In veterinary orthopedics today we are fortunate to have many methods of fracture fixation at our disposal, but these choices can be daunting. The appropriate decision is personalized for each patient, fracture, and owner combination. When assessing any animal with a fracture, multiple factors must be taken into account when deciding on the best method of fixation. These include mechanical, biologic, and clinical factors that can all influence outcome. By assessing these three influential aspects of treatment, a fixation method that will balance these appropriately for each patient can be determined.

Mechanical factors are those that affect the degree of implant loading and interfragmentary strain (motion). Implant loading is determined by its intended function. An implant that shares weight-bearing loads with a reconstructed bone column is under different loads than one that bridges the fracture and carries the entire load placed across the limb. If the fractured column of bone is anatomically reduced and the interfragmentary fracture lines are stabilized with compression, the bone shares postoperative weight-bearing loads with the implant(s). This is termed direct reduction. Examples include compression and neutralization plates, interlocking nail/cerclage wire combination, or an intramedullary pin/cerclage wire combination. If, on the other hand, the fractured column of bone is not anatomically reduced and the fracture area is bridged with an implant, the implant must carry all weight bearing until callus (biobuttress) is formed. This is called indirect reduction.

Interfragmentary stabilization is an important factor in both direct and indirect reduction. This is because the reduction technique and bone healing are directly related to interfragmentary strain. High interfragmentary strain levels slow or impede bone formation, whereas lower (< 2%) interfragmentary strain levels favor bone formation. The level of interfragmentary strain will vary depending on the size of the original fracture gap. Small fracture gaps (single fracture lines) concentrate strain, but longer fracture gaps (multiple fracture lines) lower interfragmentary strain by distributing the motion over a larger area. As is quickly evident, the method of reduction influences interfragmentary strain.

Advantages of direct reduction and load sharing between the implant and bone include lower stress on the implant system and, therefore, fewer complications. Not all fractures are best treated by direct reduction. Firstly, the fracture configuration must be such that anatomic reduction and interfragmentary stabilization are possible. Highly comminuted fractures do not qualify. Secondly, anatomic reduction and stabilization must be achieved without significant injury to the surrounding soft tissue. Iatrogenic damage to the surrounding soft tissues and therefore, the vascular supply to the bone, results in a delayed biologic response. This, in turn, prolongs bone healing and increases the likelihood of complications. Fracture configurations amendable to anatomic reduction are those with single fracture lines (transverse, oblique) or comminuted fractures having one or two large fragments. These fracture configurations also allow relatively easy interfragmentary stabilization of all fracture planes without significant disruption of the surrounding soft tissue envelope.

Direct reduction creates fracture planes with small gaps between fragments (anatomic reduction). For example, transverse fractures have small gap lengths (when reduced) and, therefore, inherently concentrate motion. Since high interfragmentary strain (motion) impedes bone formation, small gap lengths created with the use of direct reduction must be rigidly stabilized to eliminate strain.

Indirect reduction is applied if the fracture configuration is such that anatomic reconstruction and stabilization of fracture planes of the bone column are not possible. Fracture configurations, where this method of treatment is commonly employed, are highly comminuted diaphyseal fractures. The use of the implant in this situation is referred to as a bridging or buttress implant, since it is crossing an area of bone fragmentation. The implant must, therefore, be strong enough and stiff enough to withstand all weight-bearing loads until sufficient callus is formed. Implant systems useful for bridging osteosynthesis include plates, plate/IM pin combination, interlocking nails, and external skeletal fixators. Since the goal is to achieve rapid callus formation to unload the implant, the surgeon must create an environment where this will occur. There are a number of advantages of indirect reduction that help create an environment conducive to rapid callus formation. First, indirect reduction preserves the biology (soft tissue) because there is no attempt to reduce small fragments of bone in the area of comminution. Carefully preserving the injured site conserves remaining vasculature, hematoma, and inflammatory mediators needed for the induction of bone healing. Second, interfragmentary strain is low within the area of comminution. Spatial realignment of the column of bone (rotation, length, varus-valgus) instead of anatomic reduction does not create small fracture gaps that concentrate motion. Rather, a fracture zone with multiple bone fragments is maintained, which distributes strain (motion) over a larger area. This lowers strain within the fragmented zone favoring rapid bone formation.

In summary, direct reduction is indicated when the fracture configuration allows for anatomic reduction and interfragmentary stabilization. The load sharing between the implant-bone construct is a powerful method to avoid implant failures and accelerate return to function. Indirect reduction is appropriate if the fracture configuration is such that anatomic reduction is not possible or if reduction cannot be accomplished without significant injury to the soft tissue. The implant must be strong and stiff so as to bridge the fracture area until callus is formed. Do all possible to preserve the soft tissue environment and maintain an environment of low interfragmentary strain to enhance callus formation.

Biologic and clinical factors affecting bone healing are numerous. Specifically, an animal's signalment, age, and underlying medical issues must be carefully considered. An immature, healthy animal is expected to heal and form the appropriate callus much more quickly than a geriatric patient with additional endocrinopathies. Not only the animal, but the owner's situation must also be considered. As we all know, owners have different financial and situational limitations that should be considered when choosing a method for fracture repair.