Radiological landmarks for the safe extra-capsular placement of supra-acetabular half pins for external fixation

Purpose Low anterior external fixators are constructed by placing half pins in the dense bone tunnel of the supra-acetabular region in an anterior to posterior direction. Although the placement of these pins is extra-articular, they may still breach the hip capsule on the anterior inferior iliac spine and thus be intra-capsular. We aim to provide radiological markers for the most superior fibres of the capsule to allow safe extra-capsular pin placement within the supra-acetabular bone tunnel.