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KEY FACTS
Terminology
Imaging
Clinical Issues
TERMINOLOGY
Definitions
Cannulated screw: Hollow screw for placement over guidewire
Core: Central tube around which threads are wound; may be hollow or solid
Core diameter: Diameter of core portion of screw
Head: Flat surface on screw opposite tip; helps prevent inserting screw too far
Pitch: Distance between threads
Pullout strength: Amount of energy required to pullout or disengage screw from bone
Outer diameter (OD): Diameter of outer margin of threads measured from thread tip to thread tip
Shaft: Nonthreaded portion of screw
Shank: Threaded portion of screw
Tap: Instrument inserted through predrilled hole to create threads (channel) for screw
Tapping: Process of inserting tap
Thread: Inclined plane (semihorizontally oriented structure), which wraps around screw core
Converts rotational force of turning screw into linear force, driving screw forward into bone
Width of threads may vary (deep or shallow), depending on screw type
Screws may be fully or partially threaded depending on whether threads traverse part of or entire core
Tip: Distal end of screw 1st inserted into bone
May be blunt
Self-tapping: Allows screw to be advanced without tapping
Screws cut their own path through bone
Require predrilling hole size of screw core into cortex
Nonself-tapping screws require tap to be advanced after drilling hole size of core diameter
Basic Screw Types
Cancellous screw
Threads are deep, widely spaced
Relatively thin core
For same OD, weaker than cortical screws
Used for fixation of metaphyseal bone
Self-tapping or nonself-tapping
Fully or partially threaded
Cortical screw
Threads are shallow, closely spaced
Fully threaded
For same OD, stronger than cancellous screw
For fixation of diaphyseal bone
For same OD, larger central core than cancellous screw
Blunt tip, nonself-tapping
Used for plate fixation
Fixation often described by number of cortices engaged
Unicortical fixation crosses 1 cortex; may be used with locking plate
Bicortical fixation engages both cortices, protrudes into soft tissues 1-2 mm
Tricortical and quadricortical fixation also used
Specialty Screws
Arthroereisis screw
Bullet-shaped
Threads are blunted
Cannulated
Used to stabilize subtalar joint in flexible flat foot
Dynamic hip screw
Cancellous lag screw within metal cannula attached to side plate
For fixation of femoral neck and intertrochanteric fractures
With weight bearing, screw slides within metal cannula resulting in compression across fracture
Herbert screw/Acutrak screw
Cannulated screw
Distal end: Cancellous threads, smaller OD
Proximal end: Cortical threads greater OD
Produces compression as inserted into bone
With each screw turn, distal end travels further along longitudinal axis than proximal end due to greater thread pitch in distal portion of screw
Interference screws
Fixation of tendon and bone grafts within osseous tunnel
Most commonly used in ACL repair
Bullet-shaped, cannulated, and fully threaded
Related Hardware
Kirschner (K-) wires
Thin, sharp, smooth, stainless steel
Often used for temporary intraoperative fixation; allows control of fragments, aiding reduction and then maintaining reduction during placement of more definitive fixation
May be used as fracture fixation in small bones of hands and feet; often placed percutaneously for this application
Steinman pins
Threaded or nonthreaded
Larger than K-wires
Previously known as traction pins
Rarely used today
Other
Lag screw technique
Method of using screw, not specific screw type
Screw not engaged in proximal fragment; as screw is tightened in distal fragment, it draws that fragment closer to proximal fragment
Interfragmentary screws are placed as lag screws: Used to produce compression across fracture
Cortical or cancellous screws may be lag screws
With cortical screws, overdrilling proximal cortex so threads are not engaged converts to lag technique
Cannot identify lag screw by radiographic appearance
Syndesmotic screw technique
Used to immobilize distal tibiofibular syndesmosis
Placed through fibula into tibia
Tricortical fixation most common
2 fibular cortices, 1 tibial cortex
IMAGING
General Features
Radiographic Findings
CT Findings
MR Findings
CLINICAL ISSUES
Presentation
DIAGNOSTIC CHECKLIST
Image Interpretation Pearls
Selected References
Downey MW et al: Fully threaded versus partially threaded screws: determining shear in cancellous bone fixation. J Foot Ankle Surg. 54(6):1021-4, 2015
Lee MJ et al: Overcoming artifacts from metallic orthopedic implants at high-field-strength MR imaging and multi-detector CT. Radiographics. 27(3):791-803, 2007
Douglas-Akinwande AC et al: Multichannel CT: evaluating the spine in postoperative patients with orthopedic hardware. Radiographics. 26 Suppl 1:S97-110, 2006
Related Anatomy
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Related Differential Diagnoses
