1. Bony spurs resulting from erosion of the scaphoid bone and trapezium in rheumatoid arthritis can pierce the floor of the carpal tunnel and cause attrition rupture of flexor tendons—most often the flexor pollicis longus tendon.
2. It is difficult to show these spurs on conventional radiographs, but using a special tomographic technique we have been able to explore the floor of the carpal tunnel in three planes. Using this technique we have been able to guard the flexor pollicis longus against attrition rupture by early excision of a spur from the scaphoid bone.
3. Our findings support the recommendation of free division of the flexor retinaculum in rheumatoid arthritis. When doing this the surgeon should always look for a bony spur piercing the floor of the carpal tunnel.