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The use of rubber foam pads and “sensory ratios” to reduce variability in static posturography assessment
Gait & Posture 29 (2009) 158–160
Contents lists available at ScienceDirect
Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost
Short communication
The use of rubber foam pads and ‘‘sensory ratios’’ to reduce variability in static posturography assessment Di Berardino Federica a,*, Filipponi Eliana a, Barozzi Stefania a, Giordano Gianpiero a, Alpini Dario b, Cesarani Antonio a a b
Audiology Unit, I.R.C.C.S. Policlinico, Mangiagalli e Regina Elena, Department of Otolaryngology, Chair of Audiology and Phoniatrics, University of Milan, via Pace 9, Milano, Italy Otoneurology Service Sc. Institute, Scientific Institute Maria Nascente–Don Gnocchi Foundation, Milan, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 November 2007 Received in revised form 14 May 2008 Accepted 2 August 2008
Despite the numerous works published, static posturography has still a limited clinical use due to its intrinsic inter-individual high variability. For this reason, foam pads have been introduced but their use is still not standardized. Aim of the study was to define the variability of static posturography parameters in standard and foam standing. Methods: 50 healthy subjects were studied with static posturography in four standing conditions: eyes open (EO) and eyes closed (EC), with and without foam pads. Unstable tests have been performed with two different types of rubber foam pads placed on the force platform. ‘‘Sensory ratios’’ have been calculated by the ratio of sway length among the four different conditions, adapted from dynamic posturography, in order to measure the relative contributions of vestibular, visual and somatosensory inputs. Results: Static posturography in standard conditions showed unacceptable coefficients of variation (>than 15%) for all the parameters. The use of foam pads reduced the high intrinsic variability, in particular for LFS (12.6–15.4%). The use of ‘‘sensory ratios’’ led to decrease the inter-subject coefficient of variation of this measurement to about 9.47–14.42% using the bilayer foam pads. Conclusions: Further studies are needed to confirm these data by applying the ratio formulas in clinical practice. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Static posturography Variability Coefficient of variation Foam pads
1. Introduction Numerous studies have suggested that static posturography is a useful clinical tool for evaluating balance impairment in patients, nonetheless contrasting data have also been reported [1,2]. A wide range of stimulus-response parameters have been measured without any consensus regarding which of them are most useful [3]. Foam pads have been introduced so that body sways are increased and a balance disorder due to vestibular loss can be identified more easily [4]. The role of static posturography in body sway assessment still has a limited clinical use because of its intrinsic high variability which does not permit a reliable diagnosis [2]. As far as we know, there are no reports concerning the intersubject coefficients of variation (CV) of static posturography parameters in healthy subjects. The present study aimed to define the variability of static posturography parameters in standard
* Corresponding author at: Viale Piceno 14, 20129 Milan, Italy. Tel.: +39 02 70126502; fax: +39 02 70126502. E-mail address: [email protected] (D.B. Federica). 0966-6362/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2008.08.006
conditions and assess whether it is possible to reduce it by applying the ‘‘sensory ratio’’ formulas using data from the rubber foam pads standing conditions. 2. Materials and methods Fifty healthy subjects (23 females, 27 males) volunteered to participate in the study. They ranged from 20 to 61 years old (mean age 44 14) and from 44 to 78 kg of weight (mean: 59.6 9,1 kg), engaged in regular physical activity, and had no selfreported orthopedic or neurological problems that negatively affected their balance and/or mobility. Subjects with a history of vertigo and balance disorders were excluded from the study. Static posturography was performed with the subject standing on a ‘‘Svep 6.0’’ stabilometric platform (Amplifon, Milan, Italy), according to manufacturers’ instructions dated September 2006. This is a stable force-plate, mounted on 3 strain-gauge force transducers positioned at the vertices of an equilateral triangle, which is sensitive to vertical force and provides a description of body sway in terms of displacement of the patient’s center of pressure. Tests were carried out using a fixed visual target placed at a distance of 1. 20 m, at eye level. X, Y and absolute center of pressure displacements from the projection of a theoretical baricentre and the sway area (SX), sway length (LX), and sway velocity (VM) were recorded at 5 Hz, over 51.2 s, using the Svep software (Svep 6.4). All the parameters given by the Svep system were considered, in particular the correlation between length and surface (LFS).