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References
Tables
Tables
KEY FACTS
Terminology
Imaging
Clinical Issues
TERMINOLOGY
Definitions
Cannulated screw: Hollow screw for placement over guidewire
Core: Central tube around which threads are wound; may be hollow or solid
Core diameter: Diameter of core portion of screw
Head: Flat surface on screw opposite tip; helps prevent inserting screw too far
Pitch: Distance between threads
Pullout strength: Amount of energy required to pullout or disengage screw from bone
Outer diameter (OD): Diameter of outer margin of threads measured from thread tip to thread tip
Shaft: Nonthreaded portion of screw
Shank: Threaded portion of screw
Tap: Instrument inserted through predrilled hole to create threads (channel) for screw
Tapping: Process of inserting tap
Thread: Inclined plane (semihorizontally oriented structure), which wraps around screw core
Converts rotational force of turning screw into linear force, driving screw forward into bone
Width of threads may vary (deep or shallow), depending on screw type
Screws may be fully or partially threaded depending on whether threads traverse part of or entire core
Tip: Distal end of screw 1st inserted into bone
May be blunt
Self-tapping: Allows screw to be advanced without tapping
Screws cut their own path through bone
Require predrilling hole size of screw core into cortex
Nonself-tapping screws require tap to be advanced after drilling hole size of core diameter
Basic Screw Types
Cancellous screw
Threads are deep, widely spaced
Relatively thin core
For same OD, weaker than cortical screws
Used for fixation of metaphyseal bone
Self-tapping or nonself-tapping
Fully or partially threaded
Cortical screw
Threads are shallow, closely spaced
Fully threaded
For same OD, stronger than cancellous screw
For fixation of diaphyseal bone
For same OD, larger central core than cancellous screw
Blunt tip, nonself-tapping
Used for plate fixation
Fixation often described by number of cortices engaged
Unicortical fixation crosses 1 cortex; may be used with locking plate
Bicortical fixation engages both cortices, protrudes into soft tissues 1-2 mm
Tricortical and quadricortical fixation also used
Specialty Screws
Arthroereisis screw
Bullet-shaped
Threads are blunted
Cannulated
Used to stabilize subtalar joint in flexible flat foot
Dynamic hip screw
Cancellous lag screw within metal cannula attached to side plate
For fixation of femoral neck and intertrochanteric fractures
With weight bearing, screw slides within metal cannula resulting in compression across fracture
Herbert screw/Acutrak screw
Cannulated screw
Distal end: Cancellous threads, smaller OD
Proximal end: Cortical threads greater OD
Produces compression as inserted into bone
With each screw turn, distal end travels further along longitudinal axis than proximal end due to greater thread pitch in distal portion of screw
Interference screws
Fixation of tendon and bone grafts within osseous tunnel
Most commonly used in ACL repair
Bullet-shaped, cannulated, and fully threaded
Related Hardware
Kirschner (K-) wires
Thin, sharp, smooth, stainless steel
Often used for temporary intraoperative fixation; allows control of fragments, aiding reduction and then maintaining reduction during placement of more definitive fixation
May be used as fracture fixation in small bones of hands and feet; often placed percutaneously for this application
Steinman pins
Threaded or nonthreaded
Larger than K-wires
Previously known as traction pins
Rarely used today
Other
Lag screw technique
Method of using screw, not specific screw type
Screw not engaged in proximal fragment; as screw is tightened in distal fragment, it draws that fragment closer to proximal fragment
Interfragmentary screws are placed as lag screws: Used to produce compression across fracture
Cortical or cancellous screws may be lag screws
With cortical screws, overdrilling proximal cortex so threads are not engaged converts to lag technique
Cannot identify lag screw by radiographic appearance
Syndesmotic screw technique
Used to immobilize distal tibiofibular syndesmosis
Placed through fibula into tibia
Tricortical fixation most common
2 fibular cortices, 1 tibial cortex
IMAGING
General Features
Radiographic Findings
CT Findings
MR Findings
CLINICAL ISSUES
Presentation
DIAGNOSTIC CHECKLIST
Image Interpretation Pearls
Selected References
Downey MW et al: Fully threaded versus partially threaded screws: determining shear in cancellous bone fixation. J Foot Ankle Surg. 54(6):1021-4, 2015
Lee MJ et al: Overcoming artifacts from metallic orthopedic implants at high-field-strength MR imaging and multi-detector CT. Radiographics. 27(3):791-803, 2007
Douglas-Akinwande AC et al: Multichannel CT: evaluating the spine in postoperative patients with orthopedic hardware. Radiographics. 26 Suppl 1:S97-110, 2006
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