Bone screws are a basic internal fixation and might be used several or together with explicit styles of trauma implants procured from orthopedic equipment manufacturers. The common style consists of a tip, shaft, thread, and head.
A spherical screw tip needs pretapping, whereas a fluted tip is self-tapping. The screw shaft is found between the top and on the threaded portion of the screw. The thread is outlined by the subsequent variables
- Variety of threads
The root diameter decides the screw’s resistance to breakage (tensile strength). Screws are mentioned by their outer thread diameters, bone kind for supposed use (cortical or cancellous, determined by the pitch and major/minor diameters), and proportion of thread (partially or absolutely threaded).
Pitch, the space between adjacent threads, affects purchase strength in the bone. Increasing the pitch will increase bone material between the threads however decreases the number of threads per unit of distance. The lead is that the distance a screw advances with an entire flip. Lead is that the same as pitch if the screw is single-threaded, and the lead is double the pitch if the screw is double-threaded (faster screw insertion).
Screw fallback strength may be suffering from many factors. Bone composition (density) is the primary determinant of screw fixation. The entire area of thread contact to bone (root area) is another thing consider for fallback resistance.
Pretapping the screw hole, in theory, reduces microfracture at the thread-bone interface however needs an additional step for insertion. Self-tapping screws appear to possess no clinical distinction from pretapped screws for fracture or plate fixation, eliminate the sound step, and are currently the industry commonplace. As a result of the fluted portion of the screw, the tip has less thread contact with the bone, slight protrusion at the other cortex is suggested.
The business commonplace for the screw head may be a hexangular recess, that provides an oversized contact surface between the screw head and screwdriver and permits for the best transmission of force. A cross-type screw head is employed on some screws within the 2.0 and smaller (mini fragment) sets. The star style (or Torx) found in trade has been tailored to the screw heads for the Association for the Study of Internal Fixation (AO/ASIF) protection plates and has been shown to be superior for force and resistance to denudation.
Several forces are attached to screw insertion and modification. Force is applied through the screwdriver to the screw head in a very dextrorotation to advance the screw within the predrilled path or, within the case of a cannulated screw, over a guidewire; this advancement produces a circumferential force on the thread. For animal tissue screws, the drill diameter is slightly larger than the foundation (shaft) diameter of the screw.
Axial tension is created with impingement of the screw head on the cortex or plate, generating tension with the screw. To optimize these forces, screws ought to ideally be inserted at 80% of the force required to cause them to strip. Calculable 2500-3000 newtons of axial compression force may be applied to the common screw.
Over time, the quantity of compressive force decreases slowly because the living bone remodels to the stress; but, the fracture healing time is typically shorter than the time it takes for substantial loss of compression and fixation.
Two basic styles of screws are out there to be used in the bone of differing density:
- Cortical screws, designed for compact diaphyseal bone
- Cancellous screws, designed for the additional trabeculate metaphyseal bone, that is softer
Cortical screws have a smaller major (thread) diameter, a smaller pitch, and a shallower thread than cancellous screws do. Commonplace nonlocking cortical screw diameter selections embrace 1.5, 2.0, 2.7, 3.5, and 4.5 mm. Cancellous screws usually have a bigger major (thread) diameter and pitch and a bigger distinction between major and minor (shaft) diameters as compared to cortical screws, providing an additional area for bone purchase. They’re options in sizes 4.0 and 6.5 mm, and cannulated sizes vary from 4.0-7.5 mm.
Tapping isn’t sometimes necessary in metaphyseal bone, as a result of the cancellous bone is porous relative to the compact diaphysial bone and frequently needs solely the initial pilot hole or cannulated screw guidewire. With ulterior screw insertion, there’s compression of the bone on the trail of the threads, which will increase the native bone density connected with the thread, thereby probably increasing screw purchase. The tapping is also thought of in robust metaphyseal bone to avoid removal if the advancement of the screw is troublesome.
Positional or neutralization screws are to connect implant, like a plate, to the bone by compression between the plate and bone. This operating is changed once the screw is employed to lag across a fracture through the plate or when used for fracture compression, like a dynamic compression screw.