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Transcript: Locking Plates

Locking plates and screws have been on the general Orthopaedic market for the past 8 years. These have revolutionalised internal fixation of fractures, especially metaphysical fractures. They differ from conventional plates and screws and this page attempt to outline these differences as well as defining the advantages and contraindications of this new type of Orthopaedic hardware.

Use of locking plates in Orthopaedic Surgery

Over the last decade Orthopaedic plates and screws have undergone a major change.

The screw locks into a machine screw in the plate as well as being threaded into the bone. The union to the plate keeps the screw fixed rigidly at around 90 degrees to the plate. Each locked screw thus acts as a mini blade plate.

 Conventional screws rely on friction between the plate and screw to prevent movement at the fracture site – the locking pate does not!

The locking acts as a small internal fixator – the same principle as an external fixator. In other words they have rigid pins or screws into the bone and the plate is analogous to the external bar of an exfix. The plates do not have to be completely adjacent to the bone as with conventional plates. This allows preservation of soft tissue which is often the blood supply to the bone and its fragments.

The manufactures often make these locked plates anatomically specific to the area designed for. The pates come pre-contoured for various anatomical areas e.g. the proximal humerus or plateau area of the lateral proximal tibial metaphysis.

Some designs include jigs allowing the plate to be placed percutaneously onto the bone while the jig guides the surgeon to the screw holes underneath.

Preoperative Planning

As with all internal fixations, planning is essential - make sketches with pencil and paper or using a computer base CAD type system.

Locking plates are often more challenging than plates and screws, because with this modality, fracture fragments cannot easily be pulled towards the plate as they can be with conventional screws. It may be necessary to use other techniques such as "whirlybird" screw devices, or to use ordinary screws in the locking plate to achieve alignment against the plate.

 The last pull- out resistance of these locking screws is similar to that of the cortical screws.

 A single cortex locking screw has about 70% of the strength of a conventional cortical screw over two cortices.

Sharp screws such as self tapping screws need special precautions as they have sharp points. They are intended for unicortical penetration only - do not let them stick out the other side, where they can cause possible nerve or vessel damage!

 Bi-cortical fixation however, is recommended in fragility fractures and in anatomical locations exposed to high rotational forces, for example the humeral shaft.

A torque wrench type screwdriver is often very useful to prevent over tightening. Too loose a screw will also work loose and the side tapped into the plate is particularly prone to do so as it is tapered.

Screws need to be near 90 degrees to the plate – use the guide jig or sleeve provided to achieve this. Some designs allow for slight angulation (5 to 15 degrees) the more you angulate the more likely is the screw to back out.

Another principle is that to prevent stress raisers at he level of the fracture itself, rather leave empty a screw hole or two around this area. Leaving a gap without screws round about the fracture area will allow gradual bending of the plate over this increased area- thus spreading the load (stress) over a broad area rather than loading all the stresses over exactly the fracture gap. This stress raiser effect will occur when screws right up to the fracture are used. 

Disadvantages of Locking Plates

·         Locking plates are much more expensive than conventional plates.

·         The screws cannot pull in fragments towards the plate.

·         They cannot cause compression over the fracture as can conventional plates. (Eccentric hole principle)

·         The reduction has to be obtained prior to putting on the plate.

·         Excessive rigidity. The construct is sometimes so stiff that callus cannot form thus causing delayed or non-unions.

·         If a fracture is poorly reduced this alignment will persist after application of the locking plate. - The screws cannot be used to pull in the bone or fragments to achieve reduction as can be done with conventional plates.

Indications for locking plates.

Not all fractures require locking plates. For diaphyseal fractures an intramedullary pin is often more appropriate.  For simple fractures such as forearm diaphyseal breaks, conventional plates are probably even more appropriate and are cheaper! These bones have a 90 percent union rate anyway.

The prime advantage of these locking plates is when the bone is of poor quality, such as with fragility fractures and in metaphysical areas. Also in fractures where one can make use of percutaneous techniques these locking plate’s cone tom their own.

One does not aim for anatomical reduction of each fragment of a fracture – thus the blood supply is preserved by avoiding stripping.

Contraindications to locking plates

·         A simple fracture pattern requiring compression.

·         Percutaneous fixation of these simple fractures is also contraindicated as locking plates cannot compress a fracture gap, leading to non-union.

·         Cost is high and any area that can be treated with conventional plates just as effectively is a relative contraindication.

References

(1) Smith WR, Ziran BH, Anglen JO, Stahel PF. Locking Plates: Tips and Tricks. J Bone Joint Surg Am 2007;89:2298-307.

(2) AO Principlesof Fracture Management by Rüedi TP et al. (Thieme Verlag, Stuttgart, New York, 2000).