Bone Screws in Orthopaedic Surgery

Bone Screws

Rajesh Purushothaman, Associate Professor, Medical College, Kozhikode, India

• Screw is a cylinder with spiral threads running on its outer surface. It converts torsional forces into compression. The primary functional objective in the design of a screw is to dissipate and distribute the mechanical load.
• Thread design should maximise initial contact, enhance surface area, dissipate and distribute stresses at the screw-bone interface and increase the pullout strength.
• Screws can be used for attachment of implants to bone, bone to bone fixation or for soft tissue fixation or anchorage. In conventional plates they act by increasing the friction between the plate and the bone.
• Newer locking plates do not depend on the friction between the plate-bone interface; it acts as an internal fixator by using locking of screw head into the reciprocal threads of the plate to form a fixed angle construct.

History

• Archimedes is thought to have invented the screw in the third century BC. Screws were first used for irrigation.
• Use of screws for fixation was started almost a thousand years later due to technical difficulties in their manufacture.
•  William O. Sherman in 1912 published his recommendations on  the essential properties of bone screws. He designed the Sherman plate with round holes, which was the gold standard until the AO design was introduced.
• Robert Danis designed the compression plate. He advocated 3 key modifications in the machine screws to  make them suitable for bone fixation.

1. Alteration of thread diameter to  core diameter from 4:3 in machine screws to 3:2.
2. Reduction of thread surface area to one-sixth that of machine screws.
3. Buttress thread design instead of  V-shaped thread to increase pull out strength.

• In 1958, Maurice Müller and other Swiss surgeons formed the AO/ASIF (Arbeitsgemeinschaft für Osteosynthesefragen/Association for the Study of Internal Fixation) to study the bone healing and the effect of internal fixation.
• In the same year, Bagby and Janes,  designed “impacting” bone plate with low profile and oval holes that permitted eccentric screw placement.

•   Stainless steel introduced in 1926.
•   Titanium alloys introduced in the 1970s.
•   Phillips and Woodrugg screw heads made slippage at the driver–head interface a rare event.

Classification

Orthopaedics screws may be classified depending on the following characteristics.

Design

Conventional screws

Locking screws

Headless screws

Cannulated etc.

Single- vs double-lead threads.

Material

Titanium

Stainless steel

Bioabsorbable

Size

3.5 mm

4 mm

4.5 mm

6.5 mm etc.

Characteristics

Non-self tapping

Self-tapping

Self-drilling

Self tapping and self drilling

Area of application

Cortex

• Smaller pitch
• More number of threads
• Thread diameter to core diameter ratio is less
• Designed to have better purchase  in the cortical bone
• Fully threaded

Cancellous

• Greater thread depth
• Larger pitch
• Thread diameter to core diameter ratio is more
• Designed to have better purchase  in the cancellous bone
• Fully or partially threaded

Malleolar.

Function or mechanism.

•   Neutralisation screws – neutralises forces on the plate in plate fixation.
•   Lag screws – For inter-fragmentary compression.
•   Reduction screw – To reduce displaced fracture by pushing or pulling.
•   Position screw – Holds two fragments in position without compression. Eg. Syndemotic screw
•   Anchor screw – Acts as an anchor for wire or suture. In tension band wiring
•   Locking head screw – In locking plates
•   Locking screw – In interlocking nails
•   Poller screw – To guide the nail path in interlocking nailing of fractures close to the bone ends.

Structure of Screw

Screw has the following parts

• Core – Solid section from which the threads project out wards. The size of core determines the strength of screw and its fatigue resistance. The size of drill bit used is equal to the core diameter.

• Threads- Thread diameter is the maximum diameter of threads. Thread depth is half of the difference between thread diameter and core diameter. The thread depth determines the amount of contact with the bones which in turn determines the resistance to pull out. The size of tap is equal to the thread diameter.

•  Head – Prevents sinking of screw into the bone. Hemispherical in shape to increase the surface area for load transfer and to allow angulated insertion. Can be improved by use of washer. Allows attachment of screw driver. The slot for screw driver may be star, hexagonal or Philips.

•   Tip – May be blunt, corkscrew, self tapping or trocar tip.

Screw threads

• Thread geometry includes thread pitch, depth, and shape. Pitch is the  linear distance travelled by a screw for one full turn of the screw. With each full turn the screw advances by a distance equal to the distance between the threads. Cortical screws have a lower pitch and hence more number of threads. The depth of screw is more for cancellous screws to increase the surface area to improve the purchase as the bone is weaker.

•    The shape of thread may be V- thread, buttress thread, reverse buttress or square thread.

•    Buttress thread is used to increase pull out strength.

Screw tip

Three different designs are available.

Non-tapping screw – Smooth, conical tips. Needs use of tap to create a channel for insertion.

Self-tapping – Have cutting flutes for creating a channel.

Self-tapping and self-drilling- Tip will make a drill hole and will cut the channel for the thread.

Common Bone Screws

Three types of bone screws are used.

• Cortex screw thread is designed for  diaphyseal bone
• Cancellous bone screws designed for cancellous bone. The threads have larger pitch, larger outer diameter and are deeper.
• Locking head screws have larger core diameter and shallow thread with blunt edges.

 

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 with the bone depends on the thread design and number of threads

Design of screw thread

Technique of screw insertion

• Tapping improves hold
• Prevention of thermal necrosis improves fixation
• Cement augmentation
• Screw coating
• Pilot hole and tapping
• Insertional torque

Headless Screws

• Introduced by Timothy Herbert in scaphoid fractures
• Screw has different pitch at either ends.
• The pitch of the distal threads is more than that of the proximal threads resulting in 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.

•   Acutrak screw is fully threaded with a greater pitch at the tip and a smaller pitch at the head end.
• AO headless screw appears similar to a Herbert screw with a smooth shaft at the centre, but it has the same pitch at either ends. It provide interfragmentary compression by using 2 screw drivers. The first screw driver covers the proximal threads. As the screw driver is turned, the distal threads advance producing interfragmentary compression. Once adequate interfragmentary compression is achieved, the first screw driver is removed exposing the proximal threads. The second screw driver allows advancement of proximal threads leading to burial of the proximal end within the bone.

 

Bioabsorbable Screws

•  Interference screw introduce by Lambert in 1983 using AO 6.5mm screws.
•  Kurosova developed the current concept of fully threaded interference screw.

Advantages of bioabsorbable screws are the following.

• Does not interfere with MRI.
• Does not interfere with future revision surgery if needed.
• Decreased incidence of graft laceration.
• Does not need implant removal

Disadvantages of bioabsorbable screws are the following.

• Major disadvantage is screw failure during insertion. Special screw drivers that span the entire length of screw reduce incidence of screw breakage.
• Foreign body reaction may be seen in some.

1. The most common materials used are polylactic acid (PLA), poly-L-lactic acid (PLLA), and polyglycolic acid. Composite of PLA and tricalcium phosphate is also available.
2. Degree of crystallinity has significant influence on the degradation rates.
3. Greater crystallinity leads to slower reabsorption.

 

Video Courtesy: OrthNorth and AO

Further reading

1. 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
2. Danis R. The?orie et Pratique de L’Oste?osynthe?se. Paris, France: Masson; 1949.
3. Bagby GW, Janes JM. The effect of compression on the rate of fracture healing using a special plate. Am J Surg. 1958;95(5):761-771.
4. 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.
5. Sehlinger TE, Selingson D. History and development of the orthopedic
6. screw. Harvey JP, Games RF. eds. Clinical and Laboratory Performance
7. of Bone Plates. Philadelphia, Pennsylvania: ASTM; 1994:2-9.
8. Wheeler DL, McLoughlin SW. Biomechanical assessment of compression screws. Clin Orthop Relat Res 1998;237-45
9. Lambert KL: Vascularized patellar tendon graft with rigid internal ?xation for anterior cruciate ligament insuf?ciency. Clin Orthop 172:85-89, 1983
10. Guidelines for drill bits and taps of various AO Screws. http://www.rcsed.ac.uk/fellows/lvanrensburg/classification/commonfiles/Synthes%20screws.pdf