The comparison of density-elastic modulus equations for the distal ulna at multiple forearm positions: a finite element study.

The accuracy of an empirically derived density-modulus equation for bone depends upon the loading conditions and anatomic site of bone specimens used for experimentation. A recent study used FE modeling to compare the ability of three density-modulus relationships to predict strain during bending in neutral forearm rotation in the distal ulna; however, due to the inhomogeneous nature of these FE models, the performance of each equation is not necessarily consistent throughout forearm rotation. This issue is addressed in the present study, which compares the performance of these equations in pronation and supination. Strain gauge data were collected at six discreet locations of six ulna specimens loaded in bending at 40° of pronation and supination. Three FE models of each specimen were made, one for each density-modulus relation, and the strain output compared to the experimental data. The equation previously shown to be most accurate in predicting ulnar strain in neutral forearm rotation was also most accurate in pronation and supination. These results identify this one equation as the most appropriate for future FE analysis of the ulna (including adaptive remodeling, and further show that isotropic and inhomogeneous FE bone models may provide consistent results in different planes of bending.