Spine Sagittal Balance

Etiology

Sagittal imbalance can occur from anything that pushes the body’s center-of-mass forward. Several different etiologies exist, including congenital, degenerative, traumatic, and iatrogenic.[3]

Congenital scoliosis can involve fixed kyphotic segments or underdeveloped vertebra which alter spine biomechanics.[4]

Spinal trauma treated nonoperatively or operatively can cause chronic changes in spinal balance, especially with the involvement of the thoracolumbar junction (a relatively neutral transition zone from kyphosis to lordosis). Compression fractures leading to kyphotic vertebral body deformity are a typical example. 

Degenerative disease of vertebral discs causes loss of motion in those segments, which causes excess stress to other regions of the spine, which can lead to chronic loss of sagittal balance. Diseases that cause degenerative or fixed changes of the spine, such as rheumatoid arthritis or ankylosis spondylitis, exhibit this phenomenon.[5]

Fusion procedures involving the thoracolumbar junction cause a proximal lumbar hyperlordosis, which cause the rest of the lumbar and thoracic segments to compensate with either increased lordosis or kyphosis, respectively. With time, the compensatory effect is lost and results in a flat back syndrome with thoracic hyperkyphosis.

Epidemiology

Sagittal imbalance is a complex issue that results as a consequence of a multitude of etiologies of spinal deformity. The incidence of sagittal imbalance from a large cohort study was due to adolescent idiopathic scoliosis (30.4%), kyphotic deformity (20.7%), combined junctional degeneration and iatrogenic imbalance (14%), and neuromuscular, congenital, or scoliosis (25.5%).[1] 

Pathophysiology

Cervical Lordosis  

• normal 20-40°

Thoracic Kyphosis  

• Normal 20-50°. 
• This is predominantly impacted by lumbar and pelvic anatomy.  Thoracic kyphosis can generally range 30 degrees more than lumbar lordosis

Thoracolumbar Junction

• Normally a transition from thoracic kyphosis to lumbar lordosis, the T12/L1 junction should be neutral, with less than 10 degrees or regional kyphosis.

Lumbar Lordosis  

• Average 60°
• A wide range of acceptable values depending upon the pelvic anatomy
• As much as 75% of lumbar lordosis occurs between L4 and S1 with 40-50% occurring at L5/S1

Pelvic Tilt

• Normal <20 degrees

Pelvic Incidence

• Normal 55 +/- 10 degrees

Pelvic Incidence/Lumbar Lordosis Mismatch

• Normal <10 degrees

Sagittal Vertical Axis

• Normal <5mm 

C7 Plumb Line and Sagittal Vertical Axis (SVA)

The C7 plumb line is a radiographic reference to determine the sagittal vertical axis, the most traditional measurement of sagittal balance of the spine.  A vertical line is drawn from the center of the C7 vertebral body in a caudal direction. The line should connect with, or be within 5mm of, the superior-posterior endplate of S1.  This is considered within the tolerable range for health-related quality of life outcomes. However, as patients age, they lean forward and tolerate slightly more positive sagittal alignment.   Any surgical intervention must consider patient selection.

It remains vitally important to release the harmonious anatomy of sagittal plane consideration.  The cervical spine can compensate with a hyperlordotic presentation to maintain a more neutral posture. The patient can further compensate with increased pelvic tilt and knee flexion.  However, this is a physically taxing and non-anatomically normal, fatiguing position.[2] 

Pelvic Incidence

Pelvic incidence is a radiographic measurement comparing the biomechanic relationship between the lumbar spine and the pelvis in patients with differing pelvic anatomy. The pelvic incidence and lumbar lordosis may vary amongst different individuals, but their relationship is maintained to create sagittal balance in the lumbopelvic junction and maintain SVA in the global spine. Fixed sagittal deformities in the lumbar spine are often compensated by changes in pelvic and hip positioning, maintaining SVA.

PI is defined as the angle between a line drawn perpendicularly to the surface of the superior endplate of the sacrum and a line connecting the midpoint of the superior endplate of the sacrum to the center of the femoral head. The normal value for this measurement should be 55 +/- 10 degrees.[6]

Pelvic Tilt

The angle between two of the following radiographic lines

• A line from the center of the S1 endplate to the center of the femoral head
• A vertical line is drawn intersecting the center of the femoral head

The normal value should be less than 20 degrees, can vary with patient positioning and contractures.[7]

History and Physical

A thorough history of medical conditions as well as previous spine pathology must be taken, with careful respect to any prior spine surgeries. Pain must be differentiated from discogenic, mechanical, or neuropathic etiologies.

A standard complete musculoskeletal and neurologic exam should be performed. Examination of the patient must be done in both the supine and standing positions. Gait exam should be performed for secondary compensatory mechanisms.

In addition to a spinal exam, evaluate the patient's hip range of motion and pain for any possible contractures or dysfunction. Attention must also be paid to the cranium with respect to the range of motion of the cervical spine and vertical gaze functional limitations, which are a common consequence of sagittal imbalance.

Evaluation

For imaging, 36-inch standing films are the mainstay of spinal alignment imaging.  It is critical to evaluate both the pelvis, femoral heads, and the spinal anatomy to illustrate the proper alignment, symmetry, and compensation.  Certain measurements are vital. 

The EOS X-ray system is a recent breakthrough in deformity correction imaging. It is a new technology that utilizes Nobel prize-winning concepts in particle detection, allowing 2D to 3D reconstructions from biplanar X-ray images. This machine allows for accurate measurement of spinal parameters in multiple planes, which is crucial in deformity surgery. The advantage of this imaging modality is that it confers 800-1000 times less radiation dose to the patient versus CT for the same image.[8]

CT and MRI may be introduced during surgical pre-operative investigation or evaluation of the patient’s clinical disease. However, the mainstay of diagnosis remains plain films. It is important to remember the CT/MRI modalities are recombinant imaging platforms, not standing.

Treatment / Management

The overall goal is to place the body in a physiologic position so it may maintain its cone of equilibrium with minimal effort.  

Treatment may be non-operative with bracing for structural support or physical therapy for strengthening.

Operative treatment has different levels of invasiveness or bony removal to restore alignment of the spine. These procedures range from pedicle subtraction osteotomy to vertebral column resection, or further correction. Further information on operative technique is covered in the Statpearls chapter on spinal osteotomy.[9]

Differential Diagnosis

Sagittal alignment involves a harmonious relationship between the cranium, spine, and pelvis.  Changes in pelvic anatomy and pelvic alignment can greater alter sagittal plane parameters. 

Sagittal anomalies include anything that alters the regional or global alignment hypolordosis or kyphosis in the lumbar spine (flatback syndrome), kyphosis in the thoracolumbar junction, and hyperkyphosis (such as a Scheuermann’s kyphosis) in the thoracic spine.