The complex interaction of the articular facets of the talus on the calcaneus and navicular forms the talotarsal mechanism. Its function is to transfer the entire weight of the body from the tibia onto the talus where vertical forces are redirected obliquely onto four joints (posterior talocalcaneal, middle and anterior talocalcaneal, and talonavicular) formed with the calcaneus and navicular (Fig. 1).  Historically, the majority of focus on this extremely complicated structure has been limited only to the talocalcaneal joint interactions, leaving the talonavicular joint vastly ignored.  One should not think separately about these interactions, as they are tied together. 

Fig. 1:  Shows the talus on the tarsal mechanism (calcaneus and navicular)

The balance and alignment of the talotarsal mechanism is crucial for a mechanically stable foot complex.  The articular facets of the talus should remain in constant congruent contact with their counter-facets on the calcaneus and navicular during standing and walking.  This allows the normal axis of joint motion and indicates the ideal transfer of force from the talus onto the tarsal mechanism and it’s supporting soft tissue structures.  Standing, walking, and running occur in an efficient manner. 

However, an altered axis of motion indicates a loss of normal balance and alignment. In this dynamic pathologic process, the talus repeatedly dislocates on the tarsal mechanism during weightbearing. This immediately places a significant strain on the tarsal complex and compensation must occur to handle the excessive misplaced force. When there are no forces acting on the talus, its articular facets are aligned on the tarsal mechanism. Upon weightbearing or when forces are placed on the talus, it dislocates on the tarsal mechanism. This deformity is termed recurrent reducible talotarsal dislocation. This is differentiated from static non-reducible fixed dislocation where the articular facets cannot be manually realigned.

Walking is the second most common conscious function of our body.  It has been estimated that the average person takes up to 8,000 steps a day.  When the talotarsal mechanism is aligned, these steps are taken effortlessly. However, a misaligned talotarsal mechanism will lead to eventual signs and symptoms because every step taken leads to excessive abnormal forces acting on the hard and soft tissues of the foot and ankle.  These cumulative traumatic disorder(s) occur as a direct result of the dislocation of the talus on the tarsal mechanism.  This dislocation also leads to an excessive or prolonged period of pronation. 

In order to eliminate the underlying etiology of the cumulative traumatic disorder(s) we must re-establish the normal hindfoot alignment.  Keep in mind that the pathologic mal-alignment can only occur if the posterior talar facet dislocates anteriorly and/or medially from its counter-facet on the calcaneus. Partial to full obliteration of the sinus tarsi immediately occurs as a direct result of this talar dislocation.  The navicular is forced to “drop” or plantarflex due to the pathologic displacement of the talus from its normal position on the calcaneus. The axis of motion is instantly altered leading to dislocation of the remaining facets since the interaction of one of the talotarsal facets directly impacts the remaining three. The talotarsal joint complex anatomically contains completely separate articulations which physiologically act as one. A very important detail not to be over-looked is the fact that this dislocation is occurring above the bottom of the foot. 

Clinical signs of talotarsal dislocation include a pronated axis of motion leading to a prolonged period of pronation, over-pronation or hyperpronation.  The calcaneus can potentially be forced into a valgus position and when the navicular drops there is a loss of arch height.  When the rearfoot (talotarsal mechanism) is placed back into its neutral position this repositions the talotarsal mechanism back into its normal alignment and these clinical signs are also reversed. 

Weightbearing radiographic examination is the best method to diagnose talotarsal dislocation (due to inconsistency of range of motion testing from one examiner to the next).  The best views are the dorsoplantar (DP) and lateral views. It is important to only have one foot on the x-ray plate when taking the DP x-ray. The patient should try to stand only on that foot, which would show what the foot looks like during mid-stance. The best angle to measure on the DP radiograph is the talar second metatarsal over the talar first metatarsal. The reason is that in many cases of talotarsal joint dislocation there could be a co-existing metatarsus primus varus that would lead to a false-negative finding.  The second metatarsal is the reference axis of the forefoot so it makes sense to compare the bisection of the talus to the very stable and unaffected second metatarsal.  The normal talar second metatarsal angle should be less than sixteen degrees.  Any finding greater than sixteen would indicate a pathologic alignment.  The DP talar second metatarsal angle shows the severity of the transverse plane deformity.

