Modelling the elastin, collagen and smooth muscle contribution to the duodenal mechanical behaviour in patients with systemic sclerosis.

Systemic sclerosis (SS) is a connective tissue disease that involves the gastrointestinal tract. Previous experiments have shown abnormal intestinal motility, dilatation, wall stiffening and impaired smooth muscle function. Consequently, understanding the association between intestinal wall mechanics, structure and function is important. The aim was to establish a model for differentiating the biomechanical remodelling of elastin, collagen and smooth muscle in the duodenum in SS patients. A duodenal distension protocol was used in six patients and five healthy controls. A theoretical model for evaluating the mechanical contributions of elastin, collagen and smooth muscle tone was established. The tension-strain curves computed from pressure and cross-sectional area data were analysed. The elastic modulus of elastin, the relationship between the collagen recruitment, collagen density and the active tension were calculated. The model fitted the clinical data well. The material constant for elastin in the patients was 30% lower than in the control group (P = 0.005). More collagen was recruited in patients than in healthy volunteers. Eighty percent of the collagen fibres were recruited at stretch ratio 0.85-2.26 (1.61 averaged) in patients and at the stretch ratio 2.55-3.73 (2.97 averaged) in healthy controls. The maximum active muscle tension and the corresponding strain were lowest in the patients (P = 0.01). The model can be used to determine the contribution of tissue components to the mechanical behaviour of duodenum. The stiffer wall in patient was due to the small stretch ratio for the maximum collagen recruitment but the muscle activity was also impaired.