Internal Fixation of Bone Fracture – Part 2

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.

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

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 is an active and dynamic body organ and can adapt to the stresses formed by the application of the screws, allowing a gradual reduction in fixation force with time. However, the fracture is usually healed before the fixation is likely to loosen. The two main kinds of screws available are cancellous and cortical bone screws, the denser bone of the cortex being fixed with cortical screws and the more honeycomb bone of the bone ends fixed with cancellous screws. The surface areas of contact between thread and bone are greater in cancellous screws, allowing cancellous screws to achieve purchase in less dense 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 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.