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New Trends in Ring Fixators

Iobst, Christopher A. MD

Journal of Pediatric Orthopaedics: September 2017 - Volume 37 - Issue - p S18–S21
doi: 10.1097/BPO.0000000000001026
New Trends in Ring Fixators

Multiple versions of ring fixators are currently available to orthopaedic surgeons. Although the traditional Ilizarov-type ring fixator is still available, many surgeons are now using computer-assisted hexapod frames. There has been a recent surge in the number of different hexapod ring fixators on the market. This article will review some of the new trends in ring fixator hardware and software as well as discuss possible future directions of ring fixator development.

Nationwide Children’s Hospital, Columbus, OH

No funding was provided for the preparation of this manuscript.

C.I. is an educational consultant for Nuvasive, Orthofix and on the speaker’s bureau for Smith and Nephew.

Reprints: Christopher A. Iobst, MD, Nationwide Children’s Hospital, 700 Children's Drive, Columbus, OH 43205. E-mail: Christopher.iobst@nationwidechildrens.org.

Ring fixator design underwent a transformation in the mid-1990s with the advent of the hexapod frame. These frames used struts to connect the rings instead of threaded rods. The unique design of the struts allowed surgeons to correct multiplanar deformities simultaneously or sequentially without having to make time-consuming changes to the frame construct in the clinic or in the operating room. The hexapod frames were also paired with computer software that calculated deformity correction around a virtual hinge in space. With the expiration of certain design patents in 2015, the orthopaedic market has seen the release of several different types of ring fixators. This article will discuss some of the recent trends in ring fixators and review some of the new developments now available to surgeons.

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PARADIGM SHIFT

The first trend to discuss about ring fixators seems counter-intuitive but there is currently a paradigm shift occurring among deformity surgeons to move away from external fixation. Although external fixation has been and remains the standard for limb lengthening and limb reconstruction, there is no disputing that the devices are not patient friendly. The external fixators are bothersome to wear, commonly develop pin track infections, and require psychological acceptance from the patient and the family. In addition, the half pins and wires connected to the frame transfix the soft tissue envelope, which interferes with the patient’s ability to perform range of motion exercises. With the advent of locking plates, internal lengthening nails, and guided growth plates, surgical plans that correct deformity and avoid or minimize the use of external fixation are preferred. Multiplanar deformity that would require external fixation in the past can now be corrected with precision using fixator-assisted techniques in the operating room.1–3

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HARDWARE

The second trend involves improvements in the ring fixator hardware. Rings are being constructed from new materials, such as braided carbon fiber, which is designed to be stronger, lighter, and more radiolucent than traditional hexapod rings (Fig. 1). Modular rings are available that allow the surgeon to transition from a full ring to a 5/8 ring or vice versa as needed (Fig. 2). Another new ring design has slots instead of discrete holes for placement of the half pin and wire fixation elements (Fig. 3). This potentially increases the fixation options for the surgeon and improves the versatility of the system. Although not necessarily a new trend, rings are available in different colors. This feature allows the patient to become more invested in his or her treatment, which can improve the patient’s disposition during the rehabilitation process. Patients choose ring colors to coordinate with favorite outfits and even to decorate certain holidays. Finally, rings are available with tabs marked anterior and posterior to prevent the surgeon from being disoriented during the surgery and postoperative process.

FIGURE 1

FIGURE 1

FIGURE 2

FIGURE 2

FIGURE 3

FIGURE 3

Struts have also undergone some major design changes. Some ring fixators now have struts that attach to the outside of the rings rather than underneath the rings (Fig. 4). This increases the amount of space available for rotation inside the ring as well as the space available for placement of the fixation elements between the struts and the rings.4 Struts are also available with a combination of acute and gradual correction in one element (Figs. 5A, B). This helps to limit the number of strut changes necessary during a given deformity correction. Having fewer strut changes saves time for the health care provider in clinic and saves cost to the patient. The combination of acute and gradual correction in 1 strut also helps to limit the inventory of struts needed.

