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.
Reviewing The Vogler Classification Scheme For STJ Arthroereisis Implants
Vogler proposed the most commonly used classification scheme for the different STJ arthroereisis implants over 23 years ago.13 Vogler classified arthroereisis implants into three categories based on his view of how these implants functioned mechanically. These implant categories were implants that had a stable self-locking wedge, those that were axis-altering and those implants that had a direct impact.
Vogler claimed that the implants with a stable self-locking wedge work form a self-locking wedge within the sinus tarsi that “does not alter STJ axis but rather restricts its range to the neutral.”13 The axis-altering implant “elevates a pathologically low STJ axis and reduces the amount of frontal plane STJ motion,” according to Vogler. Lastly, Vogler noted the direct impact implant functions to create an “impingement effect where the talar body/process makes direct contact with the prosthesis.”
Even though Vogler’s classification scheme for STJ arthroereisis implants continues to be promoted by the podiatric medical community as the best method to describe the mechanical function of these implants, it becomes clear with a detailed analysis of the biomechanics of the STJ arthroereisis implant procedure that there are significant problems with this classification method.
Why All STJ Implants Are ‘Direct Impact’ And ‘Axis-Altering’ Implants
In order to understand why Vogler’s classification scheme may be a biomechanically inaccurate and ambiguous method by which to describe the function of STJ arthroereisis implants, it is important to understand the kinematic and kinetic functions of the STJ and its joint axis.
The STJ consists of the posterior, middle and anterior articular surfaces of the talus and calcaneus that allow the rotational and translational motions of pronation and supination. Subtalar joint pronation and supination are restrained and guided not only by the complex three-dimensional osseous geometry of the three articular facets of the talus and calcaneus, but are also restrained and guided by the strong and tight restraining ligaments within the sinus tarsi and the tarsal canal.14 With STJ pronation, the talus adducts, plantarflexes, inverts and anteriorly translates relative to the calcaneus. With STJ supination, the talus abducts, dorsiflexes, everts and posteriorly translates relative to the calcaneus.15,16 The main anatomic structures that cause the mechanical end point of STJ pronation rotational motion (i.e. maximally pronated position) are the anterior surface of the lateral process of the talus and the floor of the sinus tarsi of the calcaneus.17,18
When the STJ is in the maximally supinated position, the tip of the talar lateral process is positioned posteriorly and superiorly up on the posterior articulating facet of the calcaneus, leaving a relatively large void within the sinus tarsi. During pronation motion from the maximally supinated position, the talar lateral process will glide anteriorly and inferiorly along the posterior articular facet of the calcaneus. Once the talar lateral process directly impacts the floor of the sinus tarsi, STJ pronation will be stopped by this osseous contact of the talus on the calcaneus and one may consider the STJ to have reached its maximally pronated position. Once the talar lateral process has contacted the floor of the sinus tarsi, the sinus tarsi floor will push in a posterior-superior direction onto the talar lateral process to prevent further STJ pronation.
Therefore, the compression forces that are exerted by the floor of the sinus tarsi in an posterior-superior direction and by the talar lateral process in an inferior-anterior direction effectively cause the end point of the pronation range of motion of the subtalar joint, the maximally pronated position.17,18
With this detailed biomechanical analysis of the forces that cause the maximally pronated position of the STJ, it becomes clear that all STJ arthroereisis implants, contrary to Vogler’s classification scheme, are “direct impact” implants.
Subtalar joint arthroereisis implants, regardless of their shape, material composition or size, all rely on compression forces at the implant-calcaneus interface and implant-talus interface to create the forces that are necessary to reset the STJ maximally pronated position to a new, more supinated position postoperatively. The surgical addition of any type of spacer material within the sinus tarsi and/or tarsal canal, as long as it has sufficient resistance to compression deformation, will alter the maximally pronated position of the STJ. In other words, all STJ arthroereisis implants, since they all function as “compression spacers” within the sinus tarsi and/or tarsal canal, will cause a resetting of the maximally pronated position of the STJ postoperatively.
One should also consider all STJ arthroereisis implants, regardless of shape, material composition or size, to be “axis-altering implants,” again in conflict with Vogler’s arthroereisis classification scheme.
Due to the relatively constant exit point of the STJ axis from the dorsal talar neck anteriorly and from the posterior-lateral calcaneus posteriorly, the STJ axis will rotate and translate in space relative to the plantar foot and ground with closed kinetic chain STJ pronation and supination.16-23 When the STJ supinates, the STJ axis abducts, dorsiflexes and translates laterally relative to the plantar foot and ground. When the STJ pronates, the STJ axis adducts, plantarflexes and translates medially relative to the plantar foot and ground.17,18,24-29
When the podiatric surgeon places any arthroereisis implant into the sinus tarsi and/or tarsal canal, this resets the maximally pronated position of the STJ into a more supinated (i.e. less pronated) rotational position postoperatively. Subsequently, during weightbearing activities, the STJ spatial location at the maximally pronated position will also be altered. Since the STJ axis exits anteriorly at the dorsal neck of the talus and since its exit point stays in a relatively constant location at the dorsal talar neck during range of motion of the STJ, as the STJ is supinated away from its maximally pronated position by the STJ arthroereisis implant, the external rotation and dorsiflexion of the talar head and neck that occurs will also cause an abduction and dorsiflexion of the STJ axis.16 As a result, any arthroereisis implant will make the talus and the STJ axis become more abducted and dorsiflexed in the maximally pronated STJ position postoperatively.
