Any plate that allows the insertion of fixed-angle/angular-stable screws or
pegs can be used as a locking plate. The main biomechanical difference from
conventional plates is the fact that the latter require compression of the
plate to the bone and rely on friction at the bone-plate interface. With
increasing axial loading cycles, the screws can begin to toggle, which
decreases the friction force and leads to plate loosening. If this occurs
prematurely, fracture instability will occur, leading to implant failure.
Thus, the more difficult it is to achieve and maintain tight screw fixation
(as for example, in metaphyseal and osteoporotic bone), the more difficult it
is to maintain stability.
This biomechanical prerequisite of conventional plates is associated with
biological pitfalls due to compression of periosteal blood supply and
compromise of the vascularity of the fracture. Thus, conventional plate
osteosynthesis with rigid fixation (e.g., interfragmentary compression and lag
screws) has been associated with a substantial complication rate, including
infection, hardware failure, delayed union, and
nonunion12,16.
In contrast, locking plates follow the biomechanical principle of external
fixators and do not require friction between the plate and bone. They are
considered to be internal fixators from a biomechanical standpoint since the
angular-stable interface between the screws and the plate allows placement of
the plate without any contact to the
bone1,3,14,15,17.
In essence, however, locking plates can be considered to be external fixators
placed underneath the skin envelope, although they are more stable as a result
of the shorter distance between the plate and the bone. Many conventional
plates now have locking counterparts. There is an increasing trend by
manufacturers to supply anatomy-specific plates with locking options. Examples
include anatomically pre-shaped plates for the proximal and distal parts of
the femur, proximal and distal parts of the tibia, proximal and distal parts
of the humerus, and
calcaneus18-22.
In many cases, the design of the plate allows substantially less contact
between the plate and bone, in an attempt to preserve the periosteal blood
supply and bone perfusion. Increasingly, locking plate systems have special
features such as outriggers, jigs, and blunted ends to enhance the surgeon's
ability to pass the plate in a submuscular or subcutaneous manner for
minimally invasive
application23-27
(Fig. 1).
Most fractures undergoing operative treatment do not require a locking
plate. The majority heal with conventional plates or intramedullary nails,
provided that the principles of safe surgery are followed. There are specific
fractures, however, that are associated with a higher risk of loss of
reduction and plate or screw failure with subsequent nonunion. These are often
termed "unsolved" or "problem" fractures and include
comminuted intra-articular fractures, short-segment periarticular fractures,
and fractures in osteopenic bone. These injury patterns represent the typical
spectrum of indications for locking plates.
However, decision-making regarding the use of a locking plate must include
precise preoperative consideration of the exact principle by which the locking
plate will be used. ("What's your plan?") The main indications for
the use of a locking plate include four different "classic"
principles47: (1)
the compression principle, for osteoporotic diaphyseal fractures; (2) the
neutralization principle, also for osteoporotic diaphyseal fractures; (3) the
bridging principle ("locked internal fixator" principle), for
comminuted diaphyseal or metaphyseal extra-articular fractures; and (4) the
combination principle ("combi plate" principle), for comminuted
metaphyseal intra-articular fractures.
The surgeon using a locking plate for fracture fixation must be well aware
of the exact indication, according to these four different principles, for
which the angular-stable implant will be used
(Table I). For simple,
non-comminuted diaphyseal fractures in osteoporotic bone requiring an open
reduction and rigid internal fixation, locking plates offer the advantage of
increased pullout resistance of the locking head screws compared with that of
conventional screws. Thus, for these fractures, locking plates can be applied
according to the compression principle through eccentric placement of screws
in the dynamic compression unit of the "combi hole" or by the use
of a compression device after initial placement of one locking head screw on
the other side of the fracture. On the basis of the same rationale, locking
plates can also be used according to the neutralization principle to protect a
lag screw in osteoporotic bone, with increased pullout resistance of the
locking head screws. However, it is crucial to understand that locking head
screws can never provide interfragmentary compression. Compression can be
achieved only by the use of a compression device or by eccentric placement of
conventional screws in the "combi hole" of a combination locking
plate (lag first, then
lock)5,28,30,47.
