STRESS GOVERNS TISSUE PHENOTYPE AT THE FEMORAL INSERTION OF THE RABBIT MCL

The cells in the midsubstance portion of skeletal ligaments typically have elongated shapes, but where ligaments insert into bone the cells appear very rounded and the tissue phenotype is that of fibrocartilage . Between the midsubstance and the insertions there is a gradient in c ell shape and tissue phenotype that has been hypothesized to reflect a gradient of mechanical stresses. To test this hypothesis, cell shapes (an index of tissue phenotype) were quantified in the central part of the femoral insertion of the rabbit medial collateral ligament by com puter-assisted histomorphometry. Morphometric measurements were correl ated with the mechanical stresses and strains in the central part of t he insertion as predicted by finite element analysis. Throughout the l igament the direction of the predicted principal tensile stresses coin cides with the direction of the collagen fibers which curve from the m idsubstance to meet the femur at nearly right angles. Principal compre ssive stresses also occur within the ligament: the highest are localiz ed near the bone; the lowest in the midsubstance. The areas with the r oundest cells correspond to the areas with the highest principal compr essive stresses in the model; the areas with the flattest cells corres pond to the areas with the lowest compressive stresses in the model. A correlation between cell shape and mechanical stresses suggests that physiological loading of the MCL is important for the maintenance of t issue phenotype throughout this insertion. We theorize that the cells in ligament insertions adapt to the prevailing local mechanical enviro nment.