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Screws and plates fixation
- 1. SCREWS AND PLATES FIXATION CaoBaHuong, MD Universityof MedicineandPharmacy, HCMCity
- 2. What is a screw ? • A screw is a powerful mechanical device that converts rotation into linear motion. • 5 features Head Shaft thread core recess
- 3. Inclination of the thread Hexagonal Recess Screw Tip What is a screw ?
- 4. • Two forces : tangential and axial • A conventional 4.5 mm cortex screw during tightening : + 50% is used to overcome friction at the screw head interface + 40% is transformed into axial force + 10% overcomes the friction of the thread.
- 5. Screw threads Cortical screw Cancellous screw Locking head screw
- 6. Locking head screwConventional screw
- 9. Screw sizes
- 10. Function Name Mechanism Plate screw Preload and friction is applied to create force between the plate and the bone Lag screw The glide hole allows compression between bone fragments Position screw Holds anatomical parts in correct relation to each other without compression, ie, thread hole only, no glide hole Locking head screw Used exclusively with LCP/LISS; threads in the screw head allow mechanical coupling to a reciprocal thread in the plate and provide angular stability Interlocking screw Couples an intramedullary nail to the bone to maintain length, alignment, and rotation Anchor screw A point of fixation used to anchor a wire loop or strong suture Push-pull screw A temporary point of fixation used to reduce a fracture by distraction and/or compression Reduction screw Conventional screw used through a plate to pull fracture fragments towards the plate; the screw may be removed or exchanged once alignment is obtained Poller screw Screw used as a fulcrum to redirect an intramedullary nail
- 11. Lag screw principles • Purchase only in farcortex • Compression at fracture line
- 17. Lag screw principles Perpendicularto the fracture plane
- 19. Cancellous screw as a lag screw With partial thread A washeris often needed Usually with a buttress plate
- 20. Plate designs • DCP • LC-DCP • TubularPlates • Reconstruction plates • Anatomic Plates • LCP
- 21. Plate funtions Plate function Biomechanics Compression The plate produces compression at the fracture side to provide absolute stability Protection The plate neutralizes bending and rotation forces to protect a lag screw fixation Buttress The plate resists axial load by applying force at 900 to the axis of potential deformity Tension band The plate is attached to the tension side of a fracture and converts the tensile force into a compressive force at the cortex opposite the implant Bridging The plate provides relative stability by fixation to the two main fragments, achieving correct length, alignment, and rotation. The fracture side is left undisturbed
- 22. Dynamic Compression Plate (DCP) The screw holes in the DCP areThe screw holes in the DCP are best described as a portion of anbest described as a portion of an inclined and angled cylinder.inclined and angled cylinder. Like a ball, the screw headLike a ball, the screw head slides down the inclinedslides down the inclined shoulder of the cylindershoulder of the cylinder
- 23. Dynamic compression principle The plate is being moved horizontally when the screw is driven home
- 25. Limited Contact Dynamic Compression Plate (LC-DCP) • greatly reduced thegreatly reduced the area of the plate-bonearea of the plate-bone contactcontact • less compromised theless compromised the capillary network ofcapillary network of the periosteumthe periosteum • reduces the poroticreduces the porotic changes underneathchanges underneath the platethe plate
- 26. Limited Contact Dynamic Compression Plate (LC-DCP)
- 27. Tubular plates • 1mm thick1mm thick • useful in areas withuseful in areas with minimal soft-tissueminimal soft-tissue coveringcovering + lateral malleolus+ lateral malleolus + olecranon+ olecranon + distal end of the ulna.+ distal end of the ulna.
