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The use of plates and screws to fix fractures requires an understanding of the basic principles rather than a patterned approach. Although plates and screws are simple instruments, good technique is still required to use them correctly.
Screws
Screw cap
Screw shank: Consists of a threaded part; some threaded screws also have a non-threaded part. The diameter of the rod is used to describe the diameter of the non-threaded part. The threaded portion includes a core diameter and a thread diameter.
Pitch: Used to describe the relationship between screw rotation and axial movement (below). The pitch is related to the inclination of the thread, which determines how far the screw moves axially when the screw is rotated.
Screw point
Pitch h: the screw rotates 360°, the distance the screw moves axially; α: the inclination, the spherical part under the screw head increases the contact area with the plate or bone
When an ordinary screw is tightened, a load is created between the bottom of the screw cap and the upper surface of the thread. As the axial load increases, the holding force between the screw and the bone increases gradually. The holding force prevents the screw from pulling out, and the holding force is determined by the thread pitch. The larger the pitch, the smaller the holding force, and vice versa. The load generated by further tightening may exceed the strength of the bone, and if the strength of the bone is exceeded, the contact surface of the screw and bone will be damaged. Using a screw or washer with a larger thread reduces the strength of the screw-bone interface, reducing stress on the bone.
The drill used for pre-drilling matches the core diameter of the screw. Tapping is recommended in the cortical bone to create channels for screw threads. In addition, the taps are fluted for clear cutting of bone. Conventional screw threads do not “cut” the channels, so there is a possibility of screw-in difficulty, breakage, or slippage.
Bottom surface of the screw head
The screw head needs to be in contact with the bone and plate. Ideally, the contact area is large enough to allow the screw to be driven in at an angle. To meet these requirements, the lower surface of the screw is designed to be hemispherical (above). In cortical bone, countersinking is required for angulated screw insertion, which increases the contact area of the cortex, reduces stress, and reduces subsequent failure of the subcortical bone.
Screw groove
The screw grooves match the screwdrivers. Different designs exist clinically with the aim of providing an effective bond that is easy to use and less prone to damage when screwing in or out. In fracture fixation, hexagonal and star-shaped grooves are the most commonly used.
Threaded screw head
Currently, many manufacturers of trauma implants use threaded head screws, which allow the locking the screw head to the plate (figure). Steel plate screw construction for angular stability.
Symmetrical thread
Symmetrical thread means that the edges of the thread are symmetrical. The top and bottom edges of a symmetrical thread are the same. Screws of this design are easy to produce and are primarily used in cancellous bone (Left below).
Symmetrical Thread / Asymmetric Thread
Asymmetric thread
Asymmetric threads are mainly used for cortical bone screws (Right above). The upper surface of the thread is flatter than the lower surface of the thread. The angle between the upper surface of the thread and the axis of the screw is larger or smaller, which increases the frictional resistance. The rounded ground of the thread can reduce the resistance when screwing in the screw.
Screw point
Traditional cortical screws have blunt points and require pre-drilling and tapping. Self-tapping screws have a fluted tip that cuts the thread itself into a pre-drilled hole.
Self-drilling/self-tapping screws have a drill-like screw point that cuts holes and threads by itself.
Self-tapping screws
Self-drilling screws should only be used for monocortical fixation. If used for bicortical fixation, thread stripping may occur in the proximal cortex. In hard cortical bone, the self-drilling screw head may not advance after the thread enters the cortical bone. If bicortical fixation is required, predrilling is recommended over self-drilling screws.
Plate
Plates are designed from steel strips with circular screw holes in the middle but can be used as temporary internal fixation supports. The mechanical properties of the plate will affect the mechanical properties of the fracture site, which is determined by the length and shape of the plate. The vast majority of orthopedic implants are made of stainless steel or titanium alloys. The hardness of titanium alloy is about 50% of that of stainless steel; however, the dimension has the most influence on the mechanical properties of the steel plate, rather than the modulus of elasticity. Doubling the thickness of the steel plate can increase the hardness of the steel plate by 16%.
The strength of a fracture fixation structure depends on the mechanical connections between its constituent parts. The strength of plate-screw devices is determined by the bone-screw interface. The development of locking plates has improved the biomechanical and mechanical properties of the plate-screw, bone-plate interface.
Non-locking plate
The fixation strength of the traditional non-locking plate is determined by the friction force of the plate screw and the plate-bone interface. When the screw is tightened, the plate moves closer to the bone, which increases the friction between the plate screw and the plate-bone interface. Reduced friction at the plate-bone interface can lead to implant loosening. It can be speculated that the gradual increase in loosening will lead to the failure of internal fixation. If the technique of screwing in is correct, it can take months for the screw to loosen, allowing enough time for the fracture to heal. Over tightening of the screw will result in high pressure between the underside of the screw head and the hole in the plate, causing a mismatch in the plate screw as the plate deforms elastically. Micro motion between the plate and the articular surface will cause wear and reduce the friction between them; any loosening will lead to micro motion, bone resorption and further loosening.
Another limitation of using high tensile forces and high friction is that it can adversely affect the blood supply to the bone beneath the plate. This can lead to osteonecrosis and slowed bone healing, fracture with plate migration and infection. Low-contact plates have been used to reduce the surface area of osteonecrosis without reducing friction. These plates allow soft tissue ingrowth while avoiding large dead spaces.
Locking Plates
The screw head and plate locking screw form a stable structure with good axial and angular stability. There is no need for a tight fit between the plate and the bone surface because the maintenance of stability depends on the friction force of the plate-screw interface rather than the friction force of the plate-bone surface. Therefore, locking plate-screw functions similarly to internal fixation stents. This helps blood circulation to the plate-bone surface. The plate-locking screw interface is less likely to loosen because the locking screws are subjected to bending forces rather than tension to non-locking screws. Locking plate-screws produce stronger fracture fixation when bone defects are present. Failure at the plate-bone surface occurs only when all screws on one side of the plate fail.
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