Orthopaedics & Traumatology: Surgery & Research (2013) 99, S345—S355

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REVIEW ARTICLE

The talonavicular and subtalar joints:The ‘‘calcaneopedal unit’’ concept

R. Seringe, P. Wicart ∗, the French Society of PediatricOrthopaedics (SOFOP)1

Hôpital Necker—Enfants malades, Paris Descartes University, 149, rue de Sèvres, 75015 Paris, France

Accepted: 15 March 2013

KEYWORDSTalonavicular joint;Subtalar joint;Calcaneopedal unit;Foot segmentation;Pes cavovarus;Pes planovalgus;Congenital talipesequinovarus

Summary The talonavicular (TN) joint and the three subtalar (ST) joints are linked anatomi-cally and functionally. Together they form the subtalar joint complex, where movement occursbetween the calcaneopedal unit (CPU) (entire foot except the talus) and the talotibiofibularunit (talus held tightly by the ankle mortise). Many are unaware of the TN joint’s dual mem-bership: it is a component of the subtalar joint complex (talocalcaneonavicular joint) and alsothe transverse tarsal joint (with the calcaneal-cuboid joint). The anatomy of the articulatingsurfaces, movement of the CPU when unloaded, shifts and changes in CPU shape with weightbearing, application to clinical tests and X-ray interpretation, and the pathophysiology appli-cations to pes cavovarus, pes planovalgus and congenital talipes equinovarus (club foot) willbe reviewed here. The CPU concept corresponds to a horizontal segmentation of the foot. Thisis a useful supplement to the two other segmentation methods: frontal (hindfoot, midfoot and

forefoot) and sagittal (medial and lateral columns). This horizontal segmentation solves theissues with the ST joint complex, which straddles the hindfoot and midfoot, and also the issueswith the dual membership of the TN joint. This concept makes it easier to understand footdeformities, better interpret the clinical and radiological signs and deduce logical treatments.© 2013 Published by Elsevier Masson SAS.

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Introduction

The talonavicular (TN) joint is the only joint in the body thatbelongs to two separate joint entities: the transverse tarsaljoint (also called the midtarsal joint or Chopart’s joint)

∗ Corresponding author.E-mail address: p.wicart@nck.aphp.fr (P. Wicart).

1 56, rue Boissonade, 75014 Paris, France.

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1877-0568/$ – see front matter © 2013 Published by Elsevier Masson SAShttp://dx.doi.org/10.1016/j.otsr.2013.07.003

nd the subtalar joint complex (ST) [1—3]. The TN joint isnatomically and functionally linked to the calcaneocuboidoint to make up the transverse tarsal joint, which is at theunction between the hindfoot (talus and calcaneus) and theidfoot (cuboid, navicular and cuneiform bones). Further-ore, it is inextricably linked to the anterior ST joint toake up the acetabulum pedis [4,5]. The latter junction is

ombined with the middle and posterior ST joints to makep the ST joint complex, the site of movement between the‘calcaneopedal unit’’ (CPU) (entire foot except talus) and

.

S346 R. Seringe, P. Wicart

Figure 1 The calcaneopedal unit (CPU). A. The CPU (white) is separated from the talotibiofibular unit (TTFU) (4) by the talocal-c lcanb neon

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aneal interosseous ligament (5). B. The CPU is formed by the cay the calcaneocuboidal (3), bifurcate (2) and the plantar calca

he talus, which is held within the ankle mortise to makep the talotibiofibular unit (TTFU) [2,6] (Fig. 1). Many arenaware of this dual membership of the TN joint, whicheads to errors when interpreting X-rays [7—11] and a lack ofonsistency when explaining the relationships between theindfoot and midfoot [10,12—16] (Fig. 2).

The ST joints have a fairly simple function when theoot is not loaded. The calcaneus is said to pitch, turnnd roll under the talus [17]. The three basic move-ents of dorsiflexion/plantar flexion, abduction/adduction

nd pronation/supination are automatically associated in unique eversion/inversion movement around the Henkexis (first described in 1859). Eversion combines dorsiflex-on, abduction and pronation of the foot, while inversionombines plantar flexion, adduction and supination. How-

ver, this only applies to the unloaded foot (when examining

seated or lying subject).The function of the ST joints in the loaded foot and dur-

ng walking is completely different. They have been studied

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igure 2 Dual membership of the talonavicular joint. Left: normadduction; Right: combination of the two configurations.

eus and the midfoot and forefoot and solidly attached togetheravicular (spring)(1) ligaments.

elatively little and are relatively complex [18,19]. Some ofhe distinct features of these joints have been somewhatorgotten:

previously described interaction with the TN joint [1—3]; role in the twisting-untwisting of the foot (flattening

of the loaded foot and hollowing of the unloaded foot)[1,20];

role in the axial rotation movements of the leg [18].

