Sagittal balance describes the alignment of the spine when viewed from the side, determining whether the head and trunk are positioned directly over the pelvis. This alignment is fundamental for maintaining an upright posture with minimal muscular effort. When balance is disrupted, the body works harder to remain vertical, leading to pain and physical dysfunction. Positive sagittal balance is a common deviation where the trunk shifts forward, placing the body’s weight ahead of the hips. This forward lean is associated with a stooped appearance and increases the energy required for standing and walking. The goal of treatment is to restore this alignment to a neutral position.
The Mechanics of Spinal Alignment
The body’s ability to stand upright efficiently depends on a harmonious relationship between the curves of the spine and the orientation of the pelvis. Normal spinal alignment features a gentle “S” curve, with an inward curve in the lower back (lumbar lordosis) and an outward curve in the mid-back (thoracic kyphosis). This structure ensures that the center of gravity is maintained directly over the pelvis and lower limbs.
A standard measurement for global alignment is the C7 plumb line, a vertical line dropped from the center of the seventh cervical vertebra. In neutral sagittal balance, this line falls near the back-upper corner of the first sacral vertebra (S1). Alignment is defined as “positive” when the C7 plumb line falls more than two centimeters in front of the S1 point, indicating the upper body is leaning forward relative to the hips.
The pelvis plays a substantial role in compensating for changes in spinal curvature. Two important measurements, pelvic incidence and pelvic tilt, describe the shape and position of the pelvis and its relationship with the spine. A fixed structural issue, such as a loss of lumbar lordosis, forces the pelvis to rotate backward, increasing the pelvic tilt to pull the C7 plumb line back toward neutral. This compensatory mechanism is physically taxing and requires constant muscle contraction.
Causes and Symptoms
Positive sagittal balance primarily develops due to degenerative changes that flatten the spine’s natural curves over time. Degenerative disc disease is a frequent cause, as the collapse of intervertebral discs leads to a progressive loss of the inward curve (lordosis) in the lower back. This loss of lordosis is often a major component of flatback syndrome, which pushes the trunk forward.
Acquired kyphosis, an abnormal increase in the outward curvature of the thoracic spine, can also contribute to this forward shift. Previous spinal fusion surgeries that failed to restore or preserve lumbar lordosis are another common cause, since fused segments cannot adjust to maintain balance. Conditions causing fixed changes to the vertebral structure, such as spinal trauma or ankylosing spondylitis, can similarly disrupt the alignment.
The most debilitating symptom is chronic fatigue. The body’s inability to rest its weight over the hips forces the back and leg muscles to constantly fire to prevent falling forward. Patients often experience severe low back pain, as the posterior musculature is under continuous strain. Difficulty standing upright for extended periods is common, leading many people to develop a crouched gait by bending their hips and knees to shift their center of gravity backward.
Non-Surgical Management
For individuals with mild to moderate positive sagittal balance or those who are not surgical candidates, non-surgical management focuses on symptom relief and optimizing compensatory mechanisms. Physical therapy is a primary component, aiming to strengthen the core muscles that support the spine. Strengthening the back extensors and abdominal muscles helps increase the muscular capacity to hold the body upright.
Flexibility exercises are also important, particularly for the hamstring and hip flexor muscles, which often become tight due to the crouched posture. Targeted pain management may involve non-steroidal anti-inflammatory drugs (NSAIDs) to reduce muscle strain and inflammation. Spinal injections, such as epidural steroid injections, can treat localized nerve irritation or joint pain.
External bracing may provide temporary support and improve posture in certain situations. However, non-surgical treatments cannot correct the underlying fixed skeletal deformity. They manage associated pain and fatigue by improving muscular endurance and flexibility, but they do not fundamentally change the spinal column’s forward tilt.
Surgical Correction Procedures
Surgical intervention is typically reserved for severe, fixed positive sagittal balance that does not respond to conservative management and causes significant disability. The primary goal is to restore normal spinal alignment, shifting the C7 plumb line back over the pelvis and reducing the energy required for standing. This restoration is achieved through techniques designed to create an inward curve in the lumbar spine.
Spinal fusion is a fundamental element, stabilizing multiple vertebral segments after corrective maneuvers. The most powerful technique used to correct fixed sagittal imbalance is an osteotomy, which involves cutting and reshaping the bone to realign the spine. The Pedicle Subtraction Osteotomy (PSO) is a common and effective procedure for significant deformities.
The PSO procedure involves removing a precise, wedge-shaped section of bone from one or more vertebrae, often in the lower lumbar spine. This excision includes the posterior elements, the pedicles, and a portion of the vertebral body, allowing the surgeon to close the gap and effectively “bend” the spine backward. A single-level PSO typically achieves a substantial correction, often providing 20 to 30 degrees of lordosis necessary to restore balance.
Less extensive osteotomies, such as the Smith-Petersen osteotomy, are used for more flexible deformities or when less correction is needed, as they remove only the posterior elements of the spine. After the osteotomy, instrumentation, including rods and screws, is applied to maintain the corrected alignment while the bone segments fuse. This complex reconstruction aims to reduce chronic muscular strain and fatigue, leading to significant improvement in posture and quality of life.