- 28. Tubular plates
- 29. Reconstruction plates • Deep notches between the holes • Also dynamic compression • Useful in bone with complex 3-D geometry + Pelvis + Acetabulum + Distal humerus + Clavicle
- 30. Locking Compression Plate (LCP) • Conventional plate: - Screws in tension - Plate–bone friction - Compression at fracture site - Screw interface loosening ● External fixator: - Screws in shear - Plate–bone gap - No compression - No screw loosening
- 31. Locking Compression Plate (LCP) Pullout of regular screws Pull-out of cortexscrews byabendingload. Sequential screw loosening Pull-out of cortexscrews byabendingload. LHSprovides greater resistanceagainst bendingloads
- 32. Locking Compression Plate (LCP) DCP Locking hole
- 34. Tension band plate • The fractured bone must be eccentrically loaded • The plate must be placed on the tension side. • The plate must be able to withstand the tensile forces. • The bone must be able to withstand the compressive force
- 35. Bridge Plate • Fixed to the two main fragments only, leaving the fracture zone untouched. • Respect the biology of a complex multi-fragmentary fracture and to minimize any additional soft-tissue injury
- 36. Bridge Plate • Extramedullary splint • Provide relative stability • Maintain length, rotation, axis • Healing with calus formation
- 37. Summary “ Today, this approachis beingchallengedbyless invasive, so-calledbiological, methods of fracture fixation. Nevertheless, osteosynthesis withplates providingrigidfixationstillhas afirmplacein fracturetreatment”
Editor's Notes
- 5 features : + a central core which provides strength; +a thread which engages the bone and converts rotation into linear motion; + a tip which may be blunt or sharp; +a head which engages either the bone or the plate; +a recess to attach a screwdriver.
- The inclination of the thread—the pitch—must be small enough to provide purchase of the screw in bone, ie, to prevent the screw from unwinding and becoming loose. However, the pitch must be large enough to allow full insertion with an acceptably low number of revolutions. Screw Tip : + smooth, conical tips for insertion into a tapped drill hole; + self-tapping, which will cut a channel for the thread; + self-tapping/self-drilling, which will cut a drill hole and channel for the thread. The original, smooth, conical-tipped screws were designed for insertion after the drill hole had been tapped. Tapping may reduce the pull-out strength of screws, probably because inadvertent toggling of the tap causes the hole to be enlarged to a size greater than actually needed.
- The cortex screw thread is designed for application in diaphyseal bone. The cancellous bone screws have a deeper thread, a larger pitch, and a larger outer diameter than the cortex screws. Their applications are in metaphyseal or epiphyseal cancellous bone. locking head screws (LHS) are haracterized by a larger core diameter and a comparatively shallow thread with blunt edges.
- Screw also have different sizes in diameter, for example,4.5/6.5 millimeter for large bone, such as femur, tibia and humerus,3.5 millimeter for forearm, and 2.7 millimeter for smaller bone, such as hand and foot.
- The principle is : first, drill gliding hole at the near cortex, the drill bit’s diameter is the same as the screw, second, drill the thread hole on the far cortex, the drill bit’s diameter is the core diameter of the screw, so when the screw insert and tighten, it can create a compression force between fracture line
- For the cancellous screw, there is needn’t to drill a gliding hole.
- Since the screw head is fixed to the bone via the shaft, it can only move vertically relative to the bone. The horizontal movement of the head, as it impacts against the angled side of the hole, results in movement of the bone fragment relative to the plate, and leads to compression of the fracture
- In the DCP (a) the area at the plate holes is less stiff than the area between them. During bending, the plate tends to bend only in the areas of the hole. The LC-DCP (b) has an even stiffness without the risk of buckling at the screw holes. In cross-section the plate has a trapezoidal shape. The bony ridges which form along the edges of the plate tend to be thicker and flatter, rendering them less prone to damage during plate removal
- The collar around the hole of the one-third tubular plate prevents the screw head from protruding and secures the plate/bone contact.
- Without a collar, the screw head protrudes through the plate, preventing good fixation. Thanks to the collar the plate-screw junction is improved.
- allow accurate contouring on the flat as well as standard bending
- To prevent any sliding of the fragment, the screw is placed as proximal as possible in the hole Application of the DCP in a buttress function.To prevent any sliding of the plate, the screw hole is placed opposite to the slope in the plate hole.