Abstract
Background
Idiopathic scoliosis is described as the most common postural deformity to affect adolescents. These patients demonstrate vestibular system perturbations. Scoliosis can be treated through spinal fusion. Does spinal fusion coupled to rehabilitation program present significant effect of dynamic equilibrium?
Methods
An unstable platform was used to analyze dynamic equilibrium in patients with idiopathic scoliosis (before and one year after spinal fusion) against a population of asymptomatic subjects.
Results
A significant group and condition effect was observed on Center of Mass.
Conclusion
In relation to vestibular system, spinal fusion coupled to rehabilitation program is associated to better dynamic equilibrium.
Keywords: Scoliosis, Equilibrium, Spinal fusion, Sensorial inputs, Center of mass
Highlights
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Dynamic equilibrium for idiopathic scoliosis patients.
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Reduced base support and vestibular information.
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Influence of spinal fusion.
1. Introduction
The spinal deformity that occurs in idiopathic scoliosis (IS) involves all three spatial planes, resulting in a considerable impact on morphology and movement.1 This spinal deformation influences internal mass distribution and can alter the head position. Subjects with IS demonstrate neurological differences to asymptomatic subjects too. These differences involve: visuo-spatial perceptual impairment; motor control problem; sensory integration disorder; psychosocial difficulties (with anxiety and fears); neurodevelopmental concept and body-spatial orientation concept, all of which can affect postural control2 and dynamic equilibrium evaluated with gait analysis.3 Particularly, a malfunction of the vestibular system and/or sensorimotor integration impairment has been noticed for patients with scoliosis,4 whereas 65% of the relevant information to maintain dynamic equilibrium comes from the vestibular system.5 Study postural control and dynamic equilibrium for patients with IS could contribute to adapt treatments since the possibility of a postural disequilibrium as a contributory causative factor in adolescent idiopathic scoliosis has been previously suspected.6 Surgery is considered necessary for cases exhibiting severe scalable deformity.1 Surgical treatments could potentially have an effect on equilibrium7 and reduce movement in the segments subjected to arthrodesis.8 Spinal fusion surgery aims to correct the spinal curve, restore normal sagittal plane alignment, reduce pain, and prevent complications. About postural control, Schimmel et al. (2011) depicted none influence of spinal fusion on sensory system immaturity and motor-sensory integration with six quiet standing tasks.8 But this result could be in relation to experimental conditions: subjects were positioned directly on platforms that limited body oscillations, in particular head oscillations. The vestibular system has a role of controlling variations of head position at the quite standing posture. In function of spine deformity and spine stiffness (associated to spinal fusion), Center of Mass of the trunk is modified that influences variations of head position and vestibular system.4 So, Schimmel's results could be in relation to low differences between body oscillations before and after fusion. Specific imbalance with dynamic equilibrium and specific tests could reveal differences between these two populations.
Pre-surgery adolescents with IS present several deficiencies and impairments such as balance control and perceptual problems.1,2,8,9 However, these results dependent of studies: patients with IS exhibited only tendencies of reduced direction control during the limit of stability tests.9 Postural control and dynamic equilibrium assessments are obtained by measuring the sensory inputs through sensory organization tests under varying conditions: eyes open or closed, different foot positions, and frequency of oscillations.2,4 Free-oscillating platform with significant radius and higher frequency platform movements than dynamic stabilometry have been introduced in case of athletes with specific normative data.10 In opposition to stabilometry, tests performed on such platform may be associated with a fear of falling and important