An Evaluation of Sagittal Balance and the Influence of Spinopelvic Components on Sagittal Balance through  the Correction and Stabilization Using the Pedicle Screw Rod System on Patients with Adolescent Idiopathic Scoliosis

The definition of sagittal balance and sagittal balance measurement

Sagittal balance is the attainment of the balance of cervical lordosis, thoracic kyphosis and lumbar lordosis.¹⁰ Curves that are opposite of starting cervical lordosis, thoracic kyphosis, and lumbar lordosis allow for the absorption of load or efficient distribution of the spine and increase the efficiency of the spinal muscles. Of these curves, lumbar lordosis plays the most important role in maintaining efficient upright postures.^2,11

Broadly speaking, the sagittal balance of the body is determined by two components, namely the component of the spine (spine) and the pelvic component, which can be called spinopelvic components. Parameters in the spine as explained above include thoracic kyphosis and lumbar lordosis. Other parameters for determining the sagittal balance of the spine component include C7PL (C7 Plumb line), SCFD (Sacrofemoral distance), Sagittal global balance, thoracolumbar junction, T9 Sagittal offset, and L1 Tilt. As for determining the sagittal balance of the pelvic component, it can be measured from the pelvic tilt (PT), pelvic incidence (PI), and sacral slope (SS).

Sagittal balance measurements are obtained by measuring each sagittal balance parameter on the long standing film (standing full spine) by positioning the elbows at 90° and shoulder 45°.² The measurement locations of thoracic kyphosis and lumbar lordosis from several different pieces of literature mention that, some measure from T1 - T12 or T2 - T12 as a thoracic component and L1 - L5 as a lumbar component.¹² In the case of scoliosis, measurements of the sagittal plane components, according to King’s classification, are carried out on the thoracic spine of T4-T12. According to King’s classification, the selection of measurement of thoracic kyphosis and lumbar lordosis is carried out at this level because it is considered to have a neutral rotation.⁶ If the measurement obtained at the anterior angle of the spine is given a positive value, a negative value is obtained when measuring the posterior angle of the spine. The thoracic component measured at T4 - T12 is considered normal if the values obtained are in the range from +25° to +40°, while lumbar lordosis measured from L1 - L5 is considered normal if the values obtained are in the range from -35° to -55°.⁶ Other literature mentioned the term sagittal alignment with global sagittal alignment. Global sagittal alignment consists of global thoracic kyphosis measured from the upper endplate T2 and lower endplate T12, global lumbar lordosis is measured from the upper endplate L1 and upper endplate S1.¹³ TK/LL measurements are more often based on levels that have been determined according to the Lenke’s classification because they are more comprehensive, and these measurements are used by the authors. In accordance with Lenke’s classification, the measurement of thoracic kyphosis is at T5 - T12 with a normal mean value of 10-40 degrees,¹⁴ while lumbar lordosis is measured at the inferior end plate T12/L1 - S1 with normal values in the range of 40-60 degrees.^14,15

Sagittal spinal balance or C7PL is another parameter besides thoracic kyphosis and lumbar lordosis that can be used to determine sagittal balance. Sagittal spinal balance (C7PL) is a measure of the horizontal distance between the C7 sagittal plumb line and the posterosuperior corner of S1. The distance that is considered within the normal limit is 3 cm or less from the posterior superior corner S1 (the plumb line is located in front of or behind the L5 – S1 disc).⁵ There is positive value if the C7 sagittal plumb line is > 2 cm in the anterior promontorium and there is negative value if the C7 sagittal plumb line is > 2 cm in the posterior promontorium.^5,16,17 Global Sagittal Balance is the horizontal distance between the vertical line that passes through the HA (hip axis) and the vertical line that is parallel and passes through the promontorium and is known as the sacrofemoral distance (SCFD). The center of gravity, known as the hip axis, illustrates where the center of gravity falls along the body. The hip axis (HA) which is the center of gravity (CG) is the middle between the hip center or the middle of the line drawn from the two central head femurs. Global Sagittal Balance that can be considered within normal limits after correction is -2 to 3 cm.⁵ The closer the hip axis is, the less likely it is to translate body weight that causes compensation from the pelvis and spinal muscles.

Spinopelvic components also affects sagittal balance, wherein some recent research, it is believed spinopelvic components are the key to achieving optimal balance.¹⁸ Spinopelvic alignment is translated as the interaction between the spine and pelvis that regulates such that weight falls on the hip axis¹⁸ [Figure 2]. Sagittal sacropelvic alignment can be assessed from PT (pelvic tilt), and SS (sacral slope), which is considered an embodiment of the mechanism of pelvic compensation against imbalance.

Figure 2. Sagittal balance measurement line projection at full spine standing.¹⁸

This figure/table has been reproduced with permission from Ref. 18.

