My elderly neighbor fell and dislocated her shoulder. But it took two different doctors to figure out what the problem was. The first X-ray was normal but she still had extreme pain and couldn’t move her arm. I took her to my own doctor who diagnosed a posterior dislocation. Why didn’t it show up on the first X-ray?

Evaluating the injured or unstable shoulder can be very challenging. Posterior dislocations and some fractures just don’t show up on a standard X-ray. The most commonly used position for an X-ray of the injured shoulder is called the Velpeau axillary view.

The patient is positioned supine (lying on the back) and/or standing up and leaning slightly backwards. The X-ray beam is directed down from the top of the shoulder through the joint to the bottom of the axilla (armpit).

If the patient can’t assume either of these positions, then the wheelchair axillary view can be used. In this case, the patient sits in a standard chair or wheelchair. The arm is slightly abducted (moved away from the body). The X-ray beam is still directed down through the shoulder from top to bottom. The patient isn’t stressed or uncomfortable with this method.

It’s possible the first X-ray of your neighbor was from an oblique angle, which doesn’t always show a posterior dislocation. It’s also possible that the X-ray technician didn’t know about the wheelchair axillary view. The position used with your neighbor may have resulted in a distorted or less than clear X-ray from movement or poor positioning.

There are any number of reasons why these things happen. Imaging studies are a helpful tool, but they aren’t always 100 per cent accurate.