Understanding The Biomechanics Of Subtalar Joint Arthroereisis
Is a commonly utilized classification scheme for subtalar arthroereisis implants “biomechanically inaccurate and ambiguous”? With a thorough review of the literature, this author discusses kinematic and kinetic functions of the subtalar joint, and the biomechanical effects of the subtalar arthroereisis procedure.
The goal of the subtalar joint arthroereisis is to reduce the pronation range of motion of the subtalar joint (STJ) in order to create a positive therapeutic change in the alignment and function of the foot and lower extremity during weightbearing activities. The term “arthroereisis” comes from the Greek words arthron meaning “joint” and ereisis meaning “a raising up,” and is defined as an “operative limiting of the motion in a joint that is abnormally mobile from paralysis.”1 More specific to podiatric surgeons, Maxwell and Cerniglia have defined subtalar arthroereisis as a surgical procedure to prevent excessive pronation and preserve varus range of motion within the STJ.2
Chambers was the first author to discuss a surgical procedure that altered the geometry of the sinus tarsi in order to reduce the pronation range of motion of the STJ. Chambers described placing a bone graft anterior to the lateral process of the talus on the floor of the sinus tarsi of the calcaneus to block excessive STJ pronation in pathological flatfoot deformities.3 In 1974, Subotnick described the first modern STJ arthroereisis procedure in which an inert silicone elastomer block was shaped to form a plug with subsequent placement within the sinus tarsi to limit STJ pronation.4 Smith was the first to detail the use of an ultra-high molecular weight polyethylene plug, called the “STA-peg,” which had a stem that could be inserted into the floor of the sinus tarsi to help reduce excessive pronation.5
In 1987, Valenti described the first “screw in” cylindrical-shaped STJ arthroereisis implant, which utilized external screw threads to allow easier implantation into the sinus tarsi.6 Valenti’s threaded polyethylene threaded implant was followed in 1997 by a similarly designed, cylindrical threaded titanium Maxwell-Brancheau arthroereisis (MBA) implant.7 Other current STJ arthroereisis implants include the HyproCure (GraMedica), the Futura Conical Subtalar Implant (Tornier) and Bioarch (Wright Medical). The bioBLOCK implant (Integra) is a cylindrical threaded implant that is made of poly-L-lactic acid and is resorbable.8
From this brief historical analysis, it becomes evident that foot surgeons have been using STJ arthroereisis implant techniques for over 35 years and have routinely reported good short-term results.9-11 However, researchers recently reported on a mid- to long-term study — with a mean postoperative period of 12.6 years — which involved the use and clinical evaluation of an arthroereisis implant in 44 juvenile flatfoot deformities.12 The arthroereisis implant, which consisted of a screw inserted into the floor of the sinus tarsi, was only in use for 12 months. At the end of the study period, the study authors found normal alignment in 14 of 44 feet, mild malalignment in 26 of 44 feet and four of 44 feet showing a return to severe malalignment.