Orthopedic Lag Screws: Biomechanics, Principles, and FRCS Exam Prep MCQs
Key Takeaway
A lag screw's primary biomechanical function is to create interfragmentary compression across a fracture. It achieves this by engaging only the far cortex, while gliding freely through the near cortex. This differential engagement pulls bone fragments together, providing absolute stability crucial for optimal fracture healing, particularly in oblique or spiral fractures.
Examine this post-operative radiograph of a patient who underwent open reduction and internal fixation of a distal tibial fracture. Describe the fixation construct you see and explain the mechanical principle being utilized by the lag screw relative to the neutralization plate.
Candidate: "The radiograph shows a spiral fracture of the distal tibia fixed with an interfragmentary lag screw and a neutralization plate. The lag screw provides absolute stability by compressing the fracture fragments across the spiral plane. The neutralization plate is placed to protect the lag screw from bending, shear, and torsional forces that would otherwise lead to failure during weight-bearing."
A failing candidate often fails to distinguish the roles of the implants, stating "the screw and plate both hold the bone." They may describe the plate as "providing compression" (which it doesn't, the screw does) or fail to explain why the screw alone is insufficient (the need for protection against bending/shear).
A perfect answer structure: 1. **Identify the construct:** "This is a lag screw and neutralization plate construct." 2. **Explain the Lag Screw:** "The lag screw provides interfragmentary compression to achieve absolute stability and primary bone healing." 3. **Explain the Neutralization Plate:** "The plate serves to neutralize or protect the lag screw from torsional, bending, and shear forces. It effectively offloads the lag screw, allowing the screw to maintain compression while the plate carries the physiological load, thus preventing mechanical failure of the fixation."
You have identified that your lag screw is "stripping" while tightening it into the far cortex of the tibia. Describe the steps you would take to salvage the fixation during the index procedure.
Candidate: "I would remove the stripped screw and try to insert a screw with a larger diameter. If the hole is too damaged, I could shift the position of the lag screw slightly or consider augmenting with bone cement, though changing to a larger diameter screw is the standard first step."
Candidates often suggest "using a longer screw" or "tightening it harder," which will never work in a stripped hole. They may also immediately jump to "bone graft," which is a biological solution, whereas this is a mechanical problem requiring an immediate mechanical solution.
The Gold Standard requires a hierarchy of mechanical solutions: 1. **Larger Diameter:** Remove the screw and place a larger diameter screw (e.g., if a 3.5mm screw strips, upgrade to a 4.0mm or 4.5mm screw). 2. **Repositioning:** If the hole is catastrophically damaged, re-drill and tap in a slightly different trajectory, ensuring you aren't creating a stress riser. 3. **Augmentation:** Mention using bone cement as a last-resort mechanical filler if the bone quality is extremely poor (osteoporotic). 4. **Structural Change:** If internal fixation is no longer secure, consider a change in strategy, such as adding an external fixator or switching to intramedullary nailing.
