Bioabsorbable Implants For Flatfoot: Can They Work?

   The balance between technology and clinical practice is difficult to obtain. In the past, technology lagged far behind. Innovative thought and technique was stymied by the inability to develop practical instrumentation and implants. In recent years, however, the opposite effect has occurred. New materials and devices have inundated the marketplace. While these devices have been demonstrated to be safe, they are not necessarily any more efficacious and are certainly more costly than existing products.    Indeed, practitioners must be vigilant in evaluating the efficacy of new products. One new product that has recently emerged is the bioresorbable sinus tarsi implant. The key question to ask is do these bioabsorable implants offer advantages over the materials we currently use?

A Closer Look At The Evolution Of The Subtalar Joint Arthroereisis

   Following heel strike, the subtalar joint pronates, the calcaneus everts and the talus plantarflexes and adducts. Excessive movement in the direction of pronation results in the foot losing its ability to resupinate with ultimate collapse of the medial arch.    The purpose of arthroereisis devices is to limit excess subtalar joint motion in the direction of pronation and still allow the joint to supinate. Chambers first described the concept of using an implant to limit subtalar joint motion in 1946. He used a bone graft to elevate the floor of the sinus tarsi in order to prevent subtalar joint eversion.1    Many have followed with various arthroereisis implant devices. Smith popularized the procedure in the podiatric profession in the early 1970s with the development of the STA-Peg.2 This polyethylene implant sat in the floor of the sinus tarsi and allowed the talus to move forward but at an oblique angle to the joint surface. This limited STJ motion by its “axis-altering” properties. Although the implant was originally cemented into the calcaneus to prevent loosening, this technique was discontinued due to potential complications associated with the use of bone cement although no adverse consequences were noted.    Subsequently, a number of implants have emerged over the years. The majority of these subtalar implants occupy the soft tissue content of the sinus tarsi in order to limit the anterior movement of the talus on the calcaneus. Green, et. al., offered a comprehensive review of this procedure and other devices earlier this year.3    Maxwell and Brancheau developed the MBA implant nearly 15 years ago as an alternative arthroereisis device for people with flexible flatfoot deformity.4,5 This is a “free floating” device in the sinus tarsi that “blocks” anterior talar movement. The device’s ease of insertion, lack of complications and overall benefit has made this one of the most commonly used implants in foot surgery.    Over the years, multiple studies have documented the success of the arthroereisis procedure in children and, more recently, in adults.4-7 Needleman recently published a retrospective study of the MBA implant in the adult flatfoot and reported a 78 percent satisfaction rate.6 The most common complication in this study was sinus tarsi pain, which occurred in 13 of 28 feet (46 percent). This complication required implant removal in 11 patients but none of these implants were removed earlier than eight months postoperatively. Interestingly, radiographic parameters after implant removal remained unchanged with no loss of correction.    Perhaps the most commonly reported complication of arthroereisis procedures is sinus tarsi pain, which occurs in 5 to 10 percent of patients.8 This often resolves with rest, casting, orthotics, cortisone injection or a combination thereof. The prevailing thinking is this pain is both a reactive synovitis and bone contusion from the significant compressive forces generated by and upon the implant. Poor implant sizing, implantation and loosening may play a role as well. These findings appear to be much more common in the adult than in the juvenile patient.    Recently, Kinetikos Medical has released the resorbable bioBlock® arthroereisis implant. This is a near duplicate of the MBA implant in terms of shape, design and the method of implantation. It is made of poly-L-lactic acid (PLLA) and is therefore radiolucent. Once one has inserted the implant, the clinician can obtain an X-ray with the driver in place in order to assess the final position of the implant. Its resorption characteristics are still unknown. According to the company, an 8 mm implant placed in a saline bath to simulate the body’s environment demonstrated no loss of mechanical strength at eight months. However, much is known about PLLA and other bioresorbable materials.