Because spinopelvic alignment acts as a compensation reaction to the spine imbalance above it, a fixation construction will greatly affect the spinopelvic compensatory mechanism or even eliminate the compensatory mechanism. A pelvic tilt is an angle formed between a vertical line that passes through the hip axis and a line that connects the middle sacral end plate and the hip axis. Pelvic incidence is an angle formed from a line perpendicular to the sacral end plate and a line that passes through the hip axis (bicoxofemoral axis) and the midpoint of the sacral end plate. The sacral slope is the angle formed between the horizontal line and the sacral end plate. The normal values for each parameter considered normal are 41 ° ± 8.4 ° for sacral slope, 13 ° ± 6 ° for pelvic tilt, and 55 ° ± 10.6 ° for pelvic incidence.² Other literature mentions values that are considered within the normal range, for pelvic tilt (PT) it is 10 ° to 25 °, for pelvic incidence (PI) it is 40 ° to 65 °, and for sacral slope (SS) it is 30 ° to 50 °.¹⁶

The influence of sagittal balance

The incidences of scoliosis are known to be more common in women, with some conditions that cause a decrease in quality of life and pain, where some other conditions leads to balance disorders when standing and walking.¹⁹ This occurs because of the frontal and horizontal rotation which causes the interaction between the spine, trunk and pelvis. Delay in the correction of this deformity exacerbates pain and causes disc degeneration, joint degeneration, and flatbac.¹⁷ Patient with scoliosis have shorter stride lengths, varying muscle activation when walking, and less body stability. The correction of deformities in scoliosis remains a challenge, especially axial or rotational and sagittal. While coronal and axial or rotational will provide aesthetic effects related to the presence of a rib hump, sagittal deformity affects functionally that is more related to cervical balance, prevention of CHD (proximal junctional kyphotic), and thoracic volume effects,¹⁷ Patients with scoliosis with disturbed sagittal balance are relatively affected, experiencing defective natural backward balance. Normal sagittal balance allows one to walk upright with little effort. As a consequence of not having normal sagittal balance, the compensatory reaction will appear as being able to walk upright by reflecting one’s knee, which causes biomechanical changes in other structures.²⁰ The restoration of sagittal balance, especially for thoracal kyphosis, is not only to prevent the occurrence of junctional degeneration (proximal or distal) but also to restore pulmonary function.¹⁷

Decreased lordosis can occur due to an abnormality in the lumbar spine itself or due to a compensatory reaction due to an imbalance that results in a relative posterior translation of the vertebral disco structure²¹ [Figure 3]. Changes in abnormal lordosis do not mean that the patient is in a sagittal imbalance condition, but it can also occur as a compensatory reaction due to sagittal imbalance in another place, such as flat back or hyperkyphosis.²¹ For example if there is a change in the thoracic kyphosis outside the normal limits, which then changes the center of gravity, the body will then compensate by making the center of gravity return to the actual center of gravity, which is behind the femoral axis, by making changes in the lordosis to tolerable limits. If the compensatory reaction is not achieved, then the spinopelvic component will be included to achieve the maximum sagittal balance level [Figure 4].

Figure 3. Pelvic parameters of pelvic tilt, pelvic incidence, and sacral slope.²¹

This figure/table has been reproduced with permission from Ref. 21.

Figure 4. Sagittal imbalance and compensatory mechanisms that occur in the spine, pelvis, and lower extremities.²⁷

Achievement of sagittal balance becomes highly important because of the effects that may occur due to imbalances in the sagittal plane of the spine. Flat back is one condition that is a form of compensation for the imbalance.²² The correction of the sagittal field should be followed by ensuring optimal sagittal balance. When the sagittal plane is within normal limits but is not followed by the achievement of the sagittal balance, this condition needs to be re-evaluated.

The spinopelvic factor has a close relationship with restoring the sagittal balance, meaning that if a normal sagittal plane is achieved but is not followed by sagittal balance, it is necessary to evaluate the pelvic component, which is one of the most important factors. Vice versa, when there is an imbalance in the spine, this will affect the pelvic component itself as a form of compensation for all the effects that may arise.