The classic and ideal indications for fracture fixation with locking plates
are represented by the bridging principle and the combination principle
(Table I). Both concepts apply
to fixation of fractures with substantial comminution—either high-energy
fractures in young patients or low-energy osteoporotic fractures in elderly
patients. The bridging principle is typically represented by the concept of
minimally invasive percutaneous plate fixation (also referred to as the
"MIPO" or "MIPPO" technique), whereby the
angular-stable plate is used as an internal splint that bridges the comminuted
fracture. With this method, indirect reduction is performed by ensuring
adequate axial alignment, length, and rotation of the extremity while the
fracture fragments are not exposed or directly reduced. In contrast to the
compression and neutralization principles, which provide absolute rigid
stability leading to primary (direct) fracture-healing, the bridging concept
provides relative, elastic fixation that leads to secondary (indirect)
fracture-healing by callus formation. For adequate bridge plate fixation,
three or four holes of the plate should be left empty at the level of the
fracture, as discussed below (in the Tips, Tricks, and Pitfalls section).
The combination principle refers to a biomechanical mixture of compression
and bridging with only one implant. Although the original locking plates
available for fracture fixation, such as the point contact fixator (PC-Fix)
and the less invasive stabilization system (LISS), provided all of the
innovative biomechanical and biological properties of angular-stable devices,
surgeons expressed the desire to use a combination of both concepts, locking
and compression plate fixation, with only one implant. This option was made
available for the first time in the early twenty-first century by the locking
compression plate (LCP), which was designed by Robert Frigg (Bettlach,
Switzerland) on the basis of an idea from Prof. Michael Wagner (Vienna,
Austria)4,5,15,28.
The combination technique is indicated for fixation of fractures with a
simple pattern (e.g., an intra-articular split) at one level and comminution
(e.g., metaphyseal-diaphyseal comminution) at a different level. Under these
circumstances, the plate can be used to achieve interfragmentary compression
of the simple fracture pattern by means of a dynamic compression technique or
placement of a lag screw through the dynamic compression unit of the plate.
Thereafter, the plate can be used as a locked internal fixator to align the
articular fragment to the shaft in a bridging manner. The combination
principle is feasible only with plates that allow placement of both locking
head screws and conventional compression screws in one
implant47.
Despite the widespread use of locking plates and their wide range of
indications, a few contraindications must be acknowledged and respected
(Table II). The uncritical use
of locking plates may lead to failure of fixation and to nonunion,
particularly if the above-mentioned standard principles for use of locking
plates are violated. A typical contraindication to the use of a locking plate
as a locked internal fixator is a simple fracture pattern that requires
interfragmentary compression. For example, simple diaphyseal fractures of the
forearm fixed with a plate with a locked internal fixation technique are prone
to nonunion (Fig. 3). Similarly
contraindicated is percutaneous locking plate fixation of simple fractures
with use of a minimally invasive technique. This concept violates the
principle of the fracture gap width in relation to strain and thus leads to
nonunion, as described in an excellent review article by Stephan
Perren12. Finally,
indirect reduction and locking-plate fixation are contraindicated for
displaced intraarticular fractures, since these injuries require anatomic open
reduction and rigid interfragmentary compression.
Because of their cost, locking plates are relatively contraindicated for
fractures that can be stabilized satisfactorily with conventional plates. For
example, diaphyseal forearm fractures have healing rates in excess of 90% with
conventional plates. While there are some claims that, theoretically, the use
of unicortical locking plates should increase healing rates because of the
lack of soft-tissue stripping, this has not been validated in any type of
controlled trial, to our knowledge. Overuse of these plates in some
health-care systems may negatively impact overall patient care by consuming
resources that could be better used elsewhere.
