Biologic repair of fractures

One of the most significant advances in veterinary orthopedics in the last decade has been the promotion of “biologic” fracture repair strategies. Biologic fracture repair, or bridging osteosynthesis, seeks to disturb the fracture milieu minimally, thus turning the orthopedic surgeon into as much a gardener as a carpenter (1).

Traditional fracture repair dogma has revolved around “anatomic reduction and rigid internal fixation.” This allows the reconstructed bony column to share the load of weight-bearing with the orthopedic implant, so fatigue and failure of the implant become less likely. However, anatomic reduction requires extensive surgical exposure and intraoperative time, thereby potentially compromising the blood supply to the fracture and increasing the risk of postsurgical infection.

The principles of biologic fracture repair provide a new perspective when radiographs are viewed and surgical strategies are being planned. The question becomes, Can I reconstruct the bony column anatomically and fix it rigidly such that it will be able to share the load? If the answer is yes, such as in simple transverse or oblique fractures in healthy patients, a traditional open surgical approach and repair may be the best choice. However, if anatomic reduction and load-sharing by the bony column are not realistic expectations, as in most comminuted diaphyseal fractures, the choice would be to favor the closed or minimally invasive surgical techniques of the biologic approach. Such techniques leave the fracture's blood clot, with all its osteoinductive components, and the blood supply to the surrounding bony and soft tissue, undisturbed. The rapid, uncomplicated healing of a simple fracture in a puppy treated with external coaptation is the template for the principles of biologic repair. The young patient represents the optimum that will be available in blood supply and rapid bone healing. If the limb is aligned and provided with support, these fractures will heal remarkably quickly. While external coaptation may not be appropriate in comminuted fractures of older patients, minimally invasive principles can still be applied.

In fractures of the radius and ulna or the tibia, a closed approach will usually mean the use of external skeletal fixation (ESF). Once the fixator has been applied, the surgeon's concern is the axial alignment of the limb in all planes. Achieving normal length of the bone before tightening the fixator clamps can be facilitated by having the limb in traction from the ceiling throughout the surgical procedure (1).

The comminuted fracture fragments are not of concern and no attempt is made to reduce or stabilize them. Sequestrum formation is rare since this requires a compromised blood supply to the fragment and bacterial contamination. Both are much less likely with a closed approach (1). Adequate ESF stability and rigidity (enough pins and connecting bars of an appropriate type and size) are vital, since, in most cases, the fixator must bear the entire weight-bearing load for the first 6 to 8 wk. At this point, based on radiographic evidence of fracture callus, some pins and/or bars may be removed. This staged disassembly or “dynamization” of the fixator transfers some of the load to the bony column in a controlled manner, which further stimulates healing.

A biologic approach to fractures of the humerus or femur requires some modifications, since ESF alone is seldom applicable, especially in large dogs. In addition, a closed approach may not permit axial alignment of the proximal and distal fracture fragments. A “keyhole” (2 to 3 cm) approach is usually sufficient to allow observation of the major fragments as they are aligned.

In the femur or the humerus, ESF may be used with an intramedullary (IM) pin to facilitate alignment and augment stability. “Normograding” (introducing the pin at the proximal end of the bone, driving it to the fracture site, then into the distal fragment) the pin is preferred, because it minimizes disruption at the fracture site and allows the distal fragment to be pushed out to full length. When the pin has been normograded as far as the fracture site, the keyhole incision will allow visual confirmation that the pin is entering the distal fragment. Once this is accomplished, the chuck holding the pin is advanced with one hand while the distal fragment is not immobilized with the other hand as would normally be done in an IM pin repair. In this way, the distal fragment is pushed away from the end of the pin until the limb more closely reaches its normal length. Once the elastic limit of the soft tissues has been reached, the pin will enter and become seated in the distal fragment. The ESF may then be applied. “Tying-in” the uncut proximal end of the IM pin to the ESF connecting bar adds additional stability to the fracture repair (1).

Other “hardware” options for applying biologic techniques to comminuted fractures of the humerus or femur include interlocking nails and lengthening plates. Interlocking nails function similarly to IM pins, but they are fastened to the proximal and distal fragments by bone screws. Lengthening plates have several screw holes at each end but a solid metal area in the center that spans the area of fracture comminution; this allows the fracture area to be left relatively undisturbed and avoids open screw holes that would compromise the strength of the implant. Another means of attaining similar rigidity in a comminuted humeral or femoral fracture is the use of a conventional bone plate and an IM pin. This “plate-rod” construct reduces the axial strain in the bone plate by at least 50% and virtually eliminates the concentration of stresses at the empty screw holes (2). The IM pin is placed first, in the same manner as previously described for a pin-ESF construct, and then the plate is applied. The disadvantage of the plate-rod technique is that it may be possible to engage only the cortex adjacent to the plate with some of the bone screws, especially in the narrower areas of the diaphysis. To minimize this problem, it is advisable to use a pin that is approximately 50% of the medullary diameter. In addition, it is recommended that at least 3 monocortical screws and 1 bicortical screw are required above and below the fracture (2).

Cancellous bone grafting is another means of being a “good gardener” when a minimally invasive open approach is used (3).

The use of closed or minimally invasive biologic repair techniques are also applicable to older or debilitated patients where the intraoperative time and the blood supply at the fracture site may be concerns.

Ultimately, the orthopedic surgeon must strive to strike a balance between anatomic reduction and minimal disruption of the fracture site. With apologies to William Lyon Mackenzie King, the trend in comminuted fracture repair is “anatomic reduction if necessary, but not necessarily anatomic reduction!”

Figure 3. Postoperative radiographic view of a gunshot fracture of the radius and ulna in a Great Dane cross. A Type II external fixator has been placed without opening the fracture site.

Figure 1. Preoperative radiographic view of a severely comminuted femoral fracture in an adult cat.

Figure 2. Postoperative radiographic view of the femoral fracture displayed in Figure 1 illustrating biologic fracture repair strategies, namely, a normograded intramedullary pin plus an external fixator, placed without surgically opening the fracture site.

Figure 4. A 6-week postoperative radiographic view of the fracture displayed in Figure 3 showing prominent callus formation.