SCREWS AND PLATES FIXATION

Cao Ba Huong, MDUniversity of Medicine and Pharmacy, HCM City

What is a screw ?

• A screw is a powerful mechanical device that converts rotation into linear motion.

• 5 features

Head

Shaft

thread

core

recess

Inclination of the threadHexagonal Recess

Screw Tip

What is a screw ?

• 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.

Screw threads

Cortical screw Cancellous screw Locking head screw

Locking head screwConventional screw

Cannulated screw

Screw sizes

FunctionName 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

Lag screw principles

• Purchase only in far cortex• Compression at fracture line

Lag screw principles

Near cortex

Far cortex

Lag screw principles

Lag screw principles

Lag screw principles

Lag screw principles

Lag screw principles

Perpendicular to the fracture plane

Lag screw principles

Cancellous screw as a lag screw

With partial threadA washer is often neededUsually with a buttress plate

Plate designs

• DCP• LC-DCP• Tubular Plates• Reconstruction plates• Anatomic Plates• LCP

Plate funtionsPlate 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

Dynamic Compression Plate (DCP)

The screw holes in the DCP are The screw holes in the DCP are best described as a portion of an best described as a portion of an inclined and angled cylinder. inclined and angled cylinder. Like a ball, the screw head Like a ball, the screw head slides down the inclined slides down the inclined shoulder of the cylindershoulder of the cylinder

Dynamic compression principle

The plate is being moved horizontally

when the screw is driven home

Dynamic compression principle

Limited Contact Dynamic Compression Plate (LC-DCP)

• greatly reduced the greatly reduced the area of the plate-bone area of the plate-bone contactcontact

• less compromised the less compromised the capillary network of capillary network of the periosteumthe periosteum

• reduces the porotic reduces the porotic changes underneath changes underneath the platethe plate

Limited Contact Dynamic Compression Plate (LC-DCP)

Tubular plates

• 1mm thick1mm thick• useful in areas with useful in areas with

minimal soft-tissue minimal soft-tissue coveringcovering+ lateral malleolus+ lateral malleolus+ olecranon+ olecranon+ distal end of the ulna.+ distal end of the ulna.

Tubular plates

Reconstruction plates

• Deep notches between the holes

• Also dynamic compression• Useful in bone with

complex 3-D geometry+ Pelvis+ Acetabulum+ Distal humerus+ Clavicle

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

Locking Compression Plate (LCP)

Pullout of regular screws

Pull-out of cortex screws by a bending load.

Sequential screw loosening

Pull-out of cortex screws by a bending load. LHS provides greater resistance against bending loads

Locking Compression Plate (LCP)

DCPLocking hole

Buttress Plates

• Antiglide function

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

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

Bridge Plate

• Extramedullary splint

• Provide relative stability

• Maintain length, rotation, axis

• Healing with calus formation

Summary

“ Today, this approach is being challenged by less invasive, so-called biological, methods of fracture fixation. Nevertheless, osteosynthesis with plates providing rigid fixation still has a firm place in

fracture treatment”

Thank you for your attention !