Successful use of locking plates depends on adherence to established
principles of operative fracture care and learning the tricks of the specific
technology. Gautier and Sommer recently presented prudent guidelines that may
improve the individual learning curve of surgeons who are less familiar with
these new
implants47.
In general, successful use begins with a formal preoperative drawing. The
advent of digitized radiography at many centers requires that digital
templates be available. If plain radiographs are used, utilization of tracing
paper is still the most effective way to draw a preoperative plan. The
sequence of screw placement, the length and position of the plate, and the
surgical approach are all critical to success. A precise preoperative plan
reduces the guesswork and increases the likelihood of technical success. The
preoperative plan also ensures that the surgeon will have all necessary
implants available at the time of surgery.
Correct positioning of the patient is vital, particularly if the plan calls
for minimally invasive or percutaneous insertion of the implant. The surgeon
should ensure that all necessary images are obtainable prior to preparation
and draping of the patient. A radiolucent table is very helpful. Tightly
rolled bumps of different sizes fashioned prior to the operation can aid in
fracture reduction as well as visualization, particularly in the lateral
plane. Fracture reduction can be challenging with locking plates because the
locking screws do not pull the plate to the bone in the manner of conventional
screws. Therefore, it is essential that the surgeon have a preoperative plan
for fracture reduction. Combinations of traction to correct length and
alignment in the anteroposterior plane and placement of bumps under the
extremity to correct lateral plane deformities can successfully permit
reductions with minimal direct manipulation of the fracture fragments.
Specialized reduction clamps can be used judiciously with percutaneous
long-bone and periarticular reductions. Conventional screws or
"whirlybird" push-pull types of devices can be used to pull the
bone to the plate initially to secure fracture reduction. Once the fracture is
reduced, then locking screws can be added as needed to provide stability.
Effective use of locking plates requires an understanding of the potential
pitfalls of usage. Locking holes offer minimal opportunity for screw
angulation. More than 5° of angulation between the screw and the locking
hole can cause the screw to eventually fail. Careful technique is necessary to
ensure that the screw is perfectly lined up with the axis of the screw threads
in the plate. This may be quite difficult in a minimally invasive procedure.
Malaligned screw threads can lead to loose screws and loss of reduction
(Fig. 4). The weakest part of
the combi locking plate (e.g., the LCP) is the dynamic compression unit. This
is the part of the plate that should be used for bending, if required, and it
is the part that breaks when there is increased stress concentration and
strain on the
plate17,42.
For this reason, when a bridge plate is used to fix a comminuted fracture, at
least three or four plate holes should be left empty at the level of the
fracture, in order to achieve a larger area of stress distribution on the
plate (Fig. 5). In contrast to
conventional plates, which fail at the interface between the screws and the
plate—often leading to breakage of conventional screw heads—the
interface of the locking head screw with the threaded locking hole is the
strongest part of the locking plate system. Locking screw heads are less
likely to break since the difference between the core diameters of the screw
shaft and head is much smaller than it is with conventional screws.
Nevertheless, locking head screws can break in cases of chronic instability
and increased strain as a result of rotational forces, as is exemplified by
proximal humeral nonunion shown in Figure
4 (panel B).
Locking plates allow the use of both bicortical and unicortical locking
head screws. The choice of screw type (self-drilling/self-tapping or
self-tapping only) and screw length (unicortical or bicortical) needs to be
based on defined principles in order to avoid complications. As a general
rule, self-drilling/self-tapping screws, as are used in minimally invasive
locking plates (such as the LISS), should be employed exclusively in a
unicortical fashion. The main reason is that self-drilling screws have sharp
tips that may cause neurovascular and/or soft-tissue damage across the far
cortex. Furthermore, drilling of the far cortex with
self-drilling/self-tapping screws may lead to simultaneous disruption of the
tapped thread in the near cortex and thus reduce the overall purchase of the
locking head screws. Similarly, a pitfall with unicortical placement of
self-tapping screws is the selection of an inadequate screw length. If the
screw is too short, the threads in the near cortex will not have enough
purchase and the locking monoblock frame is prone to failure by pullout with
cyclic loading (Fig. 6). In
contrast, if the unicortical screw is slightly too long, the nondrilling screw
tip will push off from the far cortex, thus destroying the tapped thread in
the near cortex.
