Comminuted fractures can be especially challenging due to the complexity of the fracture fragments and concomitant soft tissue injury. Careful consideration should be given to decision-making prior to onset of fracture repair. Factors that should be considered include mechanical, biological and postoperative compliance. Complex fractures that are treated with a mechanically sound repair often leave the surgeon pondering what could have possibly gone wrong when a “perfect” repair fails. Often times, the answer lies in the neglect of the biological or postoperative compliance factors. Neurologic function should always be assessed because complex fractures are often associated with high-energy trauma that also can injure the brachial plexus or peripheral nerves of the forelimb. This lecture will focus on presentation of clinical cases involving complex fractures of the forelimb and hindlimb, with an emphasis on the decision-making process. A variety of fracture repair techniques will be discussed including interlocking nails, plate-rod construct and linear external fixators.

Minimally-invasive surgical approaches reduce pain and minimize trauma to the soft tissues. Biological factors important for fracture healing are preserved, enhancing the body’s ability for indirect bone healing. The technique can be used with all fracture types, but is particularly useful for stabilization of comminuted fractures. This type of bone healing is also referred to as secondary bone healing, spontaneous bone healing and callus healing. Stabilization of fractures using the principles of biologic fracture management is performed with the same type of implant systems used with traditional fracture repair, including externally and internally applied devices.

Fracture Management

Comminuted fractures of the extremities can be challenging. It is always a race between a fracture healing and an implant failing. Steps can be taken to tip the scale in the direction of early fracture healing. These steps include:

1.  Minimally invasive surgical approach

2.  Preservation of soft tissue attachments to bone fragments

3.  Use of cancellous bone grafts

4.  Rigid method of fracture stabilization

5.  Early return to function

It is always important to obtain an accurate history prior to stabilizing fractures. A complete physical exam and appropriate diagnostic tests should be performed.

Pathologic fractures are more likely to be seen in the geriatric dog and cat and should be identified preoperatively to ensure proper client education and communication

Indirect Bone Healing

Biological fracture management utilizes indirect fracture reduction to preserve the soft tissue envelope at the expense of anatomic reduction. Indirect bone healing occurs as a result. Indirect bone healing consists of three elements: first the formation of granulation tissue at the fracture site, second is fracture gap widening due to resorption of the bone ends and third is the new bone formation involving generation of a bone callus. Less disruption of the vascular supply to bone fragments is achieved through minimal handling of the fragments, promoting early callus formation.^2,3,6,7 Indirect bone healing is first associated with the formation of fibrous connective tissue and cartilage callus between the fragments.⁴ Indirect bone healing occurs due to instability at the fracture site and is partially regulated by fragment gap strain.⁴ Interfragmentary strain is a ratio of change in the gap width to the total width prior to physiological loading.^1,5 A study of the “interfragmentary strain hypothesis” using ovine osteotomy models demonstrated that the initial stages of indirect bone healing occur earlier and more extensively between gaps with lower shear strain.¹ Management of a non-reducible diaphyseal fracture with an implant system that does not utilize anatomical reconstruction and creation of subsequent small fracture gaps avoids high interfragmentary strain, favouring bone healing.

Implant Systems

External and internal implant systems can be used to achieve bone healing using biological fracture management. Examples of external devices when used in an appropriate manner include casts, splints, linear external fixators and circular fixators. Internal devices commonly used for this application include the plate-rod system, interlocking nail and bone plates. Other implant systems can also be used for biologic fracture management as long as the soft tissue envelope is preserved at the fracture site. Whatever implant system is used, its application must be possible with minimal or no handling of the comminuted fracture fragments.

External Fixator

External fixators provide rigid stabilization and can be used with minimally-invasive technique. Many fractures of the radius and tibia can be reduced closed and stabilized with an external fixator. The main disadvantage is the potential for complications with premature pin loosening and the added care needed in the postoperative period. The use of external fixators for fracture repair is not optimal if the patient or owner is likely to have poor compliance in the postoperative period. External fixator frames can be applied in one of 3 configurations—linear, circular or as a hybrid of linear and circular.

Plate-Rod Construct

The plate rod system has been found to be an ideal implant system for biological fracture management. Management of a non-reducible diaphyseal fracture with a combination of an IM Steinmann pin and bone plate can be applied without anatomical reconstruction and thus, avoids the development of small fracture gaps with high interfragmentary strain. The addition of the IM pin to the plate also significantly increases the construct stiffness and estimated number of cycles to fatigue failure when compared to a plate only construct. An IM pin serves to replace any trans-cortical defect in the bone column and acts in concert with the eccentrically positioned plate to resist bending.² Mathematical analysis of the plate-rod construct in the canine femur demonstrated that the pin and plate act most like a dual-beam structure, assuming slight motion of the pin in the canal.² Addition of an IM pin to a bone plate has been shown by Hulse et al. to decrease strain on the plate two-fold and subsequently increase the fatigue life of the plate-rod construct ten-fold compared to that of the plate alone.¹ In the canine femur, plate strain is reduced by approximately 19%, 44%, and 61% with the addition of an IM pin occupying 30%, 40% and 50% of the marrow cavity, respectively.³ Stiffness of plate-rod repairs may be as much as 40% and 78% greater when the pin occupies 40% and 50% of the marrow cavity, respectively.² Ideal diameter of the IM pin should be between 30 and 40% of the medullary canal diameter measured at the isthmus. Increasing diameter up to 50 dramatically challenge the ability to insert screws trough the plate holes.

