Concept of Biologic Osteosy Nthesis
WSAVA 2002 Congress
Don Hulse, DVM, Dip ACVS
Texas A&M University
College Station, Texas, USA

For any fracture patient, the decision as to which stabilization system to apply is made by assessing the mechanical and biologic factors that influence outcome. There are two mechanisms by which a fracture can be stabilized: (1) internal or external fixation and (2) formation of a biobuttress (biological buttress, callus). By assessing the influential aspects of treatment (mechanical, biological), the attending surgeon is able to choose a fixation method that will balance the stability gained through application of a fixation device with the stability gained by the formation of a callus (biobuttress).

Mechanical factors influencing case outcome are those affecting the degree of implant loading and those affecting interfragmentary strain (motion). Implant loading is determined by the intended function of the implant. Will the implant function to share weight bearing loads with the bone column following treatment or will the implant function to carry all the weight bearing loads until a fracture gap is bridged with callus (biobuttress). If the fractured column of bone is anatomically reduced and the interfragmentary fracture lines are stabilized with compression, the bone shares post operative weight bearing loads with the implant(s). Examples would be a compression plate, neutralization plate, interlocking nail/cerclage wire combination, or an intramedullary pin/ cerclage wire combination. When a surgeon chooses this method of fracture management (anatomic reduction), he/she is said to have applied the technique of direct reduction in the management of the fracture. If the fractured column of bone is not anatomically reduced and the fracture area bridged with an implant, the implant must carry all weight bearing until biobuttress is formed. When a surgeon chooses this method of fracture management, he/she is said to have applied the technique of indirect reduction in the management of the fracture.

The advantage of direct reduction is lower stress on the implant system and therefore, fewer complications. However, to apply the technique of direct reduction, a number of criterions must be fulfilled. First, the fracture configuration must be such that anatomic reduction and interfragmentary stabilization are possible. Second, the surgeon must be able to achieve anatomic reduction and stabilization without significant injury to the surrounding soft tissue. If the soft tissues are excessively damaged, the biologic response needed for bone union will be delayed. This 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.

Interfragmentary stabilization is an important criterion with the application of direct or indirect reduction. This is because reduction technique and bone healing relate directly to interfragmentary strain (motion). High interfragmentary strain levels slow or impede bone formation, whereas lower interfragmentary strain levels favor bone formation. The level of interfragmentary strain will vary depending on the length 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. 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.

If the fracture configuration is such that anatomic reconstruction and stabilization of fracture planes of the bone column are not possible, the surgeon should then use the technique of indirect reduction. 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 are plates, plate/IM pin combination, interlocking nails, external skeletal fixators. Since the goal is to achieve rapid callus (biobuttress) 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. Preserving the injured site conserves remaining vasculature, hematoma, and various peptides needed for induction of bone healing. Second, interfragmentary strain is low within the area of comminution. Recall that the level of interfragmentary strain will vary depending on the length of the original fracture gap. Small fracture gaps (single fracture lines) concentrate strain, but longer fracture gaps (comminuted fractures) lower interfragmentary strain by distributing motion over a larger area. 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 therefore, lowers strain within the fragmented zone favoring rapid bone formation.

In summary, choose direct reduction 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. Choose indirect reduction 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. The configuration of the implant construct is based upon the intended function (load vs non-load sharing) and the estimated time the implant must be functional. The latter factor is considered with the surgeon's biologic assessment.

Biologic assessment: Assessment of biologic factors provides the surgeon with an estimate of how rapidly (or slowly) a callus will be formed. This evaluation gives the surgeon an indication of how much she or he can rely on callus formation to provide the stability needed to achieve bone healing. Additionally this assessment gives the surgeon an indication of how long the stabilization system must remain functional, i.e., provide the majority of support. For example, in a case with strong potential for callus formation, the implant may only need to remain functional for 4-6 weeks. Conversely, in a case with poor biologic potential, the implant may need to sustain weight-bearing loads for 10 weeks or more. Improper biologic assessment may lead to implant loosening (IM pins, ESF transfixation pins) or implant breakage (bone plates).

Strength, stiffness, and implant-bone interface are factors that dictate an implants ability to withstand weight-bearing loads over time. The stronger the implant, the less likely it is to break or bend; the stiffer the implant the less stress/strain is placed at the implant-bone interface; a threaded implant (bone screw) resists stress/strain at the implant-bone interface better than a smooth surface implant (IM pin, cerclage wire). Considering all the above, one can realize that in a case where the biologic assessment is determined to be poor, the implant should have maximum strength and stiffness and have a threaded implant-bone interface.

Two biologic factorsof great importance are the age and general health of the patient. Other biologic factors include determination of open vs closed fracture and low-energy or high-energy fracture. If the fracture is an open or high-energy fracture (gunshot), the veterinarian can assume a significant degree of soft tissue injury. In terms of bone union, this simply means prolonged healing and that the implant-bone construct must remain rigid for revascularization with fragile neovascularization. Also, the implant-bone construct must remain rigid for a long period. Conversely, with a closed or low-energy fracture, less soft tissue damage is present, so bone union can proceed more rapidly. Another biologic factor is whether the fracture must have open reduction or if it can be stabilized through closed reduction. If the fracture must be opened, more vascular damage is incurred, which delays bone healing when compared with closed reduction. In addition, if the fracture requires open reduction, operative time becomes important; longer operative times translate into vascular damage and possible postoperative infection. The location of the fracture in terms of the specific bone and site of the fracture within the bone are important biological considerations as well. For example, distal radial and ulnar fractures are a Arecipe@ for nonunion in the toy canine breeds. Even though the fracture may be a load-sharing transverse fracture in a younger patient and caused by a low-velocity impact, the treatment of choice is rigid bone plating. If a fracture occurs in the metaphysis or epiphysis, the cancellous bone dictates more rapid bone union and fixation with less longevity (however, the veterinarian must be careful in making this decision also to consider articular fractures and the comfort of the patient with the implant, in that the fracture is near or involves an articular surface).

In summary, if the biologic assessment is very good or excellent (callus formed rapidly), the implant can be less stiff and the implant-bone interface need not be threaded interface (IM pin, cerclage). Intermediate biologic assessment warrants moderate strength and stiffness and, at a minimum, a combination of a smooth and a threaded interface.

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Donald Hulse, DVM, Dip ACVS
Texas A&M University
College Station, Texas, USA

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