oscillations. Fear of falling may accentuate balance control strategies (essentially based on vision, proprioception or vestibular information) and reflect an accurate knowledge about the skills used to avoid falling.11 So, free-oscillating platform with significant radius could reveal more specific balance control strategies than stabilometry, and could quantify the evolution of these balance control strategies in function of spinal fusion for patients with IS. For instance, in case of vision privation while subject resent a fear of falling with platform tilt, this dynamic condition could help to describe alternative sensory strategies (essentially vestibular system) to control dynamic equilibrium without the help of vision. Likewise, dynamic equilibrium with feet in tandem position let to evaluate specifically vestibular system.5 However, Mesure et al. (1995) uses a protocol which can be influenced by motor control learning.5 Furthermore, it seems necessary to evaluate Center of Mass (CoM) oscillations for such dynamical analysis and compute variables from CoM.12 A previously analysis revealed significant differences on displacement of CoM between patients with IS (before spinal fusion) and asymptomatic population while anteroposterior oscillations were studied and subjects had eyes open and feet in a parallel position on a surface defined by a rectangle (as detailed on previously personal communication). Since patients with IS demonstrate a malfunction of the vestibular system and/or a sensorimotor integration impairment, other conditions of study in relation to sensory organization could be impacted too. Patients should describe higher displacements and higher velocity of CoM, with lower duration than controls. The present study aimed to investigate the consequence of scoliosis and spinal fusion on dynamic equilibrium with specific conditions (surface stability and visual deprivation) in reference to asymptomatic subjects. With retrospective approach, the present two-phase study (before and one year after spinal fusion) tested three hypotheses: (1) displacement of Center of Mass (CoM) is higher in dynamic equilibrium on an unstable platform oscillating according anteroposterior axis between a population with idiopathic scoliosis and a healthy population in case of vestibular testing conditions. (2) In relation to malfunction of the vestibular system, CoM velocity and duration for specific vestibular conditions differ in dynamic equilibrium between patients and controls. (3) Spinal fusion decreases the dynamic equilibrium differences between these populations particularly in case of vestibular testing conditions.
2. Method
2.1. Subjects
Following ethical approval from Ethics Committee of Angers (n° 2017/08), the population consisted of patients with scoliosis who had undergone spinal fusion (Table 1) and controls. Individuals were excluded if they presented conditions such as mental retardation, musculoskeletal or neurological diseases, pain, or use of drugs that could influence their dynamic equilibrium. The treatment group was recruited from patients scheduled for spinal fusion in a rehabilitation center, between January 2008 and October 2015. All patients were operated on by the same orthopedic surgeon. Radiographic analysis helped define the levels of fusion in order to correct torsion and avoid compensation of the shoulders which may occur with upper fusion, and reduction of mobility between spine and pelvic, a form of pelvic compensations in condition of lower fusion. After surgery, all the patients followed an identical rehabilitation program. Their medical follow-up included a movement analysis. The treatment group was analyzed before and one year after surgery. The control group was composed of students (with low/moderate sports activity). This group was free from conditions such as musculoskeletal and neurological problems, a diagnosis of scoliosis or back pain, and the use of drugs that could influence their dynamic equilibrium. Control subjects and patients gave their informed consent to the use of anonymous data.
Table 1.
Anthropometric measurements (weight and height) of patients and scoliosis characteristics before and one year after arthrodesis. Statistical results indicate pre- and post-surgery differences. Legend: woman (W); man (M); body mass index (BMI); posterior surgery (Post); anterior surgery (Ant).
