Splay foot (pes transversoplanus)

Abstract
Splay foot (pes transversoplanus) is a three-dimensional structural deformity of the foot characterized by a fan-like spreading out or splaying of the metatarsal bones (MT1–MT5) and a prominence of the heads of MT1 and MT5. In addition to a rare, congenital cause, splay foot mainly develops from obesity, but also from shortened calf muscles and ligament instabilities among others.Because the forefoot is broadened, an increased pressure load is exerted on the medial metatarsophalangeal joint of the big toe and on the lateral metatarsophalangeal joint of the little toe. The pressure redistribution overloads the heads of MT2–MT4 and can cause painful calluses to form. If hammertoes develop along with the plantar pad shifting anterior to the metatarsal heads, the associated calluses can become painful and inflamed.
Treatment is primarily conservative, involving physical therapy, foot orthoses and shoe modifications. Surgical procedures are only indicated if the condition doesn’t respond to therapy.

Splay foot is one of the most common diagnoses rendered in orthopedic medicine. That said, there are hardly any other diagnoses in which such a lack of tangible scienti­fic evidence prevails. Even textbooks on the subject abound with controversial views on the deformity's pathobiomechanics, pathogenesis and treatment. Meanwhile, the otherwise rather sophisticated literature of American podiatry seems to have lost interest in publishing articles on splay foot. Partial aspects of splay foot are addressed under the categories fallen arches, flatfoot, hallux valgus and tailor’s bunions.That is why it would seem to make sense at this juncture to break down splay foot and its subclassifications and work from the ground up.
A general consensus does indeed exist that splay foot is unique in its fan-like spreading of the metatarsal bones. However, there have been no descriptions to date of the exact angles of divergence delineating the normal from the pathological.

The transverse arch
Any discussion about splay foot must also take the foot's transverse arch into consideration. However, there is no consensus on this subject either, with some orthopedists doubting that the transverse arch of the foot even exists. Those wishing to read up on the subject should consult Lanz and Wachsmuth's “Atlas der Praktischen Anatomie” [Atlas of Practical Anatomy] or Kapandji's “Funktionelle Anatomie der Gelenke” [Functional Anatomy of the Joints]. Further recommended reading is Hohmann's book on the foot and leg and Morton's beautiful discourse written back in 1935. Finally, innumerable radiological studies by magnetic resonance imaging (MRI) or computed tomography (CT) have demonstrated the existence of a posterior bulge and an anterior bulge traversing the foot. The posterior transverse arch is made up of the cuneiform bones and the cuboid bone; the anterior transverse arch is formed by the metatarsal bones.
Some foot experts do not regard this description as correct because the metatarsal bones – apart from their bases – no longer have any direct contact around their shafts and heads, but rather are held in place by ligaments. Therefore, the terms "anterior transverse suspension” or “transverse curvature of the foot" might actually be more precise since here it is the ligaments that absorb the thrust force, in contrast to a static arch construction where the acting forces are redistributed over the individual components.
Naturally, all metatarsal heads in the healthy foot extend equally to the plantar pad, so that there is no longer any relevant transverse foot arch spanning this area. Since the mid-foot plantar fat pad is a few millimeters thicker under the metatarsal heads, one might even refer to a transverse bulge there as well.

The natural geometry of the foot
A look at a standing radiograph of the foot taken from above reveals how the metatarsal bones spread out naturally. This configuration originates at the ­divergence of the talus and calcaneus. In normal cases, the talus-calcaneus divergence angle is maintained and the bones of the foot do not "fan out" until they reach the Lisfranc joint line between the tarsus and the metatarsal bones. Here, MT1 and MT5 form an ­angle of greater than 30°. Usually, the longitudinal axis of the talus will extend between MT1 and MT5, while the longitudinal calcaneal axis is parallel with the longitudinal axis of MT5 (Fig. 1a). In a lateral radiograph of the healthy foot, the longitudinal axis of the talus continues in MT1 (Fig. 1b). The shape of the calcaneus, which carries the talus, essentially produces the inner arch of the foot. On the outer side of the foot, the calcaneus, the cuboid bone and MT5 join together in a relatively low external arch. The bones of the foot consisting of the calcaneus, tarsus and metatarsus form longitudinal and transverse arches.
These arches are supported by both passive and dynamic structures. Passive structures include, on the one hand, the foot bones with their special formations, which are mostly wedge-shaped, thereby supporting the arches themselves. On the other hand, the passive structures also include the very strong and tight ligaments that transform the compressive forces into tensile forces. In par­ticular, the plantar fascia is of great importance for both longitudinal and transverse tension.

