Sagittal Alignment of the Neutral Spine

The purpose of this page is to demonstrate the scientific support of the geometric configuration chosen for our 3D model of the spine. This alignment is also selected as the default spine position for the application "Biomechanics of the Spine."

The sagittal alignment of the spine describes the curvature of the spine in the sagittal plane. The key points of that description are the spine's physiological curves–cervical/lumbar lordosis and thoracic/sacral kyphosis. However, it is a simplified representation that could be compared to the iceberg size estimation by looking at its top part seen above the water. The more robust perception also includes the geometry and spatial orientation of the sacrum and pelvis, the global sagittal balance of an axial skeleton, and the projection of gravity centerline to the spine curves:

Basic measurements for the asessment of the sagital alignment of the spine

Summary of measurements necessary for assessing the sagittal alignment of the spine

The set of measurements used to build the model of the human spine.

It is surprising, but there is no universal formula for the neutral sagittal alignment of the spine. Each angle and distance is with a relatively large variance. The best way to illustrate that phenomenon is to cite one of the most relevant sources–the textbook "Clinical biomechanics of the spine" by White and Panjabi–"The normal lumbar lordosis angle supported by the 20–40-year-old literature data lies within large range from 20° to 70° and an analogous thoracic kyphosis angle–from 20° to 50°".

Since these words were written, the medical imaging technique and methodology applied in research evolved significantly. Now we know more about multiple factors affecting the physiological curves of the spine, like differences between genders and ethnic groups (Roopnarian 2011, Zhu 2014), changes of spine curves during a lifetime (Iorio 2018), and an effect of physical activity (Todd 2015). However, even after selecting the study data by sex and age, we see highly variable results. This is why the choice of a particular configuration for our model was navigated by the range of standard deviations reported in multiple research papers and not by the data from the single trustable source.

The literature review presented in this webpage is in the form of graphs that were composed following the uniform logic:

The review is arranged in 3 parts. The first focuses on the spine's physiological curves (kyphoses and lordoses), the second part reviews the geometry and spatial orientation of the pelvis, and, finally, the third is dedicated to the global sagittal balance of the spine as a whole.

The Neutral Spine Curves for an Adult Man in Standing Standard Anatomical Position

The Cervical Lordosis

An evaluation of the cervical lordosis angle usually includes at least two levels: (1) angle between C1–C2 and (2) cervical lordosis from C2 to C7. The angle between C1–C2 in our model is equal to 23.5° that is in agreement with literature data.

C1-C2 angle — The C1–C2 angle was subtended by a line drawn parallel to the inferior aspect of C1 and a line drawn parallel to the inferior endplate of C2

C1-C2 angle measurement technique and the scientific basis for 23.5° value — The C1–C2 angle was subtended by a line drawn parallel to the inferior aspect of C1 and a line drawn parallel to the inferior endplate of C2

The cervical lordosis from C2 to C7 in our model is 17.7°, which is close to the average reported values in multiple studies. However, this cervical lordosis angle may be higher than reported for some Asian populations (Hasegawa 2017, Ao 2019).

C2-C7 angle — The C2–C7 angle was subtended by a line drawn parallel to the posterior border of the C2 vertebral body and a line drawn parallel to the posterior border of the C7 vertebral body

C2-C7 cervical kyphosis angle measurement technique and the scientific evidence behind — The C2–C7 angle was subtended by a line drawn parallel to the posterior border of the C2 vertebral body and a line drawn parallel to the posterior border of the C7 vertebral body

The Thoracic Kyphosis

The most commonly used methods for measuring thoracic kyphosis are (1) the angle between the Th1 upper plate and the Th12 lower plate or (2) the angle between the Th4 upper plate and Th12 lower plate. The corresponding kyphosis angles for our 3D model are respectively 30.7° and 47.0°. These angles fit well into the range reported in the scientific literature.

Thoracic kyphosis measurement technique — Thoracic kyphosis angle was measured from the superior endplate of Th1 and Th4 to the inferior endplate of Th12

Thoracic kyphosis measurement technique with the scientific basis for the 30.7°/47.0° angles — Thoracic kyphosis angle was measured from the superior endplate of Th1 and Th4 to the inferior endplate of Th12

The Lumbar Lordosis

The lumbar lordosis measurement technique differs between the studies. Below we rely on the most commonly used approach – angle measurement from the L1 upper plate to the S1 upper plate. Our model lumbar lordosis is equal to 55.3° that fits within the range from 36°(Endo 2014) to 63.5°(Legaye 2008) reported in many recent studies.

Lumbar lordosis angle L1-S1 — Lumbar lordosis was measured from the superior endplate of L1 to the superior endplate of S1

Angle of the lumbar lordosis: the measurement technique and the scientific evidence behind — Lumbar lordosis was measured from the superior endplate of L1 to the superior endplate of S1

The Configuration and Spacial Orientation of the Pelvis

The Sacral Slope

The sacral slope is considered one of the primary determinants of the lumbar spine curvature (Roussouly 2011b, Duval-Beaupère 1992) and the spine as a whole. The sacral slope of the Anatomy Standard spine model is 38.8° that is in agreement with the range generally accepted as a standard for a healthy spine and allocates the spine of our 3D model to Type 3 according to the classification proposed by Roussouly and colleagues in 2003 (Roussouly 2003). This type of spine curvature being reported as the most common for young and physically active men (Todd 2015).

