Assessing The Pros And Cons Of Subtalar Implants
“Collapsing” pes planovalgus or flexible flatfoot is a complex pedal pathological condition with numerous components. In its most significant form, the condition can lead to significant disabling and an inability to ambulate efficiently. Arthroereisis is a relatively modern surgical technique one may employ for pes planovalgus correction. Arthroereisis is defined as the limitation of exogenous joint motion without complete arthrodesis. This procedure, which involves placing a motion blocking implant within the sinus tarsi, has been designed to restrict excessive subtalar joint (STJ) pronation while preserving supination. While arthroereisis was originally designed for pediatric flexible flatfoot, many surgeons have expanded the indication for this “implantable orthotic” to adults with or without posterior tibial tendon dysfunction (PTTD). Surgeons can also use it to treat certain rigid conditions if one can restore motion.1,5,9-15 A Pertinent Overview Of Subtalar Implant Types In 1977, Subotnick described arthroereisis with a free-floating sinus tarsi implant.12 Surgeons subsequently designed other implants using many different materials and techniques. Vogler’s classification categorized all of them based upon an implant’s biomechanical properties. Vogler classified the implants as an axis-altering prosthesis, an impact-blocking device or a self-locking wedge.1,16 An axis-altering device actually elevates the floor of the sinus tarsi and accordingly elevates the subtalar joint axis, reducing calcaneal eversion. In the late 1970s, Smith developed the polyethylene disk and peg implant (STA-Peg, Wright Medical) and placed the stem of these implants vertically in the floor of the calcaneus. Lundeen modified the STA-Peg by applying an inclined platform initially and subsequently created five different sizes known as the Lundeen Subtalar Implant (LSI, Sgarlato Labs).17 The disk elevates the floor of the sinus tarsi, altering the STJ axis. This will not be as effective as other techniques in correcting the transverse planar dominant foot type. The technique also requires the articular surface of the talar posterior facet to ride up on the implant surface. This makes the joint subject to more wear and tear. With this technique, it is also more difficult to limit the precise amount of pronation. The second group of subtalar implants is referred to as impact-blocking devices or direct impact implants. Without changing the subtalar joint axis, these implants limit anterior displacement of the lateral talar process. One example is the Flake-Austin insertion technique of the polyethylene disk and peg implants, which include the Angled Subtalar Implant (ASI, Nexa Orthopedics), the LSI implants modified with a longer stem and the original STA-Peg implants. One would insert the implant at an angle so the dorsal disk surface comes into complete contact with and is parallel to the lateral leading wall of the talus. This technique may allow the most precise limitation of pronation. However, placement also takes a little longer intraoperatively than many of the other implants. Failure to place the disk parallel to the leading wall of the posterior facet of the talus may lead to temporary sinus tarsitis. With over 30 years of documented success in the pediatric patient with the Flake-Austin disk and peg technique, the impact-blocking type of arthroereisis may be the most physiologic. Longer-term effects of this impact seem to be benign but are unknown.10,18 Another example in this category is the polypropylene Domed Subtalar Implant (DSI, Nexa Orthopedics). Both of these techniques block the lateral process of the talus from advancing much beyond the posterior facet of the calcaneus. A self-locking wedge is a subtalar implant of any material that one inserts into the lateral sinus tarsi. These devices prevent contact of the lateral talar process with the floor of the sinus tarsi. The implant supports the talar neck, limits plantarflexion and adduction of the talus, and restricts eversion and pronation of the subtalar joint. The MBA threaded cylinder (Kinetikos Medical Inc.) is one of the most popular implants in this category. Other implants in this category are the Conical Subtalar Implant (CSI, Nexa Orthopedics), the Kalix (Integra Life Sciences), the Endo-Orthotic Implant expanding cylinder with screw, the Horizon (BioPro), the Talar-Fit (Osteomed), the HyProCure (Gramedica) and the recently introduced absorbable bioBlock (Kinetikos Medical Inc.). A Review Of Other Pertinent Implant Characteristics Most implants are partially or fully threaded. Some have sharp threads while others have softer thread edges. Some have a hollow center to allow for soft tissue ingrowth. Many are made with titanium and some are made with ultra high molecular weight polyethylene. The