Cannulated screws are used in bone and joint surgery to repair breaks and to secure artificial implants which may be used to replace part or all of a joint. A cannulated screw is generally made of stainless steel or titanium and is self-tapping, which means it can cut its own path through bone as it is screwed into place. The central core of the screw is hollow, allowing a guidewire to be run through. This helps to maneuver the screw into position, as it can be advanced along the wire, removing the need for a pre-drilled hole. The area of medicine associated with disorders of bones and muscles is known as orthopedics.
There are many different sizes of cannulated screws available and two kinds of screw thread, one for porous bone and one for harder, more compact bone. Cannulated screws with threads suitable for porous bone are more common, and they are larger in diameter, with bigger spaces between the threads and a relatively narrow inner shaft. Since they are intended to fix softer bone, their design provides a larger surface area for the bone to grip. As cannulated screws are around ten times more expensive than non-cannulated screws, the non-cannulated type are used where possible to reduce costs.
Cannulated screws enable the performance of what are called percutaneous techniques, where a surgical procedure takes place through puncture holes in the skin rather than making a large open incision. This type of surgery may be used to treat a femoral neck fracture, where the ball-shaped head of the thigh bone breaks off at the narrow point, or neck, where it joins to the shaft. The operation can only be used in cases where the broken bone is still in place, to ensure that the head of the thigh bone has not had its blood supply disrupted and will remain alive after the screws have secured it in place.
During surgery, images of the procedure, similar to X-rays, can be seen on screen using a piece of equipment called a fluoroscope. This enables the surgeon to position the guidewires correctly before inserting the screws. Normally, no more than three cannulated screws are needed to fix the fracture, positioned in an inverted triangle shape, but sometimes four may be used.
After surgery, the aim is to get patients moving around as soon as possible, although full mobility is not always restored. While most fractures heal well, occasionally some may become unstable, requiring a further operation. It is thought that the use of washers together with cannulated screws helps to improve the outcome.