To our knowledge, no published studies have summarizedhe subtle mechanisms of the ST joint complex in a clearnd easy to understand manner, probably because of a lackf understanding about the CPU concept [2,6,21]. However,his concept was known early on. Duchenne de Boulogne

22] made clear reference to it in 1867: ‘‘The foot turnsround the leg axis such that its anterior end goes inwardsnd the heel outwards’’. Strasser [23] characterized thisoncept with diagrams in 1917 and then, MacConnail and

l foot; Bottom: isolated CPU adduction; Top: isolated midtarsal

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The talonavicular and subtalar joints: The ‘‘calcaneopedal u

Basmajian [20] revisited this concept and called it the lam-ina pedis in 1945. Hicks [1] and then Inman et al. [19,24]completely integrated the CPU concept in their functionalanatomy work, although they did not use the proper termi-nology.

But it was only in 1950 that two pediatric surgeons (Mearyand Queneau) came up with a name for the foot bonesexcluding the talus [in 2]. Conversely, Kapandji remainedsilent on this subject [12]. Only a few initiated surgeons,such as Rigault [25] and Pouliquen [26] used the term CPUto explain the pathophysiology of foot deformities in the1960s and 1970s. From then on, this concept has been usedand accepted in France by surgeons operating both on adults[21] and children [6].

The goal of this review is to show that the CPU conceptis the key to understanding, not only the physiology ofthe normal foot but also the pathophysiology of most footdeformities. This will make clinical analysis and X-ray inter-pretation easier and result in a more suitable treatmentapproach.

Anatomy of the subtalar joint complex

It consists of four inextricably linked joints: the three STjoints (posterior, middle, anterior) and the TN joint [3].These four joints work in a synergistic manner and form thejoint complex where the movements between the CPU andTTFU occur.

Talus and its articular facets

The talus, which has no muscle insertions, has articularfacets with the ankle mortise for flexion-extension move-ments and with the CPU for ST joint movements. It consistsof multiple articular facets that fit perfectly with the corre-sponding facets of the CPU.

Calcaneopedal unit and its articular facets

The CPU consists of the calcaneus, midfoot and fore-foot, which are solidly united by the calcaneocuboidalligaments, bifurcate ligament and the plantar calcaneo-navicular (spring) ligament (Fig. 1).

The posterior articular facet (previously called the tha-lamic portion) has an oval shape and an oblique long axisaimed forwards and inwards.

The anterior calcaneal facet has either one or two dis-tinct surfaces (proximal, supported by the sustentaculumtali and distal, supported by the anterior calcaneal apoph-ysis).

The dorsal side of the plantar calcaneonavicular ligamentis encased with articular cartilage; the insertion of the baseof the deltoid ligament is on its medial side. The proximalarticular facet of the navicular has a round, concave shape

that, in combination with the dorsal side of the spring lig-ament and the anterior calcaneal articular facets, forms around cavity for the head of the talus: this is the acetabulumpedis [4,5].

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igure 3 The two standard foot division methods. Left: lon-itudinal division; Right: frontal division.

oot segmentation

he CPU concept adds a horizontal division to the foot,hich supplements the other foot segmentation methods

hat cannot explain by themselves the subtalar mechanismsFig. 3):

frontal division into three segments: hindfoot (calca-neus and talus), midfoot (navicular, cuboid and cuneiformbones) and forefoot (metatarsals and toes) (separated bythe Chopart and Lisfranc joint lines);

longitudinal division into a more flexible medial col-umn (first three metatarsals, cuneiform bones, navicularand talus) and a stiffer lateral column (lateral twometatarsals, cuboid and calcaneus).

