of the intervertebral space and the coronal/ saggital angles of the facet joints were also measured on the 3D models. Subjects undertake dual X-ray examination during seven functional positions. After imaging and system calibrations, we imported the post-processing images into the engineering software. In the virtual environment, 3D to 2D image matching was performed in order to reproduce the in-vivo three-dimensional motions of the three-joints complex at low lumbar spine. Through precise analysis of the biomechanical effect of PI angle on the lumbar three-joints complex under physiological loading, we can provide a theoretical basis for further study of the PI angle and provide scientific basis to predict the occurrence and aggravation of degenerative lumbar diseases.