Study Design. Cross-sectional study.
Objective. To establish a statistic model defining the relationship between L5 incidence (L5I) and pelvic incidence (PI) in normal population and to illustrate the importance of L5I in maintaining the normal sagittal profile of spine.
Summary of Background Data. L5I was proposed for the evaluation of postoperative improvements in high-grade isthmic spondylolisthesis. It has been proven that a postoperative improvement in L5I was correlated with a better surgical outcome, indicating that the restoration of L5I is important in restoring normal sagittal balance and spinal morphology.
Methods. This retrospective study recruited 138 healthy volunteers, who were stratified into 2 groups according to age: adolescent group (13–18 yr) and adult group (19–35 yr). Parameters including PI, pelvic tilt, sacral slope, lumbar lordosis (LL), upper arc of the LL, lower arc of the LL, L5I, lumbosacral angle, and height of L5–S1 disc were measured on the long-cassette standing upright lateral radiographs of the spine and pelvis. The relations between all parameters were determined via the Pearson correlation coefficient (r). Linear regression was applied to model the relationship between PI and L5I.
Results. This study demonstrated significant correlation between L5I and PI with a coefficient of 0.818, and a formula based on linear regression was established to estimate the ideal L5I from PI as follows: L5I = 0.725 PI − 12.757. Pelvic tilt, sacral slope, lumbosacral angle, LL, lower arc of the LL, and height of L5–S1 disc were also found strongly correlated with L5I, whereas no correlation was found between upper arc of the LL and L5I.
Conclusion. Strong correlations were found between L5I and spinopelvic parameters in healthy subjects, indicating that L5I was an important parameter in local lumbosacral alignment. Ideal postoperative L5I could be mathematically calculated by PI according to the formula as follows: L5I = 0.725 PI − 12.757, suggesting that calculated ideal L5I could be regarded as a guide in surgical decision making.
Level of Evidence: 3