Therefore, all STJ arthroereisis implants, regardless of design, material and size, are “axis-altering” devices since all arthroereisis implants alter the postoperative spatial location of the STJ axis during weightbearing activities.
What The Literature Reveals About The Biomechanical Effects Of STJ Arthroereisis
The result of such significant changes in the STJ axis spatial location from an extremely medially deviated location to a more normal location while the foot is functioning in the maximally pronated position can have profound biomechanical effects for the patient with a pathologic flatfoot deformity.
The increased medial deviation of the STJ axis that occurs with excessive STJ pronation in the pediatric and adult flatfoot causes increased magnitudes of STJ pronation moments not only from the external forces acting on the foot from ground reaction force (GRF), but also from the internal forces acting within the foot from muscle/tendon tensile forces.17-18,24-30 Due to its ability to significantly alter the STJ axis spatial location to a more normal spatial location, the subtalar arthroereisis implant has the potential to reduce external STJ pronation moments and increase external STJ supination moments in order to normalize gait function and reduce the frequency of injuries caused by excessive magnitudes of external STJ pronation moments.
Even though the STJ arthroereisis procedure primarily affects the relative positions of the talus and calcaneus, the procedure may also have rather profound effects on other pedal osseous structures.
Christensen and colleagues have clearly shown that the subtalar joint arthroereisis implant not only significantly alters the three-dimensional positions of the talus and calcaneus, but also significantly alters the three-dimensional positions of the cuboid and navicular.31 Due to its ability to change the osseous geometry of the foot, the arthroereisis procedure also has the potential to produce negative biomechanical effects on the foot and lower extremity.
In my own clinical experience, patients who have had an overcorrection with a subtalar joint arthroereisis procedure have experienced lateral dorsal midfoot interosseous compression syndrome.26,29,32 Excessive STJ supination with an oversized implant will cause an inverted position of the forefoot, which allows overloading of the lateral column and prevents normal plantar loading of the medial column.33 Overcorrection with arthroereisis implants may also cause an unmasking of metatarsus adductus deformities that may lead to excessive intoeing during gait.34-36
In addition, placement of arthroereisis implants into patients who are obese, have a significant gastrocnemius/soleus/ankle equinus deformity or have an excessively medially deviated STJ axis may result in the implant being subjected to extremely high magnitudes of interosseous pressures within the sinus tarsi. Such high sinus tarsi bone compression forces, over time, have the potential to lead to chronic sinus tarsalgia which is one of the most common complications from arthroereisis.37-41 Researchers have also reported pathologic bony changes in the sinus tarsi as a result of arthroereisis procedures that may cause chronic sinus tarsi symptoms and necessitate early excision of the arthroereisis implant.33,42-45 Fortunately, in the event that one of these postoperative complications do occur, one may readily remove the arthroereisis implant from the foot with minimal sequelae.
In conclusion, the modern STJ arthroereisis implant procedure has been in constant use for the surgical correction of flatfoot deformity in children and adults now for over 35 years, and is widely utilized for both the pediatric and adult flatfoot deformity. By acting as a “compression spacer” within the sinus tarsi and/or tarsal canal, the arthroereisis implant “resets” the maximally pronated STJ rotational position to a new, less pronated, postoperative STJ rotational position.
However, contrary to Vogler’s long-accepted arthroereisis implant classification scheme, all STJ arthroereisis implants are “direct impact” implants, which create compression forces between the talus and calcaneus that reduce STJ pronation. Additionally, all arthroereisis implants are “axis-altering” implants due to the change in STJ spatial location that occurs with resetting the maximally pronated STJ position to a new, more supinated postoperative rotational position.
Further scientific research and more long-term studies of arthroereisis implant patients will be necessary to better clarify both the positive and negative alterations in foot and lower extremity biomechanics that may occur from this relatively popular surgical procedure for the pathologic flatfoot deformity.
Dr. Kirby is an Adjunct Associate Professor in the Department of Biomechanics at the California School of Podiatric Medicine at Samuel Merritt University. He is the Director of Clinical Biomechanics at Precision Intricast, Inc.
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33. Banks AS, Downey MS, Martin DE, Miller SJ. McGlamry's Comprehensive Textbook of Foot and Ankle Surgery. Lippincott Williams & Wilkins, 2001, p. 913.
34. Roye DP, Raimondo RA. Surgical treatment of the child's and adolescent's flexible flatfoot. Clin Podiatr Med Surg. 2000;17(3):515-530.
35. Smith PA, Millar EA. STA-peg arthroereisis for treatment of the planovalgus foot in cerebral palsy. Clin Podiatr Med Surg. 2000;17(3):459-469.
36. Green DR, Williams M, Kim C. Assessing the pros and cons of subtalar implants. Podiatry Today. 2006;19(5):36-46.
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39. Nelson SC, Haycock DM, Little ER. Flexible flatfoot treatment with arthroereisis: radiographic improvement and child health survey analysis. J Foot Ankle Surg. 2004;43(3):144-155.
40. Zaret KI, Myerson MS. Arthroereisis of the subtalar joint. Foot Ankle Clin N Am. 8:605-617, 2003.
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43. Tompkins MH, Nigro JS, Mendicino SS. The Smith STA-Peg: a 7-year retrospective study. J Foot Ankle Surg. 1993;32(1):27-33.
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For further reading, see “Keys To Preventing Complications With Subtalar Joint Implants” in the January 2010 issue of Podiatry Today or “Assessing The Pros And Cons Of Subtalar Implants” in the May 2006 issue.