Locking plate principles

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 Locking versus nonlocking plates - Advantages to a locking plate/screw system
 Plate design
 Biomechanics
 Primary loss of reduction
 Secondary loss of reduction
Locking versus nonlocking plates - Advantages to a locking plate/screw system

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

  1. Locking plate and screw systems have advantages over the conventional screw systems. Conventional plate/screw systems require precise adaptation of the plate to the underlying bone. Without this intimate contact, tightening of the screws will draw the bone segments toward the plate, resulting in alterations in the position of the osseous segments and the occlusal relationship. Locking plate/screw systems offer certain advantages over other plates in this regard. The most significant advantage may be that it becomes unnecessary for the plate to intimately contact the underlying bone in all areas. As the screws are tightened, they "lock" to the plate, thus stabilizing the segments without the need to compress the bone to the plate. This makes it impossible for the screw insertion to alter the reduction.
  2. Another potential advantage in locking plate/screw systems is that they do not disrupt the underlying cortical bone perfusion as much as conventional plates, which compress the undersurface of the plate to the cortical bone.
  3. A third advantage to the use of locking plate/screw systems is that the screws are unlikely to loosen from the plate. This means that even if a screw is inserted into a fracture gap, loosening of the screw will not occur. Similarly, if a bone graft is screwed to the plate, a locking 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 a decreased incidence of inflammatory complications from loosening of the hardware. It is known that loose hardware propagates an inflammatory response and promotes infection. For the hardware or a locking plate/screw system to loosen, loosening of a screw from the plate or loosening of all of the screws from their bony insertions would have to occur.
  4. Locking plate/screw systems have been shown to provide more stable fixation than conventional nonlocking plate/screw systems.

Plate design

Locking plates 2.0
Locking plates 2.0 are available in four thicknesses, with or without center space:

  • Small profile
  • Medium profile
  • Large profile
  • Extra-large profile

They are also available in multiple shapes to meet a variety of clinical applications.

The threaded head of the 2.0 mm locking head screws is conical. It is therefore possible to insert locking head screws at small angles. A threaded drill guide is not necessary.


Locking reconstruction plate (LRP) 2.4
There is only one thickness of the locking reconstruction plate 2.4. However, there are multiple plate configurations to meet a variety of clinical applications.

The threaded head of the 2.4 mm locking head screws is cylindrical. Therefore, a threaded drill guide is mandatory to assure the correct perpendicular insertion of 2.4 mm locking head screws. Angulation is not possible.


Locking head screws
The locking plate has a corresponding threaded plate hole.


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


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


Biomechanics

Conventional screws
With the conventional technique, the tightening of the screws presses the plate against the bone. This pressure generates friction, which contributes significantly to primary stability.


Loading forces are transmitted from the bone to the plate, across the fracture and back into the bone. Friction between plate and bone is necessary for stability using conventional screws.


Locking head screws
However, with the locking head screws engaged in the plate, the plate is not pressed onto the bone. This reduces interference to the blood supply to the bone underlying the plate.


Loading forces are transmitted directly from the bone to the screws, then onto the plate, across the fracture and again through the screws into the bone. Friction between plate and bone is not necessary for stability.

The plate and screws provide adequate rigidity and do not depend on the underlying bone (load bearing osteosynthesis) when using a locking reconstruction plate 2.4.


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


Primary loss of reduction

Conventional plate system
When using conventional plates and screws it is essential to contour the plate precisely to the bone surface.


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

Note: in the top illustration the plate is not well adapted to the outer cortex. The lower illustration shows that when the screws are inserted, the bone will be pulled to the plate, causing malreduction of the fracture.


Locking plate system
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.


Secondary loss of reduction

Conventional plate system
In conventional plate systems, screw loosening may lead to loss of reduction.


X-ray shows the clinical situation described above.


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