A Guide To The Evolution Of Bioabsorbable Materials

   Foot and ankle surgeons have used bioabsorbables for decades. Reports of absorbable suture, suture anchors, interference screws, screws and pins have all been published in the literature. The ideal absorbable implant should have strength comparable to metallic implants, should remain inert throughout the degradation process and should gradually transfer load to healing bone and soft tissues.9 While companies are still striving for this ideal implant, current materials continue to evolve and gain widespread acceptance within the podiatric community.    Absorbable materials have been available for centuries but synthetic absorbables made their debut in 1962 with the advent of Dexon. Vicryl suture, a co-polymer of polyglycolide (PGA) and polylactide (PLA), soon followed in 1975. Widespread commercial production of absorbable fixation devices began in the late 1980s with the earliest device constructed of polydiaxanone (PDS).10,11    All of the bioabsorbable polymers exhibit unique mechanical properties. These properties are dependent on many variables, including the polymer’s molecular weight, chemical arrangement, structural organization, porosity and purity. The strongest polymers are those which are highly crystalline in structure and have a higher molecular weight.9 When one uses these polymers as orthopedic implants, they do exhibit viscoelasticity. In other words, these implants have dynamic physical properties, which are largely dependent upon load application and time from implantation.12    Early mass production of bioabsorbable materials involved casting the polymers into long, flat films. Today, the implants are made through a process of melt-molding and pouring into casts.13 This process provided the necessary stability and rigidity required for the use of bioabsorbable polymers as fixation devices in lieu of metal implants.10 Another important advancement in the production of bioabsorbable implants was the development of self-reinforced composites. These composites retained the strength characteristics required for rigid fixation while providing the flexibility to closely mirror bone’s modulus of elasticity.10    The biodegradation rate is based upon numerous factors with implant size, implant site, porosity and composition all playing roles. However, the two most important factors in the rate of degradation are a polymer’s hydrophilic properties and crystallinity.12,14 Those materials with hydrophilic components degrade at a quicker rate versus those with hydrophobic components. Also be aware that those with a higher degree of structural organization will degrade at a slower rate than those with decreased crystalline structure.11    The degradation process begins with a decrease in molecular weight. This is followed by decreased strength and ultimately decreased mass.12 Hydrolysis begins the process for PGA, PLA and PDS materials. This causes a decrease in the size of the polymeric chains into smaller byproducts. The byproducts from PGA and PLA (glycolic acid and lactic acid respectively) are partially excreted in the urine. These byproducts are also transformed into pyruvic acid, subsequently used in the Kreb’s cycle, and are ultimately excreted as carbon dioxide and water.9 The byproducts of PDS hydrolysis are mainly excreted via the kidneys with a small amount released as carbon dioxide and in feces.10

Key Pointers On Bioabsorbable Materials

   The three most widely used materials today are PGA, PLLA, and PDS, all of which are alpha-polyesters.12 There are also numerous co-polymers with various combinations of the three polymers.    PGA offers moderately high crystalline structure. It is the most hydrophilic and the stiffest polymer of the three materials. It is also the fastest to degenerate. It loses half its strength within two weeks and complete resorption takes one year.9,11 Researchers have blamed the material’s rapid rate of degeneration for the plethora of synovitic reactions associated with PGA.12    The polymerization of lactic acid can take two forms: dextrorotatory (D) and levorotatory (L). The L form, polymerized L-lactic acid, is highly crystalline and represents the biologically active form.9 Compared with PGA, PLA is more hydrophobic and accordingly has a slower degradation rate. PLLA implants lose half their strength at around 12 weeks.11 Some authors have documented that it may take up to six years for complete degradation of PLLA implants.10    PDS is produced by polymerization of para-dioxanone. While it remains popular as a suture material, it has fallen out of favor as a biomaterial for fixation devices.12

Reviewing The Pros And Cons Of Bioabsorbable Implants

   Bioabsorbable implants have gained popularity for a number of reasons. Perhaps the leading reason is the fact that they do not require a second procedure for removal. Metallic fixation can be problematic due to corrosion, biomechanical stiffness and, most importantly, stress shielding.13 Since the absorbable implants exhibit rigidity closer to that of bone, there is a significant decrease in stress shielding.5 The initial stability is adequate for healing but the implant gradually degrades and the stresses are transferred to the surrounding tissues.13 Another benefit of bioabsorbables is their radiolucency. They will not produce the X-ray scatter on magnetic resonance images or computed tomographic scans associated with metal fixation.15    The most cited complications associated with bioabsorbable implants are sterile sinus tracts, osteolysis at the site of insertion and late foreign body reactions. Other negative attributes include a finite life span, a diminishing strength profile over time and increased cost.9    Out of the aforementioned problems, adverse tissue reactions are of most concern to the surgeon. Böstman and Pihlajamäki reported that 4.3 percent of 2,528 patients experienced a clinically significant local inflammatory, sterile tissue reaction.16 Upon further analysis, the rate of reaction to PGA implants in the study was 5.3 percent versus 0.2 percent for PLA implants. Also, PGA reactions appeared at an average of 79 days versus 4.3 years for the lone PLA reaction.    The reaction presents as a painful, erythematous, fluctuant papule at the site of the implant. Histopathologically, there is a nonspecific, inflammatory foreign body reaction with numerous mononuclear phagocytes and multinucleated foreign body giant cells.16 Radiographically, about one half of patients with an adverse tissue reaction will exhibit osteolysis at the site of implantation.17

