ORDENACIÓN DE SIERO
COMPARATIVA DE HABITANTES POR ENTIDADES COLECTIVAS Ó PARROQUIAS Y AÑOS
The pattern of bone healing varies according to the mechanical conditions present within the fracture line after reduction and stabilization of the fracture. The follow- ing four basic mechanical situations can be observed, and all may be present in one fracture7:
1. Bone immediately adjacent to a compression plate or lag screw may experience very high static (stabilizing) load, with very little dynamic (destabilizing) component.
2. A site farther from a compression plate, or a fracture stabilized with a very stiff external fixator, will experience moderately high compressive static loading with a small dynamic component. This situation could also be present in certain intramedullary pin/cerclage wire fixations.
3. A site slightly farther from a compression plate or screw, fixation with a buttress or bridging plate, or a fracture stabilized with a moderately stable external fixator will experience more even distribution between static and dynamic components. This would also be typical of many intramedullary pin fixations.
4. At the cortex opposite a plate or a unilateral external fixator, in some
buttress or bridging plate situations and in some intramedullary pin fixations, a gap is continuously present because of varying dynamic loads (tension, bending, shear) that continuously exceed the stabilizing compressive loads. In areas of intermittent bone contact, there will be resorption of the fracture surfaces to enlarge the gap, followed by indirect bone union (Figure 2-6). The sequence of events in this case may be briefly stated as (1) hemorrhage in the area, (2) clot formation, and (3) inflammation and edema, followed by (4) proliferation of pluripotential mesenchy- mal cells, (5) cartilage and bone formation, and (6) remodeling of callus back to
normal bone. The sequence of events results in a progressive replacement of the tissue in the fracture gap with stiffer and stronger tissue, going from granulation tis- sue to connective tissue to fibrous tissue to cartilage to mineralized cartilage to lamellar bone to cortical bone. This entire process is under the direction and con- trol of a host of cellularly produced active mediators, such as chemoattractants, as well as angiogenetic and growth factors.8
Callus formation may be subdivided on the basis of location into (1) medullary bridging callus, (2) periosteal bridging callus, or (3) intercortical bridging callus (see Figure 2-6). The pattern of callus formation will vary greatly in response to circumstances and stimuli present. In general, however, stabilization of fractures by external splintage, the external fixator, buttress (bridging) plates, and intra- medullary pins is characterized by the formation of callus in all three areas. Stability of the fracture fragments is not absolute, and micromotion is present. The develop- ing callus is responsible for early stabilization of the fracture and results in relatively early clinical union, that is, the point at which the bone is able to assume normal weight-bearing forces without dependence on the fixation device. In contrast, exces- sive dynamic loading is responsible for delayed union, where the transformation of callus from cartilage to bone is delayed because of the poor blood supply within the areas of excessive motion. Other than in the young growing animal, the amount of callus is in inverse relation to the degree of stability at the fracture site.
Healing in areas of contact and high compression forces, as well as in very small, stable gaps (<0.1 mm), is described as direct bone union (Figure 2-7). This type of union bypasses most of the steps previously described and goes directly to cortical remodeling. Union of the cortices is achieved by internal remodeling of the haversian system without resorption of the fracture surfaces. This intense remodeling at the fracture surface may be radiographically confused with resorption because it results in slight loss of density in the fracture zone. Thus, stabilization of fractures by use of compression plates and screws is characterized by no visible intercortical callus and small amounts of medullary bridging callus. Healing in areas of mixed compressive and dynamic loads can exhibit all three types of healing patterns.
Successful healing in areas of direct contact of the bone fragments or in areas of very small fracture gaps depends on absolute stability, because strain on individual cells filling the fracture gap is magnified by any motion at the site and can easily cause rupture of these cells.9As can be seen from Table 2-2, there is a dramatic dif-
ference in the tolerance to strain by the three major cell types found in the healing fracture gap. Figure 2-8 illustrates the effect of micromotion in a small fracture gap.10 Thus it becomes obvious that if closed reduction and interfragmentary
compression are chosen, it becomes imperative to provide absolutely stable fixation. If this cannot be guaranteed, it is better not to reduce the fragments too closely in order to ensure survival of the tissues in the fracture gap in the presence of the micromotion inevitable in such a mechanical situation. This is the basis of the concept of “bridging osteosynthesis,” as discussed in the following section.
a b
c
b a
FIGURE 2-6. Callus formation in bone healing: a, periosteal bridging callus; b, inter- cortical bridging callus; c, medullary bridging callus.
2—Fractures: Classification, Diagnosis, and Treatment 33
In summary, bone healing depends on and is influenced by blood supply at the fracture line, reduction of the fracture fragments, and the degree of stabilization of the fracture fragments.