Sacral slope measurement technique — The sacral slope was defined as the angle subtended by a horizontal reference line and the sacral endplate

The angle of the sacral slope: the measurement technique and the scientific evidence behind — The sacral slope was defined as the angle subtended by a horizontal reference line and the sacral endplate

The Pelvic Tilt

Pelvic tilt represents a pelvic rotation in the sagittal plane (Le Huec 2011) that now substituted pelvic inclination mentioned in older literature sources (Platzer 2003; Anda 1990). The pelvic tilt of our 3D model is 14.0°, and it is strongly evidence-based.

Pelvic tilt measurement technique — The pelvic tilt was defined as the angle subtended by a vertical line through the femoral heads axis and a line drawn from the midpoint of the sacral endplate to the femoral heads axis

Angle of the pelvic tilt: the measurement technique and the scientific evidence behind — The pelvic tilt was defined as the angle subtended by a vertical line through the femoral heads axis and a line drawn from the midpoint of the sacral endplate to the femoral heads axis

The Pelvic Incidence

The pelvic incidence is an integral parameter deriving from the sacral slope and pelvic tilt, affecting both the sagittal curvature of the spine and the pelvis's spatial orientation in a standing position. It is supposed to be a fundamental pelvic parameter for the spinal curves' three-dimensional regulation (Legaye 1998). Our model's pelvic incidence is 52.7° that agrees with the literature data.

Pelvic incidence measurement technique — The pelvic incidence was defined as an angle subtended by a line drawn from the femoral heads axis to the midpoint of the sacral endplate and a line perpendicular to the sacral endplate

The pelvic incidence: the measurement technique and the scientific evidence behind — The pelvic incidence was defined as an angle subtended by a line drawn from the femoral heads axis to the midpoint of the sacral endplate and a line perpendicular to the sacral endplate

The Global Sagittal Balance of an Axial Skeleton

Many methods were proposed to assess the global sagittal balance of the spine, which is a strong predictor of the health-related quality of life in adults with spine deformities (Roussouly 2011a; Protopsaltis 2014; Mac-Thiong 2009). The most commonly reported are C7-S1 sagittal vertical alignment and Th1/Th9 spinopelvic inclination (Protopsaltis 2014; Legaye 2005).

C7–S1 Sagittal Vertical Alignment (SVA)

The C7-S1 SVA distance for our spine model is 10.0 mm that generally fits in the range of the data reported in the scientific literature for young men.

Sagittal vertical alignment (SVA) of C7 — C7-S1 SVA is the horizontal distance from a vertical line centered to the C7 body to the posterosuperior corner of the S1 endplate

C7–S1 sagittal vertical alignment (SVA) measurement technique and the scientific basis behind — C7-S1 SVA is the horizontal distance from a vertical line centered to the C7 body to the posterosuperior corner of the S1 endplate

Th1 and Th9 Spinopelvic Inclination

Th1 and Th9 spinopelvic inclination reflect the global sagittal balance of the spine. The value of Th1 (3.5°) and Th9 (7.6°) spinopelvic inclination is within or very close to the standard deviation range reported in the literature.

Th1 & Th9 Spino-pelvic inclination measurement technique — Spinopelvic inclination angle was subtended by a vertical reference line through the femoral heads axis and a line drawn from the midpoint of the Th1 or Th9.

Spinopelvic inclination measurement technique and scientific evidence for selected values — Spinopelvic inclination angle was subtended by a vertical reference line through the femoral heads axis and a line drawn from the midpoint of the Th1 or Th9.

The Sagittal Projection of the Gravity Line to the Axial Skeleton

An equilibrium of the healthy human body in general and an axial skeleton specifically is closely related to the center of gravity or so-called gravity line. In recent decades a lot of evidence was accumulated about the projection of gravity line to the skeletal landmarks and the correlation of this projection with the aging process (Hasegawa 2017) and different pathological conditions (During 1985; Mac-Thiong 2009).

According to the barycentremetric studies, the gravity centerline of the well-balanced body with the kyphosis above 30° cross the body in front of Th9 ± 1.5 cm anterior to the vertebral body (Legaye 2008, Duval-Beaupère 1992). Caudally the projection of gravity center is 36.1 ± 20.6 cm behind the femoral head axis. Under such conditions, no muscular electric activity is observed in the posterior spinal muscles to maintain the body's vertical position.

Gravity line sagittal projection to the spine — It was observed that the projection of gravity centers lies anteriorly at the thoracic levels and posteriorly at the lumbar levels, when the balance is optimal, as in healthy subjects (Legaye 2005).

List of references

Anda S, et al. Pelvic inclination and spatial orientation of the acetabulum: a radiographic, computed tomographic and clinical investigation. Acta Radiologica, 1990, 31(4):389–94.

Ao S, et al. Cervical kyphosis in asymptomatic populations: incidence, risk factors, and its relationship with health-related quality of life. J Orthop Surg Res, 2019, 14(1):322.

