Standing STJ Axis Location Technique

Standing Subtalar Joint Axis Location Technique The axis of motion of the subtalar joint (STJ) should be of special interest to all podiatrists since the STJ axis spatial location relative to the plantar aspect of the foot largely determines the kinetics (i.e. forces and moments) of the STJ during weightbearing activities (Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001). The first technique described within the scientific literature that determined the plantar representation of the STJ axis was published 30 years ago, and involved the examiner using their thumb to repeatedly push on the plantar foot to determine the points where neither pronation nor supination motion was produced. Connecting these “points of no rotation” then represented the STJ axis plantar location which allowed the clinician to determine whether their patient’s STJ axis was normally located, medially or laterally deviated within the transverse plane (Kirby KA: Methods for determination of positional variations in the subtalar joint axis. JAPMA, 77: 228-234, 1987). One of the biggest problems with this STJ Axis Palpation Technique (SAPT) is that it is performed in a non-weightbearing setting. During the performance of the SAPT, the medial aspect of the patient’s plantar forefoot is completely unloaded since the examiner’s hand supports the plantar forefoot only under the 5th metatarsal head. The lack of plantar loading on the medial forefoot during the SAPT may lead to an underestimation of the amount of STJ axis medial deviation present in the weightbearing foot since the medial forefoot may dorsiflex (i.e. forefoot may invert) relative to the rearfoot upon standing and/or walking. In addition, the STJ axis location may also be difficult to determine in feet with excessive inverted forefoot deformities (i.e. forefoot varus/supinatus deformities) since the STJ will already be in the maximally pronated position in these feet during the SAPT. If the STJ is already maximally pronated during performance of the SAPT, when the examiner pushes lateral to the STJ axis, the STJ can pronate no further, thus making it difficult to determine the points of no rotation when performing the SAPT (Kirby KA, 1987). Approximately 15 years ago, due to these problems inherent in performing the SAPT, I began to develop a technique by which to assess the STJ axis spatial location while the patient was weightbearing, in relaxed bipedal stance. This new clinical method of STJ axis determination, which I call the Standing Subtalar Joint Axis Location Technique (SSALT), is based on my observations from using the clinical technique first described by Morris and Jones where pen grids on the posterior calcaneus and anterior talar neck are used to determine the posterior and anterior exit points of the STJ axis from the foot (Morris JL, Jones LJ: New techniques to establish the subtalar joint’s functional axis. Clin Podiatr Med Surg,11: 301, 1994). To perform the SSALT, the patient is asked to stand in relaxed bipedal stance while the examiner uses a pen to draw a “rectangular box” which outlines the superior-lateral quadrant of the posterior calcaneus (Fig. 1). An “X” is then placed in the center of that “box”, which represents the posterior exit point of the STJ axis. An “X” is also drawn centered over the dorsal aspect of the talar neck anteriorly which represents the anterior exit point of the STJ axis. Then, using a pen to represent the STJ axis, one end of the pin is placed on the “X” at the dorsal talar neck while the examiner then aligns the longitudinal axis of the pen so that it points, through the foot, towards the “X” located at the center of the superior-lateral quadrant of the posterior calcaneus. If performed accurately, the 3D location of the examiner’s pen now represents the approximate location of the STJ axis spatial location, allowing the visualization by the examiner of the STJ axis location relative to the structures of the dorsal foot. Not only does the SSALT allow the STJ axis to be represented visually in 3D space, showing both its transverse plane alignment and its sagittal plane inclination, but it also allows the STJ axis to be located in a weightbearing setting. Of course, the exact 3D location of the STJ axis cannot be determined using either the SSALT or the SAPT. However, the relatively new SSALT is a valuable method by which to clinically assess the STJ axis spatial location to help determine the best foot orthosis modifications and/or surgical procedures to treat our patient’s foot and lower extremity mechanically-based pathologies. [Reprinted with permission from the July 2017 Precision Intricast Newsletter in: Kirby KA: Foot and Lower Extremity Biomechanics V: Precision Intricast Newsletters, 2014-2018. Precision Intricast, Inc., Payson, AZ, 2018, pp. 87-88.]