bioBlock is made with polylevolactic acid (PLLA) absorbable material. Without ingrowth into these implants and/or threads, loosening and extrusion can be problematic. Precise appropriate sizing can be difficult. Technically, the appropriate depth of the implant insertion is required for effective control. Sharp edges can lead to sinus tarsi irritation. There is some question as to the amount of supination that may be limited by these implants. However, these implants are usually the quickest and easiest to place. Out of the locking wedge implants currently being used, the MBA has the longest follow-up of approximately 15 years. Overall, the arthroereisis procedure has been developed to restrict excessive pronation of the subtalar joint in all ages and bypass adaptive changes in children. Such a vast array of implants and numerous surgical techniques make the surgical decision of when to use a specific implant quite difficult. This concept makes it essential to evaluate the short- and long-term biomechanical effects of the implants on the subtalar joint and foot, including radiographic findings both before and after arthroereisis. What Does The Literature Reveal About Arthroereisis Procedures? In 2001, Forg, et. al., evaluated a total of 40 patients with disk and peg procedures (Flake-Austin technique) subjectively and objectively. Indications were for pediatric flexible flatfoot with an average age group of 9.7 years. Their results showed very successful outcomes with no patient’s condition worsening. Radiographic results demonstrated a restriction of excessive weightbearing pronatory forces.10 The authors noted decreases in the following positional angles: the talar declination angle, lateral talocalcaneal angle and Kite’s angle. The talonavicular congruity improved and the sinus tarsi space improved in millimeters. These results are consistent with a decrease in pronation, rendering the Flake-Austin disk and peg arthroereisis successful even in the face of transverse planar deformity.10 In 2003, Myerson, et. al., concluded that out of 43 MBA prostheses (31 children and 12 adults), 60 percent of these feet required additional correction such as a tendon Achilles lengthening (TAL) or gastrocnemius recession.19 Indications for these implants among children were a flexible flatfoot with or without association of an accessory navicular or tarsal coalition. For adults, indications were flexible flatfoot with or without posterior tibial tendon rupture or sinus tarsi pain. Myerson also explained that the procedure may benefit the elderly who have sinus tarsi pain associated with flexible flatfoot deformity, given the minimally invasive nature of the procedure. He determined that patients with a large degree of talonavicular uncovering after a calcaneal medial displacement osteotomy benefited from arthroereisis. Myerson stressed that he avoided resecting the interosseous ligament.19 Likewise in 2003, Viladot, et. al., evaluated the placement of the Kalix arthroereisis implant in the sinus tarsi for stage II posterior tibial tendon dysfunction (PTTD). The authors concluded that arthroereisis in a flexible flatfoot attributed to posterior tibial tendon pathology corrects the bone position and creates proper talar positioning.20 However, in contrast to this, the study authors cautioned that trying to normalize the talus position via arthroereisis in an uncorrected flatfoot from childhood could lead to ankle arthrosis.20 Understanding The Advantages Of Arthroereisis The advantages of arthroereisis are apparent in the operating room. Arthroereisis is easy and quick compared to calcaneal osteotomies, arthrodesis or medial column stabilization procedures.1,3,8,9,13,19,21,22 Due to relative ease, the time of the surgery will decrease and the incision remains minimal. A great advantage for patients is that arthroereisis restricts excessive pronation without significantly restricting supination or permanently locking the joint like an arthrodesis would.12 This is especially true for pediatric patients in whom bony maturation has not occurred. Fusion may disturb natural growth and cause adaptive changes of the adjacent joints in the future. Arthroereisis salvages the joint and allows the child to develop more normal osseous and soft tissue structures while the implant maintains the correction.2,4,8 Another advantage is that the procedure is reversible.12,19,23 If the patient experiences pain with the implant, one can remove it. Many surgeons have reported episodes of maintained correction with implant removal although no significant research has been reported in this regard yet.12,24 Pediatric patients with rigid flatfoot due to a coalition without arthritic changes have had successful outcomes with arthroereisis after creating mobility with resection of the involved joint.2 For example, resection