A compression plate produces a force across a fracture site to bring the fractured components closer. The effect utilizes the principle of Newton’s Third Law (action and reaction are equal and opposite). The plate is attached to a bone fragment. It is then pulled across the fracture site by device, producing tension in the plate. As a reaction to this tension, compression is produced at the fracture site across which the plate is fixed with the screws. The direction of the compression force is parallel to the plate.
Role of Compression –
The compression plate achieves any one or all the following effects:
- Compaction of the fracture to force together with the interdigitating spicules of bone and increase the stability of the construct.
- Reduction of the space between the bone fragments to decrease the gap to be bridged by the new bone.
- Protection of blood supply due to increased fracture stability.
- Friction, which at the fracture surface resists the tendency of the fragments to slide under torsion or share. This is beneficial as plates are not so effective in resisting torsion.
Further, a compression plate can also ensure superior fracture immobilization as compared to that obtained with a neutralization plate alone because it generates axial inter-fragmental compression.
Static and dynamic compression –
Compression may be static or dynamic. A plate applied under tension produces static compression at the fracture site; this compression force prevails whether the limb is at rest or is functioning. Dynamic compression is a phenomenon by which a plate transfers or modifies functional physiological forces to compression forces at the fracture site. Compression plate on the lateral side of the shaft of the femur exerts static compression, both when the limb is at rest or is functioning. When functional activity begins, the physiological forces, which are normally destabilizing for a fracture, are converted to a stabilizing and active force by the same plate, which now acts as a tension band.
Dynamic compression is thus exerted at the fracture site. When the physiological activity will cease, this dynamic compression force will be absent, but the static compression force will continue to act.
Methods of achieving compression –
Compression may be produced by one of three techniques:
- Self-compression plate. This is a device that converts the torque (turning force) applied to the screw head to a longitudinal force which compresses the fractured bone ends. The screws and plates are designed to facilitate this conversion. As the screw advances in a self-compression plate, it slides down on an inclined plane that is part of the plate screw hole The effect is to create a tension force in the plate and compression force across the fracture fragments. One or both ends of a screw hole may be sloped, thus making it possible for compression to be produced in either direction.
- Tensioning device. A special tensioning device can be attached between the bone plate and the adjacent bone cortex. A bolt is then tightened to pull the plate across the fracture site. This produces tension in the plate and large compressive forces across the fracture. The attachment of the device to the bone necessitates a larger surgical exposure. In certain situations, for example in a smaller bone, a Verbrugge forceps may be used as a tensioning device. One jaw of the forceps is fixed in the terminal hole of the plate and the other jaw abuts against the specially inserted screw. Closing the jaws produces tension in the plate and compression across the fracture site.
- Eccentric screw placement, Eccentric placement of a screw in a plate hole creates considerable shear stress in the screw. The same force is transmitted to the plate and can occasionally be used to produce interfragmentary compression. To achieve this, a screw is eccentrically placed in the hole of a plate. The technique of eccentric screw placement, however, is mechanically inefficient. The screw head is at risk and may break. Eccentric screw placement can be used to simulate a self-compression plate, but the technique has definite limitations and should not be used as a planned procedure.