The invention includes an apparatus (110) and method (152) for determining a deformation (170) of a three-dimensional, flexible structure in the form of a tubular construct (112) with a minimum of optical waveguides. An optical waveguide (116) is introduced into the tubular construct (112), where the optical waveguide (116) extends as a helical winding (118) within the tubular construct (116) and has a multiplicity of strain measuring means (126). The configuration of the apparatus and method performed reduces the average material load of the strain measuring means and improves resolution and measurement range of the sensor (optical waveguide) while maintaining readout frequency of the strain measurements. The apparatus (110) also includes an evaluation device (130) configured to determine the deformation of the tubular construct (112), in particular in relation to an axis, surface and/or volume of the tubular construct (112), from measurement values (154) from the multiplicity of strain measuring means (126).