Terminology

Imaging

Clinical Issues

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

General Features

Radiographic Findings

CT Findings

MR Findings

Presentation

Image Interpretation Pearls