Comparison of spinal sagittal parameters by time of day in a healthy working population: Do we bend during the day?

Abstract

BACKGROUND:

Prospective study.

OBJECTIVE:

To determine the change in spinal sagittal parameters which may occur throughout the day in healthy population.

METHODS:

Thirty-five healthy hospital employees were enrolled in the study. Two standing left lateral orthoroentgenograms were obtained at 8.00 a.m and at 6.00 p.m. Six spinopelvic parameters were measured on the X-rays. Thereafter, the subjects were divided into two cohorts according to their BMI as low BMI and high BMI.

RESULTS:

Thirty-five subjects with a mean age of 25.97 $\pm$ 8.21 were evaluated. No significant change was found between morning and evening measurements for any of the parameters. Direct relationship was shown between thoracic kyphosis (TK) and lumbar lordosis (LL), lumbar lordosis and sacral slope (SS), pelvic tilt (PT) and pelvic incidence (PI) minus lumbar lordosis, sagittal vertebral axis (SVA) and pelvic incidence minus lumbar lordosis. In addition an inverse relationship was found between sacral slope and pelvic tilt, sacral slope and pelvic incidence minus lumbar lordosis, thoracic kyphosis and pelvic incidence minus lumbar lordosis, sacral slope and pelvic tilt, sagittal vertebral axis and lumbar lordosis ( $p<$ 0.05). Sagittal vertebral axis were found to be higher in the high BMI group, and daily change was lower but the differences were not statistically significant. Only the change in pelvic tilt value was found to be statistically significant in low BMI group.

CONCLUSION:

Routine workload in a hospital environment does not cause significant change in the spinopelvic parameters throughout the day.

Keywords

Sagittal balance change healthy collapse spinopelvic parameters

1. Introduction

Optimal sagittal balance ensures that we spend minimum effort to stand upright [1]. The vertebral body and disc complex acts to resist the forces of physiological compression and sustains an erect posture [2]. A degenerated disc or loss of anterior bone support may cause deformation, thus leading to flexion (kyphosis) and lateral bending (scoliosis), usually coupled with torsion (rotation) [3]. Therefore, during the course of aging it is likely that one will encounter a gradual loss of spinal alignment. As new data highlights the importance of sagittal vertebral balance for better outcomes of spinal deformity surgery, research concerning spinal sagittal parameters has become more popular among spinal surgeons [4, 5]. The amount of correction in sagittal parameters has been found to be directly related to clinical improvement [6, 7]. Therefore, understanding anatomical and physiological properties along with compensatory mechanisms which allow for the maintenance of sagittal balance has become more important than ever before. Previous literature contains a great deal of research which focuses on the determination of compensatory mechanisms and classification of sagittal imbalance [8, 9]. Currently, we know that an increase in thoracic kyphosis is compensated by an increase in lumbar lordosis, or anterior shift in the center of gravity is compensated with by retroversion of the pelvis [1, 10]. Most sagittal parameter research has focused on surgical goals for the treatment of degenerative spine cases or other special conditions that cause sagittal imbalance, such as idiopathic scoliosis and Scheuermann’s disease [11, 12]. To our knowledge, there is no study which has focused on physiological sagittal changes which may occur throughout the day as a probable result of working and muscle fatigue. As a result of this deficit, we endeavored to determine the change in spinal sagittal parameters which may occur throughout the day by comparing spinal sagittal measurements made early in the morning to measurements made in the evening in healthy hospital workers.

2. Materials and methods

Thirty-seven operating room nurses who had no back pain at the time of the study, with no known spine, hip or pelvic disorders and without any contraindications for radiographic exposure were enrolled in the study. Among them two subjects with previous spinal surgery were excluded. The participants were mainly operating room nurses who work actively throughout the entire day. All participants provided informed consent and the study was approved by the ethical committee of Acibadem University (ATADEK 2016/1).

The radiographic protocol was standardized for all participants. For each subject, two standing left lateral orthoroentgenograms including the whole spine and pelvis were obtained, with the subject standing 72 inches away from the X-ray tube. The participants were instructed to stand straight and relaxed, with their knees fully extended. The elbows were parked in 90 degrees flexion, with both elbows resting on a horizontal bar at the level of their shoulders. The first X-ray was obtained at 8 o’clock in the morning just before the work shift. The second X-ray was obtained at 6 o’clock in the afternoon at the end of the work shift. Six spinopelvic parameters were measured on the X-rays: thoracic kyphosis (TK), lumbar lordosis (LL), sacral slope (SS), pelvic tilt (PT), pelvic incidence (PI) and sagittal vertebral axis (SVA) [13]. The definition of the aforementioned study variables are provided in Fig. 1 to provide better apprehension. All radiographs were analyzed by the same surgeon and checked by two other surgeons with the aid of digital graphics software (The Surgimap software New York, NY, USA).

