Master All Types of Screws in Orthopedics: FRCS Exam Guide
Key Takeaway
Your ultimate guide to Master All Types of Screws in Orthopedics: FRCS Exam Guide starts here. Common types of screws in orthopedics include cortical, cancellous, fully/partially threaded, locking, non-locking, cannulated, and Poller screws. Design features differentiate these types of screws, focusing on thread pitch, tip design, and core to outer diameter ratio. Cortical screws are ideal for dense bone, while cancellous screws suit less dense bone. Locking and cannulated screws offer specific stability and guidance functions.
A 55-year-old male presents with a complex, displaced intra-articular calcaneal fracture. You are planning an open reduction and internal fixation. During the procedure, you are tasked with placing a lag screw to achieve optimal interfragmentary compression. Explain the biomechanics of the lag screw technique and what specific technical steps you must take to ensure compression is generated rather than simple interfragmentary fixation.
Candidate: To achieve interfragmentary compression, I need the screw to 'lag' the near fragment against the far fragment. I will use a partially threaded screw. I must drill a 'gliding hole' in the near cortex that is the size of the major diameter of the screw, while the far cortex is drilled to the core diameter. Once the screw is inserted, the threads will only purchase in the far fragment, pulling the near fragment toward it as the screw head tightens.
Failing to emphasize the distinction between the gliding hole and the thread hole, or suggesting a fully threaded screw without mentioning that it must be used with a near-cortex gliding hole. A poor candidate often forgets to mention the specific diameters required for the drill bits (e.g., matching the major diameter for the near cortex and core diameter for the far cortex).
The candidate defines the biomechanical principle: the screw threads must purchase only in the far fragment. Technical Steps: 1. Drill the near cortex with a drill bit equal to the major diameter (the 'gliding hole'). 2. Drill the far cortex with a drill bit equal to the core diameter. 3. Tap the far cortex (if not self-tapping). 4. When the screw is tightened, the head engages the near cortex and, because the threads do not engage the near fragment (due to the gliding hole), it creates compressive force across the fracture site. Bonus: Mentioning the importance of countersinking to prevent hardware prominence in the thin soft tissues of the heel.
