Principles and Complications of Orthopedic Hardware

Cortical Plates

Plate and screw technology, although seemingly simple at first glance, is an apt example of the evolution of principles of orthopedic surgery. As our knowledge of physiology, pathology, and material science has progressed, plates and screws have become more sophisticated and individualized for specific tasks.

The first plates were simple cortical plates, often referred to as flat plates. Although plates have variable functions of compression, stabilization, tension, neutralization, buttressing, and bridging, they all provide at least some stability and serve to hold bones in apposition to allow healing to occur.

Cortical plates create stability by being tightly screwed to the surface of the bone, with friction at the bone surface performing the work. The screws achieve this tight application of the plate to the bone.

To accomplish this, the screws for most cortical plates are bicortical, crossing both the near and far cortex.

The simple cortical plate has undergone a remarkable evolution, incorporating designs for specific anatomic sites and designs to satisfy different physical and physiologic principles, to the point that today it is difficult to find a simple flat rectangular plate with round holes. For instance, many plates have been manufactured to fit certain anatomic sites. The one-third tubular plate ( Fig. 106-1 ) is primarily used to fix distal fibular fractures and osteotomies. Its short axis is not flat but an arc, although curiously not one third of a tube or 120 degrees. It is also a relatively thin plate. These features make the one-third tubular plate an ideal design for the distal fibula, a bone that is so small that its cortex is noticeably curved, not flat, and that is nearly subcutaneous in all but the largest persons, requiring a thin plate. In this same family of plates are the smaller one-quarter tubular plate, occasionally used in metacarpals and metatarsals, especially in children, and the larger semitubular plate, which is rarely used today.

Other cortical plates are also contoured for specific sites. This feature is especially common in buttress plates. The term buttress plate is generally applicable to any plate near a joint that flares out or widens as it nears the articular surface. They perform a buttress function, in that they provide a broad surface to support and bolster a comminuted fracture where the cortex is thin. This situation is especially common in fractures involving the distal radius, femur, and tibia and the tibial plateau. Each of these sites has several buttress plates specific to it. Buttress plates are often named by their shape and appearance ( Fig. 106-2 ).

Another specialized plate is the reconstruction plate. Sometimes called malleable plates, these plates are manufactured with notches along the sides, in between each screw hole. When viewed en face, these plates have wavy margins ( Fig. 106-3 ). This notching weakens the plate enough to allow it to be contoured to fit the exact shape of the bone at the time of implantation.

Because these plates require tools to be shaped, they are better termed contoured plates instead of malleable plates, which might imply flimsiness in situ. Reconstruction plate is the preferred term, because these plates are employed to provide significant structural support to fracture sites, most commonly in the pelvis.

Another modification of some plates is to use them to apply compression across a fracture. Appropriate compression of fracture margins augments healing.

It remains uncertain whether this effect is due to the function of keeping the two ends in constant, close apposition, even in the presence of resorption of the fracture margins, or due to actual acceleration of the healing response. Either way, compression techniques allow more rapid healing when used appropriately.

However, several fracture characteristics obviate the use of compression: These include fractures with more than minimal comminution; fractures involving the metaphysis, osteoporotic bone, or otherwise weak cortex; pathologic fractures; and fractures involving an articular surface. Hence, the ideal fracture in which to use compression is a traumatic diaphyseal fracture in a younger adult patient. Fracture compression can be achieved in several ways. First, human anatomy and physiology provide several sites where stabilization of a fracture naturally creates tension on one side of a bone but resultant compression on the other side. The classic example of this is the femur, about which the lateral musculature is stronger than the medial musculature, and placement of a plate on the lateral side converts the tension there to compression on the medial side. This is truly a dynamic compression whenever the muscles are in use. Another way to provide compression is to use axial loading from weight bearing and gravity to compress a fracture that is not fully fixed in the longitudinal plane, such as femoral neck fractures secured by a dynamic compression screw, spine fusions with plates that allow some craniocaudal translation, and long bone fractures stabilized by nails or rods without locking screws (all discussed later in the chapter). Compression also may be applied with special devices at the time of open reduction. A plate then secures the fragments in static compression.

A final method of fracture compression involves a specific design feature of some plates. These plates contain oval screw holes. The holes have sloped margins. If the surgeon places the screw eccentrically in the hole, farthest from the fracture, the rounded head of the screw will contact the slope of the hole, forcing the screw (and the bone it is entering) toward the fracture. When this eccentric screw placement technique is carried out on either side of the fracture, the fracture margins are brought into compression ( Fig. 106-4 ). Despite the fact that this compression is static, these plates are called dynamic compression (DC) plates.

The latest advances in plate designs have been based more on biologic principles. These plates are occasionally grouped under the term biologic plates .

The first significant improvement based on biology was to limit the surface contact of the plate. Cortical plates pressed onto the surface of bones create a zone of devitalization, primarily involving the periosteum. Optimal fracture (or osteotomy) healing requires maximum vascularity, but cortical plates reduce vascularity. The concept that improving vascularity would improve bone healing and reduce infection rates led to the design of cortical plates that only contact bone where they must, adjacent to the screw holes.

Unlike reconstruction plates, these plates are notched or cut out along the deep surface, not the sides. This creates an undulating profile to these plates when one views them from the side ( Fig. 106-5 ). This plate design is termed limited contact or low contact . This feature is often present in DC plates, creating the low-contact dynamic compression (LCDC) plate.

