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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 SurgeryOver
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 PlanningAs 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 Principles
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