Orthopedic screws
Orthopedic screws
Orthopedic screws have been used to fix broken bone fragments since the mid-19th century. It should be noted that orthopedic screws follow the same mechanical principles as other common screws. Due to its threads, the screw provides a mechanism that produces linear movement when rotated. Orthopedic screws consist of a spiral groove on the rod. By rotating one end, called the screw head, the screw moves through a fixed object in a hole, which can either be grooved to mate with the screw threads, or the grooves can be made by the screw itself. As the screw goes through, its threads hold the material that is screwed into it tight. The main use of screws is to fasten two objects by pressing them against each other.
The material of orthopedic screws is medical grade titanium or 316 stainless steel, which has been adapted to the human body.
Orthopedic screws have several advantages, including:
• Properties of integration with bone
• Absence of body reaction to foreign body
• Higher mechanical strength to fix fractures
• The screws are made in a way that increases the accumulation of mesenchymal stem cells from the bone marrow to increase cartilage repair.
Anatomy of orthopedic screws
The main components of the screw include head, body and tip.
Head
Screw heads have two functions:
• Provides a barrier to stop the screw sinking into the bone. This buffer can be enlarged by placing a washer between the screw head and the bone to spread it out. This is done in very soft bone.
• The screw head has a slot in which the screwdriver turns the screw. The screw rotates and moves forward in response to the torsional force, so it is important to apply the torque effectively.
The groove in the screw head may have different shapes. Previously, machine screws (screws with a linear groove in the head) were used.
Modern orthopedic screws use a hexagonal groove. This shape creates an effective connection that makes it unlikely to be damaged in the twisting process.
Despite the hexagonal groove, no axial force is needed to keep the actuator in the page head.
body
The body is the solid part from which the threads are pushed out. The size of the body determines the strength of the screw and its fatigue resistance. The size of the drill used to make the hole is equal to the diameter of the screw body.
reserved
The thread may be seen as an inclined plate that rotates inside a slotted hole in the bone to move the screw forward in response to the screw.
Thread design should maximize initial contact, increase surface area, and increase resistance to pull-out.
There are two important aspects to threads:
1) Orthopedic screws are smooth on the upper surface in contact with the bone and rounded below. This results in a wide surface area on the drag side and low frictional resistance on the underside.
2) The depth of the thread is the distance between the body and the edge of the thread. This value is obtained as half the distance between the diameter of the thread and the diameter of the page body. Spongy bones are softer and a deeper thread is desirable to get more material between the threads and increase the resistance against pulling out.
Thread pitch
The screw pitch is the linear distance traveled by the screw when the screw is turned completely (360 degrees).
Top
Preparation means the process of creating grooves in the bone. A screw with a cutting tip creates the desired grooves for mating with the threads by entering the bone. This type of screw is called automatic.
Types of orthopedic screws
1) Cortical screws:
The cortical bone needs to be prepared first and then insert the page. Direct screw insertion or self-tapping screws are not usually used because the cortical bone is hard and requires a lot of torque, and there is a risk of screw jamming or breakage and damage to the bone groove. This preparation makes screw insertion easier.
Preparation tools and self-tapping screws have grooves. A cortical screw has no grooves for bone to grow into, making it difficult to remove the screw in the future.
Cortical orthopedic surgical screws, which are designed with thin threads throughout their entire length, hold and fasten the damaged bones, which are the densest material in the human skeleton. Cortical screws hold fractured areas in compact bone and less porous cortical in long bones such as the femur and humerus.
2) Cancellous screws:
All sponge screws are automatic. Orthopedic cancellous screws have a corkscrew-shaped tip that engages the screw in the bone. Since there is no pre-drilled hole, the soft bone material is compressed as the screw is inserted, increasing the resistance to jamming in very weak bone. The holding strength of orthopedic plates is measured based on the ratio of metal to bone contact. Unlike the cortical type, cancellous bone contains porous soft material, which requires a special design of cancellous screw with larger threads. These longer, coarser threads in cancellous screws have the same amount of metal-to-bone contact as thinner cortical threads in denser cortical bone, and therefore provide the same holding strength.
These screws are produced in the form of full thread and half thread or so-called partial brittle screws, which in partial brittle screws, the length of the screw is 16 or 32 mm from the tip of the threaded screw.
The difference between cortical and cancellous screws
Ves: In cortical screws, the number of threads is large and the distance between two threads is small, the blind is bigger than the cancellous screw. The tip of the screw is designed in the form of a self-tab and a self-drill, which is special for cortical bones.
But in cancellous screws, the number of threads is small and the distance between two threads is large, the blind is smaller, which is special for bones with spongy tissue.
3) Locking screws:
Locking screws are not used alone and must be used with a locking plate. For better fixation on the plate, the flower part of the screw is designed as a thread, which is locked on the locking plate.
4) Cannulated screw:
A hollow or cannulated surgical screw, preferred for its precise placement feature, works with a guide wire that the surgeon inserts into the affected bone through a pre-drilled hole. Having fixed the desired location of the wire, the surgeon places the screw inside the bone along the axis of the wire. After the screw is in place, the surgeon pulls out the wire.
5) Herbert Screw:
A hollow Herbert screw, used when the use of a standard surgical page invades adjacent tissue, has two threaded ends that point in the same direction. The Herbert screw thread design stabilizes broken bones attached to ligaments such as the wrist, foot, and jaw. Threads located near the bone move faster than the other end of the page, causing the two bones to come closer together for repair.
6) Interference Screw:
The interference screw used in knee reconstruction surgery holds the end bone repair pieces attached to the anterior cruciate ligament and taken from a cadaver. The end of the bone is inserted into a hole made in the femur and lower leg, allowing the replacement of the anterior cruciate ligament. This screw, which is placed along the two implanted bone units, keeps the graft site immobile.
7) Ecotrack screw:
Using the Acutrak screw helps to repair a navicular or wrist fracture. Since there is little surrounding bone tissue in wrist fractures and the use of surgical screws is usually possible, fully grooved headless screws allow for implantation below the surface of the bone. This screw improves retention for fractures or bone removal through osteotomy by allowing the screw to be placed anywhere in the surgical field.
8) Malleolar screw:
The malleolar screw is self-drilling, meaning it can create a hole without the need for a separate drill. Its tip is designed as a trocar (nail-like) to easily enter the bone and fix the fracture.
A special type of partial cancellous screw is used for the malleoli (ankle) of the foot.
Today, this screw is less used
