Bone plates are used in internal fixation of fractures. The plate is fixed to the bone by application of screws on both sides of the fracture.
Plating is widely used for fixation of diaphyseal fractures of upper extremity and metaphyseal and articular fractures.
Plates for fixation of long bone fractures were first recorded by Hansmann, of Heidelberg University, Germany in 1886.
Plates are now widely accepted with different standard techniques of osteosynthesis, throughout the skeleton. Different anatomical locations demand different shapes and sizes of plates.
For a plate to be effective means of fixation it should meet the following requirements
- It should be of appropriate width and thickness for the given bone
- The plate must have adequate and symmetric hold on either side of the fracture
- The plate must be closely opposed to the contour of bone. A prebent plate or over-contoured plate is an exception.
- The plate must neutralize all forces acting on the fracture i.e bending, compression, shear and torque
Indications for Use of Bone Plates
The use of plate is indicated:
- When anatomical alignment must be restored accurately
- Where the use of screws alone is inadequate
- When load sharing may be achieved with confidence. If this cannot be guaranteed, then a bone graft may be added at the site of any deficit.
The areas of the body where the conditions described above arise commonly are:
- Around joints – failure to restore a joint surface to its normal shape may lead to osteoarthritis because of uneven wear of the joint surfaces.
- In the bone of the forearm, which rotates about each other.
- On the pelvis especially around the acetabulum
- On the face and jaw
Other areas where plates are sometimes used include the tibia and the femur. However, these long bone are subject to large forces, so alternative techniques using intramedullary nails have been developed. Non-weight bearing long bones such as the humerus may be plated although nails are sometimes used in this site also.
Tension Band Principle of Plating
Bones are not always loaded evenly along their axes – if they are not this is called eccentric loading. This also can occur in a fracture if the soft tissues are stripped off one side while remaining intact on the other. These factors produce a tendency for a loaded bone to distort more or one side than the other.
If a plate is fixed on the side tending to open – the tension side – then this will counteract the eccentric load, compressing the fragments together at the side under the plate. The eccentric load will continue to compress the bone fragments together on the side of the bone opposite to the plate. In this situation, the fracture is compressed throughout the bone cross-section and the plate suffers an equal and opposite force – tension.
The following criteria must be fulfilled for a plate to act as a tension band:
- The fractured bone must be eccentrically loaded.
- The plate must be placed on the tension side.
- The plate must be able to withstand the tensile forces.
- The bone must be able to withstand the compressive force which results from the conversion of distraction forces by the plate.
- There must be a bony buttress opposite to the plate to prevent cyclic bending.
The plate is acting as a tension device or band. Bone compression may be achieved using wires and this technique is commonly used around the olecranon of the elbow and across the patella at the knee. In this case, the wires are acting as a tension device.
The plate is placed opposite the tension side of the bone and will easily be deformed by opening of the fracture when eccentric loading is applied.
Wherever possible plates should be put on the tension side of a fracture. This is not always possible for anatomical reasons, such as the need to respect the blood supply of the bony fragments or the risk of tethering mobile structures such as tendons.
Types of Modes of Bone Plating
A plate may be positioned so that it may be used as one of the following types of structural support:
- Compression Mode
- Neutralization Mode
- Buttress plate
- Antiglide Plate
- Span Plating or Bridge Plating
- Tension Band
Plates come in different sizes and shapes depending on the size of the bone and its anatomical shape. Different sizes of plates require different sizes of screws.
Depending upon the fracture location and type, a plate can be used in any one of the following modes
The word compression in fracture fixation means bringing the two fragments closer to each other so that the fragments oppose well.
When the plate is used to achieve compression in addition to fixation of a fracture, it is said to be used in compression mode. This mode is used on transverse and oblique fractures
In this type of plating, a lag screw is used to neutralize bending, shear and rotational forces. The aim of this type of plating is not to achieve compression [This has already been achieved by the lag screw.]
This kind of plating is also used as an adjunct where screws are used alone in long bones an accurate reconstruction with interfragmentary compression can be achieved, but screws are not particularly strong in resisting bending or twisting forces. The addition of a plate provides further resistance to such forces.
The fibular fracture commonly seen in ankle injuries is often fixed with lag screws protected by a neutralization plate.
The plate is put on the tensile surface of the bone but it is not prebent. [see below for prebending]
A buttress is an architectural structure built against or projecting from a wall which serves to support or reinforce the wall.
A buttress plate is used to prevent a collapse in fracture patterns which are unstable under compressive forces.
A buttress plate is very thin as compared to compression plate and is mostly applied on the compressive surface of the bone.
This is the most common way that plates are used around joints.
Here there is a relatively soft cancellous bone core surrounded by a thin layer of together cortical bone. Accurate osteosynthesis may be achieved by lag screws alone but under load, there may be a risk of the screws cutting out of the bone or bending.
