Orthopedic hardware and equipment for the beginner. Part 3: External skeletal fixators

External skeletal fixation (ESF) as a means of fracture stabilization is defined as the placement of pins transversely into bone which are then held in place by extracutaneous fixation (1).

The first development of ESF principles goes back over 170 years to the work of French physician Jean-François Malgaigne. Practical application of the technique in fracture patients increased around the turn of the 20th century and peaked in the treatment of casualties during World War II. Complication rates were high, and infection, pin loosening, and non-unions caused ESF to begin falling out of favor around 1950. Before then, however, there were 2 developments that had lasting implications for the use of the technique in veterinary orthopedic surgery. First, Otto Stader described the use of a full-pin fixator in the veterinary literature. Then in 1947, Dr. E.A. Ehmer and the Kirschner Manufacturing Company introduced an external fixation device, known as the Kirschner-Ehmer Splint, variations of which were used in veterinary orthopedics for more than 35 years (1–5).

External skeletal fixation systems consist of 3 primary components:

1. Fixator pins that are inserted transversely into the bone.

2. An extracutaneous “connecting bar” to span the fixator pins.

3. Clamps or some other means to firmly connect the pins to the bar.

In its earliest conception, the fixator pins were variations of smooth intramedullary pins. Connecting bars were either larger pins or other stainless steel rods, and clamps were fashioned to connect the two. Smooth pins loosened rapidly, connecting bars were not strong enough to bear up under weight-bearing, and the clamps were prone to failure.

In the late 1970’s and early 1980’s a rediscovery of ESF techniques and improvement in the components began and lead to a revival of the technique, which continues to this day. Research into the causes of ESF failure and resulting dramatic improvements in hardware, specifically by the IMEX (Longview, Texas) and Securos (Fiskdale, Maryland) companies have drastically improved clinical outcomes.

External skeletal fixation pins may be the most critical component in the system since the bone-pin interface is the limiting factor in success or failure. Arguably, the most significant development in ESF technology has been the introduction of positive profile threaded pins. When smooth pins were used it was noted that premature pin loosening was a chronic problem and was the most common reason for fixation failure. The initial answer to this was the cutting of threads into the end of the fixation pins so that the threads would provide increased holding power in cortical bone. While this positive effect was realized, it came at the cost of a weakened pin that was prone to breakage at the thread-shank junction. The solution was to manufacture a pin with the threads machined onto the shank of the pin. In other words, the threads had a larger diameter than the remainder of the pin. Initial testing showed that while the positive profile pins were resistant to breakage, they did not seem to have any greater holding power than negative-threaded pin (4). Subsequent testing and alterations in pin insertion techniques have confirmed that positive profile threaded pins do, in fact, provide superior holding power and greater strength. Their availability and economical cost means that there is no longer any justification for the use of smooth or negative-threaded pins in ESF constructs.

Several types of ESF pins have been developed in a number of different sizes. These include end and centrally threaded pins with the latter intended for so-called “full pin” usage where the pin is drilled completely through the limb and connecting bars are attached on both sides. Very small fixation pins are also available for use in small patients, birds, and exotics.

While, at first glance, the connecting bar would seem to be the most “low tech” part of the ESF system, several advances in design have been made. In the original Kirschner-Ehmer system most of the elasticity in the construct arose from the stainless steel connecting bar. The development of titanium, and especially aluminum and carbon fiber rods have dramatically improved the stiffness of ESF constructs, so much so that many cases that required 2 or more stainless steel bars now achieve superior rigidity with a single bar (6).

External skeletal fixation clamps come in slightly different styles, depending on the manufacturer, and have added significantly more security to the constructs than was the case with the old “K-E” clamps. Not only are the newer clamp designs stronger but they are more convenient and versatile in that they can accommodate different sizes and types of pins and can be added to a connecting bar “in situ” without disassembly of other clamps on the bar as was previously required. Most surgeons reuse ESF clamps as a means of decreasing cost inputs. While there is evidence to suggest that repeated tightening and loosening of the clamp does lessen holding power with time, this does not appear to be a clinically significant problem in most cases (7).

Polymethylmethacrylate (PMMA) or epoxy putty has been used to fashion a connecting bar between ESF pin ends. This is a simple and economical alternative as it eliminates the need for connecting bars and clamps. However, this technique decreases the surgeon’s versatility in adapting the fixator construct as addition or removal of single pins is much more difficult (8).