of the talocalcaneal coalition at the middle facet will result in a more flexible rearfoot range of motion. However, due to soft tissue adaptation, the rearfoot may maintain the pronated deformity. Using the arthroereisis in this situation to realign the rearfoot will allow patients to have stability and eliminate the excess pronation.2,19 Furthermore, arthroereisis can reduce deforming forces following tendon repair in stage II posterior tibial tendon dysfunction.20 Arthroereisis prevented the talus from displacing onto the calcaneus and accordingly eliminated applied tension to the repaired tendon.20 The psychosocial effect of the surgery, especially for pediatric patients, is an important preoperative consideration. There is almost instantaneous gait improvement and pain reduction as opposed to long-term orthotic treatment. With the short recovery period relative to the reconstructive surgery, the psychological health of the patient can also improve. With a “normal” appearance of the foot and the ankle, the pediatric patients are especially less self-conscious. The relationship between the parents and children reportedly improves as well.6 Other Key Considerations Significant arthritic changes that have already developed in the subtalar joint or the adjacent joints are relative contraindications for an arthroereisis alone. If there is restriction of joint movement with the possible presence of ankylosis, it would be difficult to obtain adequate correction with arthroereisis. Additionally, one cannot correct rigid flatfoot with arthroereisis alone.1,4,12,15 Many researchers suggest using this procedure in conjunction with other procedures for the treatment of flexible flatfoot.6 One may need to perform TAL, midfoot or rearfoot osteotomies, or posterior tibial tendon advancements.19 Husain and Fallat found the tendo-Achilles to be contracted with most flexible flatfeet.11 Likewise, the authors found that moving the subtalar joint into a more inverted position increased tension on the tendo-Achilles. The procedure proves to be easier with repetition as there is a learning curve with each technique. This may prevent previously documented complications such as malposition, undercorrection, overcorrection, incorrect implant size, sinus tarsi pain and cystic formation. With this knowledge and training, a surgeon may elect to cut or not cut the interosseous ligament. Complete transection of the talocalcaneal ligament may reduce complications such as restricted excursion or motion postoperatively.22 On the other hand, release of the ligament may result in subtalar joint instability or pain.19 There are varying opinions on technique, implant type and size, and results. However, the literature generally supports arthroereisis in restricting pronatory forces and successfully treating flexible flatfoot. How To Reduce The Risk Of Post-Op Surprises And Other Potential Complications Addressing metatarsus adductus is crucial. Keep in mind that rearfoot correction may unmask metatarsus adductus and cause significant intoeing.1,7,8,13,19 Obtaining preoperative neutral position X-rays can help one unmask the metatarsus adductus and avoid such postoperative surprises. Similar to other surgical flatfoot correction, one should ensure proper positioning of the rearfoot (i.e. a rectus heel or slight everted position in relaxed stance).2,3,10,12,19 Arthroereisis implants come with different sizes for proper positioning. However, be aware that overcorrections do occur and can result in varus rearfoot, leading to postural symptoms and fatigue.1,10 To prevent over- or undercorrection, Forg, et. al., suggest using “alignment markers,” which one should map out intraoperatively in order to position the rearfoot properly.10 Dickerson, et. al., indicated that preoperative weightbearing positioning is essential to estimate the desired position from the maximally pronated position and from the neutral position.18 With these guidelines, a surgeon can more precisely eliminate excessive pronation while allowing adequate pronation for shock absorption. Sinus tarsalgia is by far the most common complication with arthroereisis, whether it is due to improper positioning, implant irritation or excessive restriction of the motion of the joint.1,4-6,12,19,23 Researchers have also documented cases of impingements or irritation of the nerve.21,23 However, the pain is usually self-limiting and symptoms often successfully resolve with local injections or antiinflammatories.1,19,23 If symptoms have not resolved, one can remove the implant with relative ease and relieve the pain. Many of the newer implants have more rounded edges and are reported to reduce the frequency of these symptoms. Although implants are generally designed with a suitable biomaterial for long-term wear, fatigue failure may still occur.1,10,19,21-23 These