The addition of a plate will prevent such deformity under axial load. The plate should be situated where the maximum load is predicted to be. Proximal tibia, for example, plates may be required on one side or another to protect a tibial plateau fracture fixation.
There are specially shaped plates for the most common anatomical buttressing requirements, and these make application of the plate easier.
In antiglide plating, the plate is fixed to the bone in such a way that it prevents the distal fragment from overriding when force is applied along the long axis of the bone. ‘
The distal fragment is firmly wedged between the plate and the fracture surface of the proximal plane.
This principle is mostly applied in fracture of the distal fibula. It can be called a variation of compression plating.
Span Plating or Bridge Plating
It is a modification of neutralization mode which is applied when there is comminution and one does not want to dissect the fracture area to avoid devitalization of the fragments.
This kind of plating is done under x-ray control or C-arm image intensifier. The plate is contoured slightly to bypass the fracture zone and fixed only to major proximal and distal segments.
This not only minimizes the devitalization of the fracture zone but also promotes the formation of periosteal callus.
In this method of plating, farthermost screws in the plate are applied on either side and then fixation moves towards the center on either side of a fracture zone with holes of plate spanning left unused for fixation.
A plate can also be solely used as a tension band apparatus.
The plate is attached to the tension side of a fracture and converts the tensile force into a compressive force at the cortex opposite the implant. An example of such use is in olecranon fracture
Prebending of Bone Plates
What is the prebending of plates and why it is done? Various types of plates are used in fracture fixation. One of the common mode of use is compression mode where apart from the fixation of fractures, the application of the plate aims for closer apposition of the fracture fragments.
To understand the concept of prebending, we first need to understand terms far cortex and near cortex.
Long bones are like hollow metallic cylindrical pipes in the diaphysis. Let us take an example of a hollow pipe. Imagine drilling a transverse screw in the pipe from one end to other so that.
The screw would enter the pipe surface at one point and enter the hollow after it has pierced metallic layer on the entry side. After traversing the hollow, it would enter the opposite metallic layer from within and then come out.
The side where screw made its first entry is called near cortex [As it is nearer to you]. The opposite side where the screw made exit is called far cortex.
Why is Prebending of Bone plates required?
Prebending means bending a straight plate before application to the bone. The bend is such that the midsection of the plate is elevated from the bone surface prior to fixation to the bone.
If the plate is applied as such it produces asymmetric compression because the near cortex is compressed more than the far cortex, or the far cortex may not be compressed at all;
When prebent plate is fixed to the bone, due to its elastic recoil, the plate has a tendency to recover the bend.
This exerts a exerts a bending moment at the fracture site which results in closure of the far fracture gap.
Thus prebending aims at balancing forces on the near and far cortices as far cortex compression produces stability enough to maintain contact under physiologic loading.
It should be noted that it is not an aim of prebending to produce equal compression.
How Bend Should Be?
A distance between the two innermost screws is the most active portion of bending and therefore a sharp bend is preferable to smooth curve. The aim should be to elevate the bend about 1-2 mm from the bone surface.
Lesser bend would be ineffective in achieving proper far cortex compression.
To fix the prebent plate to the bone, inner screws should be applied first and the outer last;
Sometimes a straight plate is applied to the curved bone. [Relative prebending]
In such a scenario, outer screw holes are inserted first. As the plate conforms to the bone, it has the effect of shortening the plate resulting in compression across the fracture site
Plate Fatigue and Failure
Bone Plate is a load-bearing device. That means when it is used to fix the fracture, it bears the load transmission. This is, in contrast, to load sharing devices like interlock nails which share the load with bone and do not completely bear it.
In cases where the fracture well reduced and not comminuted, the transfer of load can be shared by the plated bone as they act as a unit.
But if there is a defect or gap at the fracture site the plate will be prone to bending. This usually happens because the fractured bone is not accurately re-assembled. In some cases of limb loading, the plate may bend backward and forwards as the incomplete bone-plate construct is loaded. This backward and forwards cyclical movement is likely to result in early fatigue failure of the plate.
In order to minimize this problem a number of factors should be considered when designing a plate:
- The system of fixation with plates and screw should, together with the bone, from as stable a construct as possible.
- As little damage as possible should be done to the blood supply to the bone, either by the surgical technique or by the surgical technique or by the implant. So that healing of the fracture complex takes place as quickly as possible.
- The plate should be placed relative to the broken bone so that it is minimally stressed.
- The plate should be placed relative to the soft tissues so that uninjured tissues are not unduly damaged. Ensuring as little damage as possible to the remaining blood supply to the bone.
- The plate should be made of materials which are as strong as possible and which can tolerate fatigue effects of stress reversals.
Bone Plates Material
The most commonly used bone plates are made of stainless steel which is strong, inexpensive and easy to machine in the manufacturing process. However, stainless steel plates do not tolerate stress reversals very well.
Titanium is material which is more biocompatible and allows for bending and shaping plates during surgery.
