The Dynamic Compression Plate

7C.1 Introduction

The dynamic compression plate (DCP) is a bone plate with a specially shaped screw hole. When a screw is tightened at one end of the hole, the screw head slides down a slope inside the hole. This movement pushes the bone fragment toward the fracture. The fracture is compressed by the plate and its own screws, and no other instrument is needed.

The DCP was introduced in 1969 by Stephan Perren and the Swiss AO group (Perren 1969; Allgöwer 1970). It is an orthopedic implant, but it matters to plastic surgeons for three reasons:

1. Compression plating was the first form of rigid internal fixation used in the mandible. Miniplates, reconstruction plates and locking plates were all developed in response to it.
2. The DCP hole is still present in the plates we use today, including small plates for the hand and the "combination hole" of locking plates.
3. The story of the DCP explains two ideas that we use every day: absolute stability and primary bone healing.

This chapter answers four questions, in this order: What was the problem? What was tried before? How does the DCP work and why was it an innovation? What were its limits and what replaced it?

7C.2 The problem before the DCP

To understand why the DCP was important, we first need to understand what fracture treatment was like before it. There were two separate problems. The first was the result of treating fractures without surgery. The second was the failure of the early plates.

7C.2.1 Fracture disease

Until the 1950s most fractures were treated in plaster or in traction for many weeks. The bone usually united. The limb often did not recover. Long immobilization caused stiff joints, muscle wasting, disuse osteoporosis and chronic swelling. This group of problems was called fracture disease. Malunion and shortening were also common.

The solution seemed simple: fix the bone so firmly that the patient can move the joints on the first day, without a cast. For this the fixation must be strong enough to work alone. The early plates were not.

7C.2.2 Why a plate without compression fails

A plain plate with round holes holds the fragments in line. It does not press them together. Three things then happen.

1. A small gap remains. Even a well reduced fracture has a gap of a fraction of a millimeter. Strain is movement divided by gap width, so a very small movement in a very small gap gives a high strain. Bone cannot form when strain is above about 2%.
2. The bone ends resorb. The body responds to high strain by resorbing the bone ends. This widens the gap and lowers the strain, but now the fragments no longer touch.
3. The plate carries all the load. With no contact between the fragments, every load passes through the plate. The plate bends a little with each movement. After many thousands of cycles it loosens or breaks by fatigue.

Compression solves all three. When the two fragments are pressed together, the gap is closed. Friction between the rough bone surfaces stops them from sliding. The bone carries load again and the plate is protected. This is absolute stability.

The compression applied at surgery is called preload. The fracture stays closed as long as the preload is greater than the forces that try to pull it open during function. This is the same principle as a row of books carried by pressing them together from both ends: the row behaves as one solid block until the pressing force is released.

So by the 1950s the aim was clear. Surgeons needed a plate that could compress the fracture. The remaining question was how to produce that compression in a simple and reliable way.

7C.3 History leading to the DCP

The DCP was the end of an 80-year series of attempts. Each attempt solved one problem and showed the next one. The table lists them in order.

Figures 7C.1 to 7C.6: Uhthoff HK, Poitras P, Backman DS. J Orthop Sci 2006;11:118–126 (open access). Reproduced for non-commercial teaching.

7C.3.1 The scientific basis: primary bone healing

Danis had observed healing without callus on radiographs. The AO group wanted to know what happened in the bone itself. Schenk and Willenegger compressed osteotomies of the dog radius with plates and examined them under the microscope (Schenk and Willenegger 1963). They found that:

• Where the cortices were in contact, osteons grew directly across the osteotomy line. Osteoclasts at the tip of a cutting cone made a tunnel across the line, and osteoblasts behind them filled it with lamellar bone.
• Where a small stable gap remained, the gap first filled with lamellar bone laid down across it. This bone was later remodeled by osteons along the axis of the bone.
• There was no cartilage stage and no external callus.

The same finding was repeated in the rabbit (Rahn 1971). This gave compression plating a biological explanation: under absolute stability, bone heals by the same process that it uses for normal remodeling.

