screws and plate

screws and plate

1. IMPLANTS –SCREWS AND PLATE

2. A screw is a device which converts rotational forces into linear motion Screws

3. Screws

4. PARTS-HEAD,SHAFT,THREAD,TIP

5. HEAD OF SCREW

6. countersink Undersurface of head. Either-conical,hemispherical.

7. Function of screw head

8. Shaft/shank Smooth link between head and thread

9. Run out Between area between shaft and thread represents a location of significant stress concentration by presence of sharp corners .

10. Pitch

11. The tip

12. Screws Diameter Diameter of the core determines the minimal hole size for the screw to be accommodated in the bone and determines the drill used to create the pilot hole for the screw. In other words the drill to be used will be the same (approximately the same) diameter as the core of the screw.

13. Screw Length The nominal length of the screw is from the top of the head to the screw tip. An appropriate length of screw needs to be chosen.

14. Screws Too short - it will not gain full purchase in the bone. Too long - it may cause problems by irritating the soft tissues, or protruding subcutaneously.

15. Screws Pitch The pitch of the screw is the length travelled by the screw with each 360° turn of the spiral.

16. Screws Shorter the distance - “ finer ” the pitch. Longer the distance - “ coarser ” the pitch. Cortical bone screws have a fine pitch . Cancellous bone screws have a coarse pitch .

17. Screws Pitch distance between threads Lead distance advanced with one revolution Screw working distance (length) defined as the length of bone traversed by the screw Outer diameter-d iameter across the maximum thread width . Root (inner) diameter

18. Types of screws:

19. Types of screws Cortical screws The threads are smaller (in diameter) and are closely placed (lower pitch). smaller pitch increases the holding power of the screw. The modulus of elasticity screw is more than 10 times that of bone; therefore, much of the elastic deformation occurs in the bone.

20. Cancellous screws Its tip is not tapered. It has larger threads and a higher pitch as compared to the cortical screw. An increase in the thread diameter of a cancellous screw increases its pull-out strength. The spring reaction comes from the cancellous bone as it is deformed during the thread forming process.

21. Self tapping screw Refers to a screw which is inserted directly into a pre-drilled hole without first tapping a thread. Self-tapping screw may further be subdivided into thread-forming and thread-cutting screws. The thread-forming type moulds (i.e. forms) its own elastic-plastic deformation or by local destruction of the bone. The thread-cutting screw cuts its threads through the bone over which it advances. The cancellous bone screw is a thread-forming, self-tapping screw. The screw thread forms its own mating bone thread by compressing the soft cancellous bone.

22. NON SELF TAPPING SCREW A non-self-tapping screw allows precision placement in hard cortical bone, particularly if one is trying to insert a screw obliquely into the bone to lag two bone fragments together. The NST screw is incapable of cutting a channel in cortical bone and can be removed and reinserted without the fear of inadvertent damage.

23. FULLY AND PARTIALLY THREADED SCREWS A fully threaded cortical screw can function as lag screw only when the near cortex is over-drilled. A fully threaded cortical screw may be self-tapping or non-self-tapping. A partially threaded cortical screw is called a shaft screw. The shaft diameter corresponds to the outer diameter of the thread. This screw has better strength and stiffness than a fully threaded screw which is an advantage when it is used as a lag screw and as an axial compression screw. It is a non-self-tapping screw

24. CANNULATED SCREW Precise insertion in metaphyseal or epiphyseal site over a guide wire reducing the problem of having to remove and reposition an incorrectly placed screw. A guide wire accurately visualizes the path of the screw. If guide wire position must be changed, it can be done without enlarging the hole and sacrificing purchase strength of the bone. Final placement of the screw requires use of cannulated drill and occasional use of a cannulated tap. The screwdriver is also cannulated. Cancellous cannulated screws come in large and small sizes.

25. HERBERT SCREW For interfragmentary compression. no head and threads are present at both ends of the screw, with a pitch differential between the leading and trailing threads. intention is for the screw to be buried beneath a bony surface.

26. PLATES

27. 1). Neutralization plates 2. Compression plates 3. Buttress plates 4. Condylar plates

28. NEUTRALIZATION PLATE acts as a ‘bridge’. It transmits various forces from one end of the bone to the other, bypassing the area of the fracture. Its main function is to act as a mechanical link between the healthy segments of bone above and below the fracture. Such a plate does not produce any compression at the fracture site. A plate used in combination with a lag screw is also a neutralization plate merely protects the lag screw, allowing mobilization of the extremity.

29. COMPRESSION PLATE produces a locking force across a fracture site to which it is applied. The effect occurs according to Newton’s Third Law (action and reaction are equal and opposite). The plate is attached to a bone fragment. It is then pulled across the fracture site by a device, producing tension in the plate. As a reaction to this tension, compression is produced at the fracture site across which the plate is fixed with the screws. The direction of the compression force is parallel to the plate.

30. Role of compression What does a compression plate achieve?

31. Methods of achieving compression SELF COMPRESSION PLATE: This is a device that converts the torque (turning force) applied to the screw head to a longitudinal force which compresses the fractured bone ends. The screws and plates are designed to facilitate this conversion. As the screw advances in a self-compressing plate it slides down on an inclined plane that is part of the plate’s screw hole

32. Tensioning device A special tensioning device can be attached between the bone plate .A bolt is then tightened to pull the plate across the fracture site. This produces tension in the plate and large compressive forces-across the fracture.

33. Eccentric screw placement Eccentric placement of a screw in a plate hole creates considerable shear stress in the screw. The same force is transmitted to the plate and can occasionally be used to produce interfragmental compression. To achieve this, a screw is eccentrically placed in the hole of a plate.

34. Compression plates Work by placing a cortical screw eccentrically into an oval hole in the plate. During the final tightening of the screw, the screw head will be forced into the center of the hole squeezing the fracture together beneath the plate. Creates compression at the fracture site .

35. BUTTRESS PLATE is to strengthen (buttress) a weakened area of cortex. The plate prevents the bone from collapsing during the healing process. It is usually designed with a large surface area to facilitate wider distribution of the load. Such a plate acts as a buttress or retaining wall. A buttress plate applies a force to the bone which is perpendicular (normal) to the flat surface of the plate. A buttress plate must be firmly anchored to the main fragment It is commonly used in fixing epiphyseal and metaphyseal fractures

36. CONDYLAR PLATE Has been in the treatment of intra-articular distal femoral fractures. It has two mechanical functions. It maintains the reduction of the major intra-articular fragments, hence restoring the anatomy of the joint surface. It also rigidly fixes the metaphyseal components to the diaphyseal shaft, permitting early movement of the extremity. This plate functions both as a neutralization plate and as a buttress plate. Since the plate can be attached to a tensioning device and has specially designed screw holes, it also functions as a compression plate. A condylar plate is used to fix a proximal femoral osteotomy and intercondylar fracture of the femur

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