D.B. Federica et al. / Gait & Posture 29 (2009) 158–160
159
Fig. 1. Monolayer (A) and bilayer (B) rubber foam pads.
All subjects underwent static posturography in each of four conditions: stable condition with visual fixation (eyes open, EO), without visual fixation (eyes closed, EC), unstable condition on foam pads with visual fixation (PAD EO) and without visual fixation (PAD EC). Unstable condition tests have been performed with two different types of rubber foam pads: a ‘‘monolayer’’ with a thickness of 10 cm and a density of 25 kg/m3 (BROGGINI s.r.l. Varese, Italy) and a ‘‘bilayer’’ with a thickness of 8 cm and a density of 100 kg/m3 (ORSAFOAM, Gorla Minore, Italy) (Fig. 1). In order to investigate the relative contributions of vestibular, visual and somatosensory inputs, the analysis based on the Clinical Test of Sensory Interaction of Balance or ‘‘Foam and Dome’’ suggested by Shumway-Cook and Horak [5] and first proposed by Nashner et al. in 1982 for dynamic posturography [6] was applied to static posturography. The ‘‘sensory ratios’’ were obtained by modifying the mathematical modelling for clinical evaluation proposed by Contarino et al. [7], and calculated using the following formulas:
Somatosensory (Som): LX EO/LX EC
Table 2 Sensory equilibrium organization Somatosensory (%)
Visual (%)
Vestibular (%)
Monolayer Mean S.D. CV
44.91 7.10 15.80
35.09 5.63 16.05
20.00 4.12 20.61
Bilayer Mean S.D. CV
35.52 5.12 14.42
40.92 4.91 11.99
23.57 2.23 9.47
t-Test
<0.001
0.002
0.002
Visual (Vis): LX EO/LX PAD EO Vestibular (Vest): LX EO/LX PAD EC
Analysis of variability among all the parameters was performed: a CV of variation in excess of 15% was considered unacceptable. Parametric univariate t-test was performed, values of p 0.05 were considered statistically significant. We used the SPSS software package for all statistical analyses (version 13.0, SPSS Inc., Chicago, IL).
system have not been reported since the inter-subjects CV were more than 20%. LFS that gives the ratio between the length measured and that statistically expected in normal subjects of the same age group for every surface always had the lowest variability. In particular, LFS measured with subjects standing on the monolayer rubber foam pads with eyes open (PAD EO, LFSAO) had the lowest variation. The difference between the lengths observed by the use of the two foam pads was significant only in the EC condition (PAD EC, p = 0.004). Table 2 shows the results of the sensory tests using each of the two foam pads. Sensory ratio analysis gave the lowest variation when bilayer foam pads were used. The CV of these new parameters calculated by the length, remain high but lower than 15%. The difference between the results with the two foam pads was statically significant.
4. Results
5. Discussion
The CV of the parameters obtained from different tests are shown in Table 1. Many of the values given by the Svep postural
Despite the numerous works published on static posturography, this method cannot yet be considered completely validated. The parameters in the EO and EC conditions gave excessive CV, thus confirming reports suggesting that static posturography alone (without the use of foam pads) is not a useful clinical tool for evaluating balance problems [8]. In fact, in 1996, a meta-analysis by Di Fabio et al. [9] reported that the sensitivity and specificity of platform posturography were about 50%. We observed a large inter-individual variability that suggests that static posturography parameters are subjected to factors that make them difficult to reproduce; however, an increase of 35–40% could generally be considered a positive response. Research on postural responses to surface translations has shown that balance is not based on a fixed set of equilibrium reflexes but on a flexible, functional motor skill that can adapt with training and experience [10]. For this reason, we agree that the role of static posturography in body sway assessment has still a limited clinical use, not for topodiagnosis,
The values obtained were converted into percentages.
% Somatosensory: Som/(Som + Vis + Vest) 100 % Visual: Vis/(Som + Vis + Vest) 100 % Vestibular: Vest/(Som + Vis + Vest) 100.
