Basics of Screws in Orthopedics

Basic Screws used in orthopaedics
Dr.Hari Prasath

1. SCREWS IN ORTHOPAEDICS DR.HARI PRASATH P 1 ST YEAR PG - CHENNAI

2. DEFINITION Screw is a device which converts rotational Forces into linear motion

3. A screw has a core about which is wrapped a spiral surface Most surgical screws are made of Titanium or Stainless Steel or Bioabsorbable

4. PARTS OF SCREW Head Shaft Thread Tip

5. HEAD Functions Attachment of a screwdriver – to apply torque Arrest forward motion Slotted Cruciate Philips Hexagonal / Allen Torx Stardrive

6. THREADED HEAD – Head engages in plate which has similar Threads Used in Locking plates The screw cannot be pulled out Locks efficiently against any tilting motion

7. SHAFT Smooth Link Not present in standard cortex screw Present in cortical shaft screws Or Cancellous screws

8. RUN OUT Transition between shaft and thread Site of most stress riser Screw Break: Incorrectly centered hole Hole not perpendicular to the plate

9. THREAD A thread can be visualized as a long wedge encircling the core Standard screw has single thread, but a screw can have 2 or more threads Double threaded screw advances twice as fast as single thread PROFILES: V-Thread Profile: more stress at sharp corner Butress thread Profile: Less Stress at the rounded corner The two main thread types of surgical screws are for cortical finer thread cancellous coarse thread

10. DIAMETER Thread diameter: Diameter across the maximum thread width Cancellous have larger thread diameter Affects the pull out strength

11. Diameter The Maximum width of the Thread Narrowest diameter of the screw across base of Thread Thread depth more in Cancellous screws Less in Cortical screws

12. CORE Solid section from which the threads project outwards 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

13. PITCH & LEAD The distance between two adjacent threads CORTICAL Screws – Small pitch 40.5 TPI CANCELLOUS Screws – Larger Pitch 9.2 TPI The Lead of the screw is the length travelled by the screw with each 360° turn of the spiral

14. TIP There are five types of tips of bone screws. Non-self-tapping tip Self-tapping tip Corkscrew tip Trocar tip Self drilling Self Tapping

15. Non Self Tapping Tip Thread extends to the tip Threads must be precut in the pilot hole before a screw is inserted With a tool called Tap Higher interfragmental compression

16. Self Tapping Tip A thread cutting device called ‘Flute’ Cuts threads in bones through which screw advances self-tapping screw needs pilot hole The length of the screw should be selected so that entire fluted segment protrudes beyond the distal cortex

17. Corkscrew Tip Used in cancellous bone The tips of cancellous screws are designed as a tapering spiral. it compresses the trabecular spongy cancellous bone to thread its way into the bone.

18. Trocar Tip A trocar tip does not produce a true thread but rather displaces the bone as it advances ‘Malleolar’ Screw has a trocar tip suited for soft cancellous bone of distal tibia Schanz screws, locking bolts for IMIL nails

19. Self Drilling Self Tapping This screw compliments MIPPO (minimally invasive percutaneous plate osteosynthesis) Sharpened tip and Tap followed by it Monocortical insertion since no way to measure exact screw length Mainly used in disphyseal area Better purchase in osteoporotic bone

20. TYPES OF SCREWS CORTICAL SCREWS CANCELLOUS SCREWS

21. CORTICAL SCREWS Machine type Smaller thread Lower pitch Larger core diameter Smaller pitch higher holding power Higher surface area Better hold in cortical bone

22. CORTICAL SCREWS Fully threaded screws Shaft screws CORTICAL Screws – Small pitch 40.5 TPI 1.5 mm – Phalanx 2.7 mm - MetaCarpals & Phalanx 3.5 mm – Radius, Ulna, Fibula, Clavicle 4.5 mm – Humerus , Tibia, Femur

23. CANCELLOUS SCREWS Wood type Core diameter is less – Larger threads Higher Pitch Greater surface area for Purchase No need for tap Pilot hole equals core diameter Better fixation in Soft cancellous Bone

24. Cancellous Screws FULLY THREADED - Cannulated or Non Cannulated PARTIALLY THREADED – Cannulated or Non Cannulated 4.0 MM – Humeral Condyle 6.5 MM – Tibial And Femoral Condyle

25. Lag screw principle Lag screws / the lag technique compresses the fracture fragments together  The cancellous bone screw and the cortical screw through a gliding hole

26. Cancellous bone screw Cancellous bone screws have threads only on the terminal end of the screw the threads of the tip of the screw engages in the cancellous bone of the metaphyseal area compression of the fracture fragments upon tightening  washers are used to prevent sinking of the screw head into the bone

27. Cortical screw through gliding hole overdrilling the near cortex to the external diameter of the screw When the screw is inserted, it glides through this hole and the threads only engage the far cortex

28. SPECIAL SCREWS

29. HERBERT SCREW Introduced by Timothy Herbert in scaphoid fractures Specialized implant to achieve interfragmentary compression. No head and threads on both ends with a pitch differential In small bones, such as the scaphoid, self compressing, double pitch screws can be Used These are usually cannulated. Used in scaphoid capitellum fractures navicular fractures malleolar fractures talus fractures arthrodesis of small joints radial head fractures

30. HERBERT SCREW VARIABLE PITCH

31. AcuTrak Screw System Fully threaded Screw has a variable thread pitch Self tapping Pitch differential causes compression at fracture site

32. DYNAMIC HIP SCREW Controlled Dynamic Screw – Richards screw Dynamic Action prevents screw cut out and penetration into hip joint Dynamic Hip Screw With Compression Screw

33. MALLEOLAR SCREW: Smooth Shaft Partially Threaded - LAG Effect Trephine Tip – No Tap needed & in osteoporotic bone without predrilling for cancellous Lag Screw for Medial Malleoli fixation Distal humerus and Lesser Trochanter SIZE : 25MM – 75MM the smaller cancellous bone screws have taken its place in this area

34. MALLEOLAR SCREW: SIZE : 4.5MM

35. LOCKING BOLTS To secure Intra Medullary nails

36. INTERFERENCE SCREW Interference screw introduce by Lambert in 1983 using AO 6.5mm screws

37. Pedicle Screw

38. BIOABSORBABLE SCREWS POLYGLYCOLIDE – Absorbs easily , Early loosening CRYSTALLINE POLYLACTIDE – Years to absorb Composite of PLA and tricalcium phosphate

39. ADVANTAGES: MRI Compatibility Easier Revision surgery No implant removal Decreased incidence of graft laceration DISADVANTAGES: Screw failure –entire length screw Foreign body reaction

40. THANK YOU