FIGURE 4

FIGURE 4

FIGURE 5

FIGURE 5

Struts now have the ability to be adjusted in increments <1 mm. Depending on the type of strut, the adjustments can be as small as ¼ or ½ mm. Struts are also designed with knobs to prevent inadvertent adjustments from occurring when the patient moves his/her limb around in bed. Some struts have a ball and socket design that allows the frame to be very rigid. The rings can even be placed very close together without losing stability, which can be important in short pediatric limbs with large deformities.5 Some strut designs allow controlled dynamization to be performed anywhere from 1 to 5 mm, which can be helpful in patients with fractures. A new trend in half-pin placement is to avoid having all of the pins placed orthogonal to the bone. Fixation elements that allow pins to be placed obliquely while still keeping the rings orthogonal to the bone are available. The oblique half pin placement creates more stability in the frame construct and may decrease the risk of fracture through a pin track after frame removal.6,7

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SOFTWARE

The third trend involves improvements in the software that is paired with the ring systems. Preoperative planning applications are available for tablets that allow the surgeon to comprehensively analyze the radiographic deformity. A surgeon can then try different osteotomy levels, different hardware choices, and perform virtual surgery on the patient before going to the operating room. This preoperative visualization can help to decrease unanticipated problems intraoperatively and makes the operative time more efficient. Some new software programs allow the preoperative planning to be based around the apex of the deformity. This concept may be more intuitive for the surgeon than previous software programs. Apex of deformity planning eliminates the need for calculating translational deformity, which can be difficult to estimate in congenital deformities. New software programs also encourage preoperative planning so that the number of strut changes during the course of treatment can be minimized or avoided altogether.

One of the major improvements in the new software is the dampening of the bone movement that occurs during the correction. Older software brought the bone ends from point A to point B in a series of spiral movements until the bone ends reached their final destination. Newer software models dampen the amount of bone spiraling that occurs and move the bone ends to the final destination in more of a straight line.8 This decreased bone travel may help to improve healing times. New software packages now allow the surgeon multiple options to control the correction process. The surgeon can choose to move the bone in millimeters per day, degrees of angulation, degrees of rotation, or simply by the desired number of days. The software can also divide the program into 2 independent components running at different rates. For example, if the surgeon wants to distract the bone fragments a certain amount before attempting angular correction, the correction program can be devised to perform 1 mm per day lengthening and then, upon completion of the distraction, start the angular correction at 2 degrees per day. There is no longer any need to create separate programs for each of these waypoints. New software also has the ability to account for imperfect frame mounting by the surgeon. At the end of the case, the surgeon can simply input that the frame was angled or rotated and the computer will make the adjustments without having to change the deformity or mounting parameters. Finally, in a world dominated by smartphones, it is only natural that the computer software should be compatible with our phones. Applications now exist allowing the patient’s prescription to be loaded onto his or her phone. The patient can follow the progress of his or her daily adjustments and receive reminders on the phone when it is time to perform the adjustments. The surgeon can also choose to monitor the patient’s progress remotely.

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MISCELLANEOUS

The fourth category of ring fixator trends represents a miscellaneous group of changes. For limb lengthening patients, a hybrid frame built using hexapod rings with straight threaded rods combines the best of the old and new techniques. During lengthening, it is not uncommon for the muscle forces to create some deformity in the regenerate bone as the lengthening progresses. For example, in the tibia, valgus, and apex anterior deformity is known to occur because of pull from the lateral and posterior muscle groups. Using straight threaded rods to lengthen allows the patient to distract in ¼ mm increments, creating a reliable rate and rhythm to produce regenerate bone. Combining this lengthening method with hexapod rings allows the surgeon to correct any accumulated deformity at the end of lengthening by exchanging the threaded rods for struts.

Ring fixator systems are now designed to be increasingly more compatible with other types of external fixators. When spanning the knee is necessary, there are now systems that permit seamless connection between a uniplanar fixator on the femur and a ring fixator on the tibia while protecting and maintaining range of motion at the knee. Ring fixators are also now available in a preassembled, “peel-packed” form that allows the surgeon to rapidly apply the fixator to a patient. These frames are designed for trauma situations in which there may be limited surgical time to achieve stable fixation of a fracture. These preassembled frames can be used as temporary or even definitive fixation in some cases. The last item is an attachment to the undersurface of the foot ring that allows the patient to more easily weight bear on a fixator that includes the foot. These nonskid, rocker-bottom shaped foot attachments can be applied independently to the medial and lateral side of the foot ring. They are also adjustable in height so that if the ring is obliquely oriented to the plantar surface of the foot, the attachments can accommodate this obliquity and create a weight-bearing surface parallel to the floor.