3. Statistical analysis
Table 1 Coefficients of variation (expressed as percentage) LX
VM
SX
LFSAO
LFSAF
LFSEO
LFSEF
EO EC
24.7 36.7
24.7 36.7
84.2 86.9
21.4 33.8
21.2 33.7
21.5 33.9
21.8 34.3
Monolayer PAD EO Monolayer PAD EC
17.8 22.7
17.8 22.7
41.5 29.8
12.6 20.7
12.6 20.6
13.7 20.6
14.3 20.6
Bilayer PAD EO Bilayer PAD EC
15.0 15.8
15.0 15.8
17.3 25.0
15.4 15.0
15.2 15.2
15.5 14.9
14.9 15.0
LX = length, VM = mean velocity, SX = surface, and LFS = length values compared to the expected normal values, as given by Svep system. [LFSAO = LX/ (396.00 EXP(0.0008 SX)); LFSAF = LX/(485.00 EXP(0.0008 SX)); LFSEO = LX/ (831.00 EXP(0.0006 SX)); LFSEF = LX/(973.00 EXP(0.0005 SX)).]
160
D.B. Federica et al. / Gait & Posture 29 (2009) 158–160
but principally because of its intrinsic inter-individual high variability. In our healthy subjects, the only Svep parameters that had an acceptable CV were the LFS in the EO condition on monolayer foam pads, because LFS values are a ratio (length values compared to the expected normal values) that reduces its intrinsic variability. Recently, it has been reported that 44–60-year old subjects were more unstable than young subjects, especially in the dark. This analysis was made on the basis of sensory tests on thick foam pads [11]. In our study, the variability of static posturography parameters was significantly reduced by the use of both foam pads. However, the comparison of the two different types of foam pad was also statistically significant, thus suggesting to compare the materials used in static posturography, not only in terms of reproducibility and internal consistency but also in terms of overall feasibility, responsiveness, and predictive validity for a given population. In 1995, Wooley et al. [12] first carried out laboratory tests to define four different types of foam pads used in posturography and suggested to take select in the foam surface used in order to avoid inaccurate diagnosis of a patient’s reliance on a particular sensory input. Our results confirmed the fact that different types of foam pads give different results also on clinical evaluation with the stabilometric platform, thus underlining the importance of knowing and specifying the characteristics of the foam pads used. In 2005, Riley et al. [13] found that the center of pressure determinism in posturography increased when participants stood on a compliant, foam surface rather then on a rigid surface. A recent work by the same group showed that rendering the sensory environment unstable by manipulating sensory information by support surface or visual surroundings during posturography also influences the amount, relative variability and temporal structure of the center of pressure trajectories [14]. In order to make static posturography a reliable clinical tool, it is necessary to use parameters with a CV lower than 15% [15]. In our study, this was only possible using ‘‘sensory ratio’’ formulas, including values from the unstable condition on foam pads. The most reliable parameters in static posturography were given by the use of bilayer rubber foam pads. The CV of the visual and vestibular percentages were about 10%. Therefore, the lowest variability suggests that these percentages derived from the ‘‘sensory ratios’’ using bilayer foam pads, together with somatosensory one that was about 14.4%, are the most acceptable parameters for clinical use. Furthermore, the role
of ‘‘sway-related’’ tests with foam pads, so called by Baloh et al. [2] and proposed by Allum et al. [4] is fundamental in order to make static posturography more useful. Further studies are needed in order to define the use of foam pads in static posturography, to verify the use of ‘‘sensory ratio’’ with other stabilometric platform systems and to evaluate the clinical role of these new parameters. Conflict of interest None. References [1] Forth KE, Metter JE, Paloski WH. Age associated differences in postural equilibrium control: a comparison between EQscore and minimum time to contact (TTCmin). Gait Posture 2007;25:56–62. [2] Baloh RW, Jacobson KM, Beykirch K. Static and dynamic posturography in patients with vestibular and cerebellar lesions. Arch Neurol 1998;55:649–54. [3] Nelson SR, D Fabio RP, Handerson GH. Vestibular and sensory interaction deficit assessed by dynamic platform posturography in patients with multiple