Emerging Insights On Fixation For Austin/Chevron Bunionectomies
Drawing from the literature as well as their clinical experience, these authors review the evolution of fixation options for Austin/Chevron bunionectomies. In addition to exploring the pros and cons of resorbable implants, they assess the advantages of allografts and offer key surgical insights.
The Austin/Chevron bunionectomy for the distal first metatarsal has long been a popular and stable procedure for the treatment of mild to moderate hallux valgus since Gill and colleagues first described it in 1976.1 In the 1980s, Austin popularized the bunionectomy, which became the desired procedure for the treatment of mild to moderate hallux valgus deformities.2
Although surgeons had historically performed the distal Chevron bunionectomy successfully without the use of fixation, many surgeons now prefer to utilize some form of rigid fixation to reduce the likelihood of displacement of the osteotomy site with possible malunion.1 Austin initially advocated manual compression of the distal metaphyseal osteotomy augmented by bandage, splintage and, at times, rigid casting.2 However, many surgeons faced inconsistent results including osteotomy subluxation, dislocation, nonunion, iatrogenic distal fragment fractures and many more.
Small and colleagues reported an 8 percent increase of displacement at the osteotomy site for distal Chevron bunionectomies without the use of fixation.3 This concern lead to the popularization of internal fixation as an adjunct to ensure stability of the osteotomy.
Since the introduction of the Austin bunionectomy, surgeons have employed numerous methods and materials for stabilization and fixation. Studies have found fixation with Kirschner wires provide adequate stability.4 Use of one or more K-wires appeared to decrease the incidence of many of the previously mentioned complications. However, such fixation can prove to be unpleasant for the patient with increased risks.
K-wires tend to cause pain in the surrounding skin as well as unwanted skin traction. They also may provide a conduit for the introduction of bacteria to the operative site. The wires also limit early range of motion at the metatarsophalangeal joint (MPJ) due to the irritation of the skin and associated pain with the tethering effect of the protruding wires during the motion of the joint.1 In addition to the potential for pin migration, another disadvantage is that patients must take extra care to avoid “bumping” or catching the wires on a multitude of surrounding obstacles.3
In order to avoid some of the disadvantages and complications of utilizing K-wires, many surgeons began to insert the K-wire and bury it in the subperiosteal layer, either indicating they would remove the K-wire at a later date, requiring an additional procedure, or that they would not remove it at all.
K-wires may be the least expensive of all implants for fixation but the low cost may not justify the minimal degree of compression and the long list of potential risks and complications.
What You Should Know About Bioabsorbable Implants
Johnson and Johnson introduced the first commercially available resorbable pin, OrthoSorb (poly-p-dioxanone) and in the late 1970s, it became a very popular subsititue for metal pins. Bionex (polyglycolic acid) then joined Orthosorb. Bionex had a difficult introduction to the market due to adverse patient reaction, which an investigation later determined to have been caused by a dye agent in the material. Other materials and designs for pins and resorbable screws from several manufacturers such as Bionx, Arthrex, Biomet and Tornier began to enter the market with wider acceptance due to increased strength and resistance to movement and migration.5
The rate of degradation has long been considered a potentially important factor in the development of adverse tissue reactions. Differences in degradation depend on a number of factors involving the specific polymer being used. In addition to molecular weight, mass and strength, the porosity, crystallinity, hydrophobicity or hydrophilicity and thermal history also influence degradation.1 The volume of breakdown in products per unit of time in local surrounding tissues is greater for an implant that degrades rapidly, such as a polyglycolic acid device, than for an implant of the same size that degrades more slowly, such as one made of polylactic acid.1 Polyglycolic acid degrades over a period of a few months whereas polylactic acid completely resorbs over a period of one and a half to four years.1,6 Poly-p-dioxanone degradation is slightly longer than that for comparably sized polyglycolic acid implants and loses its strength within two months.7
Resorption of polymers generally occurs in two phases.8 In phase one, the polymer chains break down through hydrolysis. In this particular phase, the molecular weight drops first, is followed by mechanical strength loss and finally by a loss of mass.9 In phase two, the implant loses its form and breaks down into particles, which macrophages then attack. Depending on the size of the particulates, they are phagocytosed and the kidneys and lungs excrete the byproducts.10 The corresponding biological response to the degradating polymer is thought to happen as a result of either a buildup of acidic degradation products or as a response to the particulates of the polymer.11
