| It is well known that the function of the esophagus is to propel food from pharynx to stomach by peristaltic motion, which is a result of the interaction of the tissue forces in the wall and the hydrodynamic forces in the food bolus. To understand the tissue forces of the esophagus, it is necessary to know the stress-strain relationships and the zero-stress state that is sensitive to structural and mechanical remodeling. Achalasia is a typical motor disorder of the esophagus in clinical. Caused by neural lesion the lower esophageal sphincter (LES) poorly relaxing resulting in an increased pressure in LES is generally accepted as the primary mechanisms of achalasia. To know more about the changing of the esophageal function in the progress of episode and developing, it is necessary to know the stress-strain relations and the zero-stress state of esophageal tissue, and the changes of these states and relationships due to biological remodeling of the tissue under stress.Gastroscope benzyldimethyltelradecylammonium chloride (BAC) or saline were injected circumferentially into the lower esophagus sphincter (LES) of 20 healthy cats, which were divided into 4 groups (4, 6, 8 weeks groups and control group, n = 5) . Manometries were performed before and after injection. The pressure-volume (P-V) data of upper and lower 1/3 parts of esophagus at various axial extension ratios were measured and fitted by exponential function. Then the variations of zero-compliance and compliance with pressure in each group were measured. Finally the tested esophagus was sectioned into rings with about 2 mm in length in Krebs solution. The rings were photographed in the no-load state and cut radically to obtain the zero-stress state. Equilibrium was awaited for 30 inin and the specimens were photographed again. Inner and extra circumferences, wall thickness and the opening angle were measured from the digitized images and esophageal circumferential residual strain was calculated. Strain energy density function presented by Fung, which describes mechanical property of soft tissues was introduce to esophagus to obtain the constitutive function between circumferential stress andcircumferential strain. V/ith the calculated residual strain in esophageal wall, the residual stress distribution in esophageal wall was got. LESP in BAG injection group were significantly higher than those of the control group 4, 6 and 8 weeks later [(31.2 + 5.11) mmHg> (30.6+3.61) mmHg> (32.2+3.76) mmHg vs (17.0 + 3.10) mmHg, (17.8+2.79) mrnHg, (17.8+1.33) mmHg, P<0.05, P<0.01 and P<0.01). BAG injection groups showed less relax degree and relax ratio than those of the control group 6 and 8 weeks later (relax ratio 6 weeks later P< 0.05, else P<0.01). Contraction amplitudes in esophageal body showed no difference between BAG injection groups and control group. When the esophagi were elongated to approach to the in vivo length, the lower esophagus of BAC-treated cats showed lower compliance compared to those of the control while the upper esophagus showed no difference. The numerical results showed that residual stress in esophagus was negative at inner wall, but positive at outer wall, and the absolute values of residual stress at inner wall and outer wall are very close. In BAC-treated cats, except for the upper esophagus in BAC-treated groups 8 weeks later both the upper and lower esophagus of each groups had smaller opening angles and residual stress than those of the control (upper esophagus of BAC-treated 6 weeks later P<0.05, else P<0.01). But the difference reduced with time consuming. Compared to those of the control, the upper esophagus of 8 weeks after BAC injection showed larger opening angles (P<0.01) and had no difference in residual stress. It is suggested that both the upper and lower esophagus in cat models of achalasiahad significant abnormalities of mechanics property.
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