Fig S1: Radiographs of the fetlock of a horse with a P1 fracture. The radiographs are made with the limb immobilised in a cast. Lateromedial projection (A), dorsopalmar projection (B), dorsomedial-palmarolateral oblique projection (C) and dorsolateral-palmaromedial oblique projection, both with a slight different angle (D; left is medial). On the dorsopalmar view (B), a midsagittal radiolucent line is detected in the proximal aspect of P1 running proximally towards the fetlock joint and sagitally splits in 2 radiolucent lines, one line runs parasagittal towards the proximal interphalangeal joint. This line runs distally more parasagittal involving the lateral condyle of P1 (D). The second line runs in a proximomedial-distolateral direction over half of the length of P1. Both lines are well defined and sharply bordered. On Images A and C the fracture is not visible.

Movie S1: Computed tomography study of the distal limb of a horse with a P1 fracture. (PSB = proximal sesamoid bone; NB = navicular bone). A fracture line (red arrows) (<1 mm) is visible, starting in the fetlock joint in the sagittal groove of P1, running distally in a spiral and ending in the pastern joint in the palmar half of the lateral condyle of P1. At the palmar and dorsal aspect of P1 several coalescing cracks (additional red arrows), surrounding the fracture line, are visible at the level of the proximal and distal ends of P1.

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