The invention relates to a surgical cannula for introducing instruments, scopes or tubing into body cavities or organs. The cannula comprises a conical tubular stem which is formed of thin sheet material, such as beryllium copper, the sheet material being coiled or partially coiled so that if is capable of opening up from a closed condition by reducing the cone angle, thereby increasing the diameter of the stem at the distal end.
In the preferred arrangement, the proximal end of the stem is clamped between two tubular elements fitted one inside the other.
The invention also includes the combination of the expansible cannula and a guide rod adapted to pass through the stem and bored throughout its length to receive a guide wire. The guide rod may have a slightly enlarged distal end and the arrangement be such that passage of the enlarged distal end through the distal end of the stem causes a slight opening out of the distal end to allow passage of the enlargement.
When a blunt instrument such as a nephroscope is forced through the bore of the stem from the relatively large proximal end, the stem will expand by uncoiling until the stem provides a substantially parallel-sided bore through which the blunt instrument can pass.
According to a second aspect of the invention an expansible cannular has a stem formed by a plurality of thin sheet material leaves each of arcuate cross-section and overlapped so that together they form a tube which can be increased in diameter by reducing the overlap between the leaves.