Abstract
Introduction
Application of an external fixator for type B and C pelvic fractures can be life saving. Anteriorly the fixator half pins can be placed in the long and thick corridor of bone in the supra-acetabular region often referred to as the low anterior ex-fix. Pins in this location are favoured as they are more stable biomechanically. The bone tunnel for the low anterior ex-fix can be visualised with an iliac oblique projection intra-operatively. In some cases despite being outside the articular surface it may still be low enough to pass through the capsular attachment of the hip joint on the anterior inferior iliac spine. We aim to provide radiological markers for the most superior fibres of the capsule to help accurate extra-capsular pin placement within the supra-acetabular bone tunnel.
Materials and Methods
Thirteen cadaveric pelves, embalmed with the method of Thiel, were used for this study. An image intensifier was positioned to acquire an iliac oblique outlet view, such that the supra acetabular bone tunnel was visualised. This was achieved by positioning the beam 30 degrees cephalad and 20 degrees medial. Both left and right hemipelves were examined in this way. A standard size metallic disc was included in all images with in the acetabulum to allow for image calibration. 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 using ImageJ Software to estimate the height and maximum width of the bone tunnel as seen on the images and the vertical distance of the superior most fibres of the capsule from the dome of the acetabulum.
Results
The mean height of the bone tunnel was 24.9 mm (SD 4.3 mm, Range 18.9–33.2 mm) and the maximum width of the tunnel was 11.7 mm (SD 2.6 mm, Range 7.6–16.3 mm).
The inferior margin of the bone tunnel was on average 7.4 mm (SD 3.4 mm, Range 1.1–14.4 mm) superior to the acetabular dome and the most proximal fibres of the capsule were on average 9.2 mm (SD 2.4 mm, Range 4.7–16.1 mm) superior to the acetabular dome. This meant that on average 3.6 mm (SD 2.1 mm, Range 0.3–8.9 mm) of the inferior portion of the tunnel is within the joint. There was no statistically significant difference between the left and right sides.
Conclusion
There is adequate space for two long external fixator pins within the described tunnel. These should be placed in the upper half of the anterior inferior iliac spine. Below this level there is risk of being intra-capsular which can lead to septic arthritis. For this reason we recommend that supra-acetabular pins should be placed at least 16 mm superior to the acetabular dome as visualised on the iliac oblique outlet view.
