Composition, cortical structure and mechanical properties of chicken tibiotarsi: effect of growth rate.

1. Comparisons of tibial growth in slowly (X) and rapidly (F) growing chicks were carried out weekly until the birds weighed 500 g. 2. Bone volume and percentage of ash per dry weight increased with age and were similar at equal body weights between crossbreeds. In birds of the same age, the ratio of tibial dry weight to hydrated weight was greater in crossbreed X as compared with crossbreed F. Consequently, the tibial density and the mineral content per volume were higher, at equal body weights, in crossbreed X. 3. Cortical cross-sectional area was greater in crossbreed F at the different ages, but was similar in both crossbreeds at equal body weights. A peripheral active zone of greater porosity was predominantly characterised by radial fibrolamellar tissue in fast growing birds and covered a larger area in crossbreed F than in crossbreed X. 4. Biomechanical properties of the tibiotarsi were evaluated in a three-point flexure test. The area moment of inertia was equal at similar body weights in both crossbreeds, but the yield or maximal forces and stiffness of tibiotarsi were lower in the rapidly growing crossbreed F compared with crossbreed X. 5. It is concluded that an increased rate of growth is associated with a lower mineral density and higher porosity of the tibial cortices resulting in reduced bending resistance.