ovements of the CPU

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he basic CPU movements relative to the TTFU occur aroundenke’s axis (angled downwards, backwards and outwards).he inversion movement, which results in the tip of theoot pointing downwards and inwards, is accompanied byn opposite movement of the posterior calcaneal tuberosityhat moves upwards and outwards. The pivot point is thenterosseous talocalcaneal ligament (Fig. 1). During inver-ion, active movements between the CPU and the TTFU areriven by posterior tibialis and triceps surae muscles; duringversion, the fibularis (peroneus) brevis, extensor hallucisongus, fibularis tertius and extensor digitorum longus par-icipate.

oaded (weight bearing)

simplified functional model of the subtalar joint complex

n a weight bearing subject was developed by Close et al.19]. It consisted of two pieces of wood attached at a rightngle with a hinge at a 45◦ angle; the axial rotation of oneoard leads to the rotation of the other. If we equate one

S348 R. Seringe, P. Wicart

F weight bearing when the patient turns to the right (A. Front view,B comes pes cavo-varus and the left one, pes plano-valgus.

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remarkable plasticity required by the movements of each ofits constituent joints.

Change in CPU shape during loading of the foot

This is the twisting-untwisting phenomenon of the laminapedis described by MacConnail et al. (Fig. 5). Not knowing

Figure 5 Lamina pedis helix according to MacConnail during

igure 4 The movements and shape changes of the CPU upon. Posterior view). The two feet change shape: the right one be

oard to the foot and the other to the leg skeleton, it is easyo see how supination or pronation of the foot automaticallyeads to external rotation (with foot supination) or internalotation (with foot pronation) of the leg.

Another way to visualize this phenomenon is to ask atanding subject to keep the feet firmly planted on theround, then turn completely to the right until the pec-oral girdle is pointed 90◦ relative to the initial frontaleference plane. The person’s right leg undergoes externalotation above the CPU with the foot becoming arched, evenavovarus. The left leg undergoes internal rotation abovehe CPU with the foot simultaneously flattening into valguslanus (Fig. 4).

These basic points must be fully understood. During walk-ng, the lower legs continuously rotate around the leg axis,lternating from one side to another. The ST joint complexllows the foot to conform to the ground. The CPU trans-orms the axial rotation movements above it through itsongitudinal axis.

ther features of the CPU and its clinicalmplications

lasticity

his is multifactorial.Variable geometry of the calcaneonavicular (spring) liga-

ent: this ligament, which participates in the formation ofhe talocalaneonavicular joint, can stretch out and accept

larger part of the head of the talus when the latter movesedially into the sole of the foot. Conversely, the ligament

rea can become smaller when the head of the talus risesp on the anterior calcaneal apophysis [27].

Multisegmental makeup of the CPU: the numerous liga-ents that join the CPU bones together supply it with the

weight bearing. Top: when the leg (TTFU) internally rotates (IR),the CPU untwists with calcaneal pronation (PRO) and forefootsupination (SUP). Bottom: when the leg externally rotates (ER),the opposite happens — the CPU twists more.

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Figure 6 Plantar fascia placed under tension when the firstphalanx is extended (according to Hicks).

the CPU terminology, he named the part of the foot withoutthe talus the lamina pedis [20]. This cabled plate com-bines with a helix and is flattened from top to bottom atthe metatarsal heads and from outside to inside at thecalcaneus. It is flexible and untwists itself during weightbearing with forefoot supination and calcaneus pronation.Conversely, the lamina pedis can accentuate this twistingphenomenon with forefoot pronation and heel supination ofthe foot during weight bearing, when the talus turns intoexternal rotation (or abduction) because of the motion ofthe leg (TTFU).

From a practical point of view, at the start of the sup-port phase, when the entire body weight rests on the CPU,untwisting occurs with an increase in calcaneal valgus andrelative supination of the midfoot and forefoot. At the endof the support phase when the body rises and the calcaneushas left the ground, a reverse twisting of the CPU occurs withexternal rotation of the leg, calcaneal supination and mid-forefoot pronation. During the swing phase, the CPU twistingis reduced and then completely reverses itself during thenext support phase.

The work performed on insertions by Hicks [1] was instru-mental in understanding how the plantar fascia contributesto changes in foot shape. The proximal insertion is locatedon the calcaneal tuberosity and the distal insertions areat the base of the first toe’s phalanges (Fig. 6). When themetatarsophalangeal joints are dorsiflexed, the plantar fas-cia coils up around the metatarsal heads and draws itselftight. The distance between the metatarsal heads and thecalcaneal tuberosity is reduced and the foot arches.

