Metaphyseal fractures occur close to joints. A large fragment may include a significant portion of the joint surface. If this is loaded, there will be a tendency for the fragment to displace.
To counter this shearing effect, the orientation of a lag screw may be adjusted so that it is not quite perpendicular to the fracture plane but lies more perpendicular to the long axis of the bone. If the screw is far from perpendicular, as it is tightened, there will be a shearing force, which risks displacing the fracture. The compressive effect occurs between the head of the screw in segment A and the threads at the far end of the screw in segment B as the screw is tightened.
To achieve this the screw must be able to move in the near hole without the thread obtaining purchase. The screw with the appropriate unthreaded shaft length should be selected. The cortex tends to be very thin near the ends of the bone, particularly in osteoporosis. As the screw is tightened, there is a risk that the head will sink. This can be prevented by using a washer. Cancellous screws have a greater difference between the thread diameter and the core diameter than cortical screws.
Drill the pilot hole perpendicular to the fracture plane, with a diameter corresponding to the core diameter of the screw. In cancellous bone, it is often not necessary to drill a pilot hole once the cortex is penetrated.
As there is minimal cortical bone in the metaphyseal region, and the threads of the screw are holding in the cancellous bone, there is no need for the screw to penetrate the far side of the bone.
In addition, there are often important soft tissue structures adjacent to the bone in the metaphyseal region, which may be damaged or irritated by a protruding screw.
Moreover, a very tight fit of screw thread to the bone is ensured because the bone screw cuts its own precise thread. Cortical and cancellous screws are available as fully as well as partially threaded screws.
A cortical bone screw is often fully threaded. In plate fixation, a bone screw must purchase firmly on both the cortices. The purchase on the near cortex contributes 80 percent of the grip and the distal cortex contributes around 20 percent. A fully threaded cortical screw may function as lag screw only once the near cortex is over-drilled. A fully threaded cortical screw can be either self-tapping or non-self-tapping. A partially threaded cortical screw is known as a shaft screw.
The shaft diameter resembles the outer diameter of the thread. A locking cancellous screw can be fully threaded or partially threaded. A fully threaded cancellous screw is utilized as a placement screw to fix a bone plate in metaphyseal as well as epiphyseal regions. A partially threaded cancellous screw is utilized as a lag screw. Siora Surgicals Pvt. We have a comprehensive portfolio of trauma products numbering more than two thousand.
Cancellous Screws A cancellous bone screw has larger threads and a higher pitch in comparison to the cortical screw. Cortical Screw The cortical bone screw is a machine-type screw. A cortical bone screw can be either self-tapping or non-self-tapping. Fully and Partially Threaded Screws Cortical and cancellous screws are available as fully as well as partially threaded screws.
Pullout strength and maximum insertion torque were both found to be greater for cancellous screws than cortical screws in all synthetic densities tested. The magnitude of difference in pullout strength between cortical and cancellous screws increased with decreasing synthetic bone density.
Screw displacement prior to failure and total energy absorbed during pullout strength testing were also significantly greater for cancellous screws in osteoporotic models.
0コメント