Figure 1.

Thoracic kyphosis (TK), the angle between the superior endplate of T4 and the inferior endplate of T12; lumbar lordosis (LL), the angle between the superior endplate of L1 and the superior endplate of S1; sacral slope (SS), the angle between the superior endplate of S1 and the horizontal line; pelvic incidence (PI), the angle between the line perpendicular to the superior endplate of S1 and the line connecting the midpoint of the superior endplate of S1 to the hip axis (HA, the midpoint of the line connecting the centers of two femoral heads); pelvic tilt (PT), the angle between the vertical line and the line connecting the midpoint of the superior endplate of S1 to HA (considered positive if angulated behind the vertical line and otherwise negative); sagittal vertebral axis (SVA), the distance between the C7 plumb line and the posterosuperior corner of S1 in the sagittal plane.

Table 1

Descriptive values for all parameters

	Morning measurement		Evening measurement		P values
N $=$ 35 (All patients)
Thoracic kyphosis	35.88	(51-18)	35.88	(52-16)	0.851
Lumbar lordosis	53.34	(71-37)	53.94	(76-34)	0.332
Sacral slope	35.60	(47-22)	35.74	(47-24)	0.822
Pelvic tilt	11.91	(30-( $-$ 3))	10.74	(25-( $-$ 4))	0.083
Pelvic incidence	47.71	(68-27)	47.11	(64-27)	0.062
PI minus LL	$-$ 5.62	(15-( $-$ 25))	$-$ 6.82	(13-( $-$ 29))	0.196
Sagittal vertebral axis	9.65	(83-( $-$ 52))	8.52	(87-( $-$ 70))	0.870
N $=$ 24 (low BMI)
Thoracic kyphosis	35.04	(51-18)	34.54	(52-16)	0.852
Lumbar lordosis	52.95	(71-38)	54.16	(76-37)	0.179
Sacral slope	35.37	(47-22)	36.29	(47-24)	0.189
Pelvic tilt	11.70	(30-( $-$ 3))	9.54	(25-( $-$ 4))	0.012 ${}^{*}$
Pelvic incidence	47.37	(68-27)	46.41	(64-27)	0.013 ${}^{*}$
PI minus LL	$-$ 5.58	(15-( $-$ 25))	$-$ 7.75	(13-( $-$ 29))	0.090
Sagittal vertebral axis	5.50	(53-( $-$ 52))	3.89	(87-( $-$ 70))	0.887
N $=$ 11 (High BMI)
Thoracic kyphosis	37.72	(46-28)	38.81	(48-28)	0.385
Lumbar lordosis	54.18	(68-37)	53.45	(66-34)	0.878
Sacral slope	36.09	(45-24)	34.54	(42-24)	0.124
Pelvic tilt	12.36	(19-5)	13.37	(19-8)	0.322
Pelvic incidence	48.45	(61-38)	48.63	(61-39)	0.567
PI minus LL	$-$ 5.72	(11-( $-$ 20))	$-$ 4.81	(12-( $-$ 18))	0.759
Sagittal vertebral axis	18.72	(83-( $-$ 34))	18.62	(45-( $-$ 19))	0.921

Table 2

Correlation matrix between parameters and their related P values for all subjects

	TK_diff	LL_diff	SS_diff	PT diff	PI_diff	PI-LL diff	SVA_diff
TK diff
Pearson correlation	1	0.470( ${}^{*}$ )	$-$ 0.190	0.126	$-$ 0.054	$-$ 0.497( ${}^{*}$ )	$-$ 0.084
Sig. (2-tailed)		0.004	0.273	0.470	0.758	0.002	0.632
N		35	35	35	35	35	35
LL diff
Pearson correlation		1	0.421( ${}^{*}$ )	$-$ 0.287	0.223	$-$ 0.953( ${}^{*}$ )	$-$ 0.561( ${}^{*}$ )
Sig. (2-tailed)			0.012	0.095	0.198	0.000	0.000
N			35	35	35	35	35
SS diff
Pearson correlation			1	$-$ 0.591( ${}^{*}$ )	0.129	$-$ 0.391( ${}^{*}$ )	$-$ 0.203
Sig. (2-tailed)				0.000	0.461	0.020	0.242
N				35	35	35	35
PT diff
Pearson correlation				1	0.474( ${}^{*}$ )	0.441( ${}^{*}$ )	$-$ 0.073
Sig. (2-tailed)					0.004	0.008	0.679
N					35	35	35
PI diff
Pearson correlation					1	0.083	$-$ 0.237
Sig. (2-tailed)						0.634	0.171
N						35	35
PI-LL diff
Pearson correlation						1	0.500( ${}^{*}$ )
Sig. (2-tailed)							0.002
N							35
SVA diff
Pearson correlation							1
Sig. (2-tailed)
N							35

${}^{*}$ Correlation is significant at the 0.05 level (2-tailed).

