Global Sagittal balance of the Spine
The overall alignment of the spine, known as the Global Sagittal Balance (GSB), is an essential factor in clinical evaluations and surgical outcomes. Evaluating both regional and global sagittal spinal alignment is crucial in minimizing potential complications, including adjacent segment disease, pseudarthrosis, sagittal imbalance, and progressive deformity.
The evaluation of GSB of the spine in asymptomatic controls and symptomatic individuals involves several parameters:
- SSA (spino-sacral angle): measures the angle formed between the center of the C7 vertebra and the center of the S1 endplate along with the line passing through the sacral plateau.
- SVA (sagittal vertical axis): measure the horizontal distance between the C7 plumb line and the posterior superior S1 corner.
*A positive SVA is noted when the C7 plumb line is anterior to the posterosuperior corner of the sacrum, while a negative SVA is observed when the C7 plumb line is posterior to the posterosuperior corner of the sacrum. The SVA parameter is used to assess the sagittal balance of the spine, where a positive value indicates a kyphotic curve and a negative value represents a lordotic curve.
- TPA (T1 Pelvic Angle): measures the angle between the line from the femoral head to the centroid of T1 and the line from the femoral head to the middle of the S1 endplate.
- T1SPI (T1 spinopelvic inclination): measured by calculating the angle between a vertical plumb line and a line drawn from the vertebral body centroid of T1 to the centroid of the bi-coxo-femoral axis.
- SFD (C7/sacro-femoral distance) ratio, also known as the Barrey index, is determined by dividing the distance between the midpoint of the C7 vertebra to the posterior superior corner of the sacrum along the plumb line by the SFD. This is an additional parameter used to evaluate the global sagittal balance of the spine.
- OD-HA (Odontoid Hip Axis): is measured as the angle between the vertical line and a line connecting the highest point of the odontoid to the midpoint of the bi-coxo-femoral axis.
- FBI (Full Balance Index): calculated using three parameters: the C7-translation angle (C7TA), the femoral obliqueness angle (FOA), and the pelvic compensation angle (PTCA). By adding these three values, the FBI can determine the amount of sagittal angle correction needed to achieve a balance that is tailored to the patient’s specific needs.
Since there is no universally ideal sagittal shape, determining optimal values for GSB parameters can be challenging in some cases. However, understanding the typical range of GSB values in asymptomatic individuals is crucial for evaluating and managing spinal pathologies in our patients.
Literature has reported different results for GSB outcomes and it’s hard to determine accurate reference ranges for “normal” values. Many factors like age, gender, health, BMI, ethnicity/race, and others can affect these values.
The goal of an ideal spinal balance is to reduce stress on different parts of the spine such as the vertebral bodies, disks, and facet joints. Currently, various parameters have been developed to assess spinal balance, including measurements of distance or angles. It is recommended to consider all of these parameters for assessing spinal balance.
Literature shows that angular parameters, such as SSA, are preferred over distance parameters, such as SVA, due to being less affected by differences in radiographic techniques. However, the best parameters for assessing spinal balance are still uncertain and require more research.
Although literature focuses on the geometry of spinal balance, the biomechanics of spinal balance are not fully understood, and new parameters are still being introduced.
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I am Vedant Vaksha, Fellowship trained Spine, Sports and Arthroscopic Surgeon at Complete Orthopedics. I take care of patients with ailments of the neck, back, shoulder, knee, elbow and ankle. I personally approve this content and have written most of it myself.
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