implants have to endure a prolonged period of time in the foot with significant repetitive reactive forces on them. In an attempt to reduce long-term risks, Forg, et. al., recommended removing the implants at the age of maturity (20 to 22 years of age) for the pediatric patients.10 Although arthroereisis is designed with relatively inert materials, some foreign body reaction is inevitable in the long run. Due to the location of the implant, the lateral talus and the sinus tarsi area of the calcaneus are usually affected.15,23 Flattening of the lateral talar process has occasionally occurred, especially with the traditional Smith STA-Peg type arthroereisis technique.15 The lateral talar process begins to have chronic trauma to the area and the bony erosion is likely responsible for this phenomenon. Researchers have also documented other bony changes, including sclerosis of the talar neck and a combination of radiolucency and sclerosis around the implant in the calcaneus.14,15 These changes may be attributed to long-term use of the implant and the stress shielding effect. This is where osteopenic changes take place with decreased compression in the area. Rockett, et. al., also documented subtalar cysts with the use of the STA-Peg implant.24 The exact mechanism of this complication is uncertain but the authors postulate that the chronic trauma of the lateral talus causes synovial proliferation within the talar medullary cavity. In a much more severe case, researchers reported avascular necrosis of the talus as a rare possible complication.25 The earlier models of the arthroereisis, especially the STA-Peg type, were stabilized in the sinus tarsi with the use of polymethylmethacrylate (PMMA). The so-called “bone cement” caused inflammatory reaction in certain patients.9,14,15 However, more recently, the modification of the arthroereisis and the new designs reduced the need for the bone cement and accordingly reduced possible adverse reaction.12,13 The newer implants also have a wider variety of sizes to facilitate more precise alignment of the maximally pronated rearfoot position. In Conclusion Arthroereisis is a valuable and relatively easy procedure for the treatment of flexible flatfoot. Utilizing this procedure can improve the quality of life of patients via a less traumatic approach and faster recovery. Although there are varying opinions on the benefits of arthroereisis, research does support its ability to restrict excessive pronation and reposition the talus over the calcaneus. Performing the arthroereisis also provides the pediatric population with an easily reversible procedure, which has proven to be successful in bypassing adaptive changes consistent with excessive pronation. When it comes to adults, this procedure does alleviate tension from the medial soft tissue structures but does not appear to be quite as universally free of complications. Following an advancement of the posterior tibial tendon, arthroereisis proved to aid in reducing tension and pronatory forces. Accordingly, the procedure does benefit both adults and children who have failed conservative treatment and maintain a flexible deformity. As with all surgical procedures, arthroereisis does have associated complications. However, with increasing experience in performing the procedure, knowledge of the sinus tarsi, obtaining preoperative neutral position X-rays, ensuring a thorough preoperative stance evaluation and performing a proper evaluation of the triplanar deformity, the small number of complications should decrease even more. Future research will have to evaluate the long-term effects of arthroereisis as an isolated procedure and in combination with other procedures. Since surgeons utilize various designs, sizes, techniques and materials for the arthroereisis procedure, there is no universal set guideline or perfect implant at the present time. Many modifications have taken place and the doctors are continuing to observe success with the implants. However, the arthroereisis, in general, has already proven to be a valuable technique in treating flexible pes valgo planus. Dr. Green is the Director of Podiatric Surgical Residency at the Scripps Mercy Medical Center in San Diego. He is a Clinical Professor at the California School of Podiatric Medicine at Samuel Merritt College, and is a Fellow of the American College of Foot and Ankle Surgeons. Dr. Green is also a faculty member of the Podiatry Institute and has a private practice in San Diego. Dr. Williams is a first-year resident at Scripps Mercy Hospital/Kaiser Sacramento. Mr. Kim is a fourth-year student at Scholl College at Rosalind Franklin University/The Chicago Medical School. For related articles, see “A Closer Look At Subtalar Joint Implants,” a supplement to the June 2005 issue of Podiatry Today.
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