We would like to demonstrate the scientific support of the geometric configuration chosen for our 3D model of the spine:
Sagittal alignment of the spine describes the curvature of the spine in the sagittal plane. In the perception of the majority of healthcare professionals, the description of this alignment is limited by physiological curves of the spine—cervical/lumbar lordosis and thoracic/sacral kyphosis—,which, however, is oversimplified. It is like describing the shape of an iceberg by just looking at the 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:
The following parameters were taken into consideration:
neutral spine curves for an adult man in standing standard anatomical position
the configuration of the pelvis to support spinal curves
the global sagittal balance of an axial skeleton
the sagittal projection of the gravity line to the axial skeleton
It is surprising, but in many aspects, there is no universal formula for the neutral sagittal alignment of the spine. Each angle and distance is with a relatively broad range of variance. The best way to illustrate that phenomenon is to cite one of the most relevant sources—the textbook "Clinical biomechanics of the spine." At the end of XX century, White & Pinjabi wrote: "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°" (White and Panjabi 1990). Since that time, 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 ethnical 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 very variable results. This is the reason why the choise of a particular configuration for our model was navigated by the range of standard deviations in multiple research papers, rather than reported median/mean result in one trustable source.
The lumbar lordosis measurement technique differs from study to study. Below we rely on the most commonly used approach — angle measurement from L1 upper plate to the S1 upper plate. Our model lumbar lordosis is equal to 55.3° that is within the range from 36°(Endo 2014) to 63.5°(Legaye 2008) reported in many recent studies.
The most commonly used methods for measurement of the thoracic kyphosis are (1)angle measurement from Th1 upper plate to Th12 lower plate and (2) measurement from Th4 upper plate to 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.
An evaluation of the cervical lordosis angle usually is subdivided into 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.
The cervical lordosis from C2 to C7 in our model is 17.7° that 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).
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 our 3D 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).
Pelvic tilt represents 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 incidence is an integral parameter deriving from the sacral slope and pelvic tilt, affecting both — the sagittal curvature of the spine and spatial orientation of pelvis in standing position. It is supposed to be a fundamental pelvic parameter for three-dimensional regulation of spinal sagittal curves (Legaye 1998). Our model pelvic incidence is 52.7° that agrees with the literature data.
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.
Th1 and Th9 spinopelvic inclination (SPI) reflect the global sagittal balance of the spine. The value of Th1 (3.5°) and Th9 (7.6°) spinopelvic inclination for our model are within, or very close to the range of standard deviation reported in the literature.
An equilibrium of the healthy human body in general and an axial skeleton specifically is in close relation to the 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 ageing process (Hasegawa 2017) and different pathological conditions (During 1985; Mac-Thiong 2009).
According to the barycentremetric studies the gravity centre of the well balanced body with the kyphosis above 30° is located in front of Th9 ± 1.5 cm anterior to the vertebral body (Legaye 2008; Duval-Beaupère 1992). Caudally the projection of gravity centre is 36.1 ± 20.6 cm behind the femoral heads axis. Under such conditions no muscular electric activity is observed in the posterior spinal muscles to maintain the vertical position of the body.
First published: 26/Nov/2019
Last update: 19/Aug/2020
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