Table 3

Correlation matrix between parameters and their related P values for low BMI group

	PI-LL diff	TK_diff	LLdiff	SS_diff	PT_diff	PI_diff	SVA diff
PI-LL diff
Pearson correlation	1	$-$ 0.691( ${}^{*}$ )	$-$ 0.962( ${}^{*}$ )	$-$ 0.248	0.351	0.165	0.493( ${}^{*}$ )
Sig. (2-tailed)		0.000	0.000	0.242	0.093	0.440	0.014
N		24	24	24	24	24	24
TK diff
Pearson correlation		1	0.668( ${}^{*}$ )	$-$ 0.149	0.070	$-$ 0.105	$-$ 0.185
Sig. (2-tailed)			0.000	0.488	0.744	0.626	0.388
N			24	24	24	24	24
LL diff
Pearson correlation			1	0.257	$-$ 0.213	0.109	$-$ 0.592( ${}^{*}$ )
Sig. (2-tailed)				0.226	0.317	0.614	0.002
N				24	24	24	24
SS diff
Pearson correlation				1	$-$ 0.604( ${}^{*}$ )	0.023	$-$ 0.122
Sig. (2-tailed)					0.002	0.915	0.570
N					24	24	24
PT diff
Pearson correlation					1	0.509( ${}^{*}$ )	$-$ 0.189
Sig. (2-tailed)						0.011	0.377
N						24	24
PI diff
Pearson correlation						1	$-$ 0.346
Sig. (2-tailed)							0.097
N							24
SVA diff
Pearson correlation							1
Sig. (2-tailed)
N

${}^{*}$ Correlation is significant at the 0.05 level (2-tailed).

Table 4

Correlation matrix between parameters and their related P values for high BMI group

	PI-LL diff	TK diff	LL diff	SS_diff	PT_diff	PI_diff	SVA diff
PI-LL diff
Pearson correlation	1	0.075	$-$ 0.949( ${}^{*}$ )	$-$ 0.659( ${}^{*}$ )	0.546	$-$ 0.412	0.578
Sig. (2-tailed)		0.826	0.000	0.027	0.082	0.208	0.062
N		11	11	11	11	11	11
TK diff
Pearson correlation		1	$-$ 0.080	$-$ 0.207	0.157	$-$ 0.056	0.269
Sig. (2-tailed)			0.815	0.541	0.645	0.869	0.423
N			11	11	11	11	11
LL diff
Pearson correlation			1	0.789( ${}^{*}$ )	$-$ 0.379	0.679( ${}^{*}$ )	$-$ 0.476
Sig. (2-tailed)				0.004	0.250	0.022	0.139
N				11	11	11	11
SS diff
Pearson correlation				1	$-$ 0.300	0.744( ${}^{**}$ )	$-$ 0.465
Sig. (2-tailed)					0.371	0.009	0.149
N					11	11	11
PT diff
Pearson correlation					1	0.173	0.266
Sig. (2-tailed)						0.610	0.430
N						11	11
PI diff
Pearson correlation						1	$-$ 0.029
Sig. (2-tailed)							0.932
N							11
SVA diff
Pearson correlation							1
Sig. (2-tailed)
N							11

${}^{*}$ Correlation is significant at the 0.05 level (2-tailed).

Thereafter, the subjects were divided into two cohorts according to their body mass indexes (BMI) as low BMI ( $<$ 30) and high BMI ( $\geqslant$ 30).

Data were analyzed using SPSS 14.0 software (SPSS Inc., Chicago, IL). Application of the Kolmogorov-Smirnov test showed no normal distribution of data sets, thus Wilcoxon test was utilized and median and range values were calculated and used instead of mean values. The Independent Samples test was utilized to assess comparison of the parameters. Relationships were assessed using Pearson’s coefficients. A significance value less than 0.05 was considered to be significant.

3. Results

Thirty-five subjects: 16 males (45.7%), 19 females (54.3%) with a mean age of 25.97 $\pm$ 8.21 were evaluated. The mean height and weight of volunteers were 168 $\pm$ 8.3 cm and 66.74 $\pm$ 16.9 kg and mean BMI was 23.20 $\pm$ 4.51. The number of subjects with high BMI was 11 (31.4%) and with low BMI was 24 (68.6%).

