Plates can be classified according to their design (compression/one-third tubular, reconstruction, locking) or function
(compression, neutralization, antiglide, buttress, bridging).
Date: 14.04.2014 Fig No: 3.7 Cat #/Author: K17090 - Dawson-Bowling, Achan, Briggs, Ramachandran Proof Stage: 1
Date:5.8.2014 Fig No: 3.7 Cat #/Author: K17090 - Dawson-Bowling, Achan, Briggs, Ramachandran Proof Stage: 2
Figure 3.7. Use of (a) a fully threaded and (b) partially threaded screws for fracture lagging. Note the overdrilling required with the fully threaded screw to allow the screw to glide through the near fragment.
(a) (b)
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Date: 14.04.2014 Fig No: 3.8 Cat #/Author: K17090 - Dawson-Bowling, Achan, Briggs, Ramachandran Proof Stage: 1
Date: 5.8.2014 Fig No: 3.8 Cat #/Author: K17090 - Dawson-Bowling, Achan, Briggs, Ramachandran Proof Stage: 2
Proof Stage: 3 Tension
band
Tension Compressive
force at previous tension side
Weight Weight
Figure 3.8. The tension band principle.
(a)
(b)
(c)
Figure 3.9. Preoperative (a) and postoperative (b, c) radiographs of olecranon fracture treated with tension band wire.
Compression plates
Fracture healing is promoted by compression at the fracture site. Compression plates can achieve fracture compression in one of two ways:
1. Static compression – pre-stressing of a plate can produce axial compression at the fracture site nearest the plate, but it can produce fracture distraction at the cortex opposite the plate. To reduce this fracture distraction, the plate should be contoured to ensure a concave bend, creating compressive forces on both the far and near cortices of the fracture (Fig. 3.10).
2. Dynamic compression – the screw holes in DCPs are oval and shaped with an angle of inclination orientated away from the fracture (Fig. 3.11). A screw is eccentrically placed at the end of the hole farthest from the fracture; when tightened, the screw head slides down the angle of inclination, resulting in movement of the bone relative to the plate and creating compression at the fracture site.
The plate-bone interface creates a ‘compartment’
under the plate that can result in periosteal compromise and subsequent necrosis. Limited- contact DCPs (LC-DCPs) are designed to limit stress shielding and vascular compromise from plate fixation, by decreasing plate-to-bone contact. In principle, this leads to improved cortical perfusion with increased preservation of the periosteal vascular network, potentially optimizing union.
One-third tubular plate
This plate is thin (1 mm). Its pliability allows relatively easy contouring. It is primarily used as a neutralization plate, when a lag screw has already provided fracture compression (e.g.
lateral malleolar ankle fractures). Oval holes on the plate allow for a small degree of fracture compression.
Reconstruction plates
These plates are thinner than DCPs but thicker than one-third tubular plates.
Deep notches in the side of the plate allow contouring in three planes. Oval screw holes allow limited fracture compression.
Locking plates
Since their introduction, locking plates have become increasingly popular, and indications for their use continue to expand. The holes in a locking screw plate are threaded, as are the heads of the corresponding screws, which therefore lock into the plate when tightened (Fig. 3.12). This configuration provides a rigid construct for the fixation of fractures and acts along a biomechanical principle similar to that of external fixators. Consequently, locking plate systems have been referred to as
‘internal, external fixators’.
Locking plates systems have a number of advantages over conventional non-locking plates:
• They do not require precise adaptation of the plate to the contours of underlying bone.
In non-locking systems, failure to ensure intimate contact of the plate on the bone can result in loss of reduction upon screw tightening. Because the screws lock into the plate, they can stabilize a fracture fragment without the need to compress the plate to
Date: 14.04.2014 Fig No: 3.10 Cat #/Author: K17090 - Dawson-Bowling, Achan, Briggs, Ramachandran Proof Stage: 1
Proof Stage: 2
Figure 3.10. Static compression using a precontoured, standard plate.