sclerosis. Ann Otol Rhinol Laryngol 1995;104:62–8. [4] Allum JHJ, Zamani F, Adkin AL. Differences between trunk sway characteristics on a foam support surface and on the Equitest1 ankle-sway-referenced support surface. Gait Posture 2002;16:264–70. [5] Shumway-Cook A, Horak FB. Assessing the influence of sensory interaction of balance: suggestion from the field. Phys Ther 1986;66:1548–50. [6] Nashner LM, Black FO, Wall C. 3rd. Adaptation to altered support and visual conditions during stance: patients with vestibular deficits. J Neurosci 1982;2: 536–44. [7] Contarino D, Bertora GO, Bergmann JM. Balance platform: mathematical modeling for clinical evaluation.. Int Tinnitus J 2003;9:23–5. [8] Pe´rennou D, Decavel P, Manckoundia P. Evaluation of balance in neurologic and geriatric disorders. Ann Readapt Med Phys 2005;48:317–35. [9] Di Fabio RP. Meta-analysis of the sensitivity and specificity of platform posturography. Arch Otolaryngol Head Neck Surg 1996;122:150–6. [10] Horak FB, Henry SM, Shumway-Cook A. Postural perturbations: new insights for treatment of balance disorders. Phys Ther 1997;77:517–33. [11] Poulain I, Giraudet G. Age-related changes of visual contribution in posture control. Gait Posture 2008;27:1–7. [12] Wooley SM, McCarter F B, Randolf B. An assessment of foam support surfaces used in static stabilometry. Gait Posture 1995;3:110. [13] Riley MA, Baker AA, Schmit JM, Weaver EE. Effects of visual and auditory shortterm memory tasks on the spatio-temporal dynamics and variability of postural sway. J Mot Behav 2005;37:311–24. [14] Clark S, Riley MA. Multisensory information for postural control: sway-referencing gain shapes center of pressure variability and temporal dynamics.. Exp Brain Res 2007;310:176–299. [15] Shah VP, Midha KK, Dighe S. Analytical methods evaluation: bioavailability, bioequivalence and pharmacokinetic studies. Eur J Drug Metab Pharmacokinet 1991;16:249–55.
Contents lists available at ScienceDirect
Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost
Short communication
The use of rubber foam pads and ‘‘sensory ratios’’ to reduce variability in static posturography assessment Di Berardino Federica a,*, Filipponi Eliana a, Barozzi Stefania a, Giordano Gianpiero a, Alpini Dario b, Cesarani Antonio a a b
Audiology Unit, I.R.C.C.S. Policlinico, Mangiagalli e Regina Elena, Department of Otolaryngology, Chair of Audiology and Phoniatrics, University of Milan, via Pace 9, Milano, Italy Otoneurology Service Sc. Institute, Scientific Institute Maria Nascente–Don Gnocchi Foundation, Milan, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 November 2007 Received in revised form 14 May 2008 Accepted 2 August 2008
Despite the numerous works published, static posturography has still a limited clinical use due to its intrinsic inter-individual high variability. For this reason, foam pads have been introduced but their use is still not standardized. Aim of the study was to define the variability of static posturography parameters in standard and foam standing. Methods: 50 healthy subjects were studied with static posturography in four standing conditions: eyes open (EO) and eyes closed (EC), with and without foam pads. Unstable tests have been performed with two different types of rubber foam pads placed on the force platform. ‘‘Sensory ratios’’ have been calculated by the ratio of sway length among the four different conditions, adapted from dynamic posturography, in order to measure the relative contributions of vestibular, visual and somatosensory inputs. Results: Static posturography in standard conditions showed unacceptable coefficients of variation (>than 15%) for all the parameters. The use of foam pads reduced the high intrinsic variability, in particular for LFS (12.6–15.4%). The use of ‘‘sensory ratios’’ led to decrease the inter-subject coefficient of variation of this measurement to about 9.47–14.42% using the bilayer foam pads. Conclusions: Further studies are needed to confirm these data by applying the ratio formulas in clinical practice. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Static posturography Variability Coefficient of variation Foam pads