Clinical manipulations

Passive extension of the big toe (Jack’s test) restores themedial arch. This finding can be erroneously interpreted

as being related to the hallux flexor longus tendon [28].In fact, dorsal hyperflexion of the big toe increases thetension on the plantar fascia and brings the posterior cal-caneal tuberosity forwards, which leads to the medial arch

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ecoming more prominent, while simultaneously reducinghe calcaneal valgus, inducing pronation of the midfoot andorefoot and inducing external rotation of the TTFU.

Manual rotation of the leg: in a weight bearing sub-ect, manually turning the leg inwards leads to flatteningf the foot, while turning it outwards bring the talus backbove the CPU, which leads to the medial arch becomingore prominent, calcaneal varus because of the morphol-

gy of the posterior subtalar joint and relative pronation ofhe midfoot and forefoot to maintain foot contact with theround. This test is used to evaluate CPU plasticity and SToint complex flexibility (Figs. 4 and 5).

pplication to interpreting foot x-rays

he CPU concept can be used to better analyze A/P X-rays in weight bearing subject (dorsoplantar view of foot). Threengle measurements are currently being used:

talocalcaneal angle, which directly reflects on theadduction—abduction of the calcaneus under the talus,meaning the CPU under the TTFU;

talar-first metatarsal angle, which historically reflectson forefoot adduction—abduction [7—11]. This interpre-tation is only partially correct. Because of the dualmembership of the TN joint, changes in this angle caneither come from the midtarsal area or the ST joint,in other words, indicating CPU adduction—abduction[2,6,21,29];

calcaneal-fifth metatarsal angle that, opposite to thetalar-first metatarsal angle, inherently only involves theCPU and excludes subtalar contributions. This reveals theadduction—abduction movements of the forefoot rela-tive to the hindfoot. In older children and adults, bonematuration easily allows the contribution of either thecalcaneocuboid or cuboidometatarsal joints or both, tobe determined.

As for talonavicular coverage, and contrary to standardnterpretations that deem, this a forefoot problem, thePU concept helps us realize that CPU (thus hindfoot)dduction—abduction under the TTFU can also change it.

arious CPU morphotypes

he calcaneal angle of incidence with the ground variesrom one person to another and can explain the variousPU shapes [30] (Fig. 7). With a larger angle and erectedalcaneus, the CPU is spontaneously arched: this is a physi-logical pes cavus or high instep. With a smaller angle, theateral column stays near the ground and a physiological flatoot morphotype can be observed. One must keep these twoxtreme forms (CPU with erected calcaneus and CPU withorizontal calcaneus) in mind when faced with a congenital

ant to determine what can be attributed to morphotypend what can be attributed to the deformity itself, sinceany potential combinations exist.

S350 R. Seringe, P. Wicart

Figure 7 Different CPU patterns on lateral view evaluateduh

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sing the calcaneal angle of incidence with the ground: Top:igh-arch foot type. Bottom: flat foot type.

athophysiology applications

he CPU concept sheds new light on the pathophysiology andreatment of the primary foot deformities, either acquiredpes cavus, flat foot) or congenital (club foot), and chal-enges certain ideas or commonly encountered errors.

es cavovarus (or medial cavus deformity)

athophysiology and clinical findingshis is an acquired deformity that is almost always neuro-

ogical in nature. The CPU takes a helix form (non-reducibleronation of the forefoot and calcaneal varus) in combi-ation with secondary (or adaptive) external rotation ofhe TTFU [31]. The most common form is observed dur-ng Charcot-Marie-Tooth disease with early involvement ofntrinsic muscles, especially interosseous ones. The mor-hology of the foot will typically change with growth. Theoot, normal at birth, becomes planovalgus towards 3 or

years of age, but during a heel-walking test, a dynamicedial pes cavus can already be observed. Towards 6 to

years of age, the foot gradually becomes cavovarus andhe defects can become increasingly severe until the end ofhe growth phase.

