A study of the relationship between mineral content and mechanical properties of turkey gastrocnemius tendon.

The vertebrate skeletal system undergoes adaptation in response to external forces, but the relation between the skeletal changes and such forces is not understood. In this context, the variation in the amount and location of calcification has been compared with changes in mechanical properties of the normally mineralizing turkey gastrocnemius tendon using ash weight measurements, X-ray radiography, and mechanical testing. Radiographic evidence from 12- to 17-week-old birds showed calcification in only portions of gastrocnemius tendons proximal to the tarsometatarsal joint. Mechanical testing of these dissected proximal regions demonstrated an increased ultimate stress and modulus and a decreased maximum strain that appeared to parallel calcification. Further, stress-strain curves of portions of uncalcified turkey gastrocnemius tendon were shaped similar to those of other typical unmineralized tendon curves while highly calcified tendons yielded curves resembling those of bone. The proximal portions of the gastrocnemius where mineralization begins were observed to have a decreased tendon cross-sectional area compared with distal portions which do not mineralize. Based on the resultant measures of mineral content and location and mechanical properties, it is hypothesized that increased calcification is a result of increased stresses at certain locations of the tendon, perhaps the consequence of the natural forces exerted by the large leg muscles of the bird into which the gastrocnemius inserts. More specifically, tendon calcification may be the result of stress-induced exposure of charged sites on the surfaces of collagen molecules, fibrils, or fibers so that deposition of mineral and subsequent mechanical reinforcement occur in the tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Physical properties of model viscoelastic materials.

Previous observations on polysaccharides used as viscoelastic agents in ophthalmic surgery suggest that the pseudoplasticity of solutions of hyaluronan (HA) and the low surface tension of hydroxypropylmethylcellulose (HPMC) solutions are physical properties that make these solutions useful clinically. Our laboratories are interested in correlating the physical properties of macromolecular solutions with the ability of these molecules to protect ocular structures during eye surgery. The purpose of this study is to compare the physical properties of model viscoelastics with the properties of HA and HPMC. The results of these studies suggest that polysaccharides that form extended structures in solution at low shear rates and that are characterized by large decreases in the axial ratio at high shear rates, exhibit pseudoplastic behavior. In this study pseudoplasticity is exhibited by polysaccharides with molecular weights in excess of 450 000, and is insensitive to the backbone chemistry for linear macromolecules. In addition, low surface tension is associated with charged macromolecules that have a high positive second virial coefficient.