| Patient | Sex | Age at surgery (y) | Weight (kg) |
Height (cm) |
BMI |
Risser |
Surgery | Spinal fusion | Lenke classification | Cobb Primary curve |
Cobb Secondary curve |
||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Pre | Post | Pre | Post | Pre | Post | Pre | Post | Pre | Post | Pre | Post | ||||||
| P1 | W | 14.0 | 64.0 | 64.0 | 176.0 | 178.0 | 20.7 | 20.2 | 3 | 3 | Post | T5-L2 | 1AN | 55 | 14 | / | / |
| P2 | W | 15.0 | 40.5 | 44.5 | 153.5 | 153.5 | 17.2 | 18.9 | 3 | 3 | Post | T5-L2 | 1AN | 45 | 19 | / | / |
| P3 | W | 15.0 | 55.0 | 57.0 | 167.5 | 169.0 | 19.6 | 20.0 | 4+ | 5 | Ant | L1-L4 | 5CN | 35 | 10 | / | / |
| P4 | W | 15.0 | 75.0 | 78.0 | 166.0 | 164.0 | 27.2 | 29.0 | 4 | 4 | Post | T5-L2 | 3BN | 80 | 24 | / | / |
| P5 | W | 15.0 | 48.0 | 51.0 | 155.0 | 159.0 | 20.0 | 20.2 | 4+ | 4+ | Post | T5-L4 | 5CN | 23 | 13 | 36 | / |
| P6 | W | 15.0 | 48.0 | 46.0 | 160.0 | 152.0 | 18.8 | 19.9 | 4 | 4+ | Post | T5-L3 | 3BN | 50 | 20 | 39 | 10 |
| P7 | M | 16.0 | 61.0 | 64.0 | 175.0 | 178.0 | 19.9 | 20.2 | 4 | 4 | Post | T5-L2 | 1AN | 50 | 15 | / | / |
| P8 | W | 16.0 | 40.0 | 40.5 | 155.0 | 157.5 | 16.6 | 16.3 | 4 | 4 | Post | T5-L2 | 3AN | 43 | 21 | / | / |
| P9 | W | 16.0 | 50.0 | 51.0 | 157.0 | 159.0 | 20.3 | 20.2 | 5 | 5 | Post | T4-L4 | 4CN | 50 | 20 | 35 | 25 |
| P10 | M | 16.0 | 55.0 | 55.0 | 165.0 | 166.0 | 20.2 | 20.0 | 5 | 5 | Post | T5-L3 | 2A- | 75 | 40 | / | / |
| P11 | W | 16.0 | 51.5 | 59.0 | 162.5 | 164.0 | 19.5 | 21.9 | 5 | 5 | Post | T5-L4 | 3C- | 60 | 20 | 54 | 20 |
| P12 | W | 17.0 | 53.5 | 54.0 | 169.5 | 172.0 | 18.6 | 18.3 | 4 | 4 | Post | T6-L2 | 2B+ | 55 | 23 | 30 | 6 |
| P13 | W | 17.0 | 55.0 | 57.0 | 155.0 | 155.0 | 18.7 | 19.6 | 5 | 5 | Ant | L1-L4 | 5CN | 42 | 26 | / | / |
| P14 | M | 17.0 | 56.0 | 58.5 | 161.5 | 163.0 | 21.5 | 22.0 | 5 | 5 | Post | T5-L2 | 1AN | 50 | 29 | / | / |
| P15 | W | 17.5 | 45.0 | 47.0 | 155.0 | 157.0 | 18.7 | 19.1 | 5 | 5 | Post | T5-L4 | 3C- | 60 | 54 | 20 | / |
| P16 | M | 17.5 | 53.5 | 54.0 | 164.5 | 170.0 | 19.8 | 18.7 | 5 | 5 | Post | T5-L2 | 1AN | 55 | 14 | / | / |
| P17 | W | 17.5 | 46.5 | 53.0 | 158.0 | 162.0 | 18.6 | 20.2 | 5 | 5 | Post | T5-L4 | 3B- | 45 | 19 | / | / |
| P18 | W | 17.5 | 55.0 | 58.0 | 162.5 | 165.2 | 20.8 | 21.2 | 5 | 5 | Post | T5-L3 | 5CN | 35 | 10 | / | / |
| P19 | W | 18.0 | 43.0 | 43.0 | 164.0 | 165.0 | 16.0 | 15.8 | 5 | 5 | Post | L1-L4 | 5CN | 50 | 20 | 35 | 25 |
| P20 | W | 18.5 | 48.0 | 49.0 | 148.5 | 166.0 | 21.8 | 17.8 | 5 | 5 | Post | L1-L4 | 5CN | 23 | 13 | 36 | / |
| P21 | W | 19.5 | 43.5 | 41.0 | 158.0 | 162.0 | 17.4 | 15.6 | 5 | 5 | Post | T5-L4 | 5AN | 52 | 20 | / | / |
| P22 | M | 21.0 | 58.0 | 57.5 | 172.5 | 172.5 | 19.5 | 19.3 | 5 | 5 | Post | T5-L2 | 1AN | 37 | 25 | 43 | 19 |
| P23 | W | 22.0 | 64.5 | 57.0 | 164.5 | 164.0 | 23.8 | 21.2 | 5 | 5 | Ant | L1-L4 | 5CN | 38 | 13 | / | / |
| P24 | M | 24.5 | 71.5 | 74.0 | 186.5 | 187.5 | 20.6 | 21.0 | 5 | 5 | Post | T5-L3 | 5AN | 50 | 20 | 39 | 10 |
| P25 | W | 25.0 | 52.0 | 52.0 | 175.5 | 175.5 | 16.9 | 16.9 | 5 | 5 | Post | T5-L2 | 3BN | 50 | 14 | 30 | / |
| Mean (sd) | 18.6 (2.9) | 53.5 (8.9) | 54.6 (9.1) | 163.5 (8.7) | 165.5 (8.4) | 19.9 (2.4) | 19.9 (2.7) | / | / | / | / | / | 48.3 (13.1) | 20.6 (9.6) | 36.1 (8.5) | 16.4 (7.7) | |
| Statistical result | / | P = 0.61 | P = 0.42 | P = 0.96 | / | / | / | / | / | P<.0001 | P<0.001 | ||||||
2.2. Experimental setup
