Fixation of Fractures " Part Two
If used for permanent fixation pins and wires are usually chosen if very little load is going through the fracture site or they are adding to the stability of a plate or an external fixator. Typical uses for wires or pins are to fix finger fractures, hand fractures, shoulder fractures and wrists. K-wires are often used to assist with the fixation in fractures of the patella, elbow and ankle. A device known as an image intensifier is often used to insert the device under x-ray guidance, allowing insertion of the pin or wire through the skin without operation.
Steinmann pins, being larger and which can be threaded are mostly used for applying skeletal traction to a fracture of the long bones. They are passed through a bone and a stirrup device attached to a weight provides the traction to keep the bone in alignment until a sufficient amount of healing occurs. This technique has largely been superseded by the use of more advanced techniques of internal fixation which means that long term traction, with its many negative side effects due to the patient being kept in bed for weeks or months, is rarely required.
Using bone screws is a basic technique of modern orthopaedic and trauma management, used either on their own or as part of another implant technique. Screws can be self tapping or need tapping beforehand. The force needed to pull a screw out of the bone is affected by various factors and the main determining factor is the density of the bone into which it is inserted. The total area of contact between the bone and the threads is also important and self tapping screws are typically used. Screws are inserted clockwise either straight in or along a path already drilled and once the screw head hits the cortical bone it generates tension with screws typically inserted at a force equivalent to 80 percent of the force which would strip them.
Bone adapts to the tension and the force applied by the screws decreases gradually with time, however the time for this to occur to loosen the compression is typically longer than the time needed for fracture healing. Cortical and cancellous screws are the two types available, with cortical screws used for the denser bone of the long bones and cancellous screws for the less dense bone of bone ends. Cortical screws have less and cancellous screws more surface area for purchase on the bone. Cancellous screws are used in the softer bone and cortical screws in harder bone.
Cancellous bone does not usually need tapping or pre-drilling, as it is less dense, more porous and can easily be screwed into. It may be advantageous to directly screw into this type of bone as this may make the bone more compressed over the insertion track and allow the screw to hold more strongly. An implant mechanism such as a plate can be held in place by positional screws and compress the metal plate against the bone. A pilot hole is typically drilled to start with and then the hole tapped with a screw thread unless self tapping screws are to be used.
Cannulated screws are another type of fixation, inserted over a guide wire which has already been inserted under x-ray control, allowing the initial wire fixation to be precisely completed by the final fixation. They can be used in a percutaneous way, without open operation, such as with hip fracture pinning. Cannulated screws can also be used in operations with limited open technique to minimise the size of the operation and the consequent soft tissue damage. Modern designs both drill and tap themselves on insertion and these hollow design screws are much more expensive than non-cannulated versions.