Internal elastic membrane in the internal mammary and left anterior descending coronary arteries and its relationship to intimal thickening

The internal mammary artery (IMA) is less prone to intimal thickening than coronary arteries and is routinely used in coronary bypass surgery. To resolve whether morphologic differences can explain why IMA develops less intimal thickening, morphometric measurement of the left anterior descending coronary artery (LAD) and IMA was carried out in 62 autopsies from individuals of various age groups. The amount of intimal thickening, the degree of folding of the internal elastic membrane, and media thickness were estimated by appropriate indices. Intimal thickening of LAD was more marked, occurred earlier and could be demonstrated even below one year of age, but was not found before 21 years of age in the IMA. A fold-index of the internal elastic membrane was significantly higher in the IMA than in the LAD and correlated negatively with an intimal thickening index in both arteries (P less than 0.0001 for both). The fold-index decreased with age, most markedly in LAD. Negative correlation was also found between a media thickness index and the intimal thickening index in LAD (P less than 0.02). The fold-index may indicate the magnitude of tangential intimal tension during life and may be of significance for development of intimal thickening and atherosclerosis. Lower fold-index may in part explain the propensity of atherosclerosis in LAD compared with IMA. Also, reduced fold-index with age accompanied the occurrence of atherosclerosis with age.