In its most advanced form, the medial cavus deformitys visible because of the specific CPU deformity: theres a non-reducible pronation of the forefoot. This pref-rential verticalization of the medial metatarsal, whichan explain the medial cavus deformity, is in keepingith an early structural deformity of the cuneiform bones,here their anterior and posterior articular facets no

onger parallel so, they converge towards the sole. Thiss the primum movens (cause) of a deformity, which ison-reducible. During the ground support phase, forefoot

ronation induces a varus seesaw motion (supination anddduction) of the entire CPU, which leads to talar varusnd excessive loading on the lateral side of the foot.hus, the CPU is the seat of a twisting motion with

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orefoot pronation and hindfoot supination. Simultaneousupination and adduction movements of the CPU lead thealus into relative abduction (that is to say external rotationf the TTFU) but also in dorsiflexion (horizontal displace-ent of talus) (Fig. 8). These components of the deformity

re secondary and can be reduced initially.In summary, the array of typical abnormal features of pes

avovarus is the following: forefoot pronation, medial cavuseformity with the intermediate cuneiform bone being athe apex, supination-varus of the hindfoot and external rota-ion of the TTFU above the CPU at the ground. The cavus,arus and claw toe deformity become worse when the foots unloaded (swing phase of gait or when lying down, espe-ially at night). In this situation, treatment with a nightealignment brace can be beneficial [31].

If not treated, the bone deformities and joint dis-lacements can become worse because of growth and thenderlying neurological pathology (often progressive). Sec-ndary abnormalities (supination-varus of the hindfoot andxternal rotation of the TTFU above the CPU) that are ini-ially reducible will gradually become non-reducible. ThePU adduction is gradually completed by midtarsal adduc-ion with convexity of the lateral side of the foot andhortening of the medial column relative to the lateral col-

mn. As the child grows, the two leg bones twist externallyo compensate for the subtalar adduction.

The talonavicular and subtalar joints: The ‘‘calcaneopedal unit’’ concept S351

ard test for right pes cavovarus.

performed [32]. A night realignment brace must be useduntil the end of the growth period to prevent recurrencebecause of the gradual nature of the causal neurologicalcondition [32]. A pes cavovarus at maturity is an indicationfor dorsal tarsectomy (associated with calcaneal osteotomy,plantar fasciotomy and often first metatarsal osteotomy);the goal is to avoid the need for a triple arthrodesis (subtalar,talonavicular and calcaneocuboid).

Pes planovalgus

Pathophysiology and clinical findingsPes planovalgus is a deformity that is typically acquiredwhen walking starts or around 3—4 years of age and is rarely

Figure 9 Lateral angled-bo

X-raysRadiological evaluation of the medial pes cavus during stand-ing is challenging because a lateral view will underestimatethe cavus deformity, even though it is clinically undeniable.This radiological paradox can be explained by abnormalitiesin the horizontal plane. Méary’s angle (formed by the lon-gitudinal axes of the talus and first metatarsal to quantifythe anterior cavus deformity) cannot be measured correctlybecause the talus and first metatarsal are no longer in thesame sagittal plane because of the CPU adduction and exter-nal TTFU rotation. To measure the medial cavus deformityon X-rays, an angled-board configuration must be used. Theentire CPU is loaded with pronation—abduction (Fig. 9) torestore the normal relationships between the TTFU andCPU so as to bring the talus back into the same sagittalplane as the first metatarsal [31]. This X-ray while stand-ing on an angled-board can be used to measure Méary’sangle if the frontal adaptations are reducible (Fig. 10). Astandard, standing dorsoplantar view shows the closure ofthe talocalcaneal angle while the angled-board test moreor less completely restores this discrepancy; the reducibleor incompletely reducible nature of the hindfoot varus-supination and the TTFU external rotation can be evaluatednow (Fig. 11).

TreatmentA pes cavovarus can be treated conservatively if startedearly enough (before the age of 12). The foot deformity iscorrected with a walking cast that untwists the CPU andexternally rotates it under the TTFU; the correction is thenmaintained with a night realignment brace.

Surgical treatment consists of untwisting the CPU know-ing that the apex of the cavus deformity is located atthe intermediate cuneiform bone. Selective plantar releasecan be combined with opening-wedge osteotomy of thethree cuneiform bones to extend the medial column, com-pletely straighten the forefoot pronation and correct thecavus deformity. A valgus calcaneal osteotomy must be per-

formed here because the calcaneal supination is rarely fullyreducible. In some cases, partial release of the midtarsaljoint with or without shortening of the lateral column byclosing wedge osteotomy of the calcaneus must also be

Figure 10 Lateral X-rays of loaded pes cavovarus: standardview (top) and with lateral angled-board (bottom).