With the subject standing on a freely oscillating platform (Fig. 1), oscillations were recorded in the sagittal plane. Three positions were observed with platform tilt: eyes open and feet in a parallel position on a surface defined by a rectangle (Condition 1-evaluation of the influence of vision and vestibular information); eyes closed and feet in a parallel position (feet apart) on the same surface (Condition 2-evaluation of the influence of vestibular information with low-sensitivity); eyes open and heel-to-toe foot position (Condition 3-evaluation of the influence of visual and vestibular information with high sensitivity).
Fig. 1.
Platform: top and lateral view (left). Experimental conditions (right).
These conditions were assigned at random. The experimenter held the platform (while manually obscuring a marker placed on the platform, thereby preventing recording), the subject mounted the platform and stood in the requested position. The experimenter let go of the platform, which defined the starting point of the trial. Each subject was asked to remain in equilibrium for 10 s in a prescribed position. The subject's oscillations were recorded during this time. Vertical posture was not compulsory, although foot movement was not permitted. After 10 s had elapsed, the experimenter held the platform while obscuring a marker placed on the platform, which represented the end of the trial. If the subject was unable to remain in equilibrium during the allotted time, then the actual test duration was recorded. Each subject was given 1 min to rest between tests.
2.3. Data collection
A motion capture system (Vicon, Oxford Metrics, Oxford, UK, 100 Hz) was used with 34 retro-reflective spherical markers (14 mm diameter). These markers were placed directly onto the subject's skin in the following Plug in Gait locations. To measure oscillations on the platform, four markers were positioned in each upper corner. The ability to observe these four markers defined the beginning (all markers are observed) and the end (three markers are observed: the experimenter steadies the platform to secure the subject's standing position and descent from the platform) of a trial. Two trials were performed for each subject and for each experimental condition. A smoothing procedure with a second order Butterworth filter (6 Hz) was applied to the collected kinematic data, which were filtered in the forward and reverse direction to remove phase lag.13
The following three main parameters were extracted from the collected data:
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CoM displacement and CoM velocity
The center of the platform, derived from the space delimited by the four markers, was considered the reference point. The relative position of each marker on the subject and the CoM were quantified in accordance with Dempster.14 The CoM was determined by observing the average velocity of the center point and the average distance covered from the start to the end of the trial.
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Duration
This duration, expressed in second, is equivalent to the trial time.
2.4. Statistical analyses
Based on preliminary personal communication, the power analysis was defined from displacement of CoM for asymptomatic population and patients with scoliosis before spinal fusion with Condition 1. To consider a power of 95% and an alpha of 0.05 to compare patients and controls, this study included data for 25 subjects per group.
