Bioabsorbable Implants For Flatfoot: Can They Work?
- Volume 19 - Issue 9 - September 2006
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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.
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: firstname.lastname@example.org.
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.