One of the primary objectives in the treatment of fractures is early return to full function of the injured limb. Bone plates are ideal for accomplishing this goal because they have the potential to restore rigid stability to the reconstructed fractured bone when properly applied.87-89 Bone plates are adaptable to many
situations, as follows:
1. Most long-bone fractures 2. Multiple and complex fractures
3. Fractures in larger dogs (especially the femur) because postoperative complications are less frequent and postoperative care is reduced when the fixation apparatus is covered with soft tissue
Although many designs and sizes of plates are available, the ASIF (Association for the Study of Internal Fixation, Synthes Ltd., Paoli, Pa) system is used here to illustrate the principles because it is the system with which the authors are most familiar (Figure 2-69). Several manufacturers now produce plates and screws very similar in design and function to the ASIF implants. For optimum results in the use of bone plates, a scientific understanding of the following areas is a prerequisite:
1. Anatomy (e.g., structure of bone; location of blood and nerve supply; muscle separations; attachments of muscles, tendons, and ligaments)
2. Principles of active forces (knowledge of compression, tension, and torsional and bending forces as they affect the bone)
3. Understanding of the mechanics of fixation in detail, and viewing and planning its application in three dimensions
4. Proper selection of a surgical approach and method of internal fixation best suited for the individual fracture
5. Bone-healing patterns (see also previous discussion in this chapter)
It is important to be able to interpret the biological response with rigid fixation, where primary or direct bone union is anticipated. Development of a “cloudy” irritation callus is a warning sign and indicates some movement occurring at the fracture site and the potential for delayed union or nonunion. When two vascular, anatomically reduced bone fragments are rigidly fixed under compression so that no shearing or torsional forces can act on them, no resorption of bone at the fracture line takes place, and a direct bony union occurs without any radio- logically visible periosteal callus.90,91 On the other hand, if a bridging osteosyn-
thesis approach was adopted to stabilize the fracture, considerable periosteal and endosteal bridging callus is anticipated, and its absence would be cause for concern.
Terminology
Plates may be inserted to function as a compression plate, a neutralization plate, bridging plate, or a buttress plate. Such names do not imply anything about the physical characteristics of the plate, but only its function.
Compression (Tension Band) Plate
When the plate is applied so that it is under tension and the fracture fragments are under compression, it is referred to as a compression plate or a tension band plate. Long bones (e.g., the femur) are subject to eccentric loading and may be compared to a bent column. The lateral side is subject to distracting or tension forces; the medial side, to impacting or compressive forces (Figure 2-70, A and B). It is vital that the plate be applied on the side of the bone that is most frequently under a dis- tracting or tension force (Figure 2-70, C). Clinically, these surfaces are the lateral surface of the femur, medial or cranial surface of the tibia, cranial or lateral surface of the humerus, and the craniomedial or cranial surface of the radius.
When a plate is applied to the lateral surface of the femur, it counteracts all ten- sion forces and creates compressive forces along the fracture line, thus providing rigid internal fixation (Figure 2-70, C). If the plate were applied on the medial sur- face, it would not give long-lasting fixation because the plate would be under exces- sive bending stress and subject to fatigue fracture (Figure 2-70, D). It is also critical to long-term stability and prevention of plate failure that the cortex opposite the plate be intact to prevent compression forces on that cortex from becoming bend- ing forces being applied to the plate (Figure 2-70, B). The cortex opposite the plate in this situation acts as a buttress against the compression forces.
Production of tension in the plate was originally accomplished by use of a ten- sioning device (see Figure 2-71, D) that was temporarily applied to the plate and attached to the bone, but the self-compressing plate has totally replaced the tension device in practice. Axial compression is accomplished at the fracture site with these plates, and the dynamic compression plate (introduced by Synthes and discussed later) has been the pattern for these plates (Figures 2-71 and 2-72).92Compression
plates are used on type A stable fractures, osteotomies, and arthrodeses. Neutralization Plate
The neutralization plate is applied on the tension side of the bone to neutralize or overcome torsional, bending, compressive, and distraction forces on fracture lines
2—Fractures: Classification, Diagnosis, and Treatment 127
A B C D
FIGURE 2-70. Principle of a compression plate. Insert the plate only on the tension side of the bone so that the bone will receive compressive forces. Because long bones are subject to eccentric loading, the side of the bone to be under tension must be known to determine where to apply the plate. The femur (A), for example, can be compared with a bent column (B). The plate that is applied to the outer or convex side can then counteract all tension forces (C) and provide rigid internal fixation. If applied on the inner or concave surface, the plate would not provide fixation (D); such a plate would come under excessive bending stresses and would soon show a fatigue fracture.
that have been stabilized by interfragmentary compression supplied by lag screws and cerclage, hemicerclage, or interfragmentary wire (see Figure 2-67, C). If possi- ble, the plate is applied to exert some axial compression. Neutralization plates are used on osteotomies or type B and some type C unstable fractures that can be anatomically reconstructed using lag screws or cerclage wire.
Buttress or Bridging Plate
This nomenclature can be somewhat confusing because the term “bridging plate” was developed to signify a buttress plate used for bridging osteosynthesis of diaphy- seal fractures. The buttress plate functions to shore up a fragment of bone, thereby maintaining length and the proper functional angle in fractures such as those involv- ing the proximal tibial plateau (Figure 2-73, A and B). The bridging plate may be considered a buttress plate used to splint or bridge the fracture area to maintain length of the bone when the fragments are left unreduced or are missing and FIGURE 2-71. Self-compressing plate (dynamic compression plate, DCP). A and B, Sagittal sections of a screw and screw hole in a DCP show the mechanical principle. C, The first screws on either side of the fracture line are inserted eccentrically (load position) and alter- nately tightened to produce compression. D, If there is a wide fracture gap, additional com- pression can be accomplished by the use of a tension device. (From Müller M et al: Manual
2—Fractures: Classification, Diagnosis, and Treatment 129
FIGURE 2-72. Drill guides for insertion of the self-compressing plate (DCP). A, Load guide. The guide is inserted in the screw hole of the plate with the arrow pointing to the frac- ture line. The screw is located eccentrically so that in tightening, it moves 1 mm (it also moves the bone fragment 1 mm). B, Neutral guide. The screw is located slightly eccentrically so that in tightening, it moves 0.1 mm. C, For oblique fractures, a lag screw may be inserted at an angle through the plate.
replaced with cancellous bone graft (Figure 2-73, C). The advantage for use of a buttress plate is that the plate is applied with minimal manipulation of the bone fragments and disruption of remaining blood supply. The disadvantage of this plate function is that the plate must completely absorb all bending, torsional, and compressive forces of weight bearing and is more likely to fatigue and fail compared with compression or neutralization plate constructs. Hulse and colleagues have
FIGURE 2-72. Continued D, E, and F, When multiple fracture lines are present, the first fracture line is compressed as the second screw in the load position is tightened. The second fracture line is compressed as the third screw in the load position is tightened. (From Allgöwer M et al: The dynamic compression plate, New York, 1973, Springer-Verlag, pp 15, 24, 34.)
suggested and described the use of an intramedullary pin, in conjunction with a buttress plate, to reduce stress on the plate used with buttress function.93The details
of intramedullary pin/buttress plate combination fixations are discussed later in this chapter.