Using spinopelvic parameters to estimate residual lumbar lordosis assuming previous lumbosacral fusion – a study of normative values

Publication date: Available online 16 August 2017
Source:The Spine Journal
Author(s): Hwee Weng Dennis HEY, Kimberly-Anne TAN, Shashidhar Bangalore KANTHARAJANNA, Alex Quok An TEO, Chloe Xiaoyun CHAN, Ka-Po Gabriel LIU, Hee-Kit WONG
Background contextPelvic Incidence (PI) = Pelvic Tilt (PT) + Sacral Slope (SS) is an established trigonometric equation which can be expanded from studying the fixed pelvis with the spine to a fixed spinopelvic complex with the remnant spine, in scenarios of spinopelvic fusion or ankylosis. For a fixed spinopelvic complex, we propose the equation termed: Lumbar Incidence (LI) = Lumbar Tilt (LT) + Lumbar Slope (LS).PurposeTo establish reference values for LI, LT and LS at each lumbar vertebral level, and to show how LI can be used to determine rLL.Study DesignThis is a cross-sectional study of prospectively-collected data, conducted at a single academic tertiary healthcare center.Patient sampleThe study included 53 healthy patients aged 19-35 with first episode mechanical low back pain for a period of <3 months. Patients with previous spinal intervention, those with known or suspected spinal pathologies, and those who were pregnant, were excluded.Outcome Measures: Radiological measurements of lumbar incidence (LI), lumbar tilt (LT), lumbar slope (LS) and residual lumbar lordosis (rLL).MethodsAll patients had full body lateral standing radiographs obtained via a slot scanner. Basic global and regional radiographic parameters, spinopelvic parameters, and the aforementioned new parameters were measured. LI was correlated with rLL at each level by plotting LI against rLL on scatter plots and drawing lines-of-best-fit through the datapoints. No funding was received for the purposes of this study, and there are no conflicts of interest to declare.ResultsThe mean value of L5I was 22.82°, L4I was 6.52°, L3I was -0.92°, L2I was -5.56° and L1I was -5.95°. LI turns negative at L3, LS turns negative at the L3/4 apex, and LT remains positive throughout the lumbar spine. We found that the relationship of LI with its corresponding rLL follows a parabolic trend. Thus, rLL can be determined from the linear equations of the tangents to the parabolic lumbar spine. We propose the LI-rLL method for determining rLL as the LI recalibrates via spinopelvic compensation post-instrumentation, and thus the predicted rLL will be based on this new equilibrium, promoting restoration of harmonized lordosis. The rLL:LI ratio is a simplified, but less accurate, method of deriving rLL from LI.ConclusionsThis study demonstrates the extended use of PI = PT + SS proposed as LI = LT + LS. These new spinopelvic reference values help us better understand the position of each vertebra relative to the hip. In situations when lumbar vertebrae are fused or ankylosed to the sacrum to form a single spinopelvic complex, LI can be used to determine rLL, to preserve spinal harmony within the limits of compensated body balance.



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