Screw and plates are most common used devices in orthopedics. However, sometimes we forget their principles, so this presentation hopes to review most their problems. Thank you for your attention!
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.
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
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
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
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.
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
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”
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.
