A Cross-disciplinary Approach to Understanding Flatfoot

Abstract

As form follows function, pedal anatomy is embedded in a history of evolution. This literature review seeks to further the understanding of physiologic and pathologic flatfoot through cross-disciplinary research of expired and extant members of the Homininae subfamily. Archaeological, anthropological, paleontological, and ontogenetic evidence presents multiple biomechanical similarities and anatomical parallels between flatfooted hominins and humans. Recognizing an evolutionary pattern in flatfoot pathologic disorders enhances anticipation and effective treatment.

A multitude of studies [1–6] have sought to broaden the understanding of flatfoot pathomechanics. Retracing the evolutionary steps in bipedal development creates a paradigm shift in the interpretation of flatfoot anatomy and biomechanics. Normal human anatomy is the result of a specific and highly selective evolutionary process [7]. Pedal adaptations are constituents of whole-body evolution as opposed to an individual selection process of distinct bones and joints [7]. Nonetheless, anthropological, paleontological, archaeological, and ontogenetic evidence reiterates evolutionary trends of the hominin foot in the development of flatfoot. The primary areas of comparison include midtarsal joint axes orientation, talus morphological features, rearfoot biomechanics, hindfoot to forefoot weight transfer, and first-ray deviation. Although hominins demonstrate a multitude of additional adaptations providing greater mobility, the following literature review demonstrates significant overlap with flatfoot presentation. The present evidence best supports the hypothesis that flatfoot, and associated pathologