SCREWS- Basic principles in fracture management .pptx

Basic Presentation in the use of screws as an implant in fracture management

1. SCREWS

2. A screw is a very efficient implant for re- pairing a fracture using interfragmentary compression, or for fixing a splinting device such as a plate, nail, or fixator to a bone.

3. Head Shaft Thread Tip Parts of Screw

4. Countersink refers to the hemispherical or conical undersurface of the screw head Shaft is the smooth part of the screw between the head and the thread Run out is the spot where the shaft ends and the thread begins Core diameter refers to the minimum diameter of the screw across the base of the thread Pitch defines the distance between the adjacent threads

5. Lead of the screw refers to the distance the screw will advance with each turn , lead is therefore equal to the pitch The thread (or outside) diameter is the widest diameter of the screw. Tip of the screw is the end opposite to the head

6. Screw head Recess Single slot head Cruciate head Phillips head Recessed hexagonal head Torx type head socket

7. Recessed hexagonal head The hexagonal head driver makes a strong and alignment insensitive connection with the screw and offers a good lateral guidance that allows blind insertion and removal The torque transmission is independent of axial thrust and thus does not compromise initially unstable reduction of the fracture fragments.

8. Functions of head It provides the means of applying torque (twisting force) to the screw It acts as a stop As the head comes in contact with the bone surface, the translational motion of the screw stops and the torque transforms to tension in the screw, w hich in turn induces compression between the two surfaces Compression develops only after the translational motion of the screw stops

9. Shaft Shaft or shank is the smooth link between the head and the thread The shaft length is variable ,in a standard cortical bone screw it is almost nonexistent but in a cortical shaft screw or in a cancellous screw it is significant Screws with long shafts are used as lag screws ,the smooth shaft has no purchase in the proximal hole and ensures compression by lagging

10. Core diameter Core diameter also known as the inside or root diameter represents the narrowest diameter of the screw across the base of the threads. It is the weakest part of the screw. Smaller the root diameter ,greater the tendency to shear off during insertion and removal Torsional strength of the screw varies with the cube of its root diameter

11. Cont Doubling the root diameter of a screw increases the extent of torque that it fan withstand by a factor of 8

12. Pitch Pitch is the distance between the adjacent threads Cortical screw has small pitch, cancellous screw has a large pitch. Stronger the bone (cortex), the smaller the pitch Weaker the bone ( cancellous ), larger the pitch Pitch also determines the rate of advance of the screw (lead) through the bone.

13. Cont It is measured as threads per inch(tpi) AO cortical screw -40.5 tpi AO large fragment cancellous screw -9.2 tpi

14. Lead The lead of the screw means the distance it travles on a complete turn Smaller the thread greater the, mechanical advantage of the screw Stronger cortical screws have a smaller lead as compared

15. Outside diameter It refers to the diameter across the maximum thread width and affects its pull-out strength. The greater the outside diameter, the greater the resistance to pull out.

16. V thread Buttress thread Buttress thread with rounded base Locking screw thread Bone screw fastner Thread design

17. Tip of screws Self tapping tip Non-self-tapping tip Corkscrew tip Trocar tip Self-drilling self-tapping tip

18. Self tapping tip It has a thread cutting device called a flute . This mechanism cuts threads in the bone over which the screw advances As the screw advances, the cutting flute chisel into the bone and direct bone chips away from the root. It has no purchase on the distal cortex due to the presence of flutes Advantages: No pretapping required, ensures very tight fit o f the screw

19. Non self tapping tip The thread extends upto the tip and lacks thread cutting device. Threads are precut using tap Advantages:Higher interfragmental compression, offers greater resistance to pull out , better purchase on the distal cortex

20. Corkscrew tip Used only in cancellous screws where the tip clears the predrilled hole. Cancellous screws forms its own threads by compressing the thin walled trabecular bone

21. Trocar tip It does not produce a true thread but rather displaces the bone as it advances. Malleolar screw has trocar tip which is well suited for soft cancellous bone of the distal tibia and medial malleolus.(schanz screws )

22. Self drilling self tapping tip The additionally sharpened tip of the screw drills a hole efficiently and reduces heat generation and the resisitance as the drillimg tip penetrates the bone. It is exclusively used for monocortical insertion because there is no possibility of measuring the exact screw length and the sharp drill bit like tip may injure the structured outside the bone. Particularly used in osteoporotic b o ne and in metaplyseal region

23. Threads are smaller in diameter and closely placed(lower pitch). Core diameter is relatively large and provides the necessary strength. Smaller pitch increase the holding power of the screw Threads are cut by tap or ST tip of the screw Cortical bone screw

24. Cancellous bone screw Tip is not tapered It has larger threads and a higher pitch. Core diameter which is smaller than that of the shaft, provides greater surface area for purchase of screw threads on bone. Increase in thread diameter increases the pull-out strength It is inserted into untapped pilot hole

25. Self tapping screw It is inserted directly into a predrilled hole without first tapping a thread. Two types -thread forming and thread cutting. Cancellous screw is a thread forming, ST screw. Cortical screws may be ST or NST

26. Fully and partially threaded screws Cortical screw is usually fully threaded. It may be NST or ST Partially threaded screw is called a shaft screw.It has better strength and stiffness than a fully threaded screw Fully threaded cancellous screw is used as a placement screw to f i x a bone plate in metaphyseal and epiphyseal regions. Partially threaded cancellous screw is used a lag screw.

27. Locking screws It is a bone screw with threads on the undersurface or countersink of the head. On tightening it locks in the matching threads in the plate hole and the screw becomes axially and angularly stable Its conical countersink offers better distribution of forces between the scew head and the threaded holes and improves locking fixation in the screw hole.

28. Herbert screw It is a specialised implant to achieve interfragmentary compression. There is no head and threads are available at both ends of the screw with a pitch difference between the leading and the trailing threads. Interfragmentary compression is achieved by the difference in the thread pitch, the coarser pitch moves the screw a greater distance through a bone with each turn than does the finer pitch.

29. As the screw is turned ,the leading thread penetrates the bone faster than the trailing threads, slowing penetration, reducing the surfaces to come together and creating compression during insertion. Indi ca tions include scaphoid fracture, capitellar fractures, radial head fractures, osteochondral fractures, osteochondritis dissecans and small joint arthrodesis.

30. DRILL BIT SCREW SIZE THREADED HOLE TAP SIZE 3.5MM 2.5MM 3.5MM 4.5MM 3.2MM 4.5MM 2.7MM 2.0MM 2.7MM

31. Drill bit

32. Measurement of screw length Depth gauge is used to measure the required screw length Measured before tapping the thread

33. Drill sleeve A drill sleeve is an instrument used to direct the placement of a drill hole and simultaneously protect the surrounding soft tissues It increases the accuracy of drilling and prevents the drill point from wandering across the bone

34. Washers It is often used with a cancellous screw to prevent the screw head from burying into the thin cortex overlying the cancellous bone. The flat side of the washer rests on the bone whereas the countersink side matches the underside of the screw head

35. Lag screw It a technique of insertion and not a screw It is the most effective way to achieve compression between two fracture fragments It pulls the fragments together producing friction across the fracture line Ir achieves this by gaining purchase on the distal fragment while being able to turn freely in the proximal

36. Position screw When the insertion of a lag screw will cause a fragment to collapse into the medullary cavity, it is preferable to use a position screw In this screw, thread holes are drilled in the cortex of both the near and far ined.fragments . Drill hole is measured and tapped. When the appropriate screw is inserted, the position of the two fragments is maintained.

37. cont In other words, it is fully threaded screw that joins two anatomical parts at a defined distance without compression. Eg.syndesmotic screw

38. Poller screw It is used as a fulcrum to redirect an intramedullary nail.

39. ACUTRAK SCREW It is a headless screw It has variable thread pitch It is self cutting and cannulated and fully threaded Used in fracture fixation of scaphoid, metatarsals, metacarpals

40. Malleolar screw It has smooth shaft and is partially threaded. It acts as a lag screw for medial malleolus fixation

41. Interference screw It is an effective technique in bone patellar tendon bone and ACL reconstruction

42. Bolt It is also an externally threaded device like screw, which is used in conjunction with a nut and is tightened or released only by twisting the nut.

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