L5 incidence: an important parameter for spinopelvic balance evaluation in high-grade spondylolisthesis

In high-grade spondylolisthesis (HGSPL), the pelvic incidence (PI) is not a reliable measurement because of doming of the sacrum. Measurement of L5 incidence (L5I) was described as a tool to measure pelvic morphology in HGSPL and for surgical follow-up.

We aimed to evaluate L5I in HGSPL and its relationship to other spinopelvic parameters.

A retrospective study of a cohort of 184 patients with HGSPL was carried out.

Whole spine radiographs were analyzed for PI, pelvic tilt (PT), sacral slope, lumbar lordosis (LL), L5I, L5 tilt (L5T), L5 slope, lumbosacral kyphosis, and slip percentage. Statistical analysis and correlation were made (Pearson correlation test; p<.05). In accordance to Cohen, statically significant correlation were considered strong if R>0.5, moderate if 0.3

A total of 184 cases were analyzed, with a female-to-male ratio of 2.35 and a mean age of 20.1 years. Sacral doming was present in 73% of the cases. Mean L5I incidence was 65.2° and strongly correlated to other sagittal parameters, especially PT (R=0.7), LL (R=0.7), L5T (R=0.77), and L5 slope (R=0.83). There was less but still strong correlation to Dubousset lumbosacral kyphosis (Dub-LSK) angle (R=-0.63) and percentage of slippage (R=0.56). L5 tilt showed nearly perfect correlation to PT (R=0.95). Forty-four percent of the cases were balanced HGSPL, whereas 56% of the cases were unbalanced HGSPL. L5 incidence was found to be a good predictor of local imbalance with a cutoff of L5I=61.

This paper describes L5I, which is a positional parameter in HGSPL. L5 incidence is a simple and reliable measure in the preoperative setting in HGSPL. Its correlation with spinal sagittal parameters (especially PT and LL) is strong and was found to be better than Dub-LSK. There is also a good correlation between L5I and HGSPL pelvic parameters (slip percentage and lumbosacral kyphosis). We recommend its use for evaluation of surgical correction and recommend the value of 60° as cutoff value to define spinopelvic balance in HGSPL.