A second question had to be answered before compression could be recommended. Many surgeons believed that pressure on bone caused it to die and resorb. If this were true, compression would be lost within days. Perren fitted plates with strain gauges and measured the compression in living sheep for several months. The compression fell slowly over weeks to months, and there was no resorption of the compressed bone surfaces (Perren 1969b). Bone tolerates static compression. What causes resorption is movement, not pressure.

7C.3.2 The tension device and its drawbacks

By the mid-1960s the AO round-hole plate with the tension device worked well, and the principle was accepted. The remaining problem was practical. The steps were:

1. Fix the plate to one fragment.
2. Hook the tension device into the last hole at the other end of the plate.
3. Fix the tension device to the bone with a separate screw, beyond the end of the plate.
4. Tighten the device with a spanner to pull the plate and compress the fracture.
5. Insert the remaining screws, then remove the device.

This is the exact problem that the DCP solved. The principle of compression was already proven. What was missing was a way to compress the fracture using only the plate and its own screws.

7C.4 The design of the DCP

The tension device pulled the plate from outside. Perren's idea was to put the mechanism inside the screw hole. The German name of the plate describes it well: Spann-Gleitloch-Platte, the "tension gliding-hole plate".

7C.4.1 The spherical gliding principle

The design has two parts that fit each other:

• The screw head is rounded underneath, like half of a ball.
• The hole is oval. One end of the hole has a sloping wall. The other end is a horizontal seat. The shape is the same as a sloping cylinder that meets a horizontal cylinder.

The screw is placed at the high end of the slope, away from the fracture. As the screw is tightened it moves down into the bone, and the rounded head meets the slope. A ball on a slope cannot go straight down. It must also move sideways. The screw is fixed in the bone, so the screw and its bone fragment move sideways together, toward the fracture, while the plate stays still. When the head reaches the horizontal seat the sideways movement stops and the screw tightens fully.

7C.4.2 Load position and neutral position

The surgeon decides what each screw does by choosing where to drill in the hole. Two drill guides are used. They look the same except for the position of the drill channel.

One load screw on each side of the fracture closes a gap of up to about 2 mm. If a gap remains, a third load screw can be added. The first load screw in that fragment is loosened, the new one is tightened, and the first is tightened again.

7C.4.3 Other features of the hole

• The screw can be tilted. Because the head is rounded, the screw can be angled inside the hole: up to about 25° along the plate and 7° across it. A screw can therefore be placed through the plate as a lag screw across an oblique fracture, or directed away from a nerve or a tooth root.
• Every hole is the same. Any hole can be used for compression or in neutral. The surgeon does not have to decide before choosing the plate.
• The holes are directional. In the original DCP the slope of each hole faces the middle of the plate. The holes on the left half compress to the right and the holes on the right half compress to the left. The middle of the plate, which lies over the fracture, has no hole.
• Sizes. The first DCP was the 4.5 mm plate for the femur, tibia and humerus. The same hole was then made smaller: 3.5 mm for the forearm, 2.7 mm and 2.0 mm for the hand, and 2.4 mm and 2.0 mm plates for the mandible.

A note on the name. "Dynamic" refers to the movement of the screw head in the hole while the compression is being produced. Once the screw is tight, the compression is static. It does not increase with function. Some authors have criticized the name for this reason (Uhthoff 2006).

7C.5 Technique of compression plating

The hole produces compression. Whether that compression gives a stable fracture depends on how the plate is applied. The principles come first, then the steps.

7C.5.1 Principles

• The fracture must be simple. Compression needs two fragments that fit together: a transverse or short oblique fracture, or an osteotomy. A comminuted fracture collapses when it is compressed.
• Reduction must be anatomical, and it comes first. The gliding hole closes a gap of 1 to 2 mm. It is not a reduction tool.
• The plate goes on the tension side of the bone. A bone under bending load has one side in tension and one side in compression. A plate on the tension side stops the fracture from opening and turns the bending load into compression at the opposite cortex.
• The plate must be over-bent. This is explained next.