Locking Plates

Locking plates have become very popular for minimally-invasive fracture repair. Many locking plate systems are available including the Synthes, FIXIN, SOP and ALPS. Locking plates have the ability to lock the screw into the hole of the plate. The mechanism for locking varies amongst manufactures. The Italian design FIXIN locking plate system has a conical locking mechanism while the Synthes system has a threaded locking mechanism. The FIXIN plate hole is tapered to match the conical nature of the head of the screw. This type of fitting is similar to the Morse taper of the head and neck fitting of the total hip replacement implant. The stability of this design is extremely secure. The Synthes locking plate has threaded holes in the hole of the plate. Corresponding threads in the head of the screw engage the threads of the hole, locking the screw to the plate. The ability to lock the screw to the plate increases pull-out strength of the screw and construct stability. Traditional plates do not have threaded holes. Screws placed in ordinary plates apply pressure to the plate, pressing it onto the bone surface. The friction between the plate and the bone provides the stability to the bone-implant construct. In contrast, the locking plate achieves stability through the concept of a fixed-angle construct. The locking plate is not pressed firmly against the bone as the screws are tightened. The locking screws and plate function more like an external fixator. Locking plates are essential “internal fixators.” The plate functions as a connecting bar and the screw functions as a threaded fixator pin. The tapered or threaded head of the locking screw engages the hole of the plate, similar to the clamp of an external fixator. The Synthes locking plate also has combi-holes which allow use of traditional or locking screws when desired. Traditional screws should be place prior to locking screw when using locking plates.

Locking plates are ideal for minimally-invasive fracture repair for several reasons. Blood supply to the bone is preserved because the plate is not pressed tightly against the bone. The plate does not require perfect anatomic contouring because the displacement of the plate will not occur as the screw is tightened into the hole of the plate. Accurate contouring is difficult with a minimally-invasive approach due to the minimal exposure to the shaft of the bone. Lastly, locking screws give fixed angle support to the non-reduced fracture, increasing stability and less chance of collapse and instability at the fracture gap.

Interlocking Nail

The Deuland interlocking nail system presently available in the U.S. (Innovative Animal Products, Inc., Rochester, MN) is a modified Steinmann pin modified by drilling one or two holes proximally and distally in the pin, which allows the placement of transverse bolts or screws through the bone and nail. The nail, bolts and screws can be applied in closed or open fashion due to the incorporation of a specific guide system that attaches to the nail. The equipment needed to place the nail includes a hand chuck, extension device, aiming device, drill sleeve, drill guide, tap guide, drill bit, tap, depth gauge, and screwdriver. Cost of the system is reasonable and each nail is approximately half the cost of a comparative bone plate. The nails are available in diameters of 4.0, 4.7, 6, 8 and 10 mm and varying lengths and hole configurations. The 4.0 and 4.7 mm nails use 2.0 mm screws or bolts. The 6 mm nail is available in two models and will accommodate either 2.7 or 3.5 mm screws or bolts. The 8 mm nail is also available in two models and will accommodate either 3.5 or 4.5 mm screws or bolts. The 10 mm nail uses 4.5 mm screws or bolts. The solid cross locking bolts have a larger diameter compared to a similar diameter screw, thus are less likely to break. Bolts also provide superior mechanical behavior compared to screws.

The interlocking nail is placed along the mechanical axis of the bone. The interlocking nail neutralizes bending, rotational and axial compressive forces due to incorporation of transfixation bolts or screws which pass through the pin and lock into the bone. This is in contrast to a single intramedullary Steinmann pin which is only effective in neutralization of bending forces. The interlocking nail has a similar bending strength compared to bone plates, but is slightly weaker in neutralization of torsional forces. The screws also prevent pin migration, a common complication seen with Steinmann pins.

