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First Major Improvement in ESF Pin Technology

by IMEX Veterinary

IMEX Duraface ESF half-pinsdemonstrate a 55% average increase in stiffness and a 3.7 fold increase in cyclic fatigue life compared to current positive-profile pins without an increase in cost. These mechanical improvements can prolong the integrity of the pin-bone interface in challenging orthopedic cases.

Increased stiffness, increased ultimate strength and prolonged cyclic fatigue life of these magnitudes were previously thought only possible with positive-profile pins of increased diameter. Unfortunately, pin diameter is often the limiting factor in pin selection when stronger pins are necessary. Before describing this new pin technology in detail, let me describe clinical scenarios where improved pins would be most valuable. 

Comparative chart demonstrating the increased stiffness, increased ultimate strength and prolonged cyclic fatigue life of Duraface ESF pins

FIGURE 1 | Comparative chart demonstrating the increased stiffness, increased ultimate strength and prolonged cyclic fatigue life of Duraface ESF pins

Tapered runout (TRO) feature of Duraface ESF pins

FIGURE 2 | Tapered runout (TRO) feature of Duraface ESF pins

A common and difficult fracture scenario is that of the short fracture fragment where traditional linear fixation techniques are hard to use because the limited amount of bone prevents use of an optimum number of fixation pins. When forced to use less than optimal pin numbers to fully share the load of weight bearing, stronger pins have potential to protect the pin-bone interface by decreasing localized end shear and axial loads at the pin-bone interface and to better withstand cyclic loading over time.

When forced to use less than optimal pin numbers to fully share the load of weight bearing, stronger pins have potential to protect the pin-bone interface by decreasing localized end shear and axial loads at the pin-bone interface and to better withstand cyclic loading over time.

Fractures of the humerus and femur are always considered difficult when repaired with external fixation. One major reason for this is the thick layer of surrounding muscle that requires the fixation clamp and external bar to be placed further from the bone compared to fractures of the radius and tibia. Since pin stiffness is inversely proportional to working length (length of the pin from the bone to the clamp) to the third power, there is a significant reduction of pin stiffness when repairing humeral or femoral fractures. The same issues exist when using ESF for repair of spinal fractures and luxations. Unfortunately, increased pin stiffness can only be achieved by increasing the diameter of the pin when using currently marketed positive-profile pins. If pin diameter increases too much, fracture of the surrounding bone may occur. The increased stiffness of Duraface fixation half-pins minimizes the negative effects of long pin working lengths without a dangerous increase in bone-hole diameters that could result in iatrogenic fracture.

Highly comminuted fractures are typically considered non-load sharing scenarios. In non-load sharing scenarios, the hardware (fixation pins, clamps and connecting bars) are required to carry all the load of weight bearing until fracture callous begins to form. As a result, there is a greater tendency for elastic deformation of the pin and loosening at the pin-bone interface due to repetitive cyclic motion. Often, in the case of highly comminuted fractures, there is also considerable soft tissue damage which decreases blood supply to the fracture site and increases the potential for delay in fracture healing. Duraface ESF half-pins with increased stiffness can reduce end shear and axial loads at the pin-bone interface, prolonging the longevity of the fixator-bone composite until bone healing allows the load of weight bearing to be shared between bone and hardware. Increased fatigue life of Duraface fixation half-pins can provide additional safety during prolonged healing events related to soft tissue and blood supply damage, old age, and systemic diseases.

Even transverse fractures, which intuitively seem stable, can benefit from Duraface fixation half-pins. Transverse fractures are often considered an “easy” fracture repair, but in reality, the single, small fracture line concentrates all movement (strain) at the fracture site (as contrasted to the same amount of strain spread out over many fracture lines in a comminuted fracture). High local strains can lead to delayed union or non-union. Duraface fixation half-pins with their increased stiffness can facilitate strain reduction at the fracture gap and provide added longevity in potentially prolonged healing situations.