The Shapiro–Wilk's test was applied to the statistical distribution (p ≤ 0.5). Descriptive statistics were used to report mean, standard deviation (SD), or median and quartiles, where adequate. Statistical analyses were performed using Statistica (version 13, Dell software, California, USA). The data were analyzed using a 3 (group) * 3 (condition) MANOVA for displacements of CoM and velocity of CoM. Post hoc tests (LSD test according Howel15) are applied in case of a significant F-test. A Mann-Whitney U test was used to identify differences on durations between patients with scoliosis (before and after spinal fusion) and healthy subjects for each condition. To evaluate an effect of spinal fusion, a Wilcoxon signed-rank test was used to compare durations and ratios before and after this fusion. The chosen level of significance was p ≤ 0.05.
3. Results
Characteristics of the IS group are shown in Table 1 in reference to Lenke's classification.16 Twenty five control subjects are studied (mean of age 20.4 years (SD 1.6); mean of height 169.3 cm (SD 7.1); mean of weight 58.3 kg (SD 6.4); mean of BMI 20.3 (SD 1.6)). The demographic data were similar between patients with scoliosis (before fusion) and asymptomatic population. Cobb angle values for the patients with scoliosis were significantly reduced after spinal fusion.
3.1. Overall postural stabilization
Displacements and velocities for each condition and oscillation are shown in Table 2. For all conditions and computed parameters of CoM, patients with scoliosis showed more variability than the healthy population. A significant group and condition effect was observed on displacements and velocity of CoM (F = 2.14, p = 0.03). Overall, displacement measurements in the IS group before spinal fusion were significantly higher than in the control group for Condition 1 (eyes open and feet in a parallel position) and Condition 3 (eyes open and heel-to-toe foot position). Displacement measurements in the IS group after spinal fusion were significantly higher than in the control group only for Condition 1. None effect of populations and conditions were noticed on velocities.
Table 2.
Displacement (mm) and velocity (mm/s) of CoM per Condition and population. Data are presented according to mean (standard deviation).
| Conditions | Controls | IS pre-surgery | IS 1 year after surgery | p-value | |
|---|---|---|---|---|---|
| Displacements of CoM | Condition 1 | 86.31 (30.00)*# | 121.76 (49.36)*† | 106.48 (52.43)#† | *:<0.001 |
| #:0.03 | |||||
| †:0.07 | |||||
| Condition 2 | 105.75 (51.35)*# | 123.97 (55.61)*† | 110.38 (63.06)#† | *:0.11 | |
| #:0.54 | |||||
| †:0.37 | |||||
| Condition 3 | 152.08 (65.54)*# | 210.46 (56.23)*† | 130.51 (67.26)#† | *:<0.001 | |
| #:0.07 | |||||
| †:<0.001 | |||||
| Velocity of CoM | Condition 1 | 7.99 (3.03)*# | 14.38 (7.79)*† | 13.75 (6.42)#† | *: 0.58 |
| #: 0.62 | |||||
| †: 0.95 | |||||
| Condition 2 | 8.08 (4.44)*# | 14.04 (6.75)*† | 16.02 (7.86)#† | *: 0.62 | |
| #: 0.46 | |||||
| †: 0.83 | |||||
| Condition 3 | 13.45 (4.83)*# | 22.11 (6.59)*† | 18.27 (8.64)#† | *: 0.45 | |
| #: 0.71 | |||||
| †: 0.71 | |||||
* Statistical difference between IS before spinal fusion and control population for the same condition. # Statistical difference between IS one year after spinal fusion and control population. † Statistical difference between before and one year after spinal fusion.
The comparison between the two clinical phases of the IS group revealed that significant differences existed only for Conditions 3 (displacement). Reduced base support specific to vestibular evaluation is sensible to new position of CoM.
3.2. Durations
Durations for each condition and oscillation are shown in Table 3. This table presents median, first and third quartiles (noticed in brackets) per population and conditions. The healthy group maintained each condition for longer than patients with scoliosis (before and after spinal fusion). Based on durations, scoliosis seemed to play a more important role in dynamic control.
Table 3.
