The article will describe the screw as a mechanical device. We will also discuss about its physical attributes.
What is a screw?
A screw is a device which converts the rotational forces into linear motion.
A screw has a core about which is wrapped a surface that is spiral. In the surgical framework, most screws have a screw-head. Most surgical screws are made of either titanium or stainless steel.
Thread design may vary according to the physical characteristics of the bone in which the bone screw is intended to gain purchase.
The two main thread types of orthopedic screws are for cancellous bone and cortical bone. Each screw type is available in partially threaded and fully threaded format.
Siora Surgicals is well known orthopedic implants manufacturer in India. It produce various type of trauma implant like bone plate, bone Screws, screws can also be cannulated. Such a screw can be introduced over a guide wire with threaded tip. The guide wire is inserted preliminarily, partly so that the final orientation can be checked radiologically, before screw insertion. These screws are self-tapping.
This is an example cortex screw’s thread.
A surgical screw is a device manufactured to high specifications and is to be used with precision and great care. To select the correct instruments and technique for insertion of any bone screw, the surgeon needs to be familiar with its dimensions. The core’s diameter determines the minimal holes size for the screw to be accommodated in the bone and determines the drill used to create the pilot hole for the screw. in other words, the drill to be used will be the same (Approx. the same) diameter as the screw’s core.
The screw’s nominal length is from the top of the head to the screw tip. An appropriate screw’s length needs to be chosen.
Too short- It will not gain full purchase in the bone.
Too long- It may cause some problems by irritating the soft tissues or protruding subcutaneously.
The screw’s pitch is the length travelled by the screw with each 360⁰turn of the spiral. The shorter the distance, the “finer” will be the pitch; the longer the distance, the “coarser will be the pitch. Cancellous bone screws have a coarse pitch; cortical bone screws have a fine pitch. The finer the pitch, the more turns the surgeon will have to make to insert the screw and more turns of the spiral thread engage in a given depth of cortex. The more threads engaged in the cortex, the greater will be the pull-out resistance.
There are two basic functions of screw head.
- One is to allow the attachment of a screwdriver to produce the rotation necessary to insert the screw. This is achieved by producing a shaped recess; this is generally star-drive, or hexagonal: some very small screws, used in phalangeal and metacarpal surgery, have cross-shaped recesses.
- The second function is to arrest forward motion.
Some screw heads have a third function. The development of the locking or threaded, head has allowed screws to lock into threaded plate holes and by creating an angularly stable construct.
Self-tapping, standard and self-drilling screws
Screw tips are of three major types:
- The round-tipped, standard screw needs predrilling of a pilot hole and then tapping (the creation of a thread in the bone using separate thread-cutting tool- a tap).
- The self-tapping screw needs pre-drilling of a pilot hole but creates its own thread in the cortical bone by cutting flutes at the tip.
- The self-drilling screw needs no preparation of the bone before insertion. It is also self-tapping.