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Adjuvant Use of Implantable Bone Growth Stimulator for the High-Risk Ankle Arthrodesis

DAVIS, W. HODGES M.D.; COHEN, BRUCE E. M.D.

Techniques in Foot & Ankle Surgery: September 2002 - Volume 1 - Issue 1 - p 69-73
TECHNIQUE
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Ankle fusion continues to be a widely used technique for treatment of degenerative conditions of the tibiotalar joint. The functional results are dependent on obtaining a solid arthrodesis. The authors have found an increased incidence of nonunion in certain patient populations. Specifically, the patients at high risk include the diagnoses: Avascular necrosis of the talus with or without collapse; nonunion of tibiotalar or subtalar arthrodesis; history of previous pseudoarthrosis from previous procedures in other anatomic locations; cigarette smoking, alcohol abuse or other metabolic disorders. The spine fusion and fracture literature show good clinical results using an implantable direct current electrical stimulator as an adjunct to internal fixation to enhance fusion and bone healing. In this manuscript the authors describe their indications and detailed technique for use of implantable bone growth stimulation as an adjuvant in selected high-risk patients groups. Early clinical results and common complications are also reported.

Miller Orthopaedic Clinic,

Charlotte, North Carolina, U.S.A.

Address correspondence and reprint requests to W. Hodges Davis, M.D., Miller Orthopaedic Clinic, 1001 Blythe Boulevard, Suite 200, Charlotte, NC 28203; e-mail: hodges.davis@millerclinic.com

Ankle fusion (tibio-talar and tibio-talo-calcaneal) is a widely used technique for treatment of tibiotalar joint degeneration and collapse. The functional results seem to be reproducible if a successful arthrodesis occurs. 1–3 In modern series, the fusion rates have been reported from 60 to 100%. 1,2,4–6 The union rate is adversely affected in patients with specific risk factors for delayed union. 4,5,7 These risk factors include avascular necrosis of the talus (AVN), previous failed arthrodesis attempts, metabolic bone disease, and cigarette smoking. 4–8 Despite rigid internal fixation and limited soft-tissue stripping exposure techniques, these high-risk patients are at risk for delayed union and nonunion. Traditionally, techniques such as Blair fusion, talectomy and tibio-talo-calcaneal fusions have been used in these challenging patients. Our results in these patient groups using described techniques have been unsatisfactory. We have looked for alternative arthrodesis techniques in the face of avascular necrosis, revision arthrodesis, and long-term tobacco use.

In reviewing reports of spinal fusions and fractures, the use of an implantable bone growth stimulator appeared to improve the fusion rate, especially in high-risk patients. 9–15 Recent reports have confirmed the benefits of direct stimulation in ankle and hindfoot fusions. 5,16 It was with this in mind that we have begun using an implantable stimulator for selected high risk ankle fusion candidates.

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TECHNOLOGY AND EQUIPMENT

The use of electromagnetic fields to promote healing of various types goes back to the first century. 17,18 Reports claiming that the medicinal powers of electricity could cure everything from headaches to menstrual cramps can be found in turn of the century advertisements and books. 17 These claims persist even today. The use of electrical stimulation in promoting bone growth has been demonstrated in vitro and in vivo. 17–21 Most of the research is in external devices using pulsed electromagnetic fields (PEMFs). 17,22 The device used in this article is an implantable device that delivers direct current to the site. The direct current appears to trigger mitosis and recruitment of osteogenic cells. The exact biologic mechanism of action in direct current stimulation is not totally agreed upon, but the clinical results of implantable stimulation in the spine fusion and fracture literature are impressive. 9,14,15,19,23,24 The advantages of the implantable stimulator for our purposes are 1 1) guaranteed patient compliance 2; 2) current is placed directly on the high risk fusion site 3; and 3) low profile battery compared with the external devices.

For our patients, we have been using the OsteoGen Implantable Stimulator (EBI Medical System, Parsippany, NJ). 25 This implantable stimulator product provides four options for the surgeon 1: 1) a single lead implantable mesh that will increase the surface area stimulated 2; 2) dual lead mesh 3; 3) a single lead straight cathode 4; and 4) a dual lead straight cathode. Each is connected to a generator (40 microamperes of current in the double lead and 20 microamperes of current in the single lead) that provides 20 microamperes of current to each lead. The generator casing is made of titanium. The straight cathodes total 40 mm in length and the mesh cathodes total 23 mm in length before the mesh is stretched.

