Lag screw versus lag technique
Lag screws and the lag technique compress the fracture fragments together. There are two methods by which to achieve this.
True lag screws (as illustrated here) have threads only on the terminal end of the screw. Therefore, when inserted across a fracture, the threads of the tip of the screw engage the far cortex and the head of the screw engages the near cortex, causing compression of the fracture fragments upon tightening.
True lag screws are not available for CMF surgery. Instead, a lag technique is used. The lag technique involves overdrilling the near cortex to the size of the external diameter of the screw. When the screw is inserted, it glides through this overdrilled hole and the threads only engage the far cortex. As the screw is tightened the head of the screw engages the near cortex and the fracture fragments are compressed together.
Pitfall: no compression without overdrilling
If the near cortex is not overdrilled, the threads of the screw will engage both near and far cortices preventing compression of the fracture fragments.
Perpendicular screw placement
Because lag screw technique compresses the fracture fragments together, the screw must be placed perpendicular to the fracture plane. Otherwise, the fracture will displace when the screw is tightened.
Contraindication - comminuted fractures
Because lag screw technique compresses the fracture fragments together, the use of this technique is contraindicated in comminuted fractures.
Drill the near cortex to the external diameter of screw
The first step is to determine that the drill is aligned perpendicular to the bevel of the fracture. The near cortex is perforated using a drill that is the same diameter as the external diameter of the screw. The gliding hole is taken to the fracture site or slightly beyond.
For example: when using a plating system 2.4, the external diameter of the screw is 2.4 mm. The drill used to drill the near cortex is therefore 2.4 mm.
Pearl: It may be difficult for the surgeon to determine when the fracture site has been reached with the gliding hole. It may be advantageous to drill past the fracture site rather than stay short of the fracture site. If the gliding hole is short of the fracture, compression of this fracture will not be obtained with lag screw technique.
Pearl: oblique surfaces
When drilling obliquely to the surface of the bone, the point of the drill can easily slide along the bone. It is helpful to first orient the drill perpendicular to the near cortex to create an initial hole before reorienting the drill perpendicular to the bevel of the fracture.
Countersink near cortex
A countersinking tool is used to create a platform in the near cortex.
Pearl: countersinking should be done by hand instrumentation. Use of power instruments can easily penetrate the outer cortex.
The hole created by the countersinking tool provides a platform into which the undersurface of the head of the screw will intimately contact when the screw is tightened.
Pitfall: no countersinking
Failure to perform proper countersinking causes an eccentric force which can displace the fracture fragments upon tightening the screws.
Pitfall: too much countersinking
The medullary bone offers no resistance to the head of the screw. Therefore, it is imperative that countersinking does not remove all of the cortical bone around the circumference of the head of the screw. Otherwise, as the screw is tightened its head will enter the medullary space and provide no compression of the fracture fragments.
Drill the far cortex to the inner diameter of the screw using “centering” drill guide
A special drill guide is used to drill through the far cortex. This drill guide has an extension on its tip that is the same diameter as the external diameter of the screw. The drill guide snugly fits into the hole previously drilled through the near cortex.
It is imperative that the fracture fragments be properly reduced prior to drilling through the far cortex.
Note: Gliding hole extends slightly past the fracture site as in the illustration, the drill guide is shown.
The drill guide centers the drill that will be used to drill the far cortex with the hole through the near cortex. This drill has the diameter that is similar to the inner diameter of the screw. For instance, when using a 2.4 mm screw, a 1.8 mm drill is used to drill the far cortex.
When drilling, it is difficult to irrigate the tip of the drill. Therefore, it is imperative that the drill be repeatedly withdrawn so that the irrigant effectively cools the tip of the drill and washes away bony debris.
Determine the screw length
A depth gauge is used to determine the screw length. It is important to assure that the tip of the screw completely engages the far cortex. Because self-tapping screws have a point on their tips, it is important that the tip of the screw completely exits the far cortex so that the screw threads engage completely. Therefore, it is always better to select a screw that is slightly longer than the measurement recorded with the depth gauge.
The proper length screw is inserted and tightened. One should observe the near cortex as the screw is tightened to assure that cracking or crazing does not occur from overtightening.
Properly applied lag screw resulting in interfragmentary compression.
In most cases, a single lag screw does not provide adequate three-dimensional stability. Additional means of fixation are therefore required. This could be in the form of another lag screw or a plate.