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Screw Fixation
Cheryl A. Petersilge, MD, MBA
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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

                    1. 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
                    2. 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
                    3. 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

                                      1. 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
                                      2. 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
                                      3. Douglas-Akinwande AC et al: Multichannel CT: evaluating the spine in postoperative patients with orthopedic hardware. Radiographics. 26 Suppl 1:S97-110, 2006