- How Does Screw Work?
- Components of A Screw
- Classification of Bone Screws
- Uses of Bone Screws
- Instruments Used for Screw Insertion
- Strength of Screw Fixation
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Bone screws have been used to fix the bony fragments since the middle of the nineteenth century. It must be noted that bone screws follow mechanical principles similar to other commonly used screws.
The screw is a cylinder with spiral threads running on its outer surface which converts torsional forces into compression.
Screws can be used in orthopedics for various purposes
- Attachment of implants to bone
- Bone to bone fixation
- Soft tissue fixation to bone
How Does Screw Work?
A screw because of its threads provides a mechanism that produces linear motion when rotated. Bone screws consist of a helix-shaped thread on a shaft. By turning one end, called the head, the screw will move through a stationary object in a hole that is either threaded to match screw thread or threads could be formed by the screw itself. As the screw passes, its thread grips the material it is being screwed into.
The main use of the screw is to fix together two objects by compressing them against each other.
Components of A Screw
The main components of a screw are the head, the shaft, and the tip.
The head has two functions:
- It provides a buttress to stop the whole screw sinking into the bone. This buttress can be made bigger by placing a washer between the head and the bone, thus spreading the load. This is used in very soft bone.
- Head bears a slot where the screwdriver fits in for rotating the screw. The screw turns and advances forwards in response to a twisting force so it is important that torque can be applied effectively.
The slot in the head of a screw may be one of several different shapes. Earlier machine screws were used [screw with a single linear slot in the head, also called Philips head].
Modern orthopedics screws use a hexagonal slot. This shape gives an effective coupling unlikely to be damaged in the screwing process.
With hexagonal slot, no axial force required to retain the driver in the head. This is a great advantage in tight corners.
The undersurface of the bone screw is rounded so that there is maximum contact between screw and bone after counter-sinking to reduce the risk of an area of excessive stress which may crack the bone.
Few definitions first
- The core diameter is the smallest diameter of the threaded section of the shaft.
- The shaft diameter which is the diameter of the shaft where there is no thread.
- The thread diameter which is the diameter of the widest part of the threaded section.
Some of the screws have a threaded diameter bigger than the shaft diameter. In others, the thread diameter is the same as the shaft diameter.
The strength of a screw is determined by the smallest diameter. Whether it is the core diameter or the shaft diameter. The greater the smallest diameter, the stronger the screw will be.
A core is the solid section from which the threads project outwards. The size of the core determines the strength of screw and its fatigue resistance. The size of drill bit used for drilling hole is equal to the core diameter.
The thread may be seen as an inclined plane which is rotated in the threaded hole in the bone so that the screw moves forward in response to being twisting.
Thread design should maximize initial contact, enhance the surface area
and increase the pullout strength.
There are two aspects of importance in the thread:
Shape of Thread
AO bone screws are flat on the upper surface in contact with the bone and rounded underneath. This enables them to provide a wide surface on the pulling side and little frictional resistance on the underside.
The depth of thread is the distance between the core and the thread edge. It is calculated as half the difference between the thread diameter and the core diameter. Cancellous bones are softer and a deeper thread is desirable to capture more material between the threads and increase the pull out resistance.
The pitch of a screw is the linear distance traveled by a screw when the screw is rotated by a full turn  turn of the screw.
Tapping is the process of cutting a thread in bone. A screw which has a cutting tip that enables it to cut its own female thread track as it is being inserted. This kind of screw is called a self-tapping screw.
All cancellous screws are self-tapping. Cancellous bone screws have a corkscrew tip to engage the screw in the bone. As there is no pretapped hole, the material of the soft bone is compressed as the screw is driven in, enhancing grip strength in otherwise quite weak bone.
The malleolar screw is self-drilling which means that it can drill a hole in the material without the need to use a separate drill bit. Its trocar shaped tip provides a suitable cutting edge to allow it to act as a drill.
Cortical bones require tapping first and then screw insertion. Direct screw insertion or self-tapping screws are generally not used as cortical bone is tough and too much torque would be required, risking jamming or breaking the screw and risk damage to the bone thread. This tapping makes the insertion of screw easier.