Duration (s) per Condition and population. Data are presented according to median (first and third quartiles).
| Conditions | Controls | IS pre-surgery | IS 1 year after surgery | p-value |
|---|---|---|---|---|
| Condition 1 | 10.00 (10.00; 10.00)*# | 8.38 (6.80; 10.00)*† | 8.57 (7.24; 10.00)#† | *:<0.01 #:<0.01 †:0.95 |
| Condition 2 | 10.00 (5.78; 10.00)*# | 3.72 (2.44; 5.80)*† | 3.50 (3.09; 6.18)#† |
*:<0.01 #:<0.01 †:0.88 |
| Condition 3 | 10.00 (10.00; 10.00)*# | 8.29 (5.79; 9.35)*† | 7.19 (5.42; 10.00)#† |
*:<0.01 #:<0.01 †:0.03 |
*Statistical difference between IS before spinal fusion and control population for the same condition. # Statistical difference between IS one year after spinal fusion and control population. † Statistical difference between before and one year after spinal fusion.
A comparison of the pre- and post-spinal-fusion durations for each condition revealed significant difference for Condition 3: with duration of 8.05s (5.93s; 9.52s) before fusion and 7.19s (5.05s; 10.00s) after fusion. Patients with IS reduced their ability to maintain dynamic equilibrium eyes open and heel-to-toe foot position.
4. Discussion
For patients with Idiopathic Scoliosis, the potential impact of scoliosis and spinal fusion were investigated by measuring CoM displacements, CoM velocity and duration of trial during dynamic equilibrium with anteroposterior oscillations in reference to asymptomatic subjects. With a free-oscillating platform presenting significant radius, the potential impact of visual deprivation and surface stability were specifically used. Considering that heel-to-toe foot position may accentuate vestibular control, and based on CoM displacements, results reveal significant positive impact of spinal fusion on dynamic equilibrium. In case of low base support, this fusion limits CoM displacements. Velocity of CoM is not influenced by population. About duration, the healthy population revealed better equilibrium than patients with IS (before and after spinal fusion). With dynamic equilibrium, these results confirm and precise Antoniadou's results4: in case of patients with IS, CoM is modified that influences variation of head position and vestibular system.
So, in contrast to Schimmel et al. (2011) who display a condition of postural equilibrium, patients with IS presented poorer dynamic equilibrium compared to healthy controls.8 With eyes closed, asymptomatic subjects maintained good dynamic control through a greater contribution of another sensorial input (vestibular, proprioception or somatossensorial system), even if information on the relative position of the body with respect to the external environment was considered as blocked.1,17 This result is phase with previous results2 in case of very slow oscillations of the support. Tandem Romberg's test was applied to Condition 3. This test, based on hip strategy, assesses the ability to maintain a steady stance. On the basis of the assumption that this test yields similar results in a posturography study with dynamic equilibrium, this test indicates the severity of the balance problem with respect to the vestibular system.5,18 Considering CoM displacements, patients with scoliosis were sensitive to impaired vestibular control and spinal fusion had a positive effect on this system: spinal tends to reduce displacements of CoM of trunk that reduces displacements of head and present positive impact on vestibular system.
Using the same low-cost platform and other variables, Mesure et al. compared dynamic equilibrium between six athletes and nine novices.10 In opposition to this study, vision privation is not based on darkness condition. Such condition is associated with an increased risk of falling. Our conditions with vision privation present a lower risk of falling which could explain our important durations obtained with asymptomatic population. However, this seems adequate to disturb dynamic equilibrium in case of patients with scoliosis. Spinal fusion has none influence on this result. This condition let to evaluate the influence of vestibular information with low-sensitivity: feet in parallel position limit displacement of trunk even if certainly more that 65% of the relevant information to maintain dynamic equilibrium comes from the vestibular system.5 Based on durations for all conditions and displacements of CoM for Condition 1, the sensory system in IS patients might still have been immature one year after surgery: information processed by the central nervous system was only partially exploited and no significant differences were noticed between patients before and after spinal fusion. Spinal fusion influenced partially CoM displacements.8 In accordance with these authors, spinal fusion might not compensate for sensory system immaturity and motor-sensory integration.8 We could hypothesize that none specific sensory system maturity or none new motor-sensory integration in relation to physiologic and natural evolution was appeared after spinal fusion (patients were captured twelve months after spinal fusion). Several compensatory mechanisms might be implicated depending on the kind of input received and how this information is then processed by the central nervous system.8
Power calculation and number of subjects were performed for describing differences from displacement of CoM for asymptomatic population and patients with scoliosis before spinal fusion in Condition 1. In relation to preliminary personal communication and De Santiago's result (subjects remain in a standing posture with their feet apart at shoulder width), this condition has been estimated as sufficient to describe significant difference between asymptomatic controls and patients before spinal fusion. Furthermore, power calculation and number of subjects are based on displacement of CoM considered as an indicator of postural performance. This performance quantifies the ability to ensure dynamic stability in challenging conditions.
