Anterior subcutaneous internal fixation of the pelvis - what rod-to-bone distance is anatomically optimal?

Anterior fixation of the pelvis using subcutaneous supra-acetabular pedicle screw internal fixation (INFIX) has proven to be a useful tool by avoiding the downsides of external fixation in patients where open fixation is not suited. The purpose of this study was to find a rod-to-bone distance for the INFIX that allows for minimal hazard to the inguinal neuro-vascular structures and, at the same time, as little as possible interference with the soft tissues of the proximal thigh when the patient is sitting.

An INFIX was applied to 10 soft-embalmed cadaver pelvises with three different rod-to-bone distances. With each configuration, the relations of the rod to the neuro-vascular and the muscular surroundings were measured in supine and sitting position.

Except for the femoral artery, vein and nerve, all investigated anatomical structures of the groin were under compression with a rod-to-bone distance of 1cm. With a rod-to-bone distance of 2cm most of the anatomical structures were safe in supine position, although less than with 3cm. With hip flexion some structures got under compression, especially the lateral femoral cutaneous nerve (LFCN, 80%) and the anterior cutaneous branches of the femoral nerve (ACBFN, 35%). With a rod-to-bone distance of 3cm almost all anatomical structures were safe in supine position, while with hip flexion most superficial structures of the proximal thigh got under compression, especially the LFCN (75%) and the ACBFN (60%).

Aiming for a rod-to-bone distance of 2cm is the safest way with regard to compression of the femoral neuro-vascular bundle and at the same time leads to the least compression of more superficial structures like the LFCN, the ACBFN, or the sartorius and the rectus femoris muscles in sitting position.