Global Sagittal balance of the Spine

Overview

Global Sagittal Balance (GSB) of the spine is a critical aspect of spinal health, playing a significant role in clinical evaluations and surgical outcomes. Understanding the parameters and measurements used to evaluate GSB helps in identifying potential complications and improving treatment strategies. This comprehensive guide aims to simplify the complex concepts related to GSB, making it accessible to everyone.

Understanding Global Sagittal Balance (GSB)

Global Sagittal Balance (GSB) refers to the overall alignment of the spine when viewed from the side. Proper spinal alignment ensures that the body’s weight is evenly distributed, reducing stress on various parts of the spine and maintaining overall balance. Evaluating both regional and global sagittal spinal alignment is crucial in preventing and managing conditions such as adjacent segment disease, pseudarthrosis (failure of bone fusion after surgery), sagittal imbalance, and progressive spinal deformity.

Key Parameters for Evaluating GSB

Several parameters are used to assess GSB, each providing insights into different aspects of spinal alignment:

Spino-Sacral Angle (SSA)

Definition: Measures the angle between the center of the C7 vertebra and the center of the S1 endplate along with the line passing through the sacral plateau.

Importance: Helps in assessing the overall curvature of the spine.

Sagittal Vertical Axis (SVA)

Definition: Measures the horizontal distance between the C7 plumb line (a vertical line through the center of the C7 vertebra) and the posterior superior corner of S1.

Significance: Indicates whether the spine has a forward (positive SVA) or backward (negative SVA) tilt. A positive SVA suggests a kyphotic curve (outward curvature), while a negative SVA indicates a lordotic curve (inward curvature).

 Pelvic Angle (TPA)

Definition: 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.

Usage: Assesses the relationship between the upper spine and the pelvis.

T1Spinopelvic Inclination (T1SPI)

Definition: 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.

Application: Evaluates the tilt of the upper spine relative to the pelvis.

Sacro-Femoral Distance (SFD) Ratio (Barrey Index)

Definition: 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.

Purpose: Another parameter used to evaluate the global sagittal balance of the spine.

Odontoid Hip Axis (OD-HA)

Definition: Measured as the angle between the vertical line and a line connecting the highest point of the odontoid (a projection from the second cervical vertebra) to the midpoint of the bi-coxo-femoral axis.

Function: Assesses the alignment of the upper cervical spine with the pelvis.

Full Balance Index (FBI)

Definition: 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.

Relevance: Provides a comprehensive view of the overall spinal balance, helping to guide corrective measures.

Evaluating GSB in Different Populations

Determining optimal values for GSB parameters can be challenging due to the variability among individuals. Factors such as age, gender, health, body mass index (BMI), and ethnicity can influence these values. Understanding the typical range of GSB values in asymptomatic individuals (those without symptoms) is crucial for evaluating and managing spinal pathologies.

Literature Insights on GSB

Recent studies and literature provide valuable insights into GSB outcomes and the challenges in determining accurate reference ranges for “normal” values. Here are some key takeaways:

1. Variability in Normal Values: There is considerable variation in the reported values for GSB parameters, making it difficult to establish a universal standard.
2. Influence of Demographics: Age, gender, BMI, and other demographic factors significantly affect GSB measurements.
3. Angular vs. Distance Parameters: Angular parameters (e.g., SSA) are generally preferred over distance parameters (e.g., SVA) because they are less affected by differences in radiographic techniques.
4. Biomechanics of Spinal Balance: While the geometry of spinal balance is well-studied, the underlying biomechanics are not fully understood. Ongoing research aims to develop new parameters and improve our understanding of spinal biomechanics.

Clinical Relevance of GSB

Accurate evaluation of GSB is essential for various clinical reasons:

1. Preventing Complications: Proper assessment of spinal alignment helps in preventing complications such as adjacent segment disease, which occurs when the segments next to a spinal fusion undergo degeneration.
2. Guiding Surgical Decisions: Surgeons rely on GSB measurements to plan and execute spinal surgeries, ensuring that the spine is properly aligned post-operation.
3. Tailoring Treatment: Understanding a patient’s specific spinal alignment helps in tailoring treatment plans, whether surgical or non-surgical, to achieve optimal outcomes.
4. Monitoring Progress: Regular assessment of GSB parameters helps in monitoring the progress of treatment and making necessary adjustments.

Practical Applications of GSB Parameters

Let’s delve deeper into how these parameters are practically applied in clinical settings:

1. SSA (Spino-Sacral Angle)
   • Used to evaluate the overall curvature of the spine, especially in cases of scoliosis or kyphosis.
   • Helps in determining the need for corrective surgery and in planning the surgical approach.
2. SVA (Sagittal Vertical Axis)
   • A key indicator of sagittal imbalance. A positive SVA often indicates a need for corrective measures to address forward spinal tilt.
   • Useful in assessing the effectiveness of post-surgical recovery and rehabilitation.
3. TPA (T1 Pelvic Angle)
   • Helps in understanding the relationship between the upper spine and the pelvis, which is crucial in cases of spinal deformities.
   • Guides the placement of surgical implants to ensure proper alignment.
4. T1SPI (T1 Spinopelvic Inclination)
   • Important for evaluating upper spinal tilt, particularly in patients with thoracic spine issues.
   • Assists in planning physiotherapy and other non-surgical treatments to improve posture and alignment.
5. SFD Ratio (Barrey Index)
   • Provides a detailed view of the global sagittal balance, aiding in comprehensive assessment and treatment planning.
   • Useful in longitudinal studies to track changes in spinal alignment over time.
6. OD-HA (Odontoid Hip Axis)
   • Critical for evaluating the alignment of the cervical spine with the pelvis, especially in cases of cervical spine surgeries.
   • Helps in ensuring that upper spinal corrections do not adversely affect overall spinal balance.
7. FBI (Full Balance Index)
   • Offers a holistic view of spinal alignment, combining multiple parameters to guide corrective measures.
   • Essential for complex cases requiring extensive surgical planning and post-operative care.

Conclusion

Understanding and evaluating the Global Sagittal Balance (GSB) of the spine is a complex yet crucial aspect of spinal health. By considering various parameters such as SSA, SVA, TPA, T1SPI, SFD ratio, OD-HA, and FBI, healthcare professionals can gain a comprehensive understanding of a patient’s spinal alignment. This knowledge is essential for preventing complications, guiding surgical decisions, tailoring treatments, and monitoring progress. While the biomechanics of spinal balance are still being explored, ongoing research and advancements in this field promise to improve our ability to manage and treat spinal conditions effectively.