The values of TK, LL, SS, PI, PT and SVA were not normally distributed in both cohorts. All spinopelvic parameters were measured twice from X-rays obtained in the morning at 8 o’clock and in the evening at 6 o’clock. Descriptive values of all parameters are shown in Table 1 for better comprehension. No significant change was found between morning and evening measurements for any of the parameters. The evaluation of the study parameters gathered from the morning X-ray revealed a significant relationship between thoracic kyphosis and lumbar lordosis ( $p=$ 0.001) and between pelvic incidence and lumbar lordosis ( $p=$ 0.003). When a correlation was sought for change of the values from morning to evening it was discovered that TK was directly related to LL, which means lumbar lordosis would increase as thoracic kyphosis increases. LL was inversely related to SVA, which means SVA would decrease as lumbar lordosis increases. Also, a direct relationship was shown between LL and SS, PT and PI minus LL, SVA and PI minus LL. In addition an inverse relationship between SS and PT, SS and PI minus LL, TK and PI minus LL, SS and PT was noted ( $p>$ 0.05). There was no significant relationship between SVA and TK, SVA and PT. A correlation matrix between dependent and independent parameters using Pearson’s correlation and their related P values is shown in Table 2.

When results are grouped for BMI, the value of morning SVA was found to be 5.50 $\pm$ 28.0 mm in the low BMI group and 18.72 $\pm$ 35.8 mm in the high BMI group, whereas evening SVA was found to be 3.89 $\pm$ 31.7 mm in the low BMI group and 18.61 $\pm$ 20.1 mm in the high BMI group. Change in SVA value from morning to evening was 1.60 $\pm$ 32.7 mm in the high BMI group and 0.11 $\pm$ 26.5 mm in the low BMI group. SVA values were found to be higher in the high BMI group, both in the morning and evening measurements, and daily change of SVA was lower in the low BMI group, but the differences were not statistically significant ( $p=$ 0.169, $p=$ 0.243, $p=$ 0.895). When corresponding values between morning and evening were compared, only the change in PT value was found to be statistically significant. The PT value was found to increase by 2.16 ${}^{\circ}$ in the low BMI group, whereas it was found to decrease 1 ${}^{\circ}$ in the high BMI group ( $p=$ 0.017).

As a result of the correlation of daily changes for study parameters in the low BMI cohort, we found a significant direct relationship between SVA and PI minus LL, TK and LL, PT and PI; on the other hand, a significant inverse relationship between TK and PI minus LL, LL and PI minus LL, SVA and LL, SS and PT (Table 3). The same comparison for the high BMI cohort showed significant direct relationship between SS and PI, LL and SS, LL and PI and an inverse relationship between LL and PI minus LL, SS and PI minus LL (Table 4).

4. Discussion

In recent years, understanding the importance of sagittal balance, which significantly affects quality of life, has led to the identification of new targets in deformity surgery [4, 6, 14, 15, 16]. Bess et al. defined new values for correction such as: PT $<$ 20, SVA $<$ 45 mm and PI minus LL $<$ 10, for spinal deformity surgery [17]. Such developments have led to an increase in number of surgeries being performed. For this reason, understanding the compensation mechanism which occurs over the course of a day is an important step in the evaluation of symptomatic patients who are admitted to the clinic at different times of the day.

When spinopelvic parameters were considered in a study by Mac-Thiong and colleagues [10] on 709 volunteers without any spinal symptoms, they declared the PI value range to be 32 ${}^{\circ}$ –74 ${}^{\circ}$ , PT value 0 ${}^{\circ}$ –27 ${}^{\circ}$ and SS 25 ${}^{\circ}$ –55 ${}^{\circ}$ . In another study by Lee and colleagues [18] LL value was found to be 49.4 ${}^{\circ}$ , SS value to be 36.3 ${}^{\circ}$ and PI 47.8 ${}^{\circ}$ . Finally, LaFage and colleagues [19] announced mean LL values of 50.7 ${}^{\circ}$ , SS 37.9 ${}^{\circ}$ and PI 50.2 ${}^{\circ}$ . Parallel to these findings, our study revealed a morning mean TK value of 35.8 ${}^{\circ}$ and 35.87 ${}^{\circ}$ in the evening, LL value 53.34 ${}^{\circ}$ and 53.94 ${}^{\circ}$ by evening, SS value 35.60 ${}^{\circ}$ in the morning and 35.74 ${}^{\circ}$ by evening, PT value 11.91 ${}^{\circ}$ and 10.74 ${}^{\circ}$ by evening, finally, PI 47.71 ${}^{\circ}$ in the morning and 47.11 ${}^{\circ}$ by the evening. Our investigation of sagittal vertebral alignment revealed a mean morning value of 9.65 which regressed to 8.52 by evening, which are both parallel to previous studies in the literature. Furthermore, the literature contains other studies parallel to ours describing a SVA to bisect just anterior of posterosuperior corner of the sacrum in a healthy population [20, 21, 22].

