Adult Spinal Deformity: The Complete Guide to Sagittal Balance

In the last two decades, the field of spinal surgery has undergone a Copernican revolution. We have moved away from focusing solely on the coronal plane (scoliosis) to recognizing that the sagittal plane is the primary driver of pain and disability.

Adult Spinal Deformity (ASD) is not just a cosmetic issue; it is a functional catastrophe. As patients lose their lumbar lordosis, they are forced to tilt their pelvis back and bend their knees to keep their head over their feet. This compensatory effort is exhausting.

This guide provides a comprehensive breakdown of the spinopelvic parameters, classification systems, and surgical strategies that define modern deformity practice.

The Concept of the "Cone of Economy"

Jean Dubousset introduced the concept of the "Cone of Economy." In a balanced spine, the body maintains an upright posture with minimal muscle activity. The center of gravity falls within a narrow cone.

• Balanced: Minimal energy expenditure.
• Imbalanced: As the trunk leans forward (positive sagittal balance), the paraspinal muscles must fire constantly to prevent falling. This leads to fatigue, mechanical back pain, and eventually, the inability to stand upright.

Spinopelvic Parameters: The Physics of the Spine

To plan a correction, you must measure the deformity. These parameters are the language of spine surgery.

1. Pelvic Incidence (PI) - The Genetic Fingerprint

The PI is a morphological parameter. It is defined by the anatomy of the pelvis and does not change with patient positioning.

• Definition: The angle between a line perpendicular to the sacral plate and a line connecting the midpoint of the sacral plate to the center of the femoral heads.
• Normal: 50° ± 10°.
• Significance: PI determines the amount of Lumbar Lordosis (LL) a patient needs.
  • Low PI (<45°): Requires less lordosis (flat back is normal for them).
  • High PI (>60°): Requires significant lordosis to be balanced.

2. Pelvic Tilt (PT) - The Compensation

The PT is a positional parameter. It measures how much the patient has rotated their pelvis backwards to compensate for a forward trunk.

• Definition: The angle between the vertical and the line connecting the midpoint of the sacral plate to the femoral head axis.
• Normal: < 20°.
• Clinical Pearl: A high PT (>25°) means the patient is recruiting their hips to stand up. It is a sign of hidden deformity.

3. Sacral Slope (SS)

• Definition: The angle between the horizontal and the sacral plate.
• Formula: PI = PT + SS.
  • This equation is fundamental. Since PI is constant, if PT goes up (retroversion), SS must go down (vertical sacrum).

4. Lumbar Lordosis (LL)

• Measurement: Cobb angle from the superior endplate of L1 to the superior endplate of S1.
• Distribution: 2/3 of the lordosis is in the lower lumbar spine (L4-S1).

5. Sagittal Vertical Axis (SVA)

• Definition: The horizontal distance between a plumb line dropped from the center of C7 and the posterior superior corner of the sacrum.
• Normal: < 5 cm.
• Significance: This is the measure of global alignment. Positive SVA = Patient is falling forward.

The SRS-Schwab Classification

The Scoliosis Research Society (SRS) and Frank Schwab developed a classification that correlates with Health-Related Quality of Life (HRQOL). It consists of a descriptive curve type (Coronal) and three sagittal modifiers.

Coronal Curve Types

• Type T: Thoracic Major Curve (>30°, apex T9 or higher).
• Type L: Lumbar/Thoracolumbar Major Curve (>30°, apex T10 or lower).
• Type D: Double Major Curve.
• Type N: No major coronal deformity.

The Sagittal Modifiers (The "0, +, ++" System)

These are the targets for surgery.

Clinical Goal: The goal of surgery is to return the patient to "Grade 0" in all modifiers.

Surgical Planning: The Math of Correction

How much correction do we need? The classic formula is: Target LL = PI ± 9°

Example: Patient has PI of 60°. Current LL is 20°. Mismatch = 40°. Target LL = 60°. Required Correction = 40° of lordosis.

How to get 40° of Lordosis?

We use osteotomies. The Schwab Osteotomy Classification grades them by resection amount:

1. Grade 1 (PCO): Facetectomy / Ponte Osteotomy. yields ~5-10° per level.
2. Grade 2 (SPO): Smith-Petersen (taking both facets + ligamentum flavum).
3. Grade 3 (PSO): Pedicle Subtraction Osteotomy. A wedge resection of the vertebral body. Yields ~30° at one level.
4. Grade 4 (B-VCR): Bone-Disc-Bone resection.
5. Grade 5 (VCR): Vertebral Column Resection. Complete removal of one or more segments.

Alternatively, Anterior Column Reconstruction (ACR) using ALIF or XLIF with hyper-lordotic cages can provide 15-20° or even 30° of lordosis per level without the morbidity of a PSO.

Complications: The Price of Correction

Deformity surgery has high complication rates (up to 40-50%).

Proximal Junctional Kyphosis (PJK)

The spine fails at the level immediately above the fusion.

• Risk Factors: Over-correction of lordosis (especially utilizing a PSO), osteoporosis, stopping the fusion at the "peak" of the kyphosis.
• Prevention: Soft landings (tethering, cement augmentation of the UIV), matching the correction to the patient's age-adjusted alignment goals.

Rod Fracture

• Cause: Pseudarthrosis (failure of bone healing).
• Prevention: Use multiple rods (3 or 4 rod constructs), Cobalt Chrome rods, and aggressive use of BMP/Bone graft.

Conclusion

Adult Spinal Deformity is a three-dimensional problem that requires a four-dimensional solution (considering the 4th dimension of time/aging). The spinopelvic parameters are not just numbers; they are the blueprint for a patient's quality of life. Understanding PI, LL, and the SVA is the first step in mastering this complex field.

Clinical Trap: Do not aim for the same alignment in a 75-year-old as you would in a 25-year-old. Elderly patients tolerate mild positive balance better than "perfect" alignment, which often leads to PJK. Use Age-Adjusted Alignment Goals.

References

1. Schwab, F., et al. (2012). "Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning surgical reconstruction." Spine.
2. Lafage, V., et al. (2009). "Pelvic tilt and truncal inclination: two key radiographic parameters in the setting of adults with spinal deformity." Spine.
3. Terran, J., et al. (2013). "The SRS-Schwab adult spinal deformity classification: assessment and clinical correlations based on a prospective operative and nonoperative cohort." Neurosurgery.