The most commonly cited advantages of resorbable implants include reduced stress shielding since the implants bear less load initially and transfer the load as they degrade.10 The polymers can be engineered to provide the optimum degradation profile for a specific application.10 Additionally, there is also elimination of the need for removal of hardware (and its related cost), reducing additional patient inconvenience as well as the potential for pin tract infection.1 Disadvantages of the use of resorbables include a lower strength and higher cost as well as the potential for the development of a sinus tract with sterile discharge.10 Studies have also documented synovitis and osteolytic changes in the bone, migration and breakage of the implant.12-14
How Human Allograft Can Be Advantageous In Fixation
Human allograft bone is harvested from an individual other than the one receiving the graft. Allograft bone is generally harvested from cadavers and is typically sourced from a bone bank.15
The use of human bone allografts has been gaining popularity in the surgical community and is increasingly utilized in a variety of surgical procedures of the lower extremity. The Musculoskeletal Transplant Foundation reports that more than 900,000 allograft procedures occur each year in the United States.15 The product has predominately been in use as bone filler for tumors that are excised or as grafts to assist in bone healing from postsurgical or post-traumatic non-unions. Although disease transmission is rare, a careful understanding of the tissue processing bank and its procedures is critical to allow the surgeon to provide patients with the tools to make an informed decision, and be an active participant in their surgical plan of care.
There are several physiological properties of bone grafts that directly affect the success or failure of graft incorporation. These properties include osteoconduction, osteoinduction and creeping substitution.
Osteoconduction occurs when the bone grafting material acts as scaffolding for new bone growth, which the native bone perpetuates. Osteoblasts utilize the bone grafting material as a framework to spread and generate new bone formation.16
Osteoinduction involves the stimulation of osteoprogenitor cells to differentiate into osteoblasts that subsequently begin the formation of new bone.17 Bone grafting materials that display both osteoconductive as well as osteoinductive properties will serve as scaffolding for currently existing osteoblasts and trigger the formation of new osteoblastic activity, theoretically promoting faster integration of the implanted graft.17
Creeping substitution is the process of bone remodeling by osteoclastic resorption and creation of new vascular channels with osteoblastic bone formation, resulting in new haversian systems. This is the method by which strong cortical bone forms from grafted material.18
Advantages of using allograft bone include its availability in plentiful supply, its limited risk of transmission of infectious organisms including hepatitis and HIV, and that it provides an osteoconductive scaffold as well as structural support.19,20 The goal of using allograft bone is to initiate a healing response from the host that will produce new bone at the host-graft interface and within the porous bone of the graft material. Some disadvantages to its use include delayed vascular penetration, slow bone formation, accelerated bone resorption, and delayed or incomplete graft incorporation.20
A Closer Look At A New Allograft Product
With the development and introduction of the TenFUSE Nail allograft (Solana Surgical), we begin a new generation of fixation. The TenFUSE Nail allograft nail (which complies with the Food and Drug Administration, American Association of Tissue Banks, and state regulatory requirements for donor screening and testing) is a cortical allograft fixation device that is partially demineralized to maintain osteoinductive and osteoconductive properties.
The goals with the development of this novel fixation device include:
• ease of availability and a long shelf life with no additional preparation;
• a reliable sterility assurance rate of 106, conforming with industry standards for implants;
• compatibility with host bone and possibly other additional hardware;
• ease of insertion;
• a patented design to resist migration or rotation;
• disposable drill guides and insertion sleeves; and
• a variety of lengths and diameters.
Step-By-Step Surgical Pearls
The primary indication for an Austin/Chevron bunionectomy is the presence of pain over the medial eminence of the first metatarsal head that is symptomatic while walking or wearing shoes. It is also indicated for patients who also have a mild to moderate deformity with a metatarsal primus adductus angle of 16 degrees or less and no radiographic evidence of metatarsophalangeal arthrosis.
Create a dorsal medial incision slightly medial to the extensor hallucis longus tendon. Then incise the underlying joint capsule utilizing the indicated capsulotomy depending on the degree of deformity severity. Take care when utilizing a medial “U” shape capsulotomy due to the possibility of overcorrection.
