Locking head screw insertion
1. Locking head screws
Introduction
The hole in the plate is threaded, and the head of the locking screw has a corresponding thread.
As the screw is tightened, the thread on the head engages with the thread of the plate and locks the screw to the plate at a 90° fixed angle, without compressing the plate against the bone.
Technique
Screw the appropriate drill guide into the plate hole and drill the screw hole.
Some systems have calibrated drill bits, so the depth of the tip is indicated. As the tip of the drill bit penetrates the far cortex, the depth can be measured from the shaft of the drill bit.
The depth of the hole will depend on whether monocortical or bicortical screws are used.
No sleeve is used if self-tapping, self-drilling screws are used. These should only be used as unicortical screws. If the screw is used to drill a hole in the far cortex, it only advances slowly as it is turned, so the threads in the near cortex are stripped.
Unless a calibrated drill bit was used, a depth gauge is used to determine the screw length.
The screw is inserted and tightened with a torque-limiting screwdriver. As the screw is tightened, it is locked to the plate.
Self-drilling screws
If the near cortex is thick, there is no tendency for the screw to toggle. If the near cortex is thin, there may be a tendency for the screw to toggle; this may be minimized by allowing the tip of the screw to make contact with the far cortex. This requires an accurate judgment of the screw length as the tip is sharp and can injure soft tissue beyond the cortex if it protrudes through the far cortex.
2. Variable angle locking head screws
When polyaxial screws are used, the design of the plate hole allows for the screw to be inserted in various angles up to 15° and still provide angular stability by a special threaded screw head and plate interface.
Variable angle locking head screws must be placed with high precision. Use the appropriate drill guide to prepare the screw hole. This may be a guide that fixes the screw in the nominal axis (central or 0° off axis), or at an angle up to 15° off the nominal axis.
Some systems have calibrated drill bits, so the depth of the tip is indicated. As the tip of the drill bit penetrates the far cortex, the depth can be measured from the shaft of the drill bit.
Generally, the closer the screw is inserted to the nominal axis, the more angular stability there will be.
The depth of the hole will depend on whether monocortical or bicortical screws are used.
Screw insertion is analogous to the procedure described above.
