The Vertical Projection of the Sum of the Ground Reactive Forces of a Standing Patient Is Not the Same as the C7 Plumb Line

In Brief Study Design. A radiographic study of 153 normal volunteers. Objectives. 1) To test the hypothesis that the vertical projection of the sum of the ground reactive forces of a standing patient is located in the same place in the sagittal plane as the C7 plumb line; 2) to determine if there are consistent geometric relationships between the location of the top of the spine and the pelvis in the sagittal plane that occur in individuals without symptoms of back pain or radiographic evidence of deformity. Summary of Background Data. Defining the optimal state of spinal balance is difficult. A full understanding of the compensatory relationships between the spine, pelvis, and lower limbs remains elusive. Methods. A total of 153 normal volunteers were subjected to radiographic examination using a digital force plate, a stabilized standing position, a standardized radiographic technique, and the computerized measurement of sagittal alignment. Results. 1) The C7 plumb line and the gravity line in a stabilized standing position are not located in the same place; 2) the association between the center of T1 and the sacral endplate may be an anatomic constant and a marker of spinal balance in individuals without symptoms of back pain or radiographic evidence of deformity, and is determined by the formula 99° − 0.1° (sacral slope). Conclusions. We speculate that this information will be very helpful in evaluating symptomatic spinal disease in the context of the overall alignment of the spine and pelvis. A radiographic study of 153 normal volunteers examination using a digital force plate, a stabilized standing position, a standardized radiographic technique, and the computerized measurement of sagittal alignment was performed. In this study, 1) the vertical projection of the sum of the ground reactive forces and the C7 plumb line were not found to be in the same position in the sagittal plane; and 2) the association between the center of T1 and the sacral endplate can be defined by a geometric formula of 99° − 0.1° (sacral slope).