The Importance of Spino-Pelvic Balance After Spinal Instrumentation for High Grade Developmental Spondylolisthesis

• (a, b - Medtronic Sofamor Danek)

The question of whether high grade L5-S1 developmental spondylolisthesis should or should not be reduced by surgery remains a very controversial subject. The gold standard of treatment has been in situ fusion, but some studies have reported a high rate of pseudarthrosis, further progression of the slip, as well as persistent cosmetic deformity . The use of modern spinal instrumentations has generated a renewed interest for attempting reduction prior to fusion, but the indications for this treatment remain poorly defined. Recently, pelvic incidence (PI), a fundamental anatomic parameter which is specific and constant for each individual and which strongly determines pelvic orientation (PI=SS+PT) as well as the size of lumbar lordosis (LL), has been shown to be significantly increased in subjects with spondylolisthesis and to be highly correlated with the sacral slope (SS), pelvic tilt (PT) and L5 incidence (IL5). The purpose of this study was to determine which of these measures of sagittal spino-pelvic balance are improved and/or associated with an improved clinical outcome after surgery, with the hypothesis that changes in position of L5 over S1 will be associated with an improvement in LL and a better outcome. We conducted a retrospective multi-centre study of 66 subjects (mean age 20±7 years) with developmental spondylolisthesis (15 grade II, 37 grade III, 12 grade IV and 2 grade V) and a minimum follow-up of 2 years after reduction and posterior fusion with spinal instrumentation. Measures of sagittal balance were obtained with a computer software designed to measure PI, SS, PT, LL, IL5, grade, slip angle (SA) and thoracic kyphosis (TK) on standing lateral digitised X-rays of the spine and pelvis, with a very high inter and intra observer reliability. Functional outcome was rated as good, fair or poor by the treating physician at last follow-up. Statistical analysis was done with Pearson correlation tests, Student tests and multivariate analysis. Results indicate that PI, SS, PT and TK remained unchanged after surgery, while grade, IL5, SA, and LL were significantly improved. A post op improvement in IL5, SA and LL was correlated with a better outcome while subjects with a poor outcome had a higher pre op grade, IL5, SS and LL. These results suggest that pelvic morphology (PI) and pelvic balance (SS, PT) are unchanged after surgery, but that spinal shape and spinal balance (LL, IL5, SA) are improved. The key to a successful outcome is the repositioning of L5 over S1 as measured by IL5 and SA: since L5 is now fused to the pelvis after surgery, IL5 takes over PI as the anatomic signature for pelvic morphology and becomes the main determinant of lumbar lordosis and improved functional outcome.

• If noted, the author indicates something of value received. The codes are identified as: a-research or institutional support; b-miscellaneous funding; c-stock or stock options; d-royalties; e-other financial or material support.