ANYONE who has had knee replacement surgery should take care when swimming - the added weight could mean they have trouble staying afloat, warn doctors from Raigmore Hospital, Inverness. They report in the British Medical Journal on a 67-year-old woman who had been a strong swimmer before developing arthritis and undergoing knee, shoulder and elbow replacements. Eighteen months after her last surgery she sank in the local swimming pool. Also, a man who developed rheumatoid arthritis was able to stay afloat after hip and shoulder replacements but sank when, in an experiment, prostheses were attached externally to each knee.

Buoyancy of the human body is chiefly due to the gas-filled cavities in the thorax and abdomen. While shoulder and hip replacements can probably be supported by the trunk, knee replacements are heavier and farther from the centre of buoyancy. Swimmers who have had knee replacements should wear rubber knee or ankle supports to keep them afloat.