only will hold fragments apart when used as a pin but has a tendency to push them apart as it crosses the fracture site (the threads in the near fragment screw it away from the far fragment during the moment or two before the threads catch in the distal fragment). On the other hand, lag screws won’t distract and can make excellent “pins” for fixation of cancellous fragments or as pins in the tension band wiring of the patella fracture above.
Screws used to distract fragments:
Occasionally, you want to hold two fragments apart, as in the distal humerus where the condyles can be split apart with so much comminution between them that when a compression screw is tightened it will extrude the comminuted central fragments and result in an overcompressed malreduction. There, the use of fully threaded screws can be quite helpful (known as a “positioning screw.” In the situation shown it would be followed by plates up the medial and lateral columns to hold the condyles to the shaft.
Plates: Plates are best at resisting tension (applying compression) also. Most plates, especially the semitubular and 1/3 tubular ones, are too thin to have much strength in bending. Some, such as the hip screw plates and distal femoral condylar plates have been thickened up enough to make them fairly strong in bending. Standard plates (often known as “compression plates”) are best placed on the “tension side” of a bone with a simple fracture. For instance with a simple femur fracture the bone tends to go into varus under weight bearing loads. This applies compression to the medial cortex and, if the plate is placed on the lateral surface, loads the plate mostly in tension which it resists well.
Buttress plates can be used as shown to resist shear in metaphyseal fractures as in these shear fractures of the distal radial metaphysis and of the volar lip. These plates are loaded in bending but the loads are not large and these cancellous bone fractures heal quickly before the plate fails.