Asai Y, et al. Sagittal spino-pelvic alignment in adults: the Wakayama spine study. PLoS One, 2017, 12(6):e0178697.

Boulay C, et al. Sagittal alignment of spine and pelvis regulated by pelvic incidence: standard values and prediction of lordosis. European Spine Journal, 2006, 15(4):415–22.

During J, et al. Toward standards for posture. Postural characteristics of the lower back system in normal and pathologic conditions. Spine, 1985, 10(1)83–87.

Duval-Beaupère G, et al. A barycentremetric study of the sagittal shape of spine and pelvis: the conditions required for an economic standing position. Annals of Biomedical Engineering, 1992, 20(4):451–62.

Endo K, et al. Characteristics of sagittal spino-pelvic alignment in japanese young adults. Asian Spine Journal, 2014, 8(5):599–604.

Endo K, et al. Measurement of whole spine sagittal alignment using the SLOT radiography of the SONIALVISION safire series clinical application. Medical Now 2015, 8(78).

Guo G, et al. Cervical lordosis in asymptomatic individuals: a meta-analysis. J Orthop Surg Res, 2018, 13(1):147.

Guo Q, et al. Relation between alignments of upper and subaxial cervical spine: a radiological study. Arch Orthop Trauma Surg, 2011, 131(6):857–862.

Hasegawa K, et al. Standing sagittal alignment of the whole axial skeleton with reference to the gravity line in humans. J Anat, 2017, 230(5):619–630.

Iorio J, et al. The effect of aging on cervical parameters in a normative north american population. Global Spine Journal, 2018, 8(7):709–15.

Jackson R & McManus A. Pelvic lordosis and pelvic incidence: the relationship of pelvic parameters to sagittal spinal profile. Curr Opin Orthop, 2004, 15(3):150–53.

Kumagai G, et al. Association between roentgenographic findings of the cervical spine and neck symptoms in a japanese community population. J Orthop Sci, 2014, 19(3):390–97.

Kuntz C, et al. Neutral upright sagittal spinal alignment from the occiput to the pelvis in asymptomatic adults: a review and resynthesis of the literature. J Neurosurg Spine, 2007, 6(2):104–112.

Legaye J, and G Duval-Beaupère. Gravitational forces and sagittal shape of the spine. Clinical estimation of their relations. Int Orthop, 2008, 32(6):809–16.

Legaye J, et al. Pelvic Incidence: A fundamental pelvic parameter for three-dimensional regulation of spinal sagittal curves. Eur Spine J, 1998, 7(2):99–103.

Legaye J, and Duval-Beaupèrem G. Sagittal plane alignment of the spine and gravity: a radiological and clinical evaluation. Acta Orthop Belg, 2005, 71(2):213–220.

Lewandowski J. Kształtowanie się krzywizn fizjologicznych i zakresów ruchomości odcinkowej kręgosłupa człowieka w wieku 3-25 lat w obrazie elektrogoniometrycznym. Poznan; 2006.

Le Huec J, et al. Pelvic parameters: origin and significance. Eur Spine J, 2011, 20 (August):564–571.

Mac-Thiong J, et al. Age- and sex-related variations in sagittal sacropelvic morphology and balance in asymptomatic adults. Eur Spine J, 2011, 20 Suppl 5:572–577.

Mac-Thiong J, et al. Can C7 plumbline and gravity line predict health related quality of life in adult scoliosis?. Spine, 2009, 34(15):E519.

Nojiri K, et al. Relationship between alignment of upper and lower cervical spine in asymptomatic individuals. J Neurosurg, 2003, 99(1):80–83.

Oh Y, et al. The comparison of sagittal spinopelvic parameters between young adult patients with L5 spondylolysis and age-matched control group. J Korean Neurosurg Soc, 2013, 54(3):207.

Pesenti S, et al. The amount of proximal lumbar lordosis is related to pelvic incidence. Clin Orthop Relat Res, 2018, 476(8):1603–11.

Platzer W, et al. Color atlas and textbook of human anatomy: locomotor system. Thieme, 2003.

Protopsaltis T, et al. The T1 pelvic angle, a novel radiographic measure of global sagittal deformity, accounts for both spinal inclination and pelvic tilt and correlates with health-related quality of life. J Bone Joint Surg, 2014, 96(19):1631–1640.

Roopnarian A. Ethnic variations of selected cervical spine radiographic parameters of males in KwaZulu-Natal. Acad Aant Int, 2011, 4(1):45–51. January 1.

Roussouly P, et al. [Geometrical and mechanical analysis of lumbar lordosis in an asymptomatic population: proposed classification]. Rev Chir Orthop Reparatrice Appar Mot, 2003, 89(7):632–39.

Roussouly P, et al. Classification of the normal variation in the sagittal alignment of the human lumbar spine and pelvis in the standing position. Spine, 2005, 30(3):346–353.

Roussouly P, Pinheiro-Franco J. Biomechanical analysis of the spino-pelvic organization and adaptation in pathology. Eur Spine J, 2011a, Suppl.5:609–618.