Influence of Selected Examination Postures on Shape of the Spine and Postural Stability in Humans

Aims: The accuracy of non-radiographic measurement of the spinal shape is infl uenced by postural sway (PS). The aim of this study was to determine whether certain examination postures prevent PS without changing key spinal characteristics. Methods: We tested 1) natural standing position (posture A), 2) fi xation postures standing with the support of the upper limbs against a wall (posture B) and 3) standing with the support of the head and chest against a wall (posture C). There were examined 60 subjects; the examination of each posture was repeated fi ve times in each individual by DTP-3 microcomputer position system. The spinal shape changes resulting from tested postures were assessed using t-test. The role of PS was assessed using standard deviations, and the signifi cance of diff erences between the individual postures was determined using the F-test. Results: Compared to posture A, the fi xation posture B did not have any signifi cant infl uence on the degree of PS, but it signifi cantly infl uenced the shift of the spine from the ideal vertical and its slope, although this did not result in spine curve deformation. The fi xation posture C signifi cantly reduced the PS compared to posture A; however, there were signifi cant changes in the spinal shape. This fi xation posture shifted signifi cantly the spine from the ideal vertical and slope characteristics. Conclusion: PS plays a role in the examination of spinal shape. Positions B and C did not meet the required criteria. Therefore, it is necessary to search for another examination position.


INTRODUCTION
To reduce cumulative radiation exposure, especially in children and adolescents, non-radiographic methods for spinal shape examination have been introduced [1][2][3][4][5][6] .However, random variations in the standing position (postural sways) of the examinee caused by postural instability or breathing can infl uence the accuracy and reliability of the methods developed for spinal shape measurement.Moreover, patients with scoliosis have much less control over postural stability than the normal population 7,8 .Taken together, there is a need for examination postures that may minimize the infl uence of postural sway on the accuracy of measurement.
From the physical point of view, postural sways may be described by the amplitude and period of spatial body defl ections, and therefore a single examination of spinal shape shows only the momentary body posture during a series of defl ections.One method for resolving this problem is to repeat spinal shape examinations within an appropriate time interval with the subjects in diff erent examination postures.However, any fi xation procedure that reduces the amplitude of postural sway may change the body posture and the spinal shape simultaneously.Therefore, it is important to determine the characteristics of selected examination postures in terms of both the postural stability and the infl uence of body posture on spinal shape.An accurate description of these characteristics of examination postures will enable standardization of such conditions as well as a comparison of results obtained with diff erent methods.
In this study the natural standing position was compared with two postures that utilize a diff erent degree of fi xation.The aim was to evaluate the infl uence of each position on the degree of postural sway and on spinal shape.Ideally, the best position is that with minimal postural sway while maintaining the spinal shape as close as possible to that of the natural standing position.

Investigated examination postures
The examination postures (Fig. 1) tested in the study were: • Posture A is the natural standing position without fi xation.The width of the position is habitual, with the upper limbs alongside the body; the examinee is looking straight ahead.• Fixation posture B is a posture in which the upper limbs are supported against a vertical wall.The width of the position is habitual, the forearms are horizontal, and the examinee touches the wall with their fi ngertips and looks straight ahead.• Fixation posture C is a posture in which the head and chest of examinee are supported against a vertical wall.The width of the position is habitual, the upper limbs lie alongside the body, and the forehead, tip of the nose, and the chest are touching the wall.

The instrument
We used the microcomputer diagnostic system, DTP-3, which allowed non-invasive examination of spinal shape in the sagittal and frontal planes by applying the contact method 9 .The DTP-3 software involved algorithms for calculating postural sway amplitudes and for determining spinal curvature by calculating the coeffi cients of approximation polynomials 3,9 .This system allowed repeated examinations within a short time interval.The coordinates of anatomic points on the skin surface of the examinee were registered with a position sensor and transferred to a personal computer, which converted these points into output records for subsequent evaluation.The postural sensor allowed measurement of the positions of points in the sphere with a diameter of 2200 mm and a standard error of 0.5 mm [9][10][11] .
When using the DTP-3 system to diagnose spinal shape (Fig. 2), we palpated and detected the skin projections of the following anatomic points: left and right lateral parts of the acromion, bilateral posterior superior iliac spine, and the processus spinosus of vertebrae C3-C7, T1-T12, and L1-L5.The positions of the measured points were converted into output records.The so-called ideal vertical (IV), i.e. mathematical simulation of a plumb line erected from the centre of the intercalcaneal line (V), was used to evaluate the spinal shape.The orientation of the 3D Cartesian coordinate system was as follows: axis z was on the ideal vertical and oriented in the caudal-cranial direction, axis x was parallel with the intercalcaneal line and in left-right direction, and the direction of axis y was in the posterior-anterior direction.As a result, frontal plane was defi ned by axes xz and the sagittal plane by axes yz.