7C.5.2 Over-bending the plate

The plate lies on one surface of the bone. When it is put under tension, it compresses the cortex directly under it. The fracture then hinges on this near cortex and opens at the far cortex. The far cortex is left with a gap and no compression.

The solution is to bend the plate slightly more than the shape of the bone, so that its middle stands 1 to 2 mm off the bone over the fracture. As the screws pull the plate down onto the bone, the plate straightens. It acts as a spring and presses the far cortex closed.

7C.5.3 Steps for a transverse fracture

1. Reduce the fracture and hold it with forceps.
2. Contour the plate to the bone, then over-bend it at the level of the fracture.
3. Insert the first screw next to the fracture in one fragment. Do not tighten it fully.
4. Insert a load screw next to the fracture in the other fragment.
5. Tighten the two screws in turn, a little at a time. The fracture closes and is compressed.
6. Check the far cortex. If a gap remains, add a third load screw.
7. Fill the remaining holes with screws in neutral position.

7C.5.4 Oblique fractures

If an oblique fracture is compressed along the bone, the two fragments slide past each other. To prevent this, the plate is first fixed to the fragment that forms an acute angle with the plate. This angle is called the axilla. The load screw is then placed in the other fragment. The pointed end of this fragment is pushed into the axilla and is trapped there, so it cannot slide. A lag screw placed through the plate across the fracture adds more compression.

7C.6 Why the DCP was an innovation

The DCP did not introduce a new principle. Compression came from Danis, and the oval hole had been drawn by Bagby. The innovation was that the DCP turned a difficult technique into a simple and repeatable one. The points below explain this.

Three further points made the DCP different from earlier inventions.

1. It was tested before it was promoted. The Bagby plate had the same idea but no supporting research. The DCP was published together with measurements of the compression it produced, proof that bone does not resorb under static compression, and histology of the healing it gave (Perren 1969a, 1969b). This combination of engineering, animal experiment and clinical follow-up became the model for implant development.

2. It was part of a system. The plate came with matching screws, drills, taps and guides, with a written technique, with hands-on courses, and with a register of cases. Uhthoff, who was a critic of rigid plating, still wrote that careful surgical technique and an excellent teaching program were a large part of its success (Uhthoff 2006).

3. The clinical results were very good in the right fractures. The best example is the forearm. The radius and ulna must heal with exact length and rotation, and callus between them blocks pronation and supination. This is the ideal indication for absolute stability.

Before compression plating, forearm shaft fractures in adults had a high rate of nonunion and stiffness. A union rate near 98% with early movement was a real change in what patients could expect.

7C.7 Limitations of the DCP

The DCP was used very widely in the 1970s and 1980s. With large numbers of cases, its problems became clear. Most of them come from the same source: to work, the plate must be pressed hard against a bone that has been widely exposed.

7C.7.1 Bone loss under the plate: two explanations

Both explanations are still quoted, and the question is not fully settled.

The AO group followed the vascular explanation. This led directly to the next generation of plates.

7C.8 The plates that followed the DCP

Each later plate kept part of the DCP and changed the part that caused harm. The direction of change was always the same: less contact with the bone, less exposure, and less dependence on friction.

Did limited contact help? The evidence is weaker than the theory. A cadaver study found no clear difference in contact area between the DCP and the LC-DCP on the femur (Field 1997). A randomized study in dogs found no difference in cortical blood flow or in the strength of the healed bone (Jain 1999). Clinical union rates with the DCP and the LC-DCP are similar (Uhthoff 2006). The larger benefit probably came from the change in surgical technique that went with the new plates: less stripping, indirect reduction, and bridging of comminuted zones instead of piece-by-piece reduction.

The change in thinking. In the DCP period, callus after plating was read as a sign of failure. With biological fixation, callus is expected and welcome. The modern position is that both are correct, for different fractures:

The common error is to mix the two: a simple fracture bridged with a gap by a stiff locking plate. The gap is narrow, the strain is high, and the fracture does not unite. A simple fracture should be compressed.

7C.9 Compression plating in the mandible

The history of the DCP in the facial skeleton is short and important. It explains why we fix the mandible the way we do today.

