The bone screw is a cylinder with spiral threads running on its outer surface. It changes the torsional forces into compression. The main functional objective in the screw design is to dissipate and distribute the mechanical load.
Screws can be used for attachment of orthopedic implants to bone, bone to bone fixation or soft tissue fixation and anchorage. In conventional plates, they act by increasing the friction between the bone and the orthopedic plate. There are various types of screws. Some of the screws will be described in this article.
Certain specialty bone screw is usually seen, provided by the bone screw supplier. The Herbert screw is intended for use of small articular bones such as the carpals. It is threaded and cannulated at both ends. These threads run in the same direction, but the proximal part has a wider pitch to its thread. Therefore, when the proximal threads engage in the bone, they tend to move through the bone faster than the threads at the distal end, causing the two ends of the bone to compress together. This screw is used where standard screw would impinge on adjacent tissues, such as in the treatment of osteoarticular or scaphoid fractures.
Orthopaedists also use Acutrak screw for the treatment of most scaphoid fractures. This screw, similar to the Cannulated Herbert Screw. It is also headless, which allows it to be implanted below the bone surface. It uses the same concept of variable thread pitch like the Herbert screw, but unlike the Herbert screw, is fully threaded. This feature may improve internal holding power, as well as osteotomy or fracture site to lie anywhere along the screw length.
Another specialty screw sometimes seen is the interference screw. This screw is occasionally used in the repair of the anterior cruciate ligament (ACL). In this type of repair, the orthopedic surgeon employs a cadaveric allograft ligament which has a block of bone still attached at both ends. A tunnel is drilled through the proximal tibia and the distal femur, and these bony blocks are placed within the tunnels. The interference screws are placed alongside the blocks of bone so that they tightly wedge them into the side of the tunnel and prevent them from moving. These screws can also be obtained from the orthopedic implant manufacturers in India.
One final word on orthopedic screws: In order to use them, you have to make a screw hole in the hardware that uses them or in the bone. This is of note because holes of screw weaken whatever material they may pass through. There are biomechanical estimates that one screw hole passing through both cortices of a femoral shaft will weaken that femur by 90% to some kinds of stress! There are many ramifications to this for the radiologist and orthopedist. First, since these holes weaken bones and orthopedic hardware, we should look closely at these areas on the films since the metal and bones will tend to fracture at these sites. Second, the orthopedic hardware is usually removed as soon as possible so that these holes can fill in with the new bone formation and bring the strength of bone back up to normal.