Need the cast if there's a screw in place?

I recently injured my lower leg and ankle in a motorcycle accident. There's a screw now holding the lower leg bones together while they heal. I have to wear a cast for six weeks. Why do I need the cast if there's a screw in place?

The two lower leg bones (tibia and fibula) have a fibrous sheath between them called the syndesmosis. It's designed to hold the two bones together while allowing the motion needed for normal ankle movement.

Screw fixation holds the area together and doesn't allow motion while it's healing. If you don't wear a cast, and if you do put weight on that foot and leg, the screw may come loose or even break.

Doctors are especially concerned about patients who are overweight or who have brittle (osteoporotic) bones. These factors increase the risk of screw failure. A new way to repair this injury is under investigation.

Doctors in Ireland are trying a flexible, plastic suture material. It holds the two bones
together while itself being held in place by buttons on the outside of each leg bone. The patient still wears a cast but can put weight on the leg sooner and get the cast off sooner. If studies show it's safe, the suture-button may become the standard way to treat a syndesmosis problem.

Brian Thornes, FRCSI, et al. Suture-Button Syndesmosis Fixation. In Clinical
Orthopaedics and Related Research. February 2005. Vol. 431. Pp. 207-212.