Geometrical Properties of the Ovine Tibia: A Suitable Animal Model to Study the Pin-Bone Interface in Fracture Fixation?

To determine whether dimensional scaling (relative to the human) is necessary for screwed pins used in externally applied fracture fixation studies on sheep, geometrical data were determined for six ovine tibiae. Each tibia was potted relative to a lengthwise reference axis and sectioned at 5 per cent length intervals over its central 80 per cent. Enlarged (280 per cent) images of each cross-section were digitized at 1 mm increments around the periphery of the periosteal and endosteal surfaces, the data were digitally filtered, and geometrical properties were computed to include cross-sectional area A, maximum and minimum second moments of area (I_max and I_min), polar second moment of area J, and effective polar second moment of area J_eff. Proportional scaling of geometrical properties with respect to bone length (L² for A, and L⁴ for second moments of area) significantly (p < 0.01) decreased the coefficient of variation in data by an average 36 per cent. From 30–90 per cent distal, J _eff for the ovine tibia is smaller but within 7 per cent of J—in stark contrast with the human tibia, where J_eff has been reported as 70–80 per cent of J over the same tibial length. While previous ovine studies involving external fixator pins have employed the same diameter of pin as has been used in humans (that is 5 or 6 mm), a ‘first-order’ approximation of the data for A, I_max, I_min and J_eff suggests these pins should be scaled down to 4 mm and 4.75 mm respectively for use on the ovine tibia over the range 25–80 per cent distal along its length.