Two or more plates are often used. They should be placed at right angles to each other to optimize the inertial properties of the fixation.
Following contouring of the plate to the bone surface, a 3.2 mm hole is drilled through the bone approximately 1 cm from the fracture surface. The plate is placed over this hole, and the depth gauge is used to determine the length of the screw taking into account the thickness of the plate and the diameter of the bone. The hole is then tapped with the 4.5 mm tap and the correct length 4.5 mm screw is chosen and inserted. This screw is not tightened at this time but only inserted until the screw makes contact with the slotted hole in the plate. The fracture is reduced and stabilized with a bone-holding forceps or other means (lag screw, K-wires, etc.) and the plate is aligned with the long axis of the bone. The plate is slid toward the fracture line, from the side stabilized with the screw, until the screw engages the end of the oval hole. A second hole is drilled through the plate hole nearest the fracture in the other fragment using the yellow load guide in its proper position. the load guide has its hole placed off center, and the arrow on this guide should point toward the fracture line which positions the screw 1.0 mm up the inclined plane of the oval DCP screw hole. The position of this guide is very important since the insertion of the screw in this second hole will, upon screw tightening, move the plate over the bone surface and pull together the fractured ends of the bone in compression. Following drilling of this hole, it is measured, and tapped and the proper length screw is inserted. As this screw starts to engage the oval hole it should be tightened, alternating with the first. In this way both screw heads are drawn down equally. Any screw left with its head high on the inclined plane will be subjected to bending loads and may fail. The remaining holes on both sides of the plate should be drilled using the green neutral drill guide. This guide has the hole centered in the guide and its use will result in a hole that is 0.1 mm up the inclined plane of the oval hole. Therefore the neutral drill guide will still position the screw so as to exert a slight compressive effect. Overuse of the load guide can place all screws on the inclined plane of the DCP hole and expose them to destructive bending forces. Once the fracture fragments are in contact the neutral guide should be used for all other screws. The load guide can theoretically be used three times on each side of the fractures so the total distance that the fractured ends of the bone can be moved is 6.0 mm before the screw heads come to lie at the end of the oval holes. In this case it is necessary to loosen the screws previously placed prior to tightening any subsequent ones, since the tightened screw will make further movement of the bone fragments impossible. Obviously, it is preferable to achieve better reduction before plating, and not to use the definitive implant(s) for this purpose. Once the fixation is complete the screws should be checked for tightness from the center outward, since any change in tightness in one screw may shift the plate slightly and leave other screws loose. This tightening procedure should be repeated several times until all the screws are tight.
Slide the plate toward the fracture.
Reduction should be almost perfect before plate application.
If great force was necessary to bring the fracture fragments together the central two screws may be exchanged for new ones since the heads of these screws may have been weakened by bending during insertion.
2.4.3 Tension device with DCP
Video TDDCP
There are times when it is necessary or desirable to move the fragment ends more than the 6.0 mm allowed by the use of the DCP when used as a self-compressing plate. In these circumstances the tension device should be used.
Following contouring of the plate a 3.2 mm hole is drilled approximately 1 cm from the fracture. The plate is applied over this hole and the measuring device is used to select a screw of the proper length. The hole is tapped and the screw is inserted but not completely tightened. The fracture is reduced and held with a boneholding forceps, and the plate is aligned with the long axis of the bone. The guide for the tension device is placed in the last screw hole and a 3.2 mm hole is drilled in just one cortex. The hole is tapped and a short screw is placed through the tension device after it has been extended and hooked into the last hole in the plate. The tension device is tightened slightly to align the plate, and the first screw that was placed into the plate is tightened. At this time all the screws (at least four) should be added in the fracture fragment containing the first screw. This is accomplished by drilling with the neutral drill guide, measuring, and tapping as previously described. All the screws in this fragment are inserted and tightened. The tension device is now tightened using the socket wrench. A pin wrench is also available and may provide additional load on the fracture fragments. The holes on this side of the fracture are now drilled using the neutral drill guide. They are measured, and tapped and their screws are inserted and tightened. The tension device is then loosened, and removed, and the final screws are inserted into the plate after proper drilling, measuring, and tapping.
Video TDDCP: Animation about tension device DCP.
Use of the tension device was the standard method of plate application when round hole plates were used. These plates are rarely used today but may be applied as described above.
Use the tension device to move the fragment ends more than 6 mm.
2.5 Mechanics of plate fixation
Success with internal fixation using plates and screws comes with good technical ability and an understanding of the mechanics of plate fixation. Plates used for internal fixation are strongest in tension and compression. They are weakest in bending. They are also weak in torsion, but this is a result of the screws that fasten the plate to the bone. Therefore, plates should be applied to bones so that tensile forces are applied and bending forces minimized. To accomplish this, the plates should be applied to the so called “tension side” of the bone. This is the surface that in vivo weight bearing and theoretical studies have shown to be subject to mainly tensile forces. When the plate is applied to compress the bone ends together the plate is already placed under tension. The loads of weight bearing will increase the tension in the plate and therefore the compression in the bone. Since bones have a tension surface they must also have a compression surface. This means that bones bend and the bending force can be converted to a tensile force in the plate if the cortex opposite the plate is intact. If a gap is present due to comminution of bone then an unstable situation may occur. If the comminution is bridged by the plate then it may be stable but if the comminution is at the cortex opposite the plate (trans cortex), then the implant may be subjected to cyclic bending rather than tension, and fatigue failure may result (Fig. F2J). Some situations dictate that the plate be applied in a less than optimal location. This may occur based on soft tissue coverage, vascularity of the skin, and the shape and extent of the fracture or loss of bone stock. Even when implants are applied under the best of conditions, failures may occur. Certain techniques can be used to optimize internal fixations. These are enumerated in the following sections of this chapter and demonstrated on the videotapes corresponding to the individual fractures.
A plate should be contoured to the exact shape of the bone to which it is to be affixed.
Bones bend and the bending force can be converted to a tensile force in the plate.
2.5.1 Contouring and prebending
A plate should be contoured to the exact shape of the bone to which it is to be affixed. The importance and difficulty of this step cannot be overstated. Some of the shortcomings of inadequate plate contouring can be overcome by so-called luting (see below). Bending and twisting the plate may be necessary to make it conform. In general, plates should be bent in one direction and not back and forth since such cycling weakens them. A single bend may actually work-harden the material and does not affect the overall strength of the implant. A bending press, pliers, and bending irons are available to accomplish this task.
Avoid cycling a plate during contouring.
There is a 1.5–2 mm gap beneath a properly prebent plate.