A STUDY OF THE RELATIONSHIP BETWEEN MINERAL-CONTENT AND MECHANICAL-PROPERTIES OF TURKEY GASTROCNEMIUS TENDON

The vertebrate skeletal system undergoes adaptation in response to ext ernal forces, but the relation between the skeletal changes and such f orces is not understood, In this context, the variation in the amount and location of calcification has been compared with changes in mechan ical properties of the normally mineralizing turkey gastrocnemius tend on using ash weight measurements, X-ray radiography, and mechanical te sting, Radiographic evidence from 12- to 17-week-oId birds showed calc ification in only portions of gastrocnemius tendons proximal to the ta rsometatarsal joint, Mechanical testing of these dissected proximal re gions demonstrated an increased ultimate stress and modulus and a decr eased 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 ten don curves while highly calcified tendons yielded curves resembling th ose of bone, The proximal portions of the gastrocnemius where minerali zation 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 resul t of increased stresses at certain locations of the tendon, perhaps th e consequence of the natural forces exerted by the large leg muscles o f the bird into which the gastrocnemius inserts, More specifically, te ndon calcification may be the result of stress-induced exposure of cha rged sites on the surfaces of collagen molecules, fibrils, or fibers s o that deposition of mineral and subsequent mechanical reinforcement o ccur in the tissue, The concept of stress-induced calcification of avi an tendon is found to be consistent with other examples of biological mineralization and supports the view that vertebrate calcification is an adaptive process mediated in part by the accommodation of natural o r applied forces.