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FUTURE

The final category of ring fixator trends examines future possibilities in ring fixators. Programmable self-adjusting struts that are controlled by wireless technology are now being developed. Prototypes of these struts already exist but cost and feasibility issues remain to be worked out. These struts would minimize the risk of patient error during the adjusting process and could be programmed to adjust slowly over the course of 24 hours in minute increments. Motorized distraction is another technology that currently exists but can be improved. The distractors slowly perform the lengthening for the patient in tiny fractions of a millimeter. The current version of motorized distractors is too bulky and too heavy to be used consistently on all patients. Newer, more streamlined designs would be valuable.

To decrease the risk of pin track infection, new pin coatings are being designed. In Japan, half pins are being manufactured from iodine-supported titanium. Preliminary study has shown substantial improvement in infection rates using these implants.9 The half pins, however, are not yet available in the United States. Another half pin coating design, extended hydroxy-apatite, is being evaluated for release in the United States. These half pins have the hydroxy-apatite coating extended up the shaft of the half pin, not just over the threaded portion. New fixation elements such as double column pin blocks will help increase the stability of the pin bone interface. The unique design may also help assist the surgeon with drilling accuracy and pin placement.

An anticipated software advancement will be the ability to upload the patient’s radiographs directly into available software systems. The surgeon would then plot points and measurements to create a 3-dimensional model of the patient’s bone deformity within the software itself. This could potentially enhance the surgeon’s accuracy and allow more sophisticated presurgical planning to be performed. Finally, ring fixators lend themselves well to quality, safety, and value research. Ring fixator surgeries often involve great quantities of medical equipment. It is not uncommon for the operating room staff to open 8 to 12 different trays of instruments for a single ring fixator case. Condensing the necessary equipment to 1 or 2 trays will increase efficiency and help to control cost. For example, a single tray containing all of the equipment needed to perform a standard Blount disease ring-fixator correction would help to improve efficiency and potentially decrease cost.

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REFERENCES

1. Eralp L, Kocaoglu M, Toker B, et al. Comparison of fixator-assisted nailing versus circular external fixator for bone realignment of lower extremity angular deformities in rickets disease. Arch Orthop Trauma Surg. 2011;131:581–589.
2. Paley D, Herzenberg JE, Bor N. Fixator assisted nailing of femoral and tibial deformities. Tech Orthop. 1997;12:260–275.
3. Eidelman M, Keren Y, Norman D. Correction of distal femoral valgus deformities in adolescents using minimally invasive fixator-assisted locking plating. J Pediatr Orthop B. 2012;21:558–562.
4. Iobst CA, Cherkashin A, Samchukov M. Comparison of hexapod frame systems. J Limb Length and Recon. 2016;2:29–34.
5. Samchukov M, Chiaramonti B, Leonchuk S, et al. Comparative conformational instability of different hexapod frames. J Limb Length and Recon. 2015;1(suppl 1):S13.
6. Green C, Cherkashin A, Samchukov M, et al. Divergent half pins maximize external fixator stability and fixation. Limb Lengthening and Reconstruction Society 25th Annual Scientific Meeting. Charleston, SC; July 22, 2016.
7. Thompson R, Iobst CA, Cherkashin A, et al. Do oblique half pins decrease fracture risk after fixator removal? Limb Lengthening and Reconstruction Society 25th Annual Scientific Meeting. Charleston, SC; July 22, 2016.
8. Cherkashin A. Mathematical modeling for the evaluation of hexapod frames stability and correction path. Limb Lengthening and Reconstruction Society 25th Annual Scientific Meeting. Charleston, SC; July 22, 2016.
9. Shirai T, Watanabe K, Matsubara H, et al. Prevention of pin tract infection with iodine-supported titanium pins. J Orthop Sci. 2014;19:598–602.
Keywords:

external fixator; hexapod; fracture; deformity; multiplanar

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