S352 R. Seringe, P. Wicart

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Figure 13 Drawings of dorsoplantar view X-rays of pesplanovalgus. In both cases, the talocalcaneal divergence isincreased with the navicular being displaced laterally. Left: thelateral edge of the foot is straight; only the CPU is abductedrelative to the TTFU. Right: the lateral edge is concave; theC

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igure 11 Dorsoplantar view of the same loaded foot:tandard view (left) and with lateral angled-board (right).

ue to a neurological condition. The idiopathic form is oftenalled hypermobile flatfoot because the deformity is visiblen a loaded foot and disappears completely when the foots unloaded.

This acquired deformity of the CPU is reversed fromhe one in pes cavovarus. In the pes planovalgus, the CPUelix untwists (Fig. 12) with forefoot supination and hind-oot pronation, and with calcaneal valgus associated withnternal TTFU rotation above the CPU (which corresponds tobduction of the CPU). Various tests can be performed on aoaded foot to determine if the pes planovalgus is reducible.

External rotation of the leg moves the talus into abduc-

ion and dorsiflexion on the CPU, which makes the calcanealalgus disappear and the medial arch of the sole reappear.

igure 12 Mechanism of pes planovalgus. Untwisting of thePU with forefoot supination (SUP), calcaneal pronation (PRO)ith the TTFU going into internal rotation (IR) and the talusiving into sole of foot.

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PU abduction is added to a midtarsal component.

Passive hyperextension of the hallux brings about theame sequence of events.

Spontaneous progression of hypermobile flatfoot oftenccurs between the age of 7 and 11, and results in partial orotal correction. In some cases, especially with neurologi-al involvement (cerebral palsy or neurological deficit), thetructural deformities become worse.

-rayshe following aspects are visible on X-rays:

lateral view — horizontal displacement of the firstmetatarsal, downward angle of the talus with an obtuseMéary’s angle dorsally and structural deformity of the nav-icular and/or cuneiform bones;

A/P view — variable increase in the talocalcaneal diver-gence corresponding to CPU abduction under the TTFU.This can be combined with midfoot and forefoot abduc-tion located at the CPU itself and characterized by theangle between the lateral edge of the calcaneus and lon-gitudinal M5 axis being smaller laterally and shortening ofthe lateral column relative to the medial column. Lateraldisplacement of the navicular bone relative to the headof the talus indicates the amount of abduction but not itslocation (subtalar or midtarsal) because of the talonavic-ular joint’s dual membership (Fig. 13).

A new radiological classification system for pes planoval-us (correlation of the A/P and lateral views) has identifiedour deformity patterns: subtalar pes planus, midtarsal peslanus, mixed pes planus (combination of the previous two

bnormalities) and pes planocavus (sag of medial columnnd cavus deformity of lateral column) [29].

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Figure 14 False supination in CTEV: correction of tibiotalartalipes equinus on a foot with significant adduction makes thesupination disappear. A. Explanatory drawings. B. Anatomicalsp

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The talonavicular and subtalar joints: The ‘‘calcaneopedal u

TreatmentThe severe forms of pes planovalgus can be surgically cor-rected using various procedures [33], with the indicationsderived from the above-described classification system:

• the Grice procedure by talocalcaneal coalition is not rec-ommended because it leads to delayed effects on thetibiotalar joint [34];

• the Judet technique, also called the ‘‘rider’’ (tempo-rary talocalcaneal fixation) is burdened by a recurrencerate that cannot be ignored. The subtalar pattern maybe its most appropriate indication [25]. Arthrorisis (sub-talar implant) is an alternative that should be carefullyevaluated [35];

• calcaneal lengthening according to Evans results in cor-rection of most of the defects, but postoperative stiffnessis sometimes observed because of variability in anteriortalocalcaneal joint anatomy [36]. The most appropriateindication would be the midtalar pattern and the mixedpes planus pattern if combined with a medial columnshortening;

• medial arch shortening (osteotomy/cuneonavicular fusionor cuneiform bone osteotomy) will be indicated in casesof pes planocavus and could be combined with the Evansprocedure in mixed forms.