What The Studies Reveal About The Efficacy Of Bioabsorable Implants

   Various authors have reported a wide range of applications for bioabsorbable implants in the literature, including the use of these modalities in osteotomies, fractures and arthrodeses. However, the surgeon must remember that in comparison to traditional metal implants, bioabsorbables possess inferior mechanical properties.18 Therefore, surgeons should reserve the use of these implants for instances when minimal load and stress will be applied, when healing will occur before the implant loses significant strength, and when surgeons would remove the implant under normal circumstances.9    Medial malleolar fractures and syndesmotic injuries are currently the most accepted applications for absorbable fixation. Bucholz, et. al., randomized 155 patients with displaced medial malleolar fractures.19 They managed one group with 4.0 mm stainless steel fixation while managing the other group with 4.0 mm PLA screw fixation. There was no significant difference between the groups in terms of fracture healing, postoperative complications or functional results. In addition, at an average follow-up of 37 months, no inflammatory reactions occurred in the PLA group.    Hovis, et. al., followed 23 patients with PLLA screw fixation of syndesmotic injuries for an average of 34 months.18 All patients returned to pre-injury levels of activity within the time of follow-up. There were no reactions to the absorbable material and no secondary procedures were required. The authors conclude that PLLA screws are ideal for fixation of these injuries because compression is not required in syndesmotic repairs.    Thordarson, et. al., compared bioabsorbable fixation to stainless steel screw fixation of syndesmotic injuries in pronation-external rotation ankle fractures.20 In a randomized study, the authors utilized fixation with a 4.5 mm PLA screw for 17 patients and employed a 4.5 mm stainless steel screw for 15 patients. After 11 months, all fractures healed uneventfully. In addition, there were no wound complications, no radiographic osteolytic changes and no inflammatory reactions in the PLA group.    The Lisfranc injury represents another clinical scenario in which approximation has more vital importance than compression. These injuries also typically require hardware removal after standard open reduction internal fixation (ORIF) with metal implants. Accordingly, Lisfranc injuries seem extremely conducive to bioabsorbable fixation. Thordarson and Hurvitz used PLA screws in 14 patients with Lisfranc fractures/dislocations.21 There was no loss of reduction after an average 20-month follow-up. There were also no reports of soft tissue or bone reaction to the implants.    Within a 10-year period, Rokkanen, et. al., performed a total of 2,500 orthopedic procedures using bioabsorbable materials.22 They reported fixation failure in 3.7 percent of the patients with PGA implant cases and encountered non-infectious inflammatory reactions in 2.3 percent of patients with PGA implants. The reaction appeared postoperatively at two to three months. However, no reactions occurred with the use of PLA implants.    Bioabsorbable implant use in pediatric patients is still under debate. Researchers published an experimental study on the effect of PDS implants placed across the growth plate of rabbits in 1989.23 A transphyseal 2.0 mm PDS implant showed no permanent growth disturbance and no histomorphometric change in comparison to a control. Böstman, et. al., published a study of PGA pins in treating 71 fractures in skeletally immature patients. According to the study, 87 percent of patients experienced anatomic healing until union.17 Still, the authors recommended long-term clinical studies before widespread use becomes commonplace for pediatric patients. However, keep in mind that these studies dealt with implanting devices into bone as opposed to soft tissues.    There are two publications on results with resorbable sinus tarsi implants. Giannini initially reported on a four-year follow-up of an absorbable sinus tarsi implant.24 The study evaluated the Stryker Howmedica PLLA bioabsorbable subtalar implant, which is not available in the United States. The authors treated 21 children with flexible flatfoot. The children ranged from 8 to 15 years of age. The researchers performed Achilles tendon lengthening in six feet and performed a modified Kidner in 12 feet.    In a four-year follow-up study, the authors reported that only 5 percent of the patients had pain as opposed to 81 percent of patients who had preoperative pain.24 Whether this pain was related to the implant is not clear. No implants had to be removed. The authors of the study obtained sequential MRIs from three months to five years postoperatively. There was no sinus formation or osseous changes. Changes in implant resorption began to show at six months and fragmentation was noted at one year. However, the overall structure appeared to remain intact. A loss of structural integrity with fragmentation occurred by 18 months. The authors of the study saw complete resorption at four years postoperatively.24    The only reported complication was impingement of implant fragments against the shoe in two patients at one and two years postoperatively. Both resolved with the complete resporption of the material. Keep in mind that the design of the Stryker Howmedica PLLA bioabsorbable subtalar implant is considerably different than the bioBlock implant, which is available in the U.S.    Most recently, Giannini, et. al., published a study involving 12 patients (14 feet). For these patients, they resected a tarsal coalition in the middle facet of the subtalar joint and subsequently implanted the Stryker absorbable sinus tarsi implant to correct the symptomatic flatfoot.25 All patients presented with hindfoot pain preoperatively and only 5 percent had this pain postoperatively. The results showed eight excellent, three good and three fair results subjectively with AOFAS scores improving in all 14 feet based on pain reduction, hindfoot alignment and ROM.25 There was no discussion of the implant other than its value in correcting a pronated foot.

