Bioabsorbable Implants For Flatfoot: Can They Work?
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.