Here at West Midlands Referrals we see this frequently in cats that have been shunted from behind by cars, often in association with other pelvic injuries.
The sacrum, a section of the lower vertebral column, separates from the iliac wing of the pelvis. The pelvic wing usually gets displaced forwards. This can affect one or both sides of the pelvis and is usually relatively straight forward to fix with a screw. The target area for the screw is small (about 5mm diameter in cats) and there are anatomical hazards to avoid like nerves in the vertebral canal, intervertebral disc spaces and vasculature in the pelvic canal.
18th August 2013