The pullout resistance of unicortical locking head screws is almost
identical to that of similar-diameter bicortical conventional screws and about
70% of that of bicortical locking head screws. Thus, how much pullout strength
is needed? There is no way to objectively judge this, nor is it necessarily
important, because these constructs rarely fail through pullout per se. Two
factors are essential for decision-making with regard to the use of
unicortical or bicortical locking head screws. These are, first, the quality
of the cortical bone and, second, the extent of rotational forces applied to
the fractured bone. Cortical thickness is of great importance in determining
the adequacy of the working length of unicortical
screws47. The
working length of a unicortical screw in good-quality cortical bone usually
provides sufficient pullout resistance equaling the pullout strength of a
bicortical conventional screw, as mentioned above. In contrast, in metaphyseal
bone and osteoporotic cortical bone, the cortex is usually very thin, thus
rendering the working length of unicortical screws insufficient. This
reduction in pullout strength is of particular importance when osteoporotic
bone is mainly loaded by torsional forces such as occurs in the humerus. Under
these conditions, an adequate working length must be achieved by using
bicortical locking head screws, as outlined in
Table III.
For these reasons, bicortical fixation is recommended for osteoporotic bone
in general and for metaphyseal fractures in bone of normal quality. Also,
unicortical screws should not be used in anatomic locations exposed to high
rotational forces, such as the humeral shaft. In fact, the only advantage of
using unicortical screws may be the lack of penetration of the bone and
periosteum on the far side of the bone, and this advantage is highly debatable
as far as its true impact on fracture-healing. Additionally, unicortical
screws are less stiff than bicortical locking screws. Unicortical screws are
indicated for periarticular fractures in which the screws are placed in the
direction of an articular surface, such as in the proximal part of the
humerus.
Locking head screws have some peculiar differences from conventional
screws. While these differences may seem intuitively obvious, they have
implications with regard to clinical application. The first difference is that
a torque-limiting screwdriver is useful during placement of the locking head
screw. The advantage of the use of a torque-limiting device is that, as long
as the screw is centered in the hole, the thread cannot be stripped or
overtightened. Deformation of the screw heads by overtightening or
cross-threading ("cold-welding") into the plate can make hardware
removal very difficult. This happens more often with the minimally invasive
technique, which can result in stripping of the screw heads and angulation of
the screws because of the difficulty in judging orientation without direct
visualization. In contrast to the situation with conventional screws, the
purchase of the screw in the bone cannot be felt, since locking head screws
always feel tight. Unfortunately, the tight feel is deceptive. Many fracture
surgeons, through years of experience and training, have developed a tactile
feedback loop based on how the screw "feels" as it tightens down.
The fixation can still fail despite "tightness" of the screws,
particularly if there is substantial malreduction
(Fig. 2) or if the screws are
cross-threaded or not adequately locked in the plate
(Fig. 4, panels A and B).
One strategy that is used to overcome this problem is to carefully place
perpendicular 2.0-mm Kirschner wires in the most distal and proximal plate
holes prior to screw insertion. To ensure that the wires are truly
perpendicular, they should be placed through the locking drill guides.
Alignment can then be checked by looking for the round
"bull's-eye" of the drill guide in the locking hole on the lateral
radiograph. These wires will maintain length and can also serve as a reference
for subsequent placement of screws. Alternatively, a drill bit can be left in
place through the drill sleeve to hold the plate in place temporarily, until
an adequate reduction is achieved.
Although minimally invasive techniques have been improved in recent years
by the introduction of locking plates, achieving and maintaining an adequate
reduction remain sources of pitfalls and failures. Sliding any minimally
invasive plate in the submuscular plane along the bone can be challenging.
