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

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.

Thirteen cadaveric pelves were used for this study. The supra-acetabular bone tunnel was visualised with an image intensifier. The proximal most fibres of the hip joint capsule were marked with a K-wire so that their relation to the bone tunnel could be clearly seen on the images. Once all images were acquired they were calibrated and analysed to estimate the dimensions of the supra-acetabular bone tunnel and the reflection of the hip capsule.

The median height of the bone tunnel was 23.6 mm (18.9-33.2) and maximum width was 11.4 mm (7.6-16.3). The inferior margin of the bone tunnel was 6.7 mm (1.1-14.5) superior to the acetabular dome, and the most proximal fibres of the capsule were 9.3 mm (4.7-6.1) superior to the acetabular dome. The inferior portion of the tunnel was 3.7 mm (0.3-8.9) within the joint.

Half pins for the construction of a pelvic external fixator should be placed in the upper half of the supra-acetabular bone corridor to minimise the risk of intra-capsular placement.