Lumbar lordotic (LL) is closely related to pelvic orientation expressed by the sacral slope, and it is strongly influenced by pelvic incidence. From this we can see a close relationship between the spinal parameters and the pelvic component. Lumbar lordotic changes, for example, will change pelvic incidence as a parameter determined by the sacral slope. As compensation for achieving pelvic incidence that remains within the normal range, the pelvic tilt will be enlarged by shifting the center sacral slope backward so that it is possible for slippage or spondylolisthesis. Another compensation is to reflect on the knee with all the consequences. Proper postoperative evaluation is highly necessary to determine the results of the correction of sagittal balance before various compensation including the appearance of slippage are made.²³ The results of a retrospective study conducted by Giovanni Andrea La Maida on 76 AIS patients obtained the results of uncorrected sagittal balance, especially for Lenke’s type 1 curves and patients with hypokyphosis.²³

All abnormalities that cause changes in sagittal balance need to consider components that affect changes in sagittal balance when operations are to performed so that normal sagittal balance is achieved. In the case of scoliosis, so far the only concern is how to achieve correction of coronal balance and sagittal balance. A corrective and instrumentation operation is said to be successful if short and long term evaluations are found with a sagittal balance within the normal range.²⁴

Jackson and Hales stated that there is a close relationship between thoracic kyphosis, lumbar lordosis, pelvic position and sagittal plane malalignment which will be seen clinically if there is a loss of lordosis from the lumbar spine. Excessive kyphosis has the potential to increase intradiscal pressure and affect the workings of the spinal erector muscles. Clinical patients with an imbalance in the sagittal plane will experience pain, faster fatigue, body imbalance, and interference with horizontal vision.¹⁴ If this condition is left untreated, then the advanced condition will be compensated for by doing hip extension, genu flexion, and this will ultimately result in increasing susceptibility to fatigue.²⁵

The influence of age, the presence of disc degeneration, or accompanying abnormalities that may be present in the hip and pelvis will further aggravate clinical complaints if found in patients with sagittal imbalance because the compensatory reactions that occur are not optimal.²⁶ The return of sagittal balance to normal or near normal condition will reduce the work of the erector muscle of the spine and hamstring muscles to achieve balance.^11,25

The effects of spinal components (TK and LL) on sagittal balance

Table 2 shows that the Global Sagittal Balance increases by 0.057 cm for each additional 1 ° of TK, and a decrease in the value of the Global Sagittal Balance of 0.04 cm for each additional 1 ° of LL. After testing the effect of a variable with the t-test, it shows that the addition of TK and LL degrees do not significantly influence or have weak effects on Global Sagittal Balance.

With regard to C7PL, every additional 1° of TK will reduce the C7PL value by 0.02 cm, and every additional 1° of LL will reduce the C7PL value by 0.03 cm. When the effect of variables is tested with the t-test, it shows that the addition of TK and LL degrees has no significant effect or has weak effects on C7PL. With respect to SCFD, for each additional 1 ° of TK, the SCFD value will decrease by 0.08 cm and for each additional 1 ° of LL, the SCFD value will increase by 0.02 cm. Based on the t-test, it showed that LL does not have a significant effect on all sagittal balance parameters and TK only had a significant effect on SCFD, while the effect of TK on Global Sagittal Balance and C7PL was not significant or had a weak effect.

The effects of spinopelvic components (PT, PI, and SS) on sagittal balance

The evaluation of the influence of spinopelvic components (PT, PI, and SS) on global sagittal balance using linear regression shows that every additional 1 ° of PT will increase the sagittal global balance by 0.02 cm, each additional 1 ° of PI will reduce the global sagittal balance by 0.02 cm, and each additional 1 ° of SS will reduce sagittal global balance by 0.02 cm. After testing the effect of the changes in each spinopelvic component (PT, PI, and SS) on the sagittal global balance, the value of each spinopelvic component (PT, PI, SS) has p> α =0.05, which means that the changes in each spinopelvic component are not significant in affecting global sagittal balance.

The evaluation of the influence of spinopelvic components (PT, PI, and SS) on C7PL using linear regression shows that each additional 1 ° of PT will increase C7PL by 0.189 cm, each additional 1 ° of PI will decrease C7PL by 0.109 cm, and each additional 1 ° of SS will increase C7PL by 0.105 cm. Then, the influence of each spinopelvic component (PT, PI, and SS) was tested on C7PL and the results show that only PT significantly affects C7PL changes with p = 0.029 < α = 0.05.

The evaluation of the influence of spinopelvic components on SCFD using linear regression shows that each additional 1 ° of PT will increase SCFD by 0.179 cm, each additional 1 ° of PI will reduce the SCFD value by 0.077 cm, and each additional 1 ° of SS will increase the SCFD value by 0.137 cm. Then, the t-test was performed to determine the effect of the changes in each spinopelvic component (PT, PI, and SS) on SCFD. From the t-test, it is found that PT significantly influences SCFD with a p-value of (0.005) < α = 0.05, SS significantly influences SCFD with a p-value of (0.034) < α = 0.05, while PI does not significantly influence SCFD with a p-value of (0.129) > α = 0.05. This means that the spinopelvic components that most influences SCFD changes are PT and SS [Table 3].

Ethics and consent

Ethical approval for this research was obtained from approval committee: The Ethics Committee of Saiful Anwar General Hospital, Malang with approval number: 400/222/K.3/302/2023. The date of approval 12 September 2023.

Written informed consent for publication of their details and clinical images was obtained from the patient.