The electrical current emanating from the cathode gives a cylinder of current of 5–8 mm radius. The cathodes should be placed to maximize the surface area stimulated by the current. The recommendation from the manufacturer is to not touch the internal fixation with the generator or cathode (may dissipate current or cause corrosion) and to remove the battery after union has occurred (Fig. 1). 25

FIG. 1.

FIG. 1.

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INDICATIONS/CONTRAINDICATIONS

The indications for ankle arthrodesis include painful degenerative arthrosis of the tibiotalar articulation. The presence of significant angular deformity, avascular necrosis of the talus, collapse of the articular structures, or significant bone defects may affect the specific approach chosen for ankle arthrodesis. The indications to use adjuvant implantable electrical stimulation include 1: avascular necrosis of the talus with or without collapse, 2 nonunion of tibiotalar or subtalar arthrodesis, 3 history of previous pseudoarthrosis from previous procedures in other anatomic locations, 4 and cigarette smoking, alcohol abuse, or other metabolic disorders. Contraindications or relative contraindications include history of significant infection, history of soft tissue/wound-healing problems, peripheral vascular disease, or immunosuppressive conditions. At present, our most common indications for this technique are in patients with severe degenerative disease of the tibiotalar joint, but may include ipsilateral subtalar degeneration and some cases of significant talar collapse that may involve the subtalar joint.

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PREOPERATIVE PLANNING

The use of the stimulator should not change the chosen arthrodesis technique used by each individual surgeon. The technique chosen should be one that allows for realignment in the face of deformity, for removal of all avascular bone in AVN of the talus, and adequate exposure to allow removal of retained hardware in revision cases. In our series we have performed the transfibular as well as the mini-arthrotomy approach as described by Myerson. An arthroscopic technique is not a contraindication; however, more extensive exposure may be required to appropriately position the stimulator leads.

It is often imperative to get a CT scan in these patients preoperatively to help delineate the bony anatomy, location of defects, severity of deformity, and identify areas of collapse and location of nonunion. Finally, the CT scan allows us to evaluate the presence of concomitant subtalar degenerative disease, which needs to be addressed in the surgical procedure. A MRI can be useful in confirming the presence of avascular necrosis and defining the area of involvement (Fig. 2). If a tibio-talo-calcaneal fusion is proposed, depending on the technique chosen, preoperative planning may include measuring the canal size of the tibia to place a retrograde IM nail.

FIG. 2.

FIG. 2.

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TECHNIQUE

In order to allow for maximum bone surface area to be exposed to electrical stimulation, we most often use the mesh stimulator lead in an M or L configuration. 9 As mentioned above, the operative technique of fusion is determined solely by the surgeon. Once the bone surface has been prepared by “fish scaling or drilling” and all of the avascular bone has been removed, the stimulator is laid between the tibia and the talus. The stimulator can be used in conjunction with both auto and allo-graft without loss of effectiveness. As these bones are compressed and internally fixed, the bones themselves will pinch the stimulator leads between them. We have found it helpful at times to use a loop of suture with a Keith needle threaded through the center of one of the mesh loops to place the apex of the stimulator lead to the posterior ankle joint with the mini-incision technique or to the posteromedial portion of the ankle with the transfibular approach. It is imperative to provide as much bone-to-lead contact as possible. 9 In AVN of the talus, we use a two-lead stimulator when attempting a tibio-talo-calcaneal fusion. In that situation we will place one lead in the subtalar joint and the other in the tibiotalar joint.

Once the lead is in place, the specific internal fixation technique chosen by the surgeon is completed. Self-tapping and self-drilling cannulated screws should not bind up or touch the lead. If the mesh is broken, the remaining lead will continue to provide adequate stimulation. In the retrograde IM nail technique for tibio-talo-calcaneal fusions we will most often place the stimulator lead after reaming the distal tibia over a guide wire. We have found that if we ream before placing the stimulator, the stimulator does not get caught up in the reamer. Once rigid internal fixation with compression across these areas is achieved, the stimulator leads should be well ensconced between the two well-prepared bone surfaces. A small pocket is then made in the soft tissue anterior to the syndesmosis. It is best tolerated above the “sock line.” The tissues are closed in the standard fashion (Fig. 3).