Obese patients with small bones represent another challenging fracture repair scenario because the animal’s extra weight calls for a larger pin diameter than the bone can accept without undue risk of fracture. In these scenarios, Duraface fixation half-pins may provide the stiffness needed to support fracture healing. A Duraface fixation half-pin approximates the stiffness of the next larger positive-profile fixation pin allowing the smaller Duraface pin to function well in mechanically challenging scenarios such as the obese patient above or even the overly active patient with a noncompliant owner.

Any orthopedic scenario involving multiple limb fractures or single limb fractures with other limbs suffering either acute or chronic injury are typically much more prone to orthopedic complications including pin loosening or mechanical failure of the fixation device. These complications occur because of the patient’s inability to protect the externally-fixated limb during daily use, leading to high levels of cyclic stress on the fixation device and bone over time. These high peak stresses create a greater likelihood of pin loosening or even pin failure. Duraface half-pins from IMEX offer greater longevity and durability in these challenging cases somewhat analogous to stacked plating or broad plates.

Surgeons performing limb lengthening report that small diameter wires used in circular external fixation frames (CESF) near the elbow and stifle create a significant amount of morbidity due to their irritation of high motion soft tissues near the joint. To avoid this complication, surgeons will create “linearized” CESF frames that use half-pins near the elbow or stifle joint in order to avoid wire placement through mobile soft tissues. When used in this fashion, half-pins are placed under significant load and subject to increased bending or failure of the pin-bone interface. Stiffer pins such as Duraface fixation half-pins support additional loads with less deformation and less potential for failure. 

Veterinary Surgeon Utilziing a Linearized Circular ESF Frame

FIGURE 3 | Linearized Circular ESF Frame

Radiograph of a Modified (simplified) Type-II ESF frame with a broken Centerface ESF pin

FIGURE 4 | Modified (simplified) Type-II ESF frame

Modified (simplified) Type-II ESF frames feature minimum (usually one) full-pins in each major bone fragment with the balance of the fixator pins being half-pins. These popular frames are considered simple to apply since they limit the need for accurate targeting of multiple full-pins. However, not commonly recognized is the imbalance between the stiffness of a full-pin and a half-pin during this mixed pin scenario. While full-pins are commonly recognized as being much stiffer than half-pins, when mixed in the same fracture fragment, the stiffer full-pins tend to carry more than their fair share of the loads and in effect reduce the load sharing by half-pins. This may result in pre-mature loosening or even cyclic failure of the full-pin. Once loosening or failure of the full-pin occurs, the load is shared more evenly among the half-pins but at the cost of reduced functional pin number and lameness associated with the pain of a loose full-pin. This is the most common reason for pin fatigue failure I currently see (note radiograph above) and I generally recommend consideration of a Type I-b frame in these cases. Use of Duraface half-pins in simplified Type-II frames has the potential to produce more uniform loading between the half- and full-pins, which would increase the life of either the pin-bone interface or the longevity of the fixation pin itself.

In reality, most difficult fracture repair situations result from combinations of the factors listed above and would potentially benefit from the improved mechanical properties of the Duraface half-pin. 

Cranial-caudal radiograph of a bilateral SK Type 1b frame

FIGURE 5 A two-year-old Whippet suffered bilateral fractures of distal radius and ulna when jumping off the porch. The surgeon originally considered bilateral hybrid ESF frames for the repair but opted for bilateral SK Type I-b frames using 2.0 mm Duraface ESF half-pins. Right and left limb fractures appeared very similar at the time of injury and exhibited similar appearances during healing. Cranio-caudal views of the left limb are shown.

8 week postop radiograph of a bilateral SK Type 1a frame

FIGURE 6 15-week view of left radius/ulna at time of ESF removal. Both right and left ESF frames were destabilized from Type I-b (Figure 5) to Type I-a frames at 8 weeks post-op.