Lateral weightbearing radiographs are also extremely useful in documenting talotarsal dislocation. It is recommended to take both neutral position and relaxed stance position to document the reducibility of the condition. The primary angle to measure is the talar declination angle. This shows the angle of plantarflexion of the talus and indicates the severity of the sagittal plane deformity. The normal talar declination angle is less than twenty-six degrees.  Any measure over twenty-six would be considered pathologic. Observation of the sinus tarsi would show significant obliteration. (Fig 3) Other indicators are the position of the navicular compared to the cuboid, as well as evaluation of the cyma line. Another interesting observation is the position of the sustentaculum tali. In a normal talotarsal mechanism the sustentaculum tali should be in a dorsal alignment (Fig. 2), but upon dislocation of the talus this forces the sustentaculum tali plantarly.

Fig. 2: Shows weightbearing AP radiographic views of a normal (a) talar second metatarsal angle and (b) abnormal talar second metatarsal angle indicating a transverse plane dislocation.

Fig. 3:  Shows weightbearing lateral radiographic views of a normal (a) talar declination angle with an “open” sinus tarsi and (b) abnormal talar declination angle with obliterated sinus tarsi.

Treatment of recurrent reducible talotarsal dislocation is essential in restoring hindfoot mechanics back to normal and eliminating the cumulative traumatic effects. Every effort should be made to realign and maintain the articular facets of the talus onto their counter-facets.  The modality to stabilize the talotarsal mechanism should be critically analyzed to verify a normalization of the pathologic radiographic angles.  It is possible, depending on the severity, that external measures such as arch supports will alter the weightbearing surface enough to ease the tension on the soft tissues.  However, I find that it is nearly impossible to adequately address the talar dislocation with an external device and to prove it radiographically. 

Extra-osseous talotarsal stabilization (EOTTS) makes the most sense. There are two different types of devices for consideration. The first are the “lateral arthroereisis” devices. These conical and cylindrically shaped devices are placed into the lateral half of the sinus tarsi (Fig. 4) and are held in placed by the lateral soft tissues within the sinus portion of the sinus tarsi. They function as an anterior extension of the lateral process of the talus. The anterior edge of the device abuts the floor of the sinus tarsi, which limits the amount of talar motion. When the foot supinates or “unwinds” the talotarsal complex, the lateral process along with the device externally rotate. As the talus pronates, the lateral process internally rotates, at which point the anterior edge of the arthroereisis devices makes contact with the floor of the sinus on the calcaneus until it blocks further motion of the lateral process. The lateral arthroereisis devices have a reported removal rate of up to nearly 40% .

Fig. 4:  Shows placement of a typical lateral arthroereisis device on the under surface of the talus within the sinus portion of the sinus tarsi.  Note that the leading edge of these devices is placed lateral to bisection of the talus.

The second type of EOTTS device is placed deeper into the sinus tarsi (Fig. 5) and is medially anchored. This device stabilizes the talus on the tarsal mechanism at the cruciate pivot point which is the exact center of the axis of motion.  It prevents the anterior dislocation of the talus, maintains the “open” position of the sinus tarsi and re-establishes normal talotarsal range of motion. This device functions in a completely different fashion than the laterally placed arthroereisis devices and therefore offers a significant advantage.  Finally, the reported removal rate of this medially anchored device is significantly lower at less than 6%.

Fig. 5:  Shows the placement of a HyProCure® type II EOTTS device into the central portion of the sinus tarsi.

The obvious benefit of internal measures is that there is no limitation when it comes to patient compliance which is a major problem countered with external measures such as arch supports.  The EOTTS procedure is a soft tissue surgery as there is no bone involvement.  Furthermore, and unlike traditional rearfoot reconstructive surgery, EOTTS devices can be reversed- there are no “burned bridges”.  There is no such thing as a complication free surgery which includes an EOTTS procedure.  However, when one considers the benefit-risk analysis, there is no doubt that EOTTS offers the best solution to this very common pathologic deformity.

Fig. 6: Shows example of talotarsal dislocation on a weightbearing lateral (7-a) and AP (7-b) views and internally stabilized with HyProCure® comparison lateral (7-c) and AP (7-d) views.