Normal Spinal Curvatures and Alignment¶

Definition¶

The normal adult vertebral column exhibits four physiologic curvatures in the sagittal plane — cervical lordosis, thoracic kyphosis, lumbar lordosis, and sacral kyphosis — that together form an S-shaped configuration. These curvatures develop during growth, provide mechanical advantages for load-bearing and shock absorption, and are essential parameters in evaluating spinal pathology and surgical planning.

Anatomy¶

Sagittal Curvatures¶

Curvature	Type	Development	Normal Range
Cervical	Lordosis (concave posteriorly)	Secondary — develops when infant holds head up (~3 months)	20°–40° (C2–C7 Cobb angle)
Thoracic	Kyphosis (convex posteriorly)	Primary — present from fetal development	20°–45° (T1–T12 Cobb angle)
Lumbar	Lordosis (concave posteriorly)	Secondary — develops when child begins walking (~12–18 months)	40°–60° (L1–S1 Cobb angle)
Sacral	Kyphosis (convex posteriorly)	Primary — fixed due to sacral fusion	Fixed; contributes to pelvic incidence

Primary vs. secondary curvatures:

Primary (kyphotic): thoracic and sacral — present at birth; related to the flexed fetal position
Secondary (lordotic): cervical and lumbar — develop postnatally in response to muscular activity and weight-bearing

Sagittal Balance Parameters¶

Global sagittal balance is assessed by the relationship between the head and pelvis:

C7 plumb line (sagittal vertical axis, SVA): a vertical line dropped from the C7 vertebral body; normally falls within 2 cm of the posterior superior corner of S1
SVA >5 cm indicates positive sagittal imbalance (forward lean), associated with pain and disability
Pelvic incidence (PI): a fixed anatomic parameter = pelvic tilt + sacral slope; determines the amount of lumbar lordosis needed for balance
PI–LL mismatch: when lumbar lordosis is insufficient for a given pelvic incidence (PI minus LL >10°), positive sagittal imbalance results

Coronal Alignment¶

The normal spine is straight in the coronal plane (AP view)
Scoliosis is defined as a lateral curvature >10° measured by the Cobb angle
Mild coronal imbalance is common and may be a normal variant if <10°

Measurement Techniques¶

Cobb angle method:

Identify the most tilted vertebra above and below the curve apex
Draw lines along the superior endplate of the upper vertebra and the inferior endplate of the lower vertebra
The angle between these lines (or their perpendiculars) is the Cobb angle

Clinical Pearl

Sagittal balance is increasingly recognized as the most important radiographic parameter in spinal deformity surgery. A positive SVA (>5 cm) correlates strongly with pain and disability, even more than the magnitude of scoliosis in the coronal plane. The concept of PI-LL mismatch is fundamental to surgical planning: the goal is to restore lumbar lordosis to within 10° of the pelvic incidence to achieve balanced alignment.

Imaging Findings¶

Radiography¶

Standing full-length (36-inch) scoliosis radiographs are the standard for alignment assessment:

AP view: Cobb angle for scoliosis; coronal balance (C7 plumb line to center sacral vertical line)
Lateral view: sagittal curvatures, SVA, pelvic parameters (PI, PT, SS)
Flexion/extension views: evaluate dynamic instability and segmental motion
Standing films are essential — supine imaging underestimates deformity

CT¶

Provides detailed osseous anatomy for surgical planning
3D reconstructions useful for complex deformities
Quantitative measurements of vertebral rotation (axial plane)

MRI¶

Finding	Significance
Loss of cervical lordosis / straightening	May indicate muscle spasm, degenerative disease, or trauma
Increased thoracic kyphosis	Scheuermann disease (>45°), osteoporotic wedge fractures, ankylosing spondylitis
Loss of lumbar lordosis	Degenerative flat back, post-surgical, muscle spasm
Scoliosis with cord abnormality	Syrinx, tethered cord, or tumor may be the underlying cause — MRI is essential in atypical or left-sided thoracic scoliosis

Key Imaging Finding

In adolescent scoliosis, a left-sided thoracic curve is considered atypical (most idiopathic scoliosis is right thoracic) and warrants MRI to exclude an underlying cause such as a syrinx, Chiari malformation, or intramedullary tumor. Similarly, any scoliosis presenting with pain, rapid progression, or neurologic signs should be evaluated with MRI regardless of curve direction.

Key Points¶

The adult spine has four sagittal curvatures: cervical and lumbar lordosis, thoracic and sacral kyphosis
Primary (kyphotic) curvatures are present from fetal life; secondary (lordotic) curvatures develop postnatally
Sagittal balance (SVA, PI-LL mismatch) is the most important alignment parameter for clinical outcomes
Scoliosis is defined as a coronal Cobb angle >10°
Standing full-length radiographs are the standard for alignment assessment
Atypical scoliosis patterns (left thoracic, painful, rapidly progressive) require MRI to exclude underlying pathology

References¶

Martini ML, Neifert SN, Chapman EK, Mroz TE, Rasouli JJ. Cervical Spine Alignment in the Sagittal Axis: A Review of the Best Validated Measures in Clinical Practice. Global Spine J. 2021;11(8):1307-1319. PMID: 33203239. https://pmc.ncbi.nlm.nih.gov/articles/PMC8453677/
Abrisham SMJ, Ardekani MRS, Mzarch MAB. Evaluation of the Normal Range of Thoracic Kyphosis and Lumbar Lordosis Angles Using EOS Imaging. Maedica (Bucur). 2020;15(1):87-91. PMID: 32419866. https://pmc.ncbi.nlm.nih.gov/articles/PMC7221279/
Philippi M, Shin C, Quevedo S, Weiner J, Chavarria J, Avramis I, Rizkalla JM. Roussouly classification of adult spinal deformity. Proc (Bayl Univ Med Cent). 2024;37(4):688-691. PMID: 38910817. https://pmc.ncbi.nlm.nih.gov/articles/PMC11188786/
Merrill RK, Kim JS, Leven DM, Kim JH, Cho SK. Beyond Pelvic Incidence-Lumbar Lordosis Mismatch: The Importance of Assessing the Entire Spine to Achieve Global Sagittal Alignment. Global Spine J. 2017;7(6):536-542. PMID: 28894683. https://pmc.ncbi.nlm.nih.gov/articles/PMC5582711/
Slattery C, Verma K. Classification in Brief: SRS-Schwab Classification of Adult Spinal Deformity. Clin Orthop Relat Res. 2018;476(9):1890-1894. PMID: 29601382. https://pmc.ncbi.nlm.nih.gov/articles/PMC6259802/
Kim D, Davis DD, Menger RP. Spine Sagittal Balance. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. https://www.ncbi.nlm.nih.gov/books/NBK534858/
Sagittal balance. Radiopaedia.org. https://radiopaedia.org/articles/sagittal-balance