Active stabilization of the transverse arch and the transverse curvature of the foot
Active structures include the extrinsic muscles with their tendons, which al­ternatingly stabilize the respectively loaded structures during locomotion in the individual phases of the gait cycle. In this context, particular importance is attached to the stirrup function of the posterior tibial and long calf muscle, which run from the medial and lateral under the entire posterior transverse arch to the contralateral side, thus forming a dynamic tensioning system. Together with the anterior tibial muscle and the short calf muscle, both the posterior tibial and the long calf muscles also have a stabilizing function in the longitudinal tensioning of the foot bones.
This can optimally implement the dynamic process of locomotion when the foot is in an intact, balanced state. So how does the structural disorder of splay foot happen there?

 

Pathogenic causes of splay foot
Certainly, there are also some people with congenital splayfeet due to an inherent "construction flaw" in their foot's design, but the majority of splayfeet develop with advancing age.

Shortened calf muscles
It would seem logical to consider shortened calf muscles as a cause for the development of splay foot. The twin muscle of the calf – a muscle that stabilizes and supports the kinematic chain – stereotypically tends to shorten in any disorder (instability, injury, inflammation, deformity etc.). This masked shortening of the calf causes various problems for the bones of foot. Since the forefoot cannot deviate backwards during the gait cycle, it must assume the entire weight of the body at a very early stage in the roll-off phase.
The ultimate consequence of this problem depends on the individual weak points in the foot.
During outward rotation, instabilities of the plantar calcaneonavicular ligament and along the Chopart joint line can lead to a flattened longitudinal foot arch and an outward deviation of the forefoot. If the skeletal structure of a foot is very rigid, weight-bearing on the forefoot causes pain under the metatarsal heads, which in turn triggers the toe extensors supporting the ante­rior tibial muscle against the calf muscles. This in turn results in a hammer toe deformity with anterior displacement of the plantar pad to in front of the metatarsal head. Both obesity and congenital ligament weakness can exacerbate these mechanisms.

Unique joint configurations
Considering the stabilizers of the metatarsal bones towards the tarsus, it is striking that the MT1 sometimes forms a joint with the first cuneiform bone. This is reminiscent of our prehistoric ancestors, whose prehensile first metatarsal enabled them to grasp objects with their feet. This spherical joint seems predisposed to the development of hallux valgus and splay foot. The bases of MT2 – MT4, by contrast, nearly consistently show a very stable connection with the tarsus. This is due, on the one hand, to osseous intermeshing and articular orientation, which is oriented at an approximately right angle to the respective metatarsal longitudinal axis and thus allows little room for lateral deviations. On the other hand, there is very tight ligament balancing in this area, which allows only the slightest of movements. Sometimes, the base of MT5 can assume an articular configuration with the cuboid bone, which can exhibit an elliptical articular surface similar to the medial side of the foot. When the forefoot load is increased, MT1 is now able to deviate inwards and upwards (Fig. 2a and b) and MT5 outwards and upwards (Fig. 3a and b, Fig. 4a and b), while MT2 – 4 initially remain in their original position. Since the first and fifth rays do not absorb the forces as intended, a two- to four-fold increase in the pressure peaks takes place under the metatarsal heads. The reflexive tensioning of the toe extensors, however, increases the pressure even further due to the backward-directed force of the proximal phalanx on the metatarsal head. This also worsens the shock-absorbing effect in this region, since the plantar pad is pulled forward from underneath the metatarsal heads and thus no longer available to absorb the shock.

