Sagittal alignment of the spine: What do you need to know?
Introduction
The human spine is a biomechanical masterpiece, enabling bipedalism through unique skeletal alterations and resulting in the “S-shape” spinal curvature. For instance, the lumbar curvature deliberately maintains the center of gravity over a narrow area between the feet, maximizing energy efficiency while minimizing the effect of gravity on joints, muscles, and ligaments.
Longstanding investigations on spinal evolution, function, and pathology have renewed a desire to study sagittal spino-pelvic alignment. Focused research has recently converted theoretical concepts into clinically relevant guidelines, establishing the role of sagittal alignment in treatment of various spectrums of spinal pathologies.
Sagittal alignment, often misrepresented as sagittal balance, describes the ideal and “normal” alignment in the sagittal plane, resulting from the interplay between various organic factors. Any pathology that alters this equilibrium instigates sagittal malalignment and its compensatory mechanisms. As a result, sagittal malalignment is not limited to adult spinal deformity; its pervasiveness extends through most spinal disorders. With this in consideration, sagittal malalignment is not a sole indication for surgery. However, the systematic assessment of sagittal plane is essential in evaluating the burden of spinal diseases on our patients.
This article aims to provide a pragmatic review of the most clinically relevant sagittal radiographic parameters in the literature. At the end of this review, the reader should be able to analyze the sagittal plane of the spine, identify compensatory mechanisms, and choose patient-specific alignment targets.
Section snippets
Pelvic to spine concept
The pelvis is the cornerstone of spinal sagittal alignment. Jean Dubousset's revolutionary concept that “the entire pelvis is a vertebra” triggered numerous investigations on the relationship between pelvic morphology and spinal sagittal alignment. Ginette Duval-Beaupere's work [1], [2] identified three geometrically interrelated pelvic parameters: pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS); where PI = PT + SS.
Because of the limited mobility of the sacroiliac joint, pelvic
Sagittal alignment is not restricted to deformity patients
In line with the growing interest of analyzing the sagittal plane, there is another growing concern about the applicability of this analysis in daily practice, especially for clinicians who deal with non-deformity patients. However, the literature reports the pervasive involvement of sagittal alignment in numerous spinal pathologies, such as degenerative disc disease, osteoporosis, arthritis and low back pain. Respecting the sagittal plane is not restricted to major deformity procedures, but
Sagittal alignment targets: an update
Based on the tight relationship between sagittal plane deterioration and quality of life measures, Schwab et al. incorporated sagittal parameters into an adult spine deformity classification system and determined cutoff values for the most clinically relevant parameters in the sagittal profile (SVA, PI-LL, and PT) based upon multi-center data [46]. Ideally, Schwab recommends a SVA <40 mm, a PI-LL within 10° and a PT <20° as the targets for sagittal alignment. However, age-related changes occur
Conclusion
Sagittal malalignment plays a respectable role in multiple if not all spinal pathologies and therefore the benefit of analyzing sagittal alignment is not limited to deformity patients. While further research is developing, the literature thus far reports a direct relationship between sagittal alignment and patient-reported outcomes in various spinal pathologies, attesting to the benefit of sagittal alignment analysis. The simplicity and generalized applicability of the SRS-Schwab classification
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