Congenital talipes equinovarus (CTEV)

PathophysiologyThe reasoning needed to understand the pathophysiologyof CTEV is different because this is a congenital deformitythat appears many months before birth. This deformity isnot affected by walking and weight bearing. Conversely,because it is a three-dimensional deformity with three dis-tinct defects (equinus, supination and adduction), it seemsessential to take into account MacConnail’s diadochal (suc-cessive) movement law [20] by integrating all the tibiotalar,subtalar and transverse tarsal joints into an enarthrosis.This law stipulates that successive movements about twoof the three reference axes, which automatically gener-ate a movement in the third plane, without any movementsbeing required about the third reference axis. Applying it toCTEV (previously untreated and independent of age) leadsto the conclusion that one of the three basic deformities(supination) is ‘‘false’’ or ‘‘relative’’ [2,37] (Fig. 14). Mostof the supination in CTEV (other than a minor component ofsubtalar origin accompanying the CPU adduction) is in factcreated automatically by the combination of tibiotalar equi-nus deformity (indisputable) and a significant adduction ofthe foot, meaning the CPU, but also a midtarsal adduction.When assessing the severity of the deformity in a newborn,it is not logical to separately evaluate the equinus deformityand the supination, as proposed by Dimeglio et al. [38]. Theprocedure used to correct the supination is a manipulationthat is almost exclusively located at the talocrural joint.The Dimeglio classification system should be modified to onlygrade the true fundamental clubfoot deformities: equinus,

CPU adduction and midtarsal adduction.

The CTEV adduction can be located in four separatejoints, not taking into account the bone deformities [2,37]:adduction in the ST joint complex, midtarsal adduction,

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pecimen of arthrogrypotic CTEV before and after posterior andosterolateral release without opening of the subtalar joints.

aviculocuneiform adduction, tarsometatarsal adduction. Inost cases, the CTEV adduction is located in two places: CPU

dduction under the TTFU and midtarsal adduction. The CPUdduction is recognized and evaluated in the Dimeglio clas-ification system, but not always well understood, becausen one case [39], the term CPU was replaced by ‘‘midfoot’’,hich leads to confusion.

ibrous knotshe dual membership of the TN joint (Fig. 2) helps us under-tand why, in CTEV, the medial end of the navicular makesontact with the medial malleolus. Talonavicular adductiononsists of the addition of two defects (midtarsal and CPU),hich explains the constitutive soft tissue retraction of

he anteromedial fibrous knots. Similarly, the CPU concept,hich revolves around the talocalcaneal interosseous liga-ent under the TTFU, helps us understand why the calcaneal

uberosity gets closer to the lateral malleolus and leads tohe description of the posterolateral fibrous knots with con-entrated soft tissue retraction [40,41]. Conversely, we havehown that in the posteromedial area of CTEV, there is noetraction. The soft tissue structures between the medial

alleolus and calcaneal have a long history of being sur-

ically released because of their suspected retraction. Weave also described an anterolateral fibrous knot. Whenhe CPU is significantly adducted under the talus, some

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igure 15 The three fibrous knots in CTEV. No retraction inhe posteromedial zone.

tructures (lateral part of the anterior subtalar joint capsulend main tibiocalcaneal fascicle of the extensor retinacu-um) can be retracted as a result of this subtalar adductionnd need to be surgically released. All the other retractedoft tissues in CTEV are located in the anteromedial fibrousnot, besides, the navicular tuberosity and medial malle-lus with adhesions of the flexor digitorum longus sheath,hortening of the tibialis posterior tendon and retraction ofll the tibionavicular, talonavicular and calcaneonavicularigament structures (Fig. 15).

reatmentne important treatment application involves the immobi-

ization of the knee in splints and casts (long leg system)elative to the current use of short leg splints and walkingasts [42]. In the short leg system, the foot seems to be heldroperly but the TTFU can turn into external rotation abovehe CPU, which is adduction and varus. To address this faultyotion of stabilization, the TTFU rotation must be neutral-zed by placing it in maximum external rotation with thenee flexed in the immobilization system. From then on, its possible to turn the CPU under the TTFU so that the CPUemains in abduction (and thereby eversion) relative to theTFU.

onclusions

he CPU concept introduces a horizontal division to the foothat adds useful information to the standard frontal and lon-itudinal divisions. If the surgeon is unaware of this concept,e/she lacks a means to understand foot deformities andisalignments, which could result in erroneous clinical and

adiological interpretations and lead to inappropriate treat-ents.

isclosure of interest

he authors declare that they have no conflicts of interestoncerning this article.

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