1. Introduction Numerous studies have suggested that static posturography is a useful clinical tool for evaluating balance impairment in patients, nonetheless contrasting data have also been reported [1,2]. A wide range of stimulus-response parameters have been measured without any consensus regarding which of them are most useful [3]. Foam pads have been introduced so that body sways are increased and a balance disorder due to vestibular loss can be identified more easily [4]. The role of static posturography in body sway assessment still has a limited clinical use because of its intrinsic high variability which does not permit a reliable diagnosis [2]. As far as we know, there are no reports concerning the intersubject coefficients of variation (CV) of static posturography parameters in healthy subjects. The present study aimed to define the variability of static posturography parameters in standard
* Corresponding author at: Viale Piceno 14, 20129 Milan, Italy. Tel.: +39 02 70126502; fax: +39 02 70126502. E-mail address: [email protected] (D.B. Federica). 0966-6362/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2008.08.006
conditions and assess whether it is possible to reduce it by applying the ‘‘sensory ratio’’ formulas using data from the rubber foam pads standing conditions. 2. Materials and methods Fifty healthy subjects (23 females, 27 males) volunteered to participate in the study. They ranged from 20 to 61 years old (mean age 44 14) and from 44 to 78 kg of weight (mean: 59.6 9,1 kg), engaged in regular physical activity, and had no selfreported orthopedic or neurological problems that negatively affected their balance and/or mobility. Subjects with a history of vertigo and balance disorders were excluded from the study. Static posturography was performed with the subject standing on a ‘‘Svep 6.0’’ stabilometric platform (Amplifon, Milan, Italy), according to manufacturers’ instructions dated September 2006. This is a stable force-plate, mounted on 3 strain-gauge force transducers positioned at the vertices of an equilateral triangle, which is sensitive to vertical force and provides a description of body sway in terms of displacement of the patient’s center of pressure. Tests were carried out using a fixed visual target placed at a distance of 1. 20 m, at eye level. X, Y and absolute center of pressure displacements from the projection of a theoretical baricentre and the sway area (SX), sway length (LX), and sway velocity (VM) were recorded at 5 Hz, over 51.2 s, using the Svep software (Svep 6.4). All the parameters given by the Svep system were considered, in particular the correlation between length and surface (LFS).
D.B. Federica et al. / Gait & Posture 29 (2009) 158–160
159
Fig. 1. Monolayer (A) and bilayer (B) rubber foam pads.
All subjects underwent static posturography in each of four conditions: stable condition with visual fixation (eyes open, EO), without visual fixation (eyes closed, EC), unstable condition on foam pads with visual fixation (PAD EO) and without visual fixation (PAD EC). Unstable condition tests have been performed with two different types of rubber foam pads: a ‘‘monolayer’’ with a thickness of 10 cm and a density of 25 kg/m3 (BROGGINI s.r.l. Varese, Italy) and a ‘‘bilayer’’ with a thickness of 8 cm and a density of 100 kg/m3 (ORSAFOAM, Gorla Minore, Italy) (Fig. 1). In order to investigate the relative contributions of vestibular, visual and somatosensory inputs, the analysis based on the Clinical Test of Sensory Interaction of Balance or ‘‘Foam and Dome’’ suggested by Shumway-Cook and Horak [5] and first proposed by Nashner et al. in 1982 for dynamic posturography [6] was applied to static posturography. The ‘‘sensory ratios’’ were obtained by modifying the mathematical modelling for clinical evaluation proposed by Contarino et al. [7], and calculated using the following formulas:
Somatosensory (Som): LX EO/LX EC
Table 2 Sensory equilibrium organization Somatosensory (%)
Visual (%)
Vestibular (%)
Monolayer Mean S.D. CV
44.91 7.10 15.80
35.09 5.63 16.05
20.00 4.12 20.61
Bilayer Mean S.D. CV
35.52 5.12 14.42
40.92 4.91 11.99
23.57 2.23 9.47
t-Test
<0.001
0.002
0.002
Visual (Vis): LX EO/LX PAD EO Vestibular (Vest): LX EO/LX PAD EC
Analysis of variability among all the parameters was performed: a CV of variation in excess of 15% was considered unacceptable. Parametric univariate t-test was performed, values of p 0.05 were considered statistically significant. We used the SPSS software package for all statistical analyses (version 13.0, SPSS Inc., Chicago, IL).
system have not been reported since the inter-subjects CV were more than 20%. LFS that gives the ratio between the length measured and that statistically expected in normal subjects of the same age group for every surface always had the lowest variability. In particular, LFS measured with subjects standing on the monolayer rubber foam pads with eyes open (PAD EO, LFSAO) had the lowest variation. The difference between the lengths observed by the use of the two foam pads was significant only in the EC condition (PAD EC, p = 0.004). Table 2 shows the results of the sensory tests using each of the two foam pads. Sensory ratio analysis gave the lowest variation when bilayer foam pads were used. The CV of these new parameters calculated by the length, remain high but lower than 15%. The difference between the results with the two foam pads was statically significant.
4. Results
5. Discussion
The CV of the parameters obtained from different tests are shown in Table 1. Many of the values given by the Svep postural
Despite the numerous works published on static posturography, this method cannot yet be considered completely validated. The parameters in the EO and EC conditions gave excessive CV, thus confirming reports suggesting that static posturography alone (without the use of foam pads) is not a useful clinical tool for evaluating balance problems [8]. In fact, in 1996, a meta-analysis by Di Fabio et al. [9] reported that the sensitivity and specificity of platform posturography were about 50%. We observed a large inter-individual variability that suggests that static posturography parameters are subjected to factors that make them difficult to reproduce; however, an increase of 35–40% could generally be considered a positive response. Research on postural responses to surface translations has shown that balance is not based on a fixed set of equilibrium reflexes but on a flexible, functional motor skill that can adapt with training and experience [10]. For this reason, we agree that the role of static posturography in body sway assessment has still a limited clinical use, not for topodiagnosis,
The values obtained were converted into percentages.
% Somatosensory: Som/(Som + Vis + Vest) 100 % Visual: Vis/(Som + Vis + Vest) 100 % Vestibular: Vest/(Som + Vis + Vest) 100.
3. Statistical analysis
Table 1 Coefficients of variation (expressed as percentage) LX
VM
SX
LFSAO
LFSAF
LFSEO
LFSEF
EO EC
24.7 36.7
24.7 36.7
84.2 86.9
21.4 33.8
21.2 33.7
21.5 33.9
21.8 34.3
Monolayer PAD EO Monolayer PAD EC
17.8 22.7
17.8 22.7
41.5 29.8
12.6 20.7
12.6 20.6
13.7 20.6
14.3 20.6
Bilayer PAD EO Bilayer PAD EC
15.0 15.8
15.0 15.8
17.3 25.0
15.4 15.0
15.2 15.2
15.5 14.9
14.9 15.0
LX = length, VM = mean velocity, SX = surface, and LFS = length values compared to the expected normal values, as given by Svep system. [LFSAO = LX/ (396.00 EXP(0.0008 SX)); LFSAF = LX/(485.00 EXP(0.0008 SX)); LFSEO = LX/ (831.00 EXP(0.0006 SX)); LFSEF = LX/(973.00 EXP(0.0005 SX)).]
160
D.B. Federica et al. / Gait & Posture 29 (2009) 158–160
but principally because of its intrinsic inter-individual high variability. In our healthy subjects, the only Svep parameters that had an acceptable CV were the LFS in the EO condition on monolayer foam pads, because LFS values are a ratio (length values compared to the expected normal values) that reduces its intrinsic variability. Recently, it has been reported that 44–60-year old subjects were more unstable than young subjects, especially in the dark. This analysis was made on the basis of sensory tests on thick foam pads [11]. In our study, the variability of static posturography parameters was significantly reduced by the use of both foam pads. However, the comparison of the two different types of foam pad was also statistically significant, thus suggesting to compare the materials used in static posturography, not only in terms of reproducibility and internal consistency but also in terms of overall feasibility, responsiveness, and predictive validity for a given population. In 1995, Wooley et al. [12] first carried out laboratory tests to define four different types of foam pads used in posturography and suggested to take select in the foam surface used in order to avoid inaccurate diagnosis of a patient’s reliance on a particular sensory input. Our results confirmed the fact that different types of foam pads give different results also on clinical evaluation with the stabilometric platform, thus underlining the importance of knowing and specifying the characteristics of the foam pads used. In 2005, Riley et al. [13] found that the center of pressure determinism in posturography increased when participants stood on a compliant, foam surface rather then on a rigid surface. A recent work by the same group showed that rendering the sensory environment unstable by manipulating sensory information by support surface or visual surroundings during posturography also influences the amount, relative variability and temporal structure of the center of pressure trajectories [14]. In order to make static posturography a reliable clinical tool, it is necessary to use parameters with a CV lower than 15% [15]. In our study, this was only possible using ‘‘sensory ratio’’ formulas, including values from the unstable condition on foam pads. The most reliable parameters in static posturography were given by the use of bilayer rubber foam pads. The CV of the visual and vestibular percentages were about 10%. Therefore, the lowest variability suggests that these percentages derived from the ‘‘sensory ratios’’ using bilayer foam pads, together with somatosensory one that was about 14.4%, are the most acceptable parameters for clinical use. Furthermore, the role
of ‘‘sway-related’’ tests with foam pads, so called by Baloh et al. [2] and proposed by Allum et al. [4] is fundamental in order to make static posturography more useful. Further studies are needed in order to define the use of foam pads in static posturography, to verify the use of ‘‘sensory ratio’’ with other stabilometric platform systems and to evaluate the clinical role of these new parameters. Conflict of interest None. References [1] Forth KE, Metter JE, Paloski WH. Age associated differences in postural equilibrium control: a comparison between EQscore and minimum time to contact (TTCmin). Gait Posture 2007;25:56–62. [2] Baloh RW, Jacobson KM, Beykirch K. Static and dynamic posturography in patients with vestibular and cerebellar lesions. Arch Neurol 1998;55:649–54. [3] Nelson SR, D Fabio RP, Handerson GH. Vestibular and sensory interaction deficit assessed by dynamic platform posturography in patients with multiple sclerosis. Ann Otol Rhinol Laryngol 1995;104:62–8. [4] Allum JHJ, Zamani F, Adkin AL. Differences between trunk sway characteristics on a foam support surface and on the Equitest1 ankle-sway-referenced support surface. Gait Posture 2002;16:264–70. [5] Shumway-Cook A, Horak FB. Assessing the influence of sensory interaction of balance: suggestion from the field. Phys Ther 1986;66:1548–50. [6] Nashner LM, Black FO, Wall C. 3rd. Adaptation to altered support and visual conditions during stance: patients with vestibular deficits. J Neurosci 1982;2: 536–44. [7] Contarino D, Bertora GO, Bergmann JM. Balance platform: mathematical modeling for clinical evaluation.. Int Tinnitus J 2003;9:23–5. [8] Pe´rennou D, Decavel P, Manckoundia P. Evaluation of balance in neurologic and geriatric disorders. Ann Readapt Med Phys 2005;48:317–35. [9] Di Fabio RP. Meta-analysis of the sensitivity and specificity of platform posturography. Arch Otolaryngol Head Neck Surg 1996;122:150–6. [10] Horak FB, Henry SM, Shumway-Cook A. Postural perturbations: new insights for treatment of balance disorders. Phys Ther 1997;77:517–33. [11] Poulain I, Giraudet G. Age-related changes of visual contribution in posture control. Gait Posture 2008;27:1–7. [12] Wooley SM, McCarter F B, Randolf B. An assessment of foam support surfaces used in static stabilometry. Gait Posture 1995;3:110. [13] Riley MA, Baker AA, Schmit JM, Weaver EE. Effects of visual and auditory shortterm memory tasks on the spatio-temporal dynamics and variability of postural sway. J Mot Behav 2005;37:311–24. [14] Clark S, Riley MA. Multisensory information for postural control: sway-referencing gain shapes center of pressure variability and temporal dynamics.. Exp Brain Res 2007;310:176–299. [15] Shah VP, Midha KK, Dighe S. Analytical methods evaluation: bioavailability, bioequivalence and pharmacokinetic studies. Eur J Drug Metab Pharmacokinet 1991;16:249–55.