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.