Pertinent Points To Consider

   To answer the question of whether bioresorbable implants work, the answer appears to be yes. Although there are only two published studies on bioresorbable implants in juveniles, both of which are favorable, the design characteristics of the bioBlock implant are nearly identical to those of the well documented MBA implant. Accordingly, one would expect similar results.    As to the question of whether the bioBlock implant is better, the answer is unclear at this time. The company states that one potential advantage is for parents who do not want a permanent metallic implant in their child’s foot. Although this is true, this concern is only raised in a small number of patients.    As noted earlier, subtalar joint pain is the most common complication of arthroereisis devices. If we assume the implant was the correct size and was properly inserted, will resorbable implants lower this complication rate? Sinus tarsi pain is, in part, a reactive inflammatory response secondary to the compressive forces between the talus, calcaneus and the interposed implant device. As the implant begins to resorb and lose its structural integrity, these forces should diminish with subsequent reduction of clinical symptoms.    Needleman noted that, in most patients, the sinus tarsi pain began shortly after weightbearing and ultimately required removal in 9 of 12 feet between seven and 12 months postoperatively.6 Therefore, if the implant does not lose its structural integrity until 18 months postoperatively in children as reported by Giannini, the implant will probably not resorb quickly enough to prevent its removal secondary to sinus tarsi pain.24 Resorption times for bioabsorbable subtalar joint implants are unknown in adults but probably exceed that of children due to decreased vascularity.    Kinetikos Medical recommends maximum patient body weights for each diameter implant. For an 8 mm implant, the maximum body weight would be 150 lbs. For a 9 mm implant, it would be 180 lbs. For a 10 mm implant, it would be 220 lbs. For an 11 mm implant, it would be 250 lbs. For a 12 mm implant, it would be 250 lbs.    Exceeding these limits may cause the implant to deform. It is unlikely this alteration of shape will result in loss of correction but it may reduce compressive forces within the sinus tarsi. Additionally, the implant, due to its viscoelastic properties, does have the capacity to deform under pressure. These factors may lessen the incidence of sinus tarsi pain.    Loosening is rarely a problem with most sinus tarsi implants that are properly inserted. Soft tissue ingrowth helps to stabilize these implants, which are not inserted into bone. Scar tissue encompasses the implant and occupies the spaces between the threads and central cannula. With resorbable implants, as the structure loses its integrity, this stability will be lost and implant loosening may occur. If removal of the implant or its fragments is required, this could prove difficult as the material is radiolucent.    Although the incidence of adverse reactions to PLLA is less than 1 percent, foreign body reactions are still possible, especially when the implant fragments and shards are dispersed into the soft tissues.    The final and perhaps most important question to consider is the long-term efficacy of resorbable sinus tarsi implants for the correction of flatfoot deformity. Needleman demonstrated no significant change in radiographic parameters after implant removal at an average follow-up time of nearly four years.6 All patients had either a heel cord lengthening and most had adjunctive forefoot procedures so it is difficult to isolate the effect of implant removal. However, these findings are consistent with other reports in the literature (among both adults and children) of persistent correction despite implant removal or resorption. An “arthrofibrosis” of the subtalar joint appears to develop and this may ultimately limit motion in the implant’s absence.

Final Notes

   The bioBlock represents another advance in implant technology. Although it does present some potential advantages over current implant devices, there are no studies demonstrating its success or complication rates. Since it is not designed to be more effective than current arthroereisis implants and until more clinical data is published, surgeons may want to reserve the device for use in cases in which they are dissatisfied with the results of other implants. Dr. Boberg is the Director of Residency Training at the Forest Park Hospital in St. Louis. He is a faculty member of the Podiatry Institute and is in private practice in St. Louis. Dr. Boberg can be reached via e-mail at: nailnpr@aol.com. Dr. Oldani is a third-year resident at Forest Park Hospital in St. Louis. Dr. Martin is a third-year resident at Forest Park Hospital in St. Louis. Editor’s note: For related articles, see “Assessing The Pros And Cons Of Subtalar Implants” in the May 2006 issue of Podiatry Today. Also check out the archives at www.podiatrytoday.com.
 

References:

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