After resection of the hypertrophied medial eminence, fashion the Austin/Chevron bunionectomy in the metatarsal neck with the use of a saggital saw. The apex of the “V” is distal, usually 1 cm from the articulation.
Upon completion of the osteotomy and lateral displacement of the metatarsal head, utilize manual compression of the metatarsal head on the shaft in addition to placing a K-wire for the purpose of temporary fixation and stabilization of the osteotomy. Follow this by remodeling all uneven bone surfaces with the use of intraoperative fluoroscopy to confirm appropriate positioning of the osteotomy.
After achieving a corrected position, utilize a 2.7 mm drill bit to create a pilot hole for the initial point of fixation. The drill bits are laser-etched for ease of use and are provided with 16 mm and 18 mm markings.
After the utilization of the drill bit, select the corresponding TenFUSE Nail allograft and place it at least two-thirds of the way into the previously created pilot hole. Take caution not to apply excessive bending force to the implant. Once the implant is partially seated, use the bone tamp to fully seat the TenFUSE Nail to the desired depth. Take care to ensure flush placement of the graft at the dorsal surface. Remove the temporary fixation and use a second allograft after redrilling the temporary fixation site. In the situation of “proud” bone on the dorsal surface, utilize a bone cutter and/or burr to shave down the excess graft. It is not recommended to use a cautery on the graft. Finally, utilize intraoperative fluoroscopy for final confirmation of the corrected position.
Place the patient in a forefoot cast and allow ambulation with the use of a controlled ankle motion (CAM) walker. Encourage first MPJ range of motion exercises on postoperative day one to limit post-op joint stiffness. Change the forefoot cast weekly for wound inspection and postoperative radiographs. The casting typically ends at postoperative week three and the patient utilizes the CAM boot for an additional three weeks.
In Conclusion
There is no argument that the use of rigid internal fixation with metal screws, staples and/or plates has been the gold standard for bone fixation. Having stated this, many foot and ankle surgeons agree anecdotally that due to the more compressible cancellous bone in the distal metaphyseal osteotomy, resorbable fixation may be as successful as metal in the amount of time to healing.
For over 30 years, the senior author has utilized various absorbable materials for Austin/Chevron bunionectomies. In a review of those cases, we have found no substantial difference in osteotomy healing in patients with resorbable fixation in comparison to those with rigid fixation, given similar variables including age, vascularity, the presence of diabetes and partial immobilization of weightbearing during the first three to six postoperative weeks.
Experience with resorbable fixation has anecdotally revealed a slight increase in postoperative inflammation and a potential decrease in restoration of range of motion of the first metatarsophalangeal joint. This appeared to be present primarily with synthetic resorbable fixation devices and there appears to be a marked reduction in postoperative inflammation with the TenFUSE Nail.
The TenFUSE Nail is fashioned with an octagonal shape containing ridges, which appears to adequately resist rotation and migration. The allograft fixation is available in both a 2.0 mm option as well as a 2.7 mm nail. In most Austin/Chevron procedures, the use of two points of fixation has proven to be the most stable construct. As standard procedure, it is also always recommended to obtain intraoperative radiographs after manual compression and after insertion of the second nail.
The usual amount of time until disappearance of the allograft on X-ray ranges from eight to 12 months.
In our clinical use and review of the TenFUSE Nail, we have been impressed with its ease of application, the stability of the fixation construct, the minimal amount of postoperative inflammation, complete healing following creeping substitution and success with early and increased range of motion.
Gary M. Lepow, DPM, MS, FACFAS, is a Senior Partner at Lepow Foot and Ankle Specialists. He is an Associate Clinical Professor at the Baylor College of Medicine and an Associate Clinical Professor at the University of Texas Medical School in Houston. Dr. Lepow is the Chief of Ambulatory Services/Podiatry with the Harris Health System based in Texas. He is also a Past President of the American College of Foot and Ankle Surgeons.
Brian D. Lepow, DPM, AACFAS, is an Associate at Lepow Foot and Ankle Specialists. He has received fellowship training in diabetic limb salvage and reconstructive surgery.
References
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