Locked Plates

The most recent advances in plate technology that take biologic principles into account are the locked plates. The primary differing feature of locked plates is that some or all of their screws are locking screws. A locking screw has a threaded head that locks the screw into place in the plate, which has threaded holes ( Fig. 106-6 ). This design feature is helpful in that it makes locking screws into rigid, fixed-angle devices, which will not toggle or back out of the plate. As such, locking plates should markedly reduce the incidence of loss of fixation and plate-screw failure. On radiographs, one can recognize locking screws by the fact that their threads are very shallow, meaning they take a small “bite” into the bone; and the thread pitch, or spacing between each thread, is quite narrow. Standard cortical and cancellous screws have a wider pitch and deeper threads ( Fig. 106-7 ). Plates that use locking screw technology include less invasive stabilization system (LISS) plates, combination plates, and locked condylar plates.

There are two other important features of these plates. First, because locking screws are fixed into the plate, they cannot compress the plate down on the periosteum and cortex. Their main principle of fixation and stabilization, then, is not friction of the plate on the cortex, as with cortical plates, but is along the shanks of the screws. These locking screws act like pins from external fixators, and this whole class of plates is often referred to as internal fixators. The fixation they provide is quite rigid. Second, many of these plates are inserted in a minimally invasive fashion; the surgeon makes a short incision and then attaches a guide to assist in sliding the plate in to its final, submuscular position. The guide is also a template for inserting the screws percutaneously. When the surgeon can choose a minimally invasive insertion, tissue damage is minimized and the local healing environment at the fracture is improved. LISS plates are best able to take advantage of minimally invasive techniques, but almost all locking plates have smooth, tapered, rounded ends to allow minimally invasive insertion, rather than open fixation, when appropriate.

LISS plates are specialized locking plates available only for the distal femur and proximal tibia. They are contoured to fit the shape of the lateral margins of these bones, and they come in three lengths and right and left versions. LISS plates are designed for intraarticular fractures about the knee with significant comminution, especially when there is associated diaphyseal involvement. The rigid fixation and fixed-angle locked screws resist the tendency of these fractures to fall into varus malalignment. These plates are especially beneficial in patients with osteoporosis, in whom fracture fixation about the knee is otherwise fraught with hardware pullout and failure of fixation. One can recognize these plates by three features ( Fig. 106-8 ): They are used only in the distal tibia and proximal femur; the screws are locking screws; and the holes in the plate are all single, circular, locking holes (unlike the combination holes described later). Also, because these plates are inserted in a submuscular fashion and act as internal fixators, there is a gap between most of the plate and the underlying cortex.

Combination (combi) plates have hybrid screw holes. These holes are usually shaped like the number 8. The smaller half of the hole is a locking hole. The larger half is a hole for a regular (cortical or cancellous) screw.

These holes allow the surgeon flexibility regarding when to place locking screws. If locking screws are in place, the surgeon cannot use the screw and plate to manipulate and further reduce a fracture, because the locking mechanism rigidly fixes the bone and plate in place. Similarly, locking screws will not allow the application of “dynamic” compression. Using a combi plate, surgeons may use regular screws to achieve a DC configuration and further reduce the fracture and then place the final locking screws to fix everything in place rigidly.

These plates are used in diaphyseal fractures and are recognized by the “8”-shaped combi holes ( Fig. 106-9 ; also see Fig. 106-7 ).

Locked condylar plates, also called locked periarticular plates, are for fixation of intraarticular and juxta-articular fractures. These plates flare out as they near the articular surface. In this, they resemble buttress cortical plates, but they often provide no significant buttressing function because of their locking screws and limited contact. The periarticular, wider portion of the plate only uses locked screws, with circular holes. In the diaphyseal portion, there are combi holes, allowing the surgeon to choose to place locking or regular screws. These design features allow easy recognition of these plates, which are now available in designs for the ends of most of the long bones ( Fig. 106-10 ).

Locking screw technology has revolutionized the techniques of internal fixation of numerous types of fractures. Although they have not supplanted cortical plates for all uses, the improved healing and reduced complication rates afforded by locking plates have brought them quick and widespread acceptance in the surgical community. Locking screw technology has even been applied to some old standbys, such as the one-third tubular plate.

Fractures

Because of the ability of younger bones to remodel significant displacements and modest angular deformities and because of the rapid healing of children's bones, most pediatric fractures are reduced in a closed fashion and treated with casts, splints, or slings. There are several exceptions to this concept in routine pediatric fracture treatment. First, the weight-bearing long bones will often be treated with one or more elastic intramedullary nails if the child is 5 to 15 years old and the fracture is transverse, oblique, or a simple spiral fracture. Second, plate and screw fixation is reserved for older teenagers and overweight children. Third, although displacement is of lesser concern in diaphyseal fractures, anatomic reduction is critical in epiphyseal and transphyseal fractures, to ensure a functional articular surface and to ward off growth disturbances resulting from growth plate disruption. These fractures are often fixed with Kirschner wires; the classic example is the Salter-Harris IV lateral condyle fracture of the distal humerus. Fourth, markedly unstable lower extremity fractures are often best served by external fixation.