In addition to the major improvements in ESF hardware, the other reason for a resurgence in the popularity of ESF as a modality for fracture repair is a better understanding of the best indications for its use and of the key principles that increase the chance of success.

While ESF techniques have been utilized in almost every bone they are best applied in the distal limb, specifically the tibia and radius/ulna (3,5). Use in the humerus and femur is hampered because the body wall restricts placement of connecting bars, but more importantly, it is difficult to place ESF pins in the humerus or femur without impinging on the major muscle mass surrounding these bones. Safe pin corridors that avoid much of this soft tissue are easier to come by in the tibia and radius/ulna. Impinging on muscle with fixator pins produces morbidity by irritating these tissues and by increasing pin tract drainage. These factors lead to pin loosening and can result in failure of the fixator construct (3,5,8).

In addition to general fracture repair, ESF has lent itself to a number of other more specialized orthopedic applications. The technique is ideal in many severely comminuted fractures where “biologic” fracture repair techniques are preferred. The term “biologic fracture repair” was coined to describe minimally invasive surgical techniques in which the fracture site is minimally disturbed or not opened at all (9), thus preserving blood supply and reducing the chance of introducing infectious organisms.

The ability to place ESF pins in virtually any plane makes the technique ideal for use in the correction of angular limb deformities, or to span joints in cases where ligamentous repairs need to be protected or surgical arthrodesis must be supported (8). Another important aspect of the technique’s versatility is that it can be used in concert with other hardware systems, most commonly with intramedullary pins. The addition of an ESF can prevent fragment rotation that occurs in transverse fractures when repaired with an intramedullary pin alone. In many applications, if the end of the pin is left exposed it can be “tied into” the rest of the ESF frame to provide added stability.

The bone/ESF pin interface is the weak point of any ESF construct and the most common potential source of failure. Vastly superior pins and associated hardware is one facet of strengthening this link in the chain that has developed in recent years. The other part of the equation is proper selection, insertion, and aftercare of the pins and fixator construct.

Selection of the proper pin size is partly a function of the animal’s overall size but also the size of the bone involved. In general, the pin diameter should be no more than 20% to 25% of the bone diameter (8). The importance of positive profile threaded pins has been stated. The number of pins in each fracture fragment is also important. Two pins is a minimum but 3 or 4 is better in avoiding premature pin loosening and increasing rigidity of the ESF construct. Increasing to 5 or more pins in a segment provides little, if any, benefit. Perhaps the most important factor is HOW the pins are inserted in the bone. Direct placement of the pin with a high-speed drill almost invariably leads to thermal necrosis, microfracture of bone, and subsequent premature pin loosening. Low speed predrilling of a pilot hole slightly smaller than the pin thread diameter virtually eliminates these problems and leads to greater pin security for longer periods (8).

Selecting an appropriate ESF frame construct is also critical to successful use of this technique. While an exhaustive description of the different frames is beyond the scope of this discussion and is knowledge acquired through study and experience, suffice it to say that the development of much stronger pins, clamps, and bars has greatly reduced the need for complex frame configurations, multiple bars, or planes of application.

A novel application of ESF techniques is the circular external skeletal fixation system (CESF). Instead of rigid pins, CESF uses small diameter (1.5 to 2.0 mm) tensioned wires and allows controlled axial micromotion at a fracture or osteotomy site. The micromotion is not enough to compromise stability but has been shown to stimulate callus formation (10). This technique is extremely versatile allowing fixation of very small bone fragments with the thin wires, dynamic distraction or compression of bone fragments, and has comparatively low soft tissue morbidity due to the thin wires. However, while CESF shares many common principles with conventional ESF, there is also a significant learning curve that must be overcome to become proficient with this technique.

Aftercare of ESF frames generally involves bandaging of the frame and pins for the first few days to absorb any drainage from pin tracts and to minimize swelling and soft tissue movement. After removal of the bandage, pin tracts must be monitored carefully for any excessive drainage. The normal course of events is the production of small amounts of serous discharge at the pin tract for the first few days followed by the formation of a dry scab. If drainage persists, increases, becomes purulent or serosanguinous, or suddenly develops well into the post-operative period the surgeon must determine the source of the problem, be it infection or soft tissue irritation. Cleaning of the draining pin tract with a cotton-tipped applicator soaked in chlorhexidine or povidone iodine solution is usually indicated. Systemic antibiotics may be required in some cases. Persistent pin tract drainage, especially if accompanied by the acute development of lameness is almost always a signal that one or more pins are loose and must be removed.