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The mechanical properties of the esophagus are important for its function because the esophagus is subjected to changes in wall stress and strains caused by the passage of boli and the action of peristalsis.

Using impedance planimetry (IP) the cross-sectional area and pressure changes of the esophagus can be measured at the same time and the esophageal wall tension and stiffness can be obtained in vivo or in vitro. Using this novel technique it was demonstrated that the circumferential wall tension and wall stiffness of the esophageal wall increased exponentially as function of pressure [Orvar et al., 1993; Rao et al., 1995; Patel et al., 1998M; Vanags et al., 2003]. The tension distribution were non-uniform along the esophagus [Patel et al.,1998; Vanags et al., 2003] and the LES and the proximal esophagus had greater wall tension and were less deformable than the mid- or distal esophagus [Patel et al., 1998]. Combining with ultrasound the wall thickness of the esophagus also can be obtained and the stress-strain relationship of the esophageal wall can be computed [Assentoft et al., 2000; Frokjaer et al., 2006a., Zhao et al., 2007a]. Using IP combining the ultrasound, it was demonstrated that the esophageal wall stress-strain curves are also exponential [Zhao et al., 2007a] meaning that the tissue is soft at physiologic pressures and stiffer in the supraphysiologic pressure range. This mechanism seems to prevent overstretch of the esophageal wall when luminal loading becomes supraphysiologic. Furthermore it was also shown that the stress-strain distribution is non-uniform across the esophageal wall, i.e., the stiffness increased throughout the wall and was highest at the outer surface [Frokjaer et al., 2006a]. However, Takeda et al [2002, 2003] showed that the stress-strain relationship of the human esophagus in vivo was linear during both isovolumic and isobaric distension, before as well as after injection of the antimuscarinic drug atropine. The stress-strain relationship of the active component (muscle contraction) was different during isovolumic and isobaric distensions but the passive components were similar.

Furthermore, the method of esophageal distension significant influences on the active but not on the passive biomechanical properties due to a strain-rate effect. Investigating the degree of stretch and tension to initiate motor and sensory responses of the esophagus showed that the stretch rather than tension appears to be the major factor influencing sensory responses to esophageal distension [Barlow et al, 2002; Drewes et al, 2003a].

Esophageal function is usually evaluated by means of manometry which is a proxy of the force in the radial direction. However, force measurements in the axial direction will provide a better measure of esophageal transport function. Gravesen et al [2008] recently developed a probe based on electrical impedance measurements to quantify the axial force generated by esophageal contractions, i.e. probe elongation was associated with the axial force. The probe showed good reproducibility and the dispersion was <0.04. The swallowed bolus size correlated with the axial force measurements (P = 0.038) but not with manometric measurements. Therefore, the new technique measuring axial force in the esophagus may in the future provide valuable information about esophageal function.

Experimental and diagnostic procedures like distension of a balloon catheter, bougie, and esophagogastroduodenoscopy can induce shear deformation in the esophageal wall. Hence, knowing the shear modulus of the esophagus is important. Yang et al [2004a] has studied the shear modulus of elasticity of the esophagus in the rat. Results were obtained using a triaxial instrument to perform simultaneous torsion, inflation, and longitudinal stretching tests (Figure 1). The shear modulus varied with the inflation pressure and the longitudinal stretch ratio. The mechanical constant of the esophagus showed that the esophageal wall was anisotropic with different stiffness in the circumferential, longitudinal, and the shear directions. The stiffness in the longitudinal direction was higher than in the circumferential direction.

To understand esophageal function, it is necessary to know the stress-strain relationship for the different layers, because the esophageal wall is composed of four layers. Studies in layered wall of the esophagus showed that the stress distribution in the different layers were nonlinear and anisotropic [Lu and Gregersen, 2001; Gregersen et al, 2008; Fan et al, 2004; Liao et al, 2003a, 2007; Zhao et al, 2007b; Yang et al, 2007a]. The submucosa-mucosa layer has the highest stiffness [Fan et al., 2004; Liao et al.,2003a]. The residual strain differs

Figure1. Torsion machine setup. 1: Linear stage, 2: Torque transducer, 3: Organ bath, 4: Specimen, 5: Force transducer, 6: Motor for rotation, 7: Pressure transducer, 8: Infusion channel, 9: Motor for linear stage, 10: Rails for linear stage, 11: CCD camera, 12: Plastic rod.

Figure 2. Residual strain distribution in the intact wall and layers of pig oesophagus. Residual strain distribution of the intact wall referenced to the zero-stress state of the intact wall Residual strain distribution of layered wall in intact wall referenced to the zero-stress state of layered sectors Residual strain distribution of separated layers referenced to the zero-stress state of layered sectors. Referenced to the zero-stress state of the intact sectors, the inner residual strain of intact rings was -0.13 ± 0.04 and outer residual strain was 0.31 ± 0.03. Referenced to the ―true‖ zero-stress state of separated three layered sectors, the inner residual strain of intact rings was -0.22 ± 0.02 and outer residual strain was 0.07 ± 0.02. The residual strain distribution of the layers in intact rings referenced to the ―true‖ zero- stress state is also shown. The inner surface residual strain was negative at mucosa-submucosa and inner muscle layers and was positive at outer muscle layer, whereas the outer surface residual strain was negative at the mucosa-submucosa layer and positive at the inner and outer muscle layers. Referenced to the ―true‖ zero-stress state, the inner residual strain was negative and outer residual strain was positive for the separated layered rings.

between the layers, and the residual strain distribution was more uniform after the layers were separated (Figure 2) [Zhao et al, 2007b]. Furthermore, experimental studies have demonstrated esophageal mucosal folds in the no-load state. This indicates that mucosal buckling must be considered in the analysis of the mechanical reference state since the material stiffness drops dramatically after tissue collapse [Liao et al, 2007; Yang et al, 2007a].

The esophageal wall is composed of different material components that contribute to the esophageal biomechanical properties. Fan et al [2005] studied the effect of collagenase and elastase on the morphological and biomechanical properties in the no-load and zero-stress states in the rat esophagus. It was demonstrated that the collagenase and elastase caused the opening angle and the residual strain in the separated mucosa-submucosa layer to decrease.

The opening angle of the separated mucosa-submucosa layer depended to some extent on the fraction of collagen and elastin. This indicates that collagen and elastin are important for passive biomechanical properties of the esophagus.