Tapping instruments and self-tapping screw have flutes. The cortical screw has no flutes into which bone can grow to make later screw removal difficult.
Classification of Bone Screws
Bone screws may be classified depending on
- Conventional screws
- Locking screws
- Headless screws – Herbert Screws
- Introduced by Timothy Herbert in scaphoid fractures
- The screw has a different pitch at either end.
- The pitch of the distal threads is more than that of the proximal threads resulting in a variable lead of screw at either end. Distal threads are advanced more within the distal bone segment with each turn of the screw; leading to interfragmental compression.
- Cannulated screws – These have a canal drilled through their core allowing a guide wire to be placed. Tapping and screw insertion is carried out over the guide wire.
The only disadvantage is that cannulated screws are marginally weaker than solid ones, but in standard sizes, this is not a serious problem.
- Stainless steel
- Bioabsorbable Screws
- Do not interfere with MRI.
- Do not need implant removal
- Higher risk of screw failure during insertion.
- Foreign body reaction may be seen in some.
- Polylactic acid, poly-L-lactic acid (PLLA), and polyglycolic acid screws are available
- 5 mm
- 4 mm
- 5 mm
- 5 mm
- Non-self tapping
- Self-tapping and self-drilling
Region of Application
- Smaller pitch, a greater number of threads
- Thread diameter to core diameter ratio is less
- Fully threaded
- For cortical bone
- Larger pitch
- Thread diameter to core diameter ratio is more
- For cancellous bone
- Fully or partially threaded
- Trocar tip – allows insertion without tapping and sometimes predrilling
- Used in the metaphyseal area, distal humerus, trochanteric area, & sometimes in the ankle, where the bone is rather dense
- Partially threaded
Uses of Bone Screws
Bone screws can be used as
These neutralize forces on the plate in plate fixation.
Screws are used for inter-fragmentary compression.
To reduce displaced fracture by pushing or pulling.
Hold two fragments in position without compression, for example, syndesmotic screw
Act as an anchor for wire or suture. Used in ligament reconstruction or in tension band wiring
These are used to fix locking plates, In these screws, the head of the screw is also threaded which fits into the threaded hole on the plate.
The term locking screw is also used when simple conventional screws are used to lock the two ends in interlocking nails.
These screws are similar to use in locking screws in interlock nails where medulla is wider so as to prevent undue placement of nail and to guide the nail path.
To Prevent Displacement of Fragments
The screw is commonly used alone around joints to hold cancellous bone fragments together. For example in fracture of the medial malleolus.
Instruments Used for Screw Insertion
A drill bit is a tool for cutting a hole in a material. It may be used on a hand drill or air powered drill.
Drill bits come in various sizes which correspond to the shaft or the thread diameter of screws.
This should always be used to provide protection for surrounding soft tissues and to help to direct the drill bit accurately.
Tapping instruments have a cutting end and a flute and are used to thread the bone.
The tap should have the same thread profile as the screw being used.
These tools permit accurate assessment of screw length.
There are two commonly used screwdrivers, 3.5 and 4.5, each with hexagonal heads of different dimensions.
Strength of Screw Fixation
It depends on the screw-bone interface which is influenced by the following factors.
- Density and quality of bone
- Strength of screw material
- Area of contact of threads
- Design of screw thread
- Timothy T. Roberts, MD; Christoph M. Prummer, BS; Dean N. Papaliodis, MD; Richard L. Uhl, MD; Theodore A. Wagner, MD. History of the Orthopedic Screw. Orthopaedics. January 2013 Volume 36 Number 1: 12-14. doi: 10.3928/01477447-20121217-02
- Chapman JR, Harrington RM, Lee KM, Anderson PA, Tencer AF, Dowalski D. Factors affecting the pullout strength of cancellous bone screws. J Biomech Eng. 1996; 118:391-398.
- Sehlinger TE, Selingson D. History and development of the orthopedic screw.
- Wheeler DL, McLoughlin SW. Biomechanical assessment of compression screws. Clin Orthop Relat Res 1998;237-45