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Date: 14.04.2014 Fig No: 3.11 Cat #/Author: K17090 - Dawson-Bowling, Achan, Briggs, Ramachandran Proof Stage: 1
Proof Stage: 2
Figure 3.11. Dynamic compression plate.
the bone. However, placement of locking screws cannot alter fracture reduction.
• Because locking plates sit slightly off the bone, the underlying periosteum is compromised much less than with conventional plates (Fig. 3.12).
• Locking plate systems have been shown to provide a more stable fracture fixation, even in poor quality bone.
• Some locking plate systems can be inserted percutaneously, creating multiple small incisions, as opposed to a single large incision (‘minimally invasive plate osteosynthesis’ – MIPO). A potential drawback of this technique is the higher chance of malunion; accurate reduction is more easily obtained via an open approach.
Many locking plates have combination holes allowing insertion of either a locking screw or a conventional screw. Non-locking screws allow fracture compression by eccentric screw placement or permit lagging through the plate.
This must be undertaken before the first locking screw is sited.
Indications for the use of locking plates include:
• Complex periarticular fractures.
• Comminuted metaphyseal or diaphyseal fractures.
• Periprosthetic fractures.
• Fractures occurring in poor quality bone (e.g. osteoporotic bone).
• Metaphyseal fractures of long bones in which intramedullary nail fixation may have a high likelihood of malalignment.
Some advocates of locking plates claim that unicortical locking screws provide adequate fixation. The authors’ preference, however, is to use bicortical locking screws where possible;
unicortical locking screws have lower torsion fixation strength than non-locking bicortical constructs. A minimum of two bicortical or three unicortical screws on either side of the fracture should be used.
The following factors maximize locking plate construct stability:
• Use of bicortical locking screws.
• Use of a large number of screws.
• Minimization of screw divergence from the screw hole (<5°).
• Use of a long plate.
Plate use
Plates are mainly used in one of six modes:
1. Neutralization – this protects lag screw fixation from bending, torsional and shearing forces.
2. Compression – plates provide compression at the fracture site.
3. Bridging – no screws are placed at the level of the fracture. This provides relative stability, relative length and alignment in fractures where there has been bone stock loss (comminuted, unstable fractures). A bridging plate preserves the blood supply to the fracture fragments.
4. Buttress – the plate counteracts compression and shear forces that often occur with fractures of the metaphysis and epiphysis.
The buttress plate is always fixed to the larger fracture fragment; however, it does not necessarily require fixation to the smaller.
5. Antiglide plate – this is a development of the buttress plate. The plate is secured at the apex of an oblique fracture, creating an
‘axilla’ which prevents shortening or angular displacement (Fig. 3.13).
6. Tension band plate – this works in
accordance with the tension band principle.
A plate applied to the tensile side of an eccentrically loaded bone converts the tension forces into compressive forces at the fracture site.
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Date: 14.04.2014 Date: 5.08.2014
Fig No: 3.12 Cat #/Author: K17090 - Dawson-Bowling, Achan, Briggs, Ramachandran Proof Stage: 1
Proof Stage: 2 Proof Stage: 3 Figure 3.12. Locking plate systems have threaded holes and threaded screw heads. The plate sits slightly off the bone and acts as an ‘internal, external fixator’.
Mechanical properties of a plate
The strength of a plate varies according to the material from which it is manufactured and the second moment of area (SMA). The SMA refers to the spatial distribution of material within the plate structure. The rigidity of a plate is therefore proportional to the third power of the plate’s thickness.
For a structure with a quadrilateral cross-section (e.g. a plate), the value of the SMA = wh3/12.
General principles of plate use
• Soft tissue stripping should be minimized.
• The selected plate should be appropriate for the function intended.
• It is important to ensure that sufficient screws are used either side of the fracture (Table 3.5).
• As the distance of the closest screws to the fracture (the working distance) increases, the plate-screw construct becomes less rigid.
• More than one plate may be required if the fracture is unstable in multiple planes.