It has been well defined by a number of studies focusing on relations of sagittal vertebral parameters that the thoracic kyphosis is balanced with hyperlordosis of the cervical or lumbar region and global kyphosis can be compensated with an increase of the pelvic tilt due to pelvic retroversion [9, 10, 17, 23, 24, 25]. Among these, Mac-thiong and colleagues [10] have demonstrated a direct relation between C7 translation amount and other spinopelvic parameters such as PT, SS and PI. Ghandhari and colleagues [26] have found a direct relationship between TK-LL, LL-PI and LL-SS and an inverse relationship between LL-SVA and PT-PI. In our study, we have found a direct relationship between TK-LL and SS-LL and an inverse relationship between SS-PT.

Furthermore, when the relationship between sagittal vertebral alignment, PI, TK and LL were investigated we discovered that only LL was inversely related to sagittal vertebral alignment. This finding supports previous studies proposing lumbar osteotomies to correct severe thoracic kyphosis [27, 28]. Further analysis revealed that PI minus LL values were related to SS with a 39% effect. Likewise, the direct relationship between SVA and PI minus LL had a 50% effect.

In this study, our main aim was to investigate change of sagittal balance between morning measurements and evening measurements, thus making inferences concerning compensation mechanisms to spinopelvic balance. When the measurements for the main cohort were taken into account, it was shown that there is no significant change in sagittal spinopelvic parameters throughout the day, but the PI minus LL were decreased from $-$ 5.62 to $-$ 6.82, which caused SVA to regress from 9.65 mm to 8.52 mm by evening. This was interpreted as the kicking in of compensation mechanisms by evening and pulling the spinal sagittal alignment to a more posterior trajectory. In other words, general assumption that the spinal muscles become fatigued by evening and the sagittal vertical axis shifts anteriorly is rejected by our study since we have seen that compensation mechanisms are effective for preserving the sagittal balance througho $\imath$ ut the day. When the same correlations were sought after the main cohort was divided into two cohorts by BMI $<$ 30 and BMI $\geqslant$ 30, it was shown that intragroup spinopelvic parameters were similar. On the other hand, it was discovered that the low BMI cohort’s increase in PT ( $p=$ 0.017) by evening was greater than the high BMI cohort. As a result of this we may conclude that the low BMI cohort was more successful in compensating by increasing pelvic retroversion due to increased pelvic tilt. The SVA value in the high BMI cohort was found to change minimally between morning and evening measurements (18.72 mm to 18.61 mm). However, the SVA value for the low BMI cohort changed from 5.50 mm in the morning to 3.89 mm by the evening, indicating compensatory mechanisms kicking in by the evening. Boulay and colleagues [29] have demonstrated a strong correlation between BMI and LL, SS and PI. Parallel to their findings we have demonstrated that compensatory mechanisms in high BMI people are less effective when compared to low BMI people, but we found this difference to be insignificant. This, we believe, may play a role in surgical decision-making, since corrective surgery in the high BMI population might increase complication rates.

This is the first study focusing on sagittal vertebral alignment changes throughout day in the literature, so we think that the results we have produced are important for the spinal research community. However, our study has its own limitations. Firstly, the number of participants is limited, but given that both morning and evening measurements of our study are parallel to the recent literature findings we find this to be of little concern. Secondly, the measurements made with computer software may have their own measurement faults. We attempted to address this problem by repeating measurements multiple times with multiple surgeons. Finally, we believe that the change of spinopelvic parameters throughout the day or by specific professional work scenarios should be further investigated with well-planned cohorts for age and body mass index with more subjects.

5. Conclusion

In the light of all this information, we observed no significant change in the spinopelvic parameters throughout the day. This showed that compensatory mechanisms will work to prevent collapse of spinal sagittal balance. Nevertheless, one should keep in mind that in extremely overweight patients these mechanisms might become insufficient.

Footnotes

Acknowledgments

We thank Funda H. Sezgin for her statistical analysis.

Conflict of interest

We all certify that there is no conflict of interest with any medical and financial organization regarding the manuscript. We all state that there is no funding received for this study.

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