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- Discussion:
    - as slip progresses, area of contact between L5 & S1 decreases & body of L5 tilts forward on sacrum;
          - this is referred to as sagittal rotation, angle of slipping, roll, L5/S1 kyphosis, or gibbus;
    - slippage is not present in pts who have mild (grade-I or II) spondylolisthesis, but is more common in pts who have a 50 % slip & is
            always present in pts w/ 75 % slip;
    - risk factors for slippage:
          - preop slip angle > 35-40 deg;
          - slip of more than 50%;
          - kyphotic slip angle
          - female;
          - dysplastic spondylolisthesis
          - back pain;
          - abnormal gait;
    - measurement:
          - angle that is formed by extending a line along anterior border of body of L5 until it intersects line that has been drawn along the 
                 posterior border of the body of first sacral vertebra;
          - slip angle is measured by drawing a line perpendicular to line drawn along posterior aspect of first sacral vertebral body and 
                 measuring angle between that and a line parallel to inferior end plate of L5