Emphasizing fundamental biomechanical principles, these authors discuss comprehensive evaluation and management of midtarsal joint pathology, including salient anatomical insights, key orthotic considerations and practical treatment applications.
As the demands of the modern medical education model shift to support a more encompassing knowledge base and a mandatory baseline understanding of surgical principles needed to pass podiatry licensing boards and secure residency placement, there is a perception that schools of podiatric medicine may be scaling back the traditional biomechanical component of the curriculum. While the extent of this phenomenon certainly varies depending on which school of podiatric medicine one attends, there is also a perception that the biomechanical understanding of new podiatric practitioners is not on par with past generations and some may even consider it inadequate.1
With this in mind, we would like to specifically focus on demystifying biomechanical principles related to the midfoot and describe simple treatment protocols you can implement in your office tomorrow to help patients suffering from midtarsal joint dysfunction that is causing or confounding midfoot pathology.
Clarifying The Role Of The Subtalar Joint In Midtarsal Joint Biomechanics
When it comes to subtalar joint (STJ) function, clinicians have historically utilized a calcaneal bisection reference to help assess the joint’s position and motion. Podiatrists have also universally viewed the STJ as a primary reference for ascertaining efficient or inefficient foot function. In contrast, static longitudinal arch height contour and, more recently, changes in arch height have been key emerging indicators for helping to determine midtarsal joint (MTJ) efficiency in our clinical experience.
The STJ does react to and help control the effectiveness of other joints proximal and distal to it. During the loading response (contact) phase, the STJ has its most significant influence on efficient weight acceptance linkages especially as this influences the ability of the midtarsal joint to adapt to the ground. Later in stance phase, the STJ assists in stabilization of the midfoot under full body weight and continues affecting midtarsal joint stability after heel-off.2
A Closer Look At The Two-Axis Model Of Midtarsal Joint Function
The longitudinal midtarsal joint axis is an imaginary line that runs axially through the plantar calcaneal tuberosity, anteriorly through the talonavicular joint and exits dorsally through the medial cuneiform. This axis lies primarily in the sagittal plane with slight deviation in the transverse plane. Accordingly, the motion associated with this axis occurs mostly in the frontal plane.2,3
The oblique midtarsal joint axis lies just proximal to the calcaneocuboid joint and runs from the plantar lateral anterior process of the calcaneus to an exit dorsomedial through the talonavicular joint.2,3 The obliquity of this axis provides some transverse plane motion in the propulsive phase of gait but sagittal plane motion dominates in the stance phase.2 The literature surrounding this phenomenon is somewhat controversial due to inconsistencies in attempts to quantify the parameters of this theory.3 Although some have suggested this theory is merely a convenient explanation of clinical observations, the two-axis theory of triplanar motion traditionally ascribed to the rearfoot, midfoot and first ray is supported by a clinically reliable understanding of typical movement patterns and continues to be a cornerstone of foot and ankle biomechanics.2,3
The STJ has particular influence on the immediately adjacent joints making up the midtarsal joint as well as the facets of the talonavicular and calcaneocuboid joints. Axes of motion around these joints are commonly recognized collectively as the longitudinal midtarsal joint axis, which corresponds to the talonavicular joint and the oblique midtarsal joint axis, which corresponds to the calcaneocuboid joint. These axes are generally recognized as having a more parallel orientation as the STJ pronates and a convergent orientation as the STJ supinates.4-6
Motion around a parallel axis orientation makes motion available at both the longitudinal and oblique midtarsal joint axes, and suppresses motion around both axes as their orientations converge with STJ supination.4-6 While illustrations of axes may best describe this relationship on paper, the actual mechanical construct of motion availability or suppression in the authors’ experience is more likely determined by the relative orientation of the midtarsal joint articular surfaces as the STJ pronates and supinates (see first figure above). The talonavicular joint (a ball and socket joint) and the calcaneal-cuboid articulation (a planar joint) change in position relative to each other as the STJ pronates or supinates, resulting in a more parallel or convergent domed medial joint surface to lateral planar/saddle surface orientation.4-6 Consider the similarity with opening a hardback book and standing it up on its covers. The book will be stable and stand on its own with the covers at a 90-degree angular orientation as opposed to the book covers being parallel to each other.
