Keys To Intraoperative Positioning For Subtalar Joint Procedures

Kelley Wallin, DPM, and Donald Green, DPM, FACFAS

Subtalar joint arthrodesis and subtalar joint arthroereisis are widely used surgical procedures for eliminating motion or limiting excessive pronation at the subtalar joint.1-3

   Although the indications for either procedure may be vastly different, the successful outcome of both relies on proper positioning of the subtalar joint. A poorly positioned fusion or arthroereisis may result in continued pain and disability, gait dysfunction, or excessive stress on adjacent joints.4,5 Careful preoperative and intraoperative planning are required in order to ensure the best possible outcome.

   Podiatric physicians should perform a thorough preoperative biomechanical evaluation on every patient they are considering for a possible subtalar joint arthrodesis or arthroereisis.6-8 Paramount in this evaluation is to determine the optimal position of the heel relative to the ground. When the patient is lying on the operating table, it is often very difficult for the surgeon to determine the proper heel position if he or she did not first obtain a weightbearing, preoperative reference point. Without this point of reference, the surgeon may fail to take into account frontal plane deformities of the hip, knee and forefoot that may ultimately result in an undesirable position of the heel.

   For subtalar joint fusion, one should ensure positioning of the heel in approximately 2 to 4 degrees of valgus relative to the ground in order to maximize the stability of the medial column and prevent rigidity and overload of the lateral column. When it comes to arthroereisis, surgeons should place the heel in a position that is approximately 4 to 6 degrees less everted than its maximally pronated position relative to the ground. This position allows some compensatory pronation for shock absorption while still therapeutically limiting the excesss pronatory motion of the subtalar joint.

   Fusing the heel in a varus position or overcorrection of an arthroereisis implant will prevent normal plantar loading of the medial column, and lead to lateral column overload. Conversely, too much valgus or under-correction of arthroereisis can lead to calcaneofibular impingement or less effectiveness of the implant correction in providing the desired effect. Accordingly, success or failure of either procedure depends heavily on obtaining a correct position of the heel relative to the ground.9,10

A Guide To Pertinent Measurements In The Preoperative Biomechanical Exam

During the preoperative biomechanical exam, the patient should be weightbearing in order to measure the neutral calcaneal stance position (NCSP), relaxed calcaneal stance position (RCSP), and the ideal final position of the heel relative to the ground (i.e. 2 to 4 degrees of valgus for fusion and 4 to 6 degrees less eversion than in maximum pronation for arthroereisis).

   With the patient standing and the posterior heel facing the examiner, one should draw a line on the distal, posterior leg, parallel to the longitudinal axis of the tibia. The examiner places the patient in a neutral calcaneal stance position and marks the posterior heel in line with the longitudinal axis of the tibia. Then shift the patient to a maximally pronated (everted) position and mark the posterior heel to reflect the eversion of the heel. This is usually the relaxed calcaneal stance position.

   The final mark represents the ideal position of the heel relative to the ground. This almost always falls between the two previous marks of both the neutral position and maximally pronated position.

   This desired final position of the heel is a reproducible “distance” (or amount of pronation/supination) relative to both the neutral calcaneal stance position and the maximally pronated position. Therefore, as long as the examiner can determine both the neutral position and the maximally pronated position intraoperatively, the examiner can reproduce the ideal final position of the heel by measuring or accurately estimating the same amount of pronation/supination that he or she obtained preoperatively. It will be the predetermined position from the neutral position and/or from the maximally pronated position.

Ensuring Optimal Intraoperative Positioning

One may place the patient in the supine or lateral decubitus position intraoperatively. It is not necessary for the patient to be prone in order to reproduce the ideal position of the heel that the surgeon obtained during the preoperative examination. Ensure that the patient’s leg is prepped and draped to the knee in order to allow visualization of the lower leg.

   First draw a line along the distal, anterior leg parallel to the longitudinal axis of the tibia. Then determine the neutral subtalar joint position by manual manipulation and palpation of the talar head. Extend the leg line onto the dorsum of the foot with the foot in its neutral position. Draw a line parallel to and in line with the mark on the distal anterior leg. In the same manner, shift the patient’s foot to a maximally pronated position of the subtalar joint and extend the leg line once again onto a corresponding mark placed on the dorsum of the foot.

   The surgeon has now reproduced the same two reference points he or she obtained in the preoperative examination (i.e. the neutral subtalar position and the maximally pronated subtalar joint position). One can now easily identify the ideal subtalar joint position for fusion or arthroereisis by pronating or supinating the foot the desired amount relative to the two previous marks. The surgeon can usually mark this position with a dotted line.

   This ideal position, which surgeons would determine preoperatively, should be equal to the same relative amount of heel eversion from the neutral position or heel inversion from the maximally pronated position that he or she measured during the weightbearing preoperative exam.

   The surgeon can now proceed with the surgical procedure, taking care to refer back to the marks for final positioning of the foot. He or she can be assured of the proper positioning of the foot to the ground postoperatively when the final placement of the foot allows the dotted line to be a straight line extension of the leg line intraoperatively.

In Conclusion

Subtalar joint arthrodesis and arthroereisis can be powerful surgical procedures to reduce pain and disability, and restore or improve functionality of the foot.

   However, successful outcomes depend on correct and accurate intraoperative positioning of the foot. A surgeon should always perform a preoperative weightbearing exam to determine the ideal position of the heel for fusion or arthroereisis relative to the ground. Failure to take weightbearing into account can lead to frontal plane deformities and an undesirable final position of the foot.

   By preoperatively determining the ideal heel position relative to the neutral position and maximally pronated position, the surgeon can accurately reproduce the proper foot position intraoperatively and ensure the best possible outcome.

   Dr. Wallin is a second-year resident with the Scripps Mercy Kaiser Podiatric Residency Program at the Scripps Mercy Medical Center in San Diego.

   Dr. Green is the Residency Director of the Scripps Mercy Kaiser Podiatric Residency Program at the Scripps Mercy Medical Center in San Diego. He is a Clinical Professor at the California School of Podiatric Medicine at Samuel Merritt University, and is a Fellow of the American College of Foot and Ankle Surgeons. Dr. Green is also a faculty member of the Podiatry Institute and has a private practice in San Diego.

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7. Kirby KA. Biomechanics of the normal and abnormal foot. J Am Podiatr Med Assoc. 2000;90(1):30-34.
8. Piazza SJ. Mechanics of the subtalar joint and its function during walking. Foot Ankle Clin N Am. 2005;10(3):425-442.
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10. Tuijthof GJ, Beimers L, Kerkhoffs GM, Dankelman J, Dijk CN. Overview of subtalar arthrodesis techniques: options, pitfalls and solutions. Foot Ankle Surg. 2010;16(3):107-16.

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   For further reading, see “Understanding The Biomechanics Of Subtalar Joint Arthroereisis” in the April 2011 issue of Podiatry Today, “How To Address Subtalar Joint Instability” in the May 2007 issue or “Assessing The Pros And Cons Of Subtalar Implants” in the May 2006 issue.

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