FIG. 3.

FIG. 3.

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RESULTS

Reports of external stimulation using PEMFs for fractured nonunion in the foot and ankle abound. 12 There is only one report of a series detailing the use of an implantable stimulator in the lower extremity. 5 Donley and Ward reported on 13 patients where an implantable stimulator was used as an adjunct to internal fixation in patients undergoing hindfoot fusions. Their indications were previous nonunion, AVN, history of infection, and major medical problems. They achieved a fusion in 11 of 13 patients (85%). Four of 13 battery packs were removed.

Our experience at the Miller Clinic is similar to Dr. Donley's, but in a pure ankle population. From January of 2001 to March of 2002, 12 patients and 13 ankles underwent tibio-talar and tibio-talo-calcaneal fusions using an implantable bone growth stimulator. The average age was 58 years (range 72–25 years). Ten were female and two male. Seven had tibio-talo-calcaneal fusions, five had tibio-talar fusions, and one had completion of a pantalar fusion. The surgical indication at the time of surgery was nonunion of a tibio-talar fusion in seven patients, primary AVN in four, a failed total ankle replacement (TAR) in a smoker, and a tibio-talar fusion in a heavy smoker. Of note, of the nonunions treated, four patients had AVN as a pre nonunion diagnosis. At this time all but the fusion after the failed TAR have complete radiographic fusion. The patient with the TAR fusion has had her rod dynamized with the proximal portion of her bulk allograft incorporated, but the distal lagging behind. Four of the stimulators have been removed. Two were removed for pain, and two for ulceration over a prominent lead wire and subsequent local infection. There were no problems after the hardware was removed.

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COMPLICATIONS

  1. Breakage or disconnection of the stimulator once implanted. This breakage is almost always caused by placement of hardware. If this happens, one is relegated to taking everything down and replacing the stimulator.
  2. Migration of the leads. Typically it is not the distal portion of the leads, but the wire that connects the lead to the power pack. This wire can migrate to the surface and create an ulcer. We have had to remove two of these and the ulcer closed without problems.
  3. Painful power pack. In especially thin ankles, the power pack can be sizeable and painful. It may have to be removed. Typically, this is easily done under local anesthesia. There is no problem leaving the leads in after the power pack is removed.
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POSTOPERATIVE MANAGEMENT

The implantation of the stimulator does not change the postoperative management used for whichever fusion technique the surgeon chooses. If fixation is solid the patient will be touch down weight bearing for 6 weeks in a cast, then 4–6 weeks in a walking cast. Fusion has usually happened by 10–11 weeks postoperatively. AVN patients remain non weight bearing for 10–12 weeks to try to prevent further collapse. After casting is complete the patients are placed in shoe with a rocker sole. A lift may be needed if significant shortening has occurred.

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PRODUCT CONCERNS AND FUTURE OF THE TECHNIQUE

At this point, our main concern is the status of the instrumentation as it pertains to the ankle. The stimulators at present are designed primarily for spine fusions and long bone fracture non-unions. This is where most of the marketing and development has been for this product. In the spine and fracture work, the stimulator can be placed under direct vision around the fusion or fracture site. In the ankle, it is often necessary to be place the lead in non visible areas It is our hope that in the future some simple disposable instrumentation will be developed to place the stimulator lead in an area, somehow anchor the lead in that area, and protect it from the internal fixation implants while the compression device is placed.

Secondly, the major complication of disconnection of the stimulator lead from the power pack needs to be addressed. It is our hope that we can provide some type of replacement lead that will be less costly to the patient. Finally, this technology continues to be quite costly. This fact encourages us to keep very narrow indications for its use. The use of this technique requires long-term studies of its efficacy compared with traditional arthrodesis techniques in the high-risk patients and in the presence of AVN.

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REFERENCES

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Keywords:

nonunion fusion; ankle fusion; bone stimulation

© 2002 Lippincott Williams & Wilkins, Inc.