The STJ and midtarsal joint also react to the mobility and stability of either joint. The two-way street of STJ/midtarsal joint efficiency begins when the heel hits the ground and the STJ pronates, making motion available to the midtarsal joint, which is necessary for efficient supple adaptation to the supporting surface. This efficiency continues as the STJ begins to supinate in response to external limb rotation, suppressing motion around both the longitudinal and oblique midtarsal joint axes during midstance while full weightbearing is pressing down on both joints. This dual action-reaction of STJ pronation allows the foot and limb to effectively attenuate ground reaction forces up the kinetic chain and then changes to provide increased midtarsal joint stability secondary to suppressed motion as the pelvis advances over the foot.4,5
Keys To Assessing And Controlling Midtarsal Joint Instability
One may treat excessive or restricted midtarsal joint motion (availability or suppression) by using foot orthoses to control both STJ pronation during the loading response as well as the orientation of both midtarsal joint axes and ultimately overall stability during midstance.
While there is currently little if any average or expected open chain midtarsal joint range of motion parameters to reference, there is an ongoing study that is using electronic gyroscopic transmitters that record position and open chain range of motion of the longitudinal midtarsal joint axis and oblique midtarsal joint axis. As subsequent data emerges, we should be able to fully develop reliable parameters for determining clinical limits and ranges of longitudinal and oblique midtarsal joint motion. However, until such data sets are ready for publication and available, we believe using physical movement techniques may currently provide the best methods of determining midtarsal joint range of motion.
One can make a reasonably reliable assessment of the global midtarsal joint range of motion by evaluating mobility and flexibility with palpation. With the patient in a supine or long sitting position, the clinician can easily evaluate non-weightbearing mobility by grasping the calcaneus with an inside hand and using it to pronate the STJ. This makes motion available at both midtarsal joint axes. With an outside hand, reach across the midfoot at the level of the midtarsal joint/lesser tarsus with your thenar webspace along the inferior lateral cuboid. Now you can assess longitudinal midtarsal joint axis mobility by inverting and everting the midfoot (see second photo above). Using the same hand hold positions, an examiner can also qualify oblique midtarsal joint axis mobility through adduction/plantarflexion and abduction/dorsiflexion of the midfoot.
Without reliably established limits of end range to end range motions, our experience suggests that the best ways to describe midtarsal open chain mobility/range of motion include the following categories.
- Restricted/tight: a feeling of tense/stiff/contracted ligaments or muscular tone.
- Within normal limits: having reasonably adequate motion around both axes without being tight or loose.
- Loose: something close to a feeling of ligamentous laxity.
Have the patient perform a sit to stand maneuver while observing navicular drop. This is a good indication of how much drop occurs during gait and one can easily measure this by placing a block labeled with a measuring scale (mm or inch) next to the medial apex of the longitudinal arch. Mark the navicular tuberosity and see how much displacement there is as the patient changes from sitting to standing posture. Once you are familiar with what appears to be a 10 to 12 mm drop, subsequent assessments will become easier to estimate. Selection of orthotic shell stiffness is less dependent on measured drop than qualifying what is an excessive drop and the need for increased rigidity to control navicular displacement.
When it comes to feet with loose longitudinal midtarsal joint axis motion exhibiting excessive navicular drop, one can best control these feet with stiffer, less flexible orthotic shells/plates that effectively restrict navicular displacement, which is commonly associated with excessive available motion in that axis.7,8 High medial flanges could be applicable when navicular drop exceeds 10 to 12 mm for males with size nine shoes or females with size 10 shoes on a Brannock Device.9 In our clinical experience, the larger the foot, the more navicular drop one can expect. The smaller the foot, the less navicular drop you would expect. Clinicians can measure or estimate navicular drop depending on how familiar they are with visually identifying displacement.
For feet with a restricted/tight longitudinal midtarsal joint axis that exhibit little or no navicular drop in midstance, one should utilize less stiff/more flexible orthotic shells, or modules made from forming closed cell foams (Plastazote™) or semi-soft ethylene-vinyl acetate (EVA) that do not incorporate a polypropylene plastic module/plate.
What Is The Impact Of Forefoot Supinatus?