Splay foot subentities
Hallux valgus
The longer these processes go untreated, the more the splay foot disorder will progress. One subentity is hallux valgus, which develops from an outward displacement of tendons caused by the inward deviation of MT1. The further MT1 is displaced inwardly, the more strain is exerted on the two-headed abductor hallucis (abductor hallucis muscle), leading to sometimes grotesque hallux valgus deviations.

Tailor’s bunion
The other subentity is a tailor’s bunion, where the prominence of the fifth metatarsal head on the lateral foot can cause pressure problems. Both hallux valgus and tailor’s bunion, however, can arise on their own without splay foot.

Clinical and radiological picture
Clinically, the condition is often observed in overweight patients with a flattened longitudinal and transverse foot arch. In these cases, the transverse arch is often convex in the plantar direction, opposite to the natural position. When examining the arch of the foot after it has been manually ­restored, a considerable shortening of the calf muscles can usually be ­observed. Calluses, and sometimes ­also pressure sores, are found on the medial side of the large metatarso­phalangeal joint, on the lateral side of the small metatarsophalangeal joint and inferior to the heads of MT2 to MT4.The standing radiograph viewed from above shows a widening of the intermetatarsal angle from MT1 to MT5 and frequently a spherically shaped first midfoot cuneiform joint. In addition, all radiological criteria of a hallux valgus may be present. In the lateral radiograph of the standing foot, a flattening of the longitudinal arch is regularly observed, which is more or less localized in the talonavicular joint, the navicular-cuneiform joint, the midfoot cuneiform joint or in all three joints. In addition, toe malalignments and dislocations may be observed.

Treatment of splay foot
The treatment of splay foot is initially conservative. The younger the patient, the more effectively the normally shortened calf muscles overloading the forefoot can be treated with physiotherapy. A prescription is issued for stretching therapy of the calf muscles with trigger point treatment (see Janet Travell and David Simons, Handbuch der Muskel-Triggerpunkte [Handbook of the Muscle Trigger Points], published with Urban Schwarzenberg), antagonist training (anterior tibial muscle), coordination and strengthening of the outer and inner foot rotators. Additionally, a prescription is issued for a pair of custom-cast, long-soled foot orthoses to support the sustentaculum with retrocapital pad and rocker sole shifted to the posterior. Surgical procedures for splay foot therapy are deemed controversial. Me­thods like ligament plasty, ligament transposition or restraint have rarely met expectations. It should be mentioned at this point that the problematic shortening of calf muscles has only been inadequately addressed. This fact may have deprived some practitioners of one or the other successful outcome.
Since the treatment of shortened calf muscles is based on the cause of the problem, the surgical procedure naturally targets this structure when conservative treatment fails. There are various methods available to achieve this objective. Depending on whether the gastrocnemius muscle only is affected, success can be achieved by the Strayer procedure – transverse severance of the gastrocnemius aponeurosis – or a z-shaped lengthening or a percutaneous Hoke procedure to treat a shortening of all three calf muscles (example 1 with Fig. 5, 6, 7a and b, 8a, b and c, 9, 10a and b). In bony stabilizations, stiffening of the first midfoot cuneiform joint using the Lapidus technique has proven its merits and become one of today's standard procedures (example 2: Fig. 11a and b). Most medical instrument manufacturers now offer an extensive variety of plates and screws specifically for this operation. Stiffening of the fifth cuboid-metatarsal joint has rarely been undertaken to date. However, as shown in the example, it can be quite helpful to treat more severe conditions.
The displacement of the metatarsal head upwards using the Weil technique is preferably and frequently carried out. Nevertheless, its approach starts at the wrong end of the pathology and rarely leads to long-term success if the intervention is used as a monotherapy.

Ludwig Schwering
Mathias-Spital Rheine, Germany

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