There are a number of strategies for aligning the plate along the bone
percutaneously. Kirschner wires can be inserted manually just anterior and
posterior to the bone to mark the boundaries for plate passage. The plate is
then passed between the wires, and the wires will prevent posterior or
anterior deviation of the plate. Another tactic is to make 4 to 6-cm incisions
at the proximal and distal sites of the plate. With use of blunt dissection
down to bone, the plate can be directly visualized as it passes into the
wound. Locking drill sleeves should be attached to the most proximal and
distal holes of the plate to form a frame for easier positioning of the plate
on the bone as depicted in Figure
1. This way, the plate can be centered on the bone at each end and
anchoring screws can be placed under direct visualization. If the fracture is
well reduced with regard to length, alignment, and rotation, then the plate
will be appropriately positioned along the entire length of bone when it is
centered at each end. The plate must be held to the bone by first placing a
conventional screw or a "whirlybird" tool, since the placement of
a self-drilling/self-tapping screw will push the plate away from the bone and
may cold-weld the screw head to the plate. Since the incisions are distant to
the fracture site, the principle of minimally invasive fixation is preserved,
as the fracture fragments and soft-tissue attachments are undisturbed.
Locking plates, particularly the specialized so-called all-locking plates,
require an approach to fracture reduction that is completely different from
what most of us have practiced. When one begins to use locking plates, a good
approach is to "start easy." One should consider initially using
combination plates that permit the use of traditional reduction techniques.
Lost in the current enthusiasm for this new technology is the recollection
that ten years ago almost no surgeons in the world were using locking plates
routinely. Most of the current high-volume experts in this field started by
practicing on Sawbones and attending workshops. This is a good approach for
anyone starting to use these techniques.
Malreduction can result in failure regardless of whether the plate is
conventional or locking. Common problems include varus in the proximal part of
the humerus and distal part of the femur and distraction in diaphyseal
fractures (Figs. 2,
3, and
4). In many cases, locking
plates are used as bridge plates in the presence of substantial comminution.
Plates that are very stiff or stiff fracture constructs with too many screws
can lead to nonunion and eventually plate failure
(Fig. 5). Bridge plates must be
longer, and fewer screws are needed. For the treatment of periarticular
fractures, few screws are needed in the diaphysis but more screws may be
required near the articular surface. The precise length of the bridge plate
and the number of screws needed for a specific fracture remain controversial.
In general, the length of the plate should be more than two times the length
of the fracture zone. Screws should be spread evenly, and ideally there should
be at least one empty hole between each pair of holes filled with screws. As
mentioned above, when the bridging principle is used, three or four screw
holes should be left empty at the level of the fracture to avoid a local
stress concentration, which may lead to breakage of the
plate42. The even
distribution of force over a long working plate length with relatively few
screws appears to provide a stable stimulus for indirect bone-healing and
callus formation.
Locking plate technology offers improved fixation stability in osteopenic
bone and for comminuted and periarticular fractures. The additional stability
per screw compared with that of conventional nonlocking fixation enhances the
application of minimally invasive fracture techniques such as use of bridge
plates and percutaneous fracture stabilization. The application of locking
plates is somewhat more difficult than the placement of conventional plates.
Fracture reductions are often done indirectly, the locking screw must be
carefully aligned along the axis of the receiving hole to ensure proper
tightness, and the length of the plate must be selected carefully. Despite the
necessity of mastering these nuances, the use of locking plates will likely
increase, particularly with the increasing prevalence of fragility fractures
in our aging population and the increase in high-energy fractures in younger
patients surviving severe trauma. While a substantial amount of biomechanical
and animal data have been published, few series have validated the long-term
advantages of fixation with locking plates. The initial results in series that
included a variety of fractures are encouraging, although it is increasingly
apparent that failures do occur. The causes of failure should be examined
carefully in both the literature and one's own practice in order to learn from
mistakes and refine our techniques.