Fixing Fractures - Part Two
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If the fracture is only experiencing a small level of force across its site then fixation pins or wires can be sufficient or can add appropriately to the fixation already provided by a fixator or plate. Fractures of the upper arm and shoulder, wrist, fingers and hands are the most commonly fixed with this method. Tension band wiring is used for fractures of the elbow, knee-cap and ankle, with k-wiring sometimes used to add to stability. By using a percutaneous technique a pin can be inserted through the skin using x-ray guidance with an image intensifier.
Steinmann pins, often threaded and thicker than K-wires, have been routinely employed to engage and maintain skeletal traction predominantly in the bones of the leg. The pin is inserted through the bone and this is attached to a traction cord via a stirrup device, holding the bone in the correct position until enough healing callus has formed to allow removal. Modern techniques of fixing fractures sooner and more accurately have meant that the application of traction for long periods, with the risks of bed rest, can now be avoided.
Screws
The use of bone screws is a fundamental technique of the modern management of trauma and orthopaedic conditions, either individually or as an adjunct to another method. Screws can be tapped or can be self tapping. A variety of factors determine the force required to pull a screw out, most important being the bony density of the screw site. Self tappers are commonly used and the fixation is affected by degree of contact area between the threads of the screw and the bone. Clockwise insertion of a screw is either performed with or without pre-drilling, with the screw head generating force once its head hits the hard outer bone layer. Eighty percent of the stresses needed to pull out the screw are used to fix it.
The tension forces imposed by insertion of the screws are adapted to by bone which is a living and dynamic tissue, leading to a reduction in the desired fixation forces with time. The fractures typically heal however before the tension reduction becomes functionally relevant. For the harder and denser bone of the cortices, the outer parts of long bones, cortical screws are used. For the less dense bone of the bone ends cancellous screws are chosen. Cancellous screws have a greater contact surface area between the threads and the bone and are designed to make an effective level of purchase in the softer structure of cancellous 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.
A degree of compression can be produced by inserting lag screws across the line of a fracture to increase alignment and stability of a long bone fracture and to produce and maintain reduction of a fracture across a joint. To provide the greatest degree of stability requires the screw to be placed at right angles to the line of the break. It is unlikely that lag screws will give sufficient stability alone so they are often supplemented with added stability from an external fixator or a plate.
Cannulated screws are often used to fix hip fractures and can be inserted percutaneously without needing a full open operative technique, inserting the screw along the track already identified by a guide wire and performed under x-ray control for positional control. To limit the potential damage to the soft tissues and the size of the operation, cannulated screws are employed in operations with limited open surgery. Modern screws are self tapping and self drilling as they are inserted and are much more costly than normal screws which are not cannulated.
Article Source: Articlelogy.com
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