Study group
The experimental part of the study included measurements of 60 subjects: 26 men and 34 women, aged 22.8 ± 1.3 years (mean±SD), weighing 67.4 ± 9.6 kg, and 174.1 ± 9.5 cm tall; the height of the spine given by the vertical distance between the processus spinosus of C3 and L5 was 52.0 ± 3.9 cm.The group included only healthy subjects.The examination of spinal shape in individuals was repeated fi ve times for each of the three postures (A, B, C), and individual measurements followed immediately one after the other.The duration of one measurement was 32.2 ± 4.6 s.Spine height was standardized so that the results of subjects of diff erent height could be compared.

Evaluation of the postural stability
The stability of selected processus spinosus of individual subjects was evaluated by standard deviations (SDs) calculated from fi ve repeated measurements of coordinates of the selected processus spinosus in all monitored postures 9,12 .Based on these, mean of SDs (MSD) in coordinates x, y, and z were calculated for the whole group.

Evaluation of the spinal shape with approximation polynomial
Spinal shape is given by the mean coordinates of all processus spinosus calculated from fi ve repeated measurements of their coordinates.The repeated measurement and averaging improves the reliability of measurements as it reduces the infl uence of postural sway on the examinee.
Direct evaluation of 3D coordinates x i , y i , and z i of all processus spinosus, i=1, 2, …, 22, for all subjects is time- consuming.Therefore, a computer algorithm was used to evaluate the spinal shape, applying coeffi cients of the approximation polynomial 9,13  A sixth-degree polynomial was applied to describe the spinal shape in the sagittal plane since it represented the best approximation to the physiological curvature of spine with two infl exion points 9 .The positions of two infl exion points might be interpreted as a cervicothoracic transition and a thoracolumbar transition.These transitions split the spine into three parts: the cervical, thoracic, and lumbar spine.The curvature of the respective spinal parts could be described with angle parameters (Fig. 3) defi ned as follows: • The angle of the cervical lordosis, α C , is the angle between the normal lines (perpendicular line to tangent of polynomial curve in the respective point) projected from the processus spinosus at C3 and the cervicothoracic transition.• The angle of the thoracic kyphosis, α T , is the angle between the normal lines projected from the cervicothoracic transition and the thoracolumbar transition.Infl uence of selected examination postures on shape of the spine and postural stability in humans • The angle of lumbar lordosis, α L , is the angle between the normal lines projected from the thoracolumbar transition and the processus spinosus at L5.

Statistics
All calculations for evaluation of postural sway and spinal shape were performed using MATLAB 7.0 and the Statistics Toolbox (The MathWorks, Natick, MA).The statistical signifi cance of changes in any parameter was assessed using the F-test and paired t-test 14 , with a cut-off for signifi cance of p=0.05.The eff ect of size of parameter change was based on the absolute size of the diff erence.For an absolute diff erence of |∆|<1 mm (1°), the parameter change was considered as an insignifi cant eff ect of size, and vice versa.

Infl uence of examination posture on postural stability
The values of MSD for coordinates x and y of the individual processus spinosus in the 60 subjects in postures A, B, and C are given in Table 1 together with the results of the statistical tests and eff ect of size of diff erences between the postures A and B and postures A and C. The values of MSD in the coordinate z did not exceed 1 mm and no statistical or eff ect size signifi cance was found, hence these values were not shown in the Table 1. Figure 4 shows the results of the MSDs of the processus spinosus including the mean position of the processus spinosus for all three studied postures.
The postural sway in coordinate x was signifi cantly lower for fi xation posture B than for posture A, and the eff ect size was within the limits of signifi cance.The reduction of postural sway in coordinate y was statistically signifi cant only for the processus spinosus between T6 and L5, and the eff ect size was signifi cant only for the processus spinosus of T8 and T9.The postural sway in coordinates x and y was signifi cantly lower for fi xation posture C than for posture A, and the eff ect size was signifi cant along the entire spine except for the processus spinosus of L4 and L5 for the coordinate x.Taken together, fi xation posture C was the only position that signifi cantly increased the stability of the standing position during contact non-radiographic examination of spine.

Infl uence of examination posture on spinal shape
The results of the evaluation of spinal shape in the sagittal plane, including mean values, SDs of the polynomial coeffi cients, and angle values corresponding to the angles of cervical lordosis, thoracic kyphosis, and lumbar lordosis in postures A, B, and C are given in Table 2.This table also lists the eff ect size and the statistical signifi cance of diff erences in coeffi cients and angles between postures A Eff ect size and statistically signifi cant diff erences in polynomial coeffi cients b 0 and b 1 were found between postures A and B, whereas diff erences in coeffi cients b 4 and b 6 were statistically signifi cant but there was no signifi cant eff ect size.The diff erence in the angles of cervical lordosis achieved signifi cant eff ect size, but this was not statistically signifi cant.It might be concluded that the spinal shift from ideal vertical and spinal slope was higher for fi xation posture B than for posture A, but the curvature of the individual sectors was the same.The results of the spinal shape analysis in the sagittal plane are shown in Fig. 3.
Eff ect size and statistically signifi cant diff erences were found between postures A and C in terms of all polynomial coeffi cients and all angle polynomial parameters except for coeffi cient b 3 which was not statistically signifi cant.Compared to posture A, fi xation posture C exhibits an increased deviation of the spinal shape from ideal vertical, a decreased spinal slope, a fl attened cervical lordosis and thoracic kyphosis, and an increased lumbar lordosis curvature.

