Bioabsorbable Implants For Flatfoot: Can They Work?
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.