Locking plate principles

There are several advantages to a locking plate/screw system:

• A locking plate does not have to precisely contact the underlying bone in all areas. When screws are tightened, they "lock" to the threaded screw holes of the plate, stabilizing the segments without pulling the bone to the plate. Locking screws make it impossible for screw insertion to alter the reduction. Nonlocking plate/screw systems require a precise adaptation of the plate to the underlying bone. Without this close contact, tightening of the screws will pull the bone segments toward the plate, resulting in loss of reduction and possibly the occlusal relationship
• Locking plate/screw systems do not disrupt the underlying cortical bone perfusion as much as conventional plates, which compress the plate to the cortical bone.

• Screws are unlikely to loosen from the plate. Similarly, if a bone graft is screwed to the plate, a locking head screw will not loosen during the phase of graft incorporation and healing. The possible advantage to this property of a locking plate/screw system is decreased risk of inflammatory complications due to hardware loosening.

• Locking plate/screw systems have been shown to provide more stable fixation than conventional nonlocking plate/screw systems.

The disadvantage of locking head screws is that a surgeon might not realize that the holes have been over-drilled because the head of the screw locks into the plate.

The locking plate has a corresponding threaded plate hole.

Note that the screw head is also threaded, unlike the conventional non-lock screw/plate.

During insertion, the locking screw head engages and locks into the threaded plate hole.

If necessary, the threaded plate hole also accepts nonlocking screws, which permit angulation.

With the conventional technique, the tightening of the screws pulls the plate against the bone. Tightening the screws "lag" the bone to the undersurface of the plate.

Locking head screws are engaged in the plate, and the plate is not pressed against the bone. This reduces interference to the blood supply to the bone underneath the plate.

The plate and screws provide adequate rigidity and do not depend on the underlying bone buttressing (load-bearing osteosynthesis).

On each side of the fracture, the screws are locked into the plate and the bone. The result is a rigid frame construct with high mechanical stability (internal-external fixator).

When using a nonlocking plate and screws, the plate must be precisely adapted to the bone; otherwise, the tightening of the screws will lead to loss of reduction.

When using a locking plate/screw system, the plate does not have to be precisely adapted to the bone. When tightening a locking head screw, the screw will not cause a primary loss of reduction as it tightens into the threaded plate hole and will not draw the bone fragments to the plate.

In nonlocking plate systems, screw loosening may lead to loss of reduction.

In a locking system, screw loosening rarely occurs because the screw head is locked to the plate.

One disadvantage of a locking screw system is if the hole has been over drilled, the screw will engage in the screw hole threads locking to the plate, and the surgeon may not be aware that the threads in the bone lack purchase.