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Design and function of surgical screws
- 1. AO Foundation Education Design andfunction of surgical screws AOTrauma Principles CourseSteve R Schelkun, Coronado, USA
- 2. 2 Learning outcomes • Identifywhat is a screw • Define the design characteristics of a screw • List what kind of screws are available • Explain how a screw works • Describe how the lag screw technique provides absolute stability • State the purpose of the countersink • Evaluate the different functions of screws
- 3. 3 What is ascrew? A simple mechanical device for turning rotational forces into linear motion.
- 4. 4 Types of screwsand material cortex screw cancellous bone screw stainless steel titanium standard self-tapping
- 5. 5 Cortex screw terminology 4.5mm cortex screw • 4.5 mm thread diameter • 8.0 mm head - Hexagonal - Cruciate - Star (Torx) • 1.25 mm pitch • 3.1 mm core diameter • 3.2 mm pilot drill bit • 4.5 mm gliding hole
- 6. 6 Cortex screws 4.5 mmcortex screw thread is designed for application in diaphyseal bone.
- 7. 7 Cancellous bone screwterminology 6.5 mm cancellous bone screw • 6.5 mm thread diameter • Thread length - Full - Partial, 32 mm or 16 mm • 8.0 mm head • 1.75 mm pitch • 4.5 mm shaft • 3.2 mm pilot drill bit 32 mm 16 mm
- 8. 8 Cancellous bone screws 6.5mm cancellous bone screw is used in cancellous or spongy bone
- 9. 9 Function of screws •Compression • Types of compression: - Plate to bone - Two bone fragments for interfragmentary compression
- 10. 10 Working concept ofscrews • Threads advance screw • Head contacts bone (plate) • Compression under head • Tension in shaft • Friction produces stability - Under plate - Between fragments
- 11. 11 Principle of thelag screw technique • It is a technique of insertion not a type of screw • Any screw can function as a lag screw • Provides interfragmentary compression • Produces 2500-3000 Newtons of force • Results in absolute stability
- 12. 12 Conditions—interfragmentary compression • Screwmust glide through near cortex • Threads hold only in far cortex • Screw head stops at near cortex • Best compression 90°
- 13. 13 Axiom Any time ascrew crosses a fracture line it must be inserted as a lag screw to provide interfragmentary compression.
- 14. 14 Bone–screw interface • Maximumstress is between screw head and bone cortex • Countersink to maximize contact between screw and bone to minimize stress
- 15. 15 When to countersink •Cortical bone • Screw outside plate • Create circular “seat” for undersurface of screw head • Oblique orientation produces oval hole
- 16. 16 Washers • Metaphyseal bone •Thin cortex • Artificial cortex • Two sides - Flat and concave
- 17. 17 Lag screw technique—step1 • Drill 4.5 mm gliding hole near cortex • Protect soft tissues, use 4.5 mm drill sleeve
- 18. 18 Lag screw technique—step2 • Drill 3.2 mm threaded hole in far cortex • Insert drill sleeve • Colinear
- 19. 19 Lag screw technique—step3 • Use 4.5 mm countersink • Create circular seat for undersurface of screw
- 20. 20 Lag screw technique—step4 Use depth gauge to measure depth of hole • Longest distance allows maximum purchase
- 21. 21 Lag screw technique—step5 Tap cortex with 4.5 mm tap • Screw and tap same size • Always use tap sleeve • Soft tissue protection • Direction and wobble control
- 22. 22 Lag screw technique—step6 Insert 4.5 mm cortex screw • Interfragmentary compression • Absolute stability
- 23. 23 Six step lagscrew technique Using 4.5 mm cortex screw: • Drill 4.5 mm gliding hole • Drill 3.2 mm threaded hole • Countersink • Measure depth • Tap • Screw
- 24. 24 Names of thedifferent screw functions • Lag/interfragmentary compression screw • Compression screw • Position screw • Locking head screw • Buttress/antiglide screw • Anchor screw • Push-pull screw • Reduction screw • Poller screw
- 25. 25 Screw functions—lag screw Cancellousscrews create interfragmentary compression.
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- 27. 27 Screw functions—position screw •Syndesmosis • No compression
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- 34. 34 Summary • Every screwhas two names • Any screw can perform different functions • Lag screw is a technique - Building block of internal fixation - Interfragmentary compression - Absolute stability • Countersink to distribute forces under the screw head • Washer in metaphyseal regions
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- 36. 36 Next • Function ofplates • Principles of stability • Practical exercise 1 - Application of these principles - Absolute stability