DISCUSSION
Accurate and timely diagnosis of type and degree of a spinal deformity is basic to correct treatment, either surgical or conservative 15,16 .The most common examination method is a radiological evaluation which involves measurement of rotation and torsion of the vertebrae 17 .Long-term follow-ups are required in the majority of cases of spinal deformity leading to increased cumulative radiation dose.Another limitation of the radiographic method is that it cannot make X-rays in the frontal and sagittal plane at the same time unless computed tomography is used which enables three-dimensional reconstruction 18 .However, computed tomography exposes the patient to a much higher level of radiation than that for simple radiography.Alternative, non-radiographic methods are therefore being researched.In addition to eliminating the disadvantages of radiological examination, these techniques should also use modern technologies with their increased reliability and accuracy, and that allow dynamic examinations 6,9 .
In theory, these methods are based on the spinal shape being suffi ciently represented by projections of anatomic points on the body surface 19 .The coordinates of these points are measured from the surface of the back using mechanical, optical, or ultrasound sensors.Subsequent computer processing enables both the 3D reconstruction of spinal shape and the calculation of other clinically important parameters 20 .On the other hand, all of these methods depend on stable posture during the examination period.In this regard, postural sway, breathing and other sources of postural instability can be a problem.
The results of this study show that the examination position used for spine diagnostics can signifi cantly in-fl uence the spinal shape and degree of postural sway.Theoretically, precise and reliable diagnostics of spinal shape require an examination position that minimizes both the postural sway and changes in spinal shape.The examination positions evaluated in our study do not simultaneously fulfi l both of these requirements.Postures A and B do not address the requirement of suffi cient postural stability, and although posture C signifi cantly reduces the degree of postural sway, it does not address the requirement of a minimal infl uence on spinal shape.
Selecting the most appropriate examination position for spinal shape diagnostics does not depend on the examination method, and hence is applicable to both non-invasive, surface examinations and radiological examinations.The infl uence of various modifi cations of examination positions applicable to the X-ray method in the frontal plane has been described previously 21 , but that study did not consider postural sway.Various examination positions were used in the various methods of surface diagnostics of spinal shape, such as the Arcometer 1 , the BACES system 2 , the Metrecom system 22 , the Ortelius800™ system 6 , Posturometr-S 23 , and the SpinalMouse system 24 .These variations preclude direct comparisons of the results of such studies.For the BACES system, the Metrecom system and Posturometr-S, the use of special fi xation devices to increase postural stability was proposed but the degree of postural sway was not quantifi ed and the infl uence of the fi xation procedure on the spinal shape was not discussed.Other reports on surface spinal shape diagnostics using X-ray 1,4 have not addressed the infl uence of various examination positions.Overall, close attention should be paid to standardizing the measurement protocol including the defi nition of examination posture.

CONCLUSIONS
We found that postural sway could strongly infl uence on the variability of method for spinal shape examination.Therefore, further research is necessary to fi nd out such examination position that would increase the reproducibility and validity of spinal examinations.In addition, this study has supported further development of non-radiographic computer-assisted assessment of spinal shape because they are three-dimensional, quick, and reliable.

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .
Fig. 1.Natural standing position (posture A) and proposed fi xation postures B and C . The clinical interpretation of calculated polynomial coeffi cients b 0 , b 1 , b 2 , b 3 , b 4 , b 5 , b 6 in the sagittal plane was as follows: • b 0 -posterior-anterior shift of the spine from the IV • b 1 -posterior-anterior sloping of the spine to the IV • b 2 -overall spinal curvature (i.e., primarily curvature of thoracic kyphosis) • b 3 and b 4 -curvature of the upper and lower parts of the spine (i.e., curvature of cervical and lumbar lordosis) • b 5 and b 6 -residual spinal curvature of just units of millimetres This study was supported by a research grant from the Ministry of Education, Youth, and Sports of the Czech Republic (No. MSM 6198959221) entitled "Physical Activity and Inactivity of the Inhabitants of the Czech Republic in the Context of Behavioral Changes."

Table 1 .
Mean of standard deviations of processus spinosus coordinates x and y in the 60 subjects for postures A, B, and C

Table 2 .
Polynomial coeffi cients and angle parameters in the sagittal plane in the 60 subjects in postures A, B, and C