Fixing an Internal Bone Fracture-Part 2

                                                                                         

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.

Screws

A basic tool in the armoury of managing orthopaedic and trauma injuries and conditions is the use of bone screws to effect fixation or to aid other techniques of fixation. Pre-drilling can be performed before insertion or a self tapping implant used. The amount of physical stress which can pull a screw out of the bone is affected by a series of matters of which the most influential is the density of the bone into which it is implanted. The surface area of contact between the bone and the screw threads determines a degree of the fixation achieved. Screw insertion is performed in a clockwise direction either along a drilled path or self tapped and produces force once the hard bone cortex is contacted by the head of the screw.

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.

In a percutaneous technique often used for hip fracture fixation, cannulated screws can be inserted along the previously inserted guide wire which has been located under the control of x-ray guidance, completing the fixation started by wiring. As surgeons always try to minimise the size of operations and the resulting damage to soft tissues and bone membranes, cannulated screws are typically used in limited open surgery. Screws are now typically self drilling and self tapping although they are much more expensive than normal screws.

About the author:
Jonathan Blood Smyth is the Superintendent of Physiotherapy at an NHS hospital in the South-West of the UK. He writes articles about back pain, neck pain, and injury management. If you are looking for Physiotherapists in Bournemouthvisit his website.

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