Fracture Fixation ” Part Two

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.

Larger than wires and able to be threaded, Steinmann pins are typically employed to apply traction skeletally for one of the long bones, mostly in the leg. They are driven through the bone and attached to a weight via a stirrup-like device which applies the traction to maintain bony alignment until sufficient callus has formed for the traction to be removed. Traction is used much less often now as this technique has been overtaken by more advanced methods of internal fixation which allows us to avoid the negative consequences of long term bed rest needed for traction.

Bone Screws

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.

Tapping the hole or drilling it before insertion may not be necessary in cancellous bone as it can usually be easily screwed being relatively porous and low in density. Screwing a screw directly into the bone may be good as it may compress the local bone track and increase the density of the interface between the screw threads and the bone, allowing an increased hold for the screw. Plates can be held in position by positional screws which compress it against the bone, pilot holes being drilled beforehand and tapped or self tapping screws 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 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.