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Bone Strength as a Trait for Assessing Mineralization in Swine: a Critical Review of Techniques Involved^1,2,

University of Nebraska, Lincoln 68583

Abstract

Lack of standardized test procedures has resulted in considerable variation in reported values for bone strength. Such variation can be attributed in part to the type of instruments used to determine physical properties of bone, procedures used to prepare the bones for testing and equations used to calculate strength. If bone strength is to be used as a major criterion of response in mineral nutrition research, standardization of procedures for measuring and reporting bone strength is essential. Traits that describe the mechanical properties of bone as determined in the commonly used flexure test in which force is applied perpendicularly to the longitudinal axis are bending moment, stress, moment of inertia, strain and modulus of elasticity. Bending moment is a measure of the amount of force withstood by the bone, whereas stress is a measure of force per unit area of bone. Stress allows comparisons to be made between bones that differ in size and shape. The moment of inertia is a measure not only of the area over which the force is applied, but also of the shape in which the area is distributed. Strain is a measure of the amount of bending per unit of length that occurs as the bone is tested. The modulus of elasticity is a measure of the rigidity of the bone or, more simply, is the stress to strain ratio. Instruments that allow the researcher to control the rate of deformation as well as to record the force and deformation are important. Since the modulus of elasticity is affected by the rate of deformation, a standard rate of 5 mm/min is suggested. Differences exist in the mechanical properties of wet and dry bones. Wet bones bend to a greater extent but withstand less ultimate force than dry bones. As little as 10 min exposure to air can result in changes in the mechanical properties of wet bones. Simplification of equations used to calculate stress may yield values that are only a reflection of bending moment if the simplifications do not account for differences in shape or size of the bone. Mechanical properties of bones respond differently to nutritional treatments, and different conclusions can be made, depending upon which trait is used. As bone mineralization increases, maximum stress and bending moment of the bone increase. At a point of optimum mineralization, stress reaches a maximum. Bending moment can increase beyond the point of optimum mineralization if the bone continues to deposit more total minerals. Conclusions about the nutrient requirements affecting bone mineralization should be based on several of the mechanical properties rather than just one.

¹ Published as Paper No. 6110, Journal Ser., Nebraska Agr. Exp. Sta.

² Dept. of Anim. Sci

³ Current address: Meat and Anim. Sci. Dept., Univ. of Wisconsin, Madison.