When using an interlocking nail, the largest diameter nail should be selected that can be accommodated by the medullary cavity at the fracture site. In most large dogs, an 8 mm nail and either 3.5 or 4.5 mm screws or bolts can be used in the femur and humerus. In medium-sized dogs, the 6 mm nail and either 2.7 or 3.5 mm screws or bolts are typically used. In small dogs and cats, the 4.7 mm nail and 2.0 mm screws are typically used. The tibia of medium and large-sized dogs will usually accommodate a 6 mm nail, but some large dogs will accept an 8 mm nail. Small dogs and some cats will accept a 4.0 mm nail for repair of tibial fractures. Dejardin et al. have developed a novel interlocking nail that provides an angle stable locking mechanism.

The advantage of angle stable locking is the elimination of torsional and bending slack, resulting in reduced interfragmentary motion. This interlocking nail system provided comparable mechanical performance to a plate system. Dejardin’s nail is currently available.

Surgical Approach

Closed reduction and stabilization is the optimal method of treatment when possible. Unfortunately, this method is rarely possible in the senior patient due to the severity of fractures seen, long time until bony union, and the tendency for patients to develop bandage sores. Open surgical approaches can be either traditional or minimally invasive. The minimally invasive approach has also been described as an “open but don’t touch” approach. The acronym, OBDT, is used to describe this technique. The advantages to using an OBDT technique is preservation of vascular supply to the fracture site and thus quicker healing, shorter intraoperative time, less postoperative pain and early return to function. Methods of stabilization that work well with an OBDT approach include the interlocking nail, plate-rod hybrid and external fixation.

The key feature of a minimally-invasive approach is the preservation of the soft tissue envelope at the fracture site. Small comminuted fragments will become quickly incorporated into the bony callus if left with a vascular pedicle. Anatomic reduction of small fragments is difficult if vascular supply to the fragment is to remain uncompromised.

Bone Grafts

Numerous sites for harvest of cancellous bone graft have been described in the dog, but the most practical are the greater tubercle of the humerus, wing of the ilium and the medial, proximal tibia. The humerus provides the greatest amount of cancellous bone, but the ilium and tibia provide sufficient amounts for most applications. All of these sites are readily accessible, have easily recognizable landmarks, have little soft tissue covering, and provide relatively large amounts of cancellous bone. The greater trochanter can also be used if other sites are not available; however, the yield of cancellous bone is markedly less. Occasionally multiple sites are required to harvest sufficient quantities of bone to fill large bone defects or during arthrodesis.

Minimal instrumentation is required for harvest of cancellous bone graft. Basic surgical instruments are used to approach the site selected for harvest. A hole is drilled through the near cortex using either a drill bit, trephine or trocar-pointed pin. A curette is used to scoop the graft out of the metaphyseal cancellous bone. The cancellous bone should be scooped out in large clumps if possible. Use a curette that can be comfortably manipulated in the medullary cavity; I prefer to use a relatively large curette as this speeds harvest and reduces trauma to the graft. Closure is performed routinely in 2–3 layers. Recently, a technique was described using an acetabular reamer to harvest large amounts of cortico-cancellous bone graft from the lateral surface of the wing of the ilium.

The graft collected should be handled gently. It is desirable to collect the graft immediately prior to usage. This increases the osteogenic properties of the graft.

As graft is harvested, it should be placed on a blood-soaked gauze until transfer to the recipient site. Extreme care should be taken to store the graft properly; do not accidentally discard the graft due to misidentification of the gauze as being used. The graft should be atraumatically packed into the recipient site. Lavage of the site should be avoided after the graft is placed.

Acknowledgements to Dr. B. Beale.

References

1.  Cheal EJ, Mansmann KA, Digioia III AM, Hayes WC, Perren SM. Role of interfragmentary strain in fracture healing: ovine model of a healing osteotomy. J Orthop Res 1991;9:131–142.

2.  Hulse D, Hyman W, Nori M, Slater M. Reduction in plate strain by addition of an intramedullary pin. Vet Surg 1997;26:451–459.

3.  Hulse D, Ferry K, Fawcett A, Gentry D, Hyman W, Geller S, Slater M. Effect of intramedullary pin size on reducing bone plate strain. Vet Comp Orthop Traumatol 2000;13:185–90.

4.  Johnson AL, Egger EL Eurell JC, Losonsky JM. Biomechanics and biology of fracture healing with external skeletal fixation. Compend Contin Educ Prac Vet 1998;20(4):487–502.

5.  Johnson AL, Seitz SE, Smith CW, Johnson JM, Schaeffer DJ. Closed reduction and type-II external fixation of comminuted fractures of the radius and tibia in dogs: 23 cases (1990–1994). JAVMA 1996;209(8):1445–1448.

6.  Palmer, RH. Biological Osteosynthesis. Veterinary Clinics of North America: Small Animal Practice 1999;29(5):1171–1185.

7.  Palmer, RH. Fracture-patient assessment score (FPAS): a new decision-making tool for orthopedists and teachers. 6th Annual American College of Veterinary Surgeons Symposium, San Francisco, 1996:155–157.