3-DIMENSIONAL GEOMETRIC AND STRUCTURAL SYMMETRY OF THE TURKEY ULNA

Structural models of long-bone preparations usually assume left-right symmetry of contralateral bones under normal (baseline) conditions. To obtain insight on how this assumption affects the detection of subtle changes (as from functional adaptation), we formally examined the thr ee-dimensional geometric and structural symmetry of paired long bones, using contemporary image reconstruction and stress analysis technique s. Nine pairs of ulnae from normal male turkeys were reconstructed com putationally from serial transverse images obtained by either (a) mech anical sectioning and digital photographic imaging or (b) computed tom ography. Computed tomography scans allowed greater precision in recons truction than did digitally imaged photographs. Left-right comparisons of parameters of geometric symmetry (from computed tomography reconst ructions) revealed average differences in whole bone volume and whole bone principal moments of inertia of 3.6 and 3.0%, respectively. Diffe rences in bone curvature were indexed as noncolinearity of left compar ed with (mirrored) right centroidal axes, giving a disparity of 0.7 +/ - 0.3 mm. Within the longitudinal central 20% of the diaphysis (the cu stomary region for histomorphometry), average left-right differences i n cross-sectional area and area principal moments of inertia for compu ted tomography images were 4.7 and 5.0%, respectively. The overlap of longitudinally paired cross sections of the mid-diaphysis, aligned at common centroids and oriented in the respective principal inertial dir ections, was greatest (as much as 95%) in the central 20% of the diaph ysis. Paired three-dimensional finite element models demonstrated near ly identical left and right stress/strain fields throughout the ulnar diaphyses for both compressive and torsional loading. Our data suggest that the assumption of contralateral geometric symmetry in long bones should be judged in the context of the specific attribute of symmetry under consideration: however, we conclude that for purposes of finite element modeling the assumption of symmetry is reasonable.