7C.9.1 Luhr and Spiessl

Until the 1960s mandibular fractures were treated with wire osteosynthesis and four to six weeks of intermaxillary fixation. The limb instruments of the AO, including the tension device, were too large for the jaw. In 1967 Hans Luhr in Germany performed the first compression plating of the mandible, using a Vitallium plate with eccentric holes that he developed for this purpose (Luhr 1968). Soon afterward Bernd Spiessl in Basel adapted the AO dynamic compression plate to the mandible (Spiessl 1972). For the first time a patient with a fractured mandible could open the mouth and eat a soft diet immediately after surgery.

7C.9.2 The special problem of the mandible

The plate should be placed on the tension side of a bone. In the mandibular body and angle, the tension side is the upper (alveolar) border and the compression side is the lower border. But a large bicortical compression plate cannot be placed on the alveolar border. The tooth roots and the inferior alveolar nerve are in the way. It has to go on the lower border, which is the wrong side.

A compression plate on the lower border closes the lower border and opens the alveolar border. The result is a gap between the teeth and a changed occlusion. Two solutions were developed:

• A tension band. An arch bar on the teeth, or a small plate at the upper border, holds the tension side closed. The compression plate is then applied at the lower border.
• The eccentric dynamic compression plate (EDCP). The two inner holes compress along the plate as usual. The two outer holes are set obliquely, so that their screws rotate the fragments and press the alveolar border together (Schmoker and Spiessl 1973). It was used where a tension band could not be applied, for example at the angle when the proximal fragment had no teeth.

7C.9.3 Results, and why miniplates replaced compression plates

Compression plating of the mandible worked, but it was demanding.

The malocclusion rate shows the weakness of a rigid compression plate in the face. The plate is stiff and the screws pull the bone to the plate. If the plate is bent with an error of a fraction of a millimeter, the fragments move to fit the plate and the teeth no longer meet. In a long bone this error is not noticed. In the mandible the patient feels it at once.

At the same time Michelet (1973) and Champy (1978) showed that the mandible does not need compression at all. A small monocortical plate on the tension side, along the ideal lines of osteosynthesis, turns the forces of chewing into compression at the lower border. The plate is placed through the mouth, without a skin scar, without risk to the nerve, and with little periosteal stripping. The bone shares the load.

Today compression plates are rarely used in facial fractures. Simple fractures are treated with miniplates. Comminuted fractures, defects and atrophic mandibles are treated with a load-bearing locking reconstruction plate. Compression is still used in the face in one form: the lag screw, for oblique fractures of the symphysis and body and for fixing onlay bone grafts.

7C.10 Where the principle is used in plastic and reconstructive surgery today

We seldom apply a large DCP, but we use its hole and its principle often.

In every row the same three conditions decide whether compression is the right choice: two surfaces that fit, bone strong enough to hold screws, and a blood supply that survives the exposure.

7C.11 Summary

• Before the 1950s, fractures were immobilized for weeks and patients were left with stiff, weak limbs. Early plates failed from corrosion and then from lack of strength.
• Danis (1949) showed that a compressed fracture heals without callus. Bagby (1956) drew a self-compressing oval hole. Müller and the AO (1958 to 1965) made compression plating a system, using a separate tension device.
• The DCP (Perren 1969) put the compression mechanism inside the screw hole. A screw placed 1.0 mm off center slides down a slope and moves its fragment toward the fracture.
• It was an innovation because it made compression simple, repeatable and possible in any size of bone, and because it was supported by experimental proof and a teaching system.
• Its limits were wide exposure, pressure on the periosteum, bone loss under the plate and poor results in comminuted fractures. These led to the LC-DCP, the PC-Fix and the locking compression plate.
• In the mandible, compression plates (Luhr 1968, Spiessl 1972) gave the first stable fixation without intermaxillary fixation, but they were replaced by miniplates and reconstruction plates.
• In plastic surgery the principle remains in small plates for the hand, in lag screws, and in the combination hole of modern locking plates.

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