So, from Center of Mass, two variables have been studied. Mean displacement and mean velocity quantified dynamic equilibrium according: the smaller the value, the better the dynamic control. Our results reveal none influence of population on velocity. Even if dynamic oscillations are more discriminating than static condition,10,19 mean velocity does not tend to more important values in case of patients with scoliosis. So, in case of dynamic equilibrium, velocity of CoM seems not to be an indicator of postural performance.
In opposition to other studies,10 none Quotient's Romberg was calculated in this first article based on dynamic imbalance. Such Quotients are calculated as the Ratio between values (durations, CoM displacements, CoM velocities) calculated from data obtained with two conditions (Closed and Open Eyes values for instance, which generally increase with the loss of functional performance). However, using ratios need a particular attention: the type of relationship between the numerator and the denominator, the potential intersection with the origin influence these ratios.20 Wrong clinical interpretations are possible. Therefore, these seductively simple ratios could be analyzed in a specific study.
Nevertheless, the present study had the following limitations. First, our approach used a particular condition of equilibrium: a freely oscillating platform with a large radius and variables were chosen in accordance with classic postural analysis. Considering that dynamic equilibrium can be quantified with Center of Mass, Center of Pressure or anchoring index applied on segments,2 this first retrospective study is only based on CoM and durations. It could be interesting to compare data obtained from such freely oscillating platform and data obtained from postural analysis. Moreover, durations, displacements and velocity of CoM could be associated to pelvic and trunk mobility from computed from kinematic data in order to define strategies. From CoM, a dynamic “Cone of Economy” could be developed in case of dynamic equilibrium in reference to asymptomatic subjects to establish the Cone of Economy boundaries.21 This macroscopic variable seems more adapted to our free-oscillating platform than center of pressure or the relative phase between CoM and center of pressure.12 This measurement of the pressure centre would require specific equipment adapted to our platform.
Second, the experimenter asked subjects to close their eyes. Contrary to other methods of dynamic equilibrium analysis,1 blindfolds were not used during our study, and subjects were not maintained in position with belts, which could have increased confidence.
Third, this pilot study is based on a limited number of subjects, the use of a healthy population as control presents a first normative approach with durations, displacements and velocities of CoM. About patients with scoliosis, only spinal fusion has been studied. It could be interesting to evaluate incidence of spinal brace.
Finally, based on anchoring index, it could be interesting to study segmental coordination and control in order to describe strategies adopted.2 Three modes of postural coordination have been depicted in relation to hip strategy, ankle strategy and ankle-hip coordination.12 Ride pattern, inverted pendulum pattern, rigid mode seems represent the overall coordinative behavior on a moving support surface. Does patient with IS adopt one of this three classical model before spinal fusion? Does spinal fusion influence the model adopted?
5. Conclusion
Spinal fusion is associated to better dynamic equilibrium in relation to vestibular information one year after surgery. We evoked postural disequilibrium as a contributory causative factor in adolescent idiopathic scoliosis6: so, our results could imply an increasing improvement after this first year. A new study to evaluate the effects of surgery at 10 years of follow-up could be very interesting.
Funding
None.
Ethical approval
Ethics Committee of Angers (n° 2017/08).
Declaration of competing interest
None.
Acknowledgements
The authors would like to thank all patients and subjects.
Contributor Information
Y. Delpierre, Email: uam@asso-prh.fr.
P. Vernet, Email: philippe.vernet@asso-prh.fr.
D. Colin, Email: denis.colin@asso-prh.fr.
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