Altered collagen concentration in mitral valve leaflets: Biochemical and finite element analysis

Background. Ischemic mitral regurgitation or ventricular wall motion abnorm alities will alter the stress distribution in the mitral valve. We hypothes ize that in response the regional collagen concentration will be altered an d will significantly impact the stress distribution in the mitral valve. Methods. Two sheep served as normal (sham) controls. Two other sheep had co ronary ligation resulting in abnormal ventricular wall motion. Four sheep u nderwent ligation to infarct the posteromedial papillary muscle, resulting in ischemic regurgitation. After 4 or 8 weeks, the mitral valves were excis ed, and the anterior leaflet sections were subjected to an assay for collag en concentration. Next, in a finite element model, to simulate changes in c ollagen concentration, the tissue stiffness was increased by 20%, and then decreased by 20%. In another model, the thickness of the tissue was increas ed by 20%, and then combined with decreased tissue stiffness. Physiologic l oading pressures were applied, and leaflet stress, chordal stress, and coap tation results were analyzed. Results. The average collagen concentration in the normal sheep leaflets wa s 59.2% (dry weight), 50.6% in the ischemic controls, and 45.8% in the papi llary muscle infarct group. Collagen concentration was greatest at the midl ine and decreased toward the commissures. Increased tissue stiffness result ed in increased leaflet and chordal stresses, as well as reduced coaptation . Decreased stiffness resulted in the opposite. Increased tissue thickness reduced leaflet and chordal stresses, but also reduced coaptation. The comb ination of increased tissue thickness and decreased stiffness demonstrated the greatest reduction in leaflet and chordal stress, while maintaining nor mal leaflet coaptation. Conclusions. The observed changes may demonstrate an early effort to compen sate for increased leaflet stress. Microstructural alterations may demonstr ate an early effort to compensate for altered physiologic loading to reduce stress and maintain coaptation. It is crucial in repairing or partially re placing thickened tissue that normal geometry and physiology be restored. ( C) 1998 by The Society of Thoracic Surgeons.