But titanium is expensive and difficult to machine. Titanium is biologically more inert less likely to cause allergies than stainless steel, which contains nickel. It is quite likely that this material will become more commonly used in the future.
It is important to remember that plates and screws must be of the same material otherwise corrosion of the implants is likely to occur.
You cannot use the plate of steel and screws of titanium or vice versa.
Different Types of Orthopedic Bone Plates
Dynamic Compression Plate
In the first designs of the bone plate, the screw holes were round but with dynamic compression plate or DCP, there was an improvement in the design. This plate has specially shaped screw holes which have an inclined plane at one end.
When the screw is placed at this end of the hole and driven home, it slides down this plane, pulling the plate and causing compression of the bone fragments.
If the screw is placed in the center of such a screw hole in a plate it does not make contact with the inclined plane and movement of the plate relative to the inserted screw does not occur – in this situation the screw in said to be placed in a neutral position.
Each screw hole in a plate is situated in a recess which is shaped to accommodate the rounded undersurface of the screw head. This has three benefits:
- By burying the screw head in the plate the whole construct has a smoother profile which does not irritate soft tissues.
- The contact between plate and screw heads is optimized preventing stress raisers.
- The screw may be slightly angled relative to the plate which may help in achieving optimal screw placement in the bone.
Limited Contact Dynamic Compression plates
Experimental work proved that the under-surface of the DCP, by contact with bone interfered with the blood supply of the underlying cortex.
The area of undersurface of the plate in contact with bone is called the footprint of the plate.
Limited contact dynamic compression plate or LCDCP was an attempt to reduce the footprint of the bone.
The LCDCP has a fluted under surface and minimizes the plate bone contact.
Locking Compression Plates
Locking compression plate is called so because it has got screws with threaded heads which lock themselves when inserted to threaded plate holes, thus locking the screw with the plate.
The plate also has the conventional holes and can be used as conventional plate system.
From the middle of the plate, the distal hole is a simple and proximal hole of combi hole is threaded.
The threaded or locking hole adjoins the conventional hole and arrangement is called a combi hole. Locking compression plate has a combi hole which permits the insertion of standard head screws and of threaded locking head screws.
The LCP is also designed with a minimal footprint.
Locking plate application provides locked plate/screw constructs compared to non-locked plate/screw constructs which are stronger especially to angular forces. It is an especially useful feature in comminuted metaphyseal fractures
The locking plate is used in the following situations
- In indirect fracture reduction
- Fractures of osteoporotic bone
- For bridging in severely comminuted fractures
- Short segment fixation
- Percutaneous fixation of fractures
The advantages of locked fixation are
- Sparing of periosteal blood supply
- Lack of need for contouring
- Improved holding power
These plates have notched edges to permit bending in any desired plane. These plates are very adaptable and are useful in complex anatomical sites, such as the distal humerus, the pelvis, the clavicle. An application of the reconstruction plate is shown in the beginning of the article.
Anatomical plates/angular Locked Plates
The plate given in above diagram is an anatomical plate for distal tibial fractures. These are specially contoured plates for different anatomical sites. For example distal humerus plate, distal tibial plate or proximal tibial plate.
These plates are typically used for intra or juxtarticular fractures because the bone anatomy is unique near the joints.
Screws Used for Plating
Screws used for plating are the same as those used for lagging and maybe cortical or cancellous depending on the bone type under the plate. Some situations demand a mixture of cortical and cancellous screws.
Bone Plating Instrumentation
Drill guides have three purposes:
- They protect the soft tissues from the rotating drill.
- They prevent the drill scratching the plate. Scratches act as stress raisers and may result in plate fatigue failure due to the concentrated load. Scratched plates should be discarded.
- They ensure accurate placement of drill holes relative to the plate holes.
The shape of the end of the drill guide should correspond to the shape of the plate holes. Oval ended guides fit the holes in the DCP plates. There are two types of guide for these plates and they are usually joined together at either end of a common handle.
At one end of the handle is a drill guide with an eccentrically placed guide hole – the compression guide. It is usually color coded yellow and has an arrow on it so that, if pointed towards the fracture, the drill hole will be placed eccentrically so as to compress the fracture when the screw is driven home.
At the other end is a guide with a centrally placed hole which will guide the drill to produce a neutral screw hole affording no compression. The neutral drill guide is color coded green.
Round drill guides may be used with oval holed plates when a lag screw is being placed through the plate or when the screw is being put in at an angle relative to the plate.
It is sometimes necessary to hold a plate against the bone during a fixation. Holders should be placed carefully so as not to scratch the plates.
Bone plates need to be bent in two situations
- To produce a concavity during compression plating to ensure compression of the opposite cortex – pre-bending.
- To contour bone plates to fit the anatomical features of the bone.
Bone plates should only be bent in one direction and not over bent and then corrected. Such a maneuver is an example of a stress reversal which will damage the crystalline structure of the metal, weaken the plate and reduce its fatigue life.