Historical Evolution of ESF Pins

A brief history of the evolution of veterinary ESF pins is outlined here to provide insight as to how a negatively threaded pin could be an improvement over positive threaded pins that are currently considered state of the art. The first ESF pins used in veterinary medicine were nothing more than smooth IM (Steinmann) pins. Veterinarians quickly realized smooth pins did not provide any “bone holding” power over time and decided to utilize partially threaded or fully threaded Steinmann pins. Unfortunately, partially threaded IM pins were very prone to bending or fatigue failure at the thread/non-thread junction and fully threaded pins were recognized as having inadequate stiffness. The acute junction at the thread/non-thread junction of the partially threaded Steinmann pins was determined to be a stress riser that often failed. Less emphasis was placed on the smaller core diameter which is responsible for pin stiffness under the threads and the apparent lack of stiffness with fully-threaded IM pins. Veterinary surgeons then attempted to use negative threaded pins with a very short thread length in order to place the stress riser in a protected, intramedullary position but this came at the cost of having threads engage only the far cortex.

After trial and error with single cortex, negative threaded pins (Ellis Pins), veterinarians began using human pins not designed for external fixation, but that featured a thread profile coined as “positive thread” meaning that the threads were “added” to the smooth shaft of pins. This design eliminated the stress riser junction of negative threaded IM pin while maintaining core diameter of the pin under the threads. In addition, positive-profile pins provided two-cortex thread engagement. IMEX was founded 20 years ago with the mission of providing positive-profile pins to veterinarians and was the first veterinary company to market positive thread profile pins in 3 sizes compatible with the then popular KE clamp. Since that time, IMEX has increased its ESF pin offering to include a number of new sizes – from 0.035” for avian and exotic patients to 6.3 mm for large animal transfixation casting. IMEX now offers a greater variety of ESF pins at a lower cost than any other veterinary company. 

Illustration of a smooth pin

FIGURE 7 | Smooth Pin – The smooth pin is the historical “starting point” for external fixation pins. This pin has an absence of stress riser but lacks bone holding power.

Illustration of a negative-profile pin

FIGURE 8 | Negative-Profile Pin – This image shows loss of core diameter under the threads and resulting acute junction of the thread/non-thread junction created when threads are cut into the smooth pin shown in figure 7.

Illustration of a positive-profile pin

FIGURE 9 | Positive-Profile Pin – Positive-profile threads are shown in red as being added to the basic smooth pin shown in gray. The constant diameter of the underlying smooth pin eliminates any stress riser effect.

Illustration of a Duraface ESF pin with tapered run out (TRO)

FIGURE 10 | Duraface Pin – The gray and red represent the positive-profile pin while the yellow indicates addition of shaft diameter and tapered run out (TRO) feature to a positive-profile pin.

The Duraface ESF pin represents a powerful step forward in the evolution of veterinary ESF pins. Smooth pins that had metal removed to provide threads were named negative threaded pins. Smooth pins that had threads added were termed positive threaded pins. Duraface pins start with a positive threaded pin and add smooth shaft diameter while at the same time blending the thread/non-thread junction to minimize the acute nature of the stress riser. This shaft diameter addition technically results in a negative thread pin, but does so without a loss in core diameter or core strength. The increased shaft diameter results in decreased deflection under load which minimizes peak stresses and increases fatigue life while providing for additional pin stiffness. This is all accomplished without a change in core or thread diameter. Therefore, drill bit sizes for pre-drilling are unchanged and the portion of the pin that engages bone is of the same size as currently marketed positive-profile pins. No potential increase for iatrogenic fracture occurs.

IMEX pioneered the concept of using half-pins coupled with strong external bars as the optimum method of limiting patient morbidity while providing proper stability for fracture healing. IMEX still maintains that full-pin use should be minimal and believes that unnecessary use of full-pins will often mandate more complex staged disassembly requirements due to excess frame stiffness as contrasted to mechanically adequate half-pin frames in most cases. If you have been hesitant to reduce your use of full-pins, the Duraface half-pin offers a greater margin of safety when doing so. The SK external fixation device was designed from the beginning not only to grip a wide variety of external fixation pin diameters, but also a wide variety of external fixation pin styles – including the Duraface half-pin. The SK external fixation device remains the external fixator depended on by more teaching hospitals and referral surgery practices than any other ESF device.

Duraface half-pins provide third generation external fixation technology at the same price as our Interface fixation pin line. 

* U.S. Patent 8,282,676