Excessive STJ pronation into and through midstance compromises midtarsal joint stability at a time when mobility should be decreasing. Resulting arch collapse is directly associated with motion around the longitudinal midtarsal joint axis, primarily inversion, a component motion of supination. Over time, the ligaments on the plantar aspect of the midfoot plastically lengthen secondary to continuous tension and shorten on the dorsal aspect in response to closer osseous proximity positioning. This produces an inverted forefoot to rearfoot relationship, which is colloquially referred to as forefoot supinatus. Many commonly misidentify this soft tissue malalignment as a forefoot varus, which is a fixed osseous deformity.
Treating forefoot supinatus with forefoot posting is contraindicated as elevating the medial column further increases the malalignment, resulting in additional soft tissue contracture on the dorsal aspect of the midtarsal joint and lengthening of the plantar ligaments.
In our clinical experience, forming the orthotic shell over a positive cast modified by filling plaster across the lateral aspect of the plane forefoot (primarily beneath the lateral column) to create a perpendicular forefoot to rearfoot alignment is an effective orthotic application to remodel the forefoot to rearfoot relationship (see third figure above). When the forefoot supinatus foot type loads onto this newly aligned plate, it applies dorsiflexory force to the lateral column. This also promotes plantarflexion of the medial column while maintaining longitudinal arch contour. This technique results in reduction of forefoot supinatus malalignment and maintains more efficient alignment over time.
While this new alignment treats the pathological forefoot position, it does not address the biomechanical condition that created the acquired deviation exhibited in a forefoot supinatus foot type. The STJ on loading response and into midstance still allows for the compromised midfoot. On loading response, a STJ that excessively pronates to compensate for limb/pelvic influences or an inverted calcaneal position (rearfoot varus) will abruptly accelerate after heel strike, causing the calcaneus to evert farther and faster than it would in feet with less compromising alignments. Rapid calcaneal displacement develops sufficient inertia to increase the eversion moment, resulting in available midtarsal joint motion into and through midstance when motion suppression should stabilize the midfoot. As the calcaneus remains everted in midstance, the arthrokinematic response at the talonavicular joint is relative inversion, a component motion of supination. Over the many thousands of steps a patient may take daily, ligamentous structures in the midfoot adapt to this inverted forefoot position, making the forefoot to rearfoot alignment appear as a forefoot varus.
It is best practice to address the biomechanical cause of forefoot supinatus by controlling rearfoot motion at heel strike/loading response. Posting the rearfoot, in combination with modifying forefoot-to-rearfoot alignment on the positive cast, is a recommended orthotic application for forefoot supinatus. A deep heel seat is highly recommended in this situation, as it increases soft tissue compression around the calcaneus, improving motion control during contact phase. One may consider skiving (via the Kirby technique) a flat spot on the medial heel area of the positive cast to increase ground reactive force beneath the calcaneal condyle for situations where the calcaneus is severely everted in midstance.
In our clinical experience, posting the rearfoot in combination with modifying the forefoot to rearfoot relationship on the positive cast is a recommended orthotic application for forefoot supinatus.
Understanding The Standing Midtarsal Joint Assessment
Currently, when it comes to determining oblique midtarsal joint axis (OMJA) position in standing, podiatrists assess this by observing for the “too many toes sign.” In regard to assessing whether the oblique midtarsal joint axis has moved into a dysfunctional position, clinicians can quantify this when they see more than one and half toes when gazing at the lateral side of the foot when being directly behind it. The reliability of this assessment has been confirmed by its inclusion in the Foot Posture Index screening protocol. As the forefoot displaces laterally on the rearfoot, the rearfoot displaces medially. One can easily see this arthrokinematic relationship as navicular protrusion in the transverse plane.
One can measure positional changes resulting from oblique midtarsal joint axis movement by placing a block alongside the navicular tuberosity with the patient sitting, a 90 degree angle of knee flexion and both heels on the floor. Then, orient the foot to leg at 90 degrees, commonly referred to as “ankle/knee 90/90.” Position a ruler alongside the block at one edge in such a way that it remains visible when the patient stands up (see photos four through six above). Motion at the oblique midtarsal joint axis produces the “too many toes” sign. It also causes the navicular to drift medially. Eichelberger and colleagues reported dynamic navicular drop ranged between negative 5.9 and negative six mm and dynamic navicular drift was between 5.4 and 5.6 mm.10 A reliable ratio of expected or excessive navicular drift to drop during a sit to stand maneuver is not yet substantiated by published study.
Navicular drop using the navicular tubercle as a reference point is marginally reliable according to research by Langley and colleagues.11 Clinical experience suggests that displacement of more than a eight mm drop is excessive for a male foot with a size 9 shoe and a female foot with a size 10 shoe as measured on a Brannock device.9 In our experience, we would expect more displacement in larger feet and conversely less in shorter feet. When observing how many millimeters the block moves medially with the patient standing, this should not exceed four to six mm.
McPoil and colleagues documented a reliable midfoot morphological measuring scheme to assess foot mobility magnitude by finding a midway point on the dorsum of the foot, between the most posterior aspect of the heel and the medial aspect of the first metatarsal head, which is commonly identified as the “ball length” in pedorthic shoe industry parlance.12 They reported the mean values for difference in arch height for left and right feet as 1.21 and 1.31 cm respectively.12 The difference in midfoot width mean values for left and right feet were 0.96 and 0.93 cm respectively. “(Foot mobility magnitude) … is based on two highly reproducible measurements of vertical (difference in arch height) and medial-lateral (difference in midfoot width) mobility of the midfoot that can be collectively analyzed to create an overall index of midfoot mobility.”12
Having a foot measurement platform specifically constructed for gathering data is effective for establishing midfoot drop to width change ratios, making those measurements highly reliable. But employing such an instrument in clinic is impractical. Knowing there is a mean of 1.2 to 1.3 cm drop in overall midfoot height at the level proximal one-third of the medial cuneiform and 0.96 cm 0.93 cm increase in width at that same area of the foot is instructive. One can utilize those ranges of measures and apply them when making visual determinations of excessive or insufficient change in foot shape between sitting and standing foot posture.
The more reliable scheme above is different from the less reliable but demonstrative observations using a block and ruler to clinically assess navicular drop to drift as we have described above (see fourth and fifth photos above). The foot mobility magnitude test method provides us with a referential schedule of linking arch height to width. Using McPoil and colleagues’ scheme as a corollary reference along with acknowledging Langley and team’s discouragement of relying on measurements using the navicular tubercle marking, clinical experience suggests that reasonable navicular drift (transverse displacement) during a sit to stand maneuver would be six mm and the legacy reference for drop at the navicular tubercle remains at eight mm.9,12
Treating drifting displacement of the navicular with a foot orthosis can be uncomfortable for the patient if his or her navicular presses medially against a longitudinal arch contour shaped with a vertical curvature, such as a medial flange. This is particularly problematic for feet that present with forefoot equinus, a sagittal plane malalignment. One may see this as a high lateral arch if the foot posture remains intact during midstance. If the forefoot collapses (dorsiflexes relative to the rearfoot) during full weightbearing, this motion around the oblique midtarsal joint axis also comes with abduction, resulting in a “too many toes” sign and navicular drift with medial displacement in the transverse plane.
To prevent possible medial arch discomfort for the patient, the clinician should request reshaping the medial aspect along the medial margin of the positive cast to a more horizontal contour, beneath the first metatarsal, cuneiform and navicular. This change in contour allows for medial drift of the navicular without impingement in the arch area. Technicians at the Biomechanical Services Inc., orthotic laboratory are instructed to apply a medial arch platform (MAP) modification on the positive cast, which results in a more horizontal-shaped medial edge (see sixth photo above). One should select a stiffer shell/module when applying this modification to the contour in order to prevent navicular drop while still allowing for medial displacement.
How Does One Evaluate Midtarsal Joint Integrity?
Our clinical experience demonstrates time and again that increased orthotic control (stiffer or closer contouring shells/modules) are effective for managing unstable midtarsal joints. When a fully supinated STJ does not produce adequately suppressed motion at the longitudinal midtarsal joint axis or complete suppression (“locking”) at the oblique midtarsal joint axis, applying a stiffer or even a rigid shell is necessary to maintain midtarsal joint integrity in midstance. Another important consideration is the minimal effect a rearfoot post will have on influencing midtarsal joint stability. When the STJ/midtarsal joint locking mechanism is not intact, posting the rearfoot to reduce STJ pronation will not influence midtarsal joint integrity in later phases of stance.
Assess longitudinal and oblique midtarsal joint axis integrity with the patient lying in a supine or long sitting position. Grasp the calcaneus along the medial side with your inside hand and use it to supinate the subtalar joint to end range of motion. With your outside hand, place your index finger on the dorsal lateral surface area of the cuboid and your thumb on the inferior medial area of the bone. Upon palpation with your index finger and thumb, attempt to articulate the cuboid on calcaneus using a firm pincer grasp. When you do this, be careful not to apply too much grasping pressure as this will make it uncomfortable for the patient. Articulate the cuboid on the calcaneus, moving it back and forth in plantarflexion with adduction and dorsiflexion with abduction) to assess available motion. There should not be any motion at the oblique midtarsal joint axis when the STJ is maximally supinated.
Change your hand hold grasp to the calcaneus along the lateral side with your outside hand and again use it to supinate the STJ to end range of motion. Place your thumb on the superior lateral aspect of the navicular. Hook your index finger around the navicular tuberosity and grasp the navicular between your index finger and thumb. Rest your ulnar eminence along the first metatarsal. Use your forearm (radial deviation) to invert and evert the navicular on talus while palpating it with your grasping fingers. Given the ball and socket articulation, motion at the longitudinal midtarsal joint axis with a maximally supinated STJ is rarely fully suppressed or “locked” up. If it is unclear that the motion suppression mechanism is intact, pronate the STJ and repeat the range of motion palpation. If detection of a change in navicular range of motion between a pronated and supinated STJ is not easily apparent, the longitudinal midtarsal joint axis is unstable.
When the subtalar joint is maximally supinated, the convergence of longitudinal midtarsal joint axis and oblique midtarsal joint axis should completely suppress motion at the oblique axis and motion at the longitudinal mid-tarsal joint axis should be significantly suppressed, which is a positive finding for the influence of STJ pronation/supination to assist with mid-tarsal joint stabilization during gait.
Another useful cast modification to help stabilize the cuboid on the calcaneus is an increased calcaneal angle on the positive cast. The technician files a concavity into the plaster along the inferior lateral side of the molding model directly beneath the calcaneocuboid joint. This cast modification produces a convexity in the shell/plate that pushes up beneath the articulation, stabilizing the cuboid and preventing it from rotating into an everted position during midstance. One can also accomplish this technique with the application of a cuboid pad (see seventh photo above). This softer version creates the same contour as an increased calcaneal angle but with a less aggressive effect needed to form a convexity beneath the cuboid. Both techniques provide resistance to a rotary force into eversion that can lead to subluxation or even dislocation of the cuboid.
It is important to consider that efficient midtarsal function involves much more than just sagittal plane deviation of the navicular, change in longitudinal arch shape. The entire midfoot, which includes frontal and transverse plane displacement of osseous alignments (inversion/eversion at the talonavicular joint and abduction/adduction at the calcaneocuboid joint will alter arthrokinematic orientation, position and motion of all midfoot structures, especially during single limb weightbearing. It is also important to carefully consider what inefficient displacement of osseous structures and articular motions focused on in this article have on the lesser tarsal bones and their articular relationships to the metatarsals. One should also evaluate those structures for increased or decreased contribution to mobility, stability and proper load distribution in the midfoot.
Managing midtarsal dysfunction is as complex as the three interdependent articular complexes involved. Determining which anatomical components and biomechanical interactions exhibit compromised positional alignments, inefficient motions, reduced or excessive mobility and insufficient stability will help clinicians in establishing or re-establishing efficient load management and distribution when using orthotic control. While treatment applications need to be effective, they also need to be comfortable for the patient. Utilizing a balance of adequate midfoot control with reasonable orthotic intervention techniques will produce favorable clinical responses.
Dr. Romansky is a Diplomate of the American Board of Foot and Ankle Surgery, and is in private practice in Media, Pa.
Dr. Anselmo is a Chief Resident at Tower Health in Phoenixville, Pa.
Mr. Wolfe is President of Biomechanical Services Inc., a custom foot orthoses laboratory in Walnut, Calif. He is a certified pedorthist by the American Board for Certification in Orthotics, Prosthetics & Pedorthics, and the Board of Certification/Accreditation: Pedorthics.
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