How can ‘blind’ participant be so if intervention can be perceived

If the intervention involves a foot orthoses how can the intervention group participant be ‘blind’ if they can perceive a difference in surface topography of material which has resistance to deformation relative to before intervention whilst the control participant would possibly perceive no difference from the ‘so-called’ sham orthoses?

If the intervention involves a foot orthoses how can the intervention group participant be ‘blind’ if they can perceive a difference in surface topography of material which has resistance to deformation relative to before intervention whilst the control participant would possibly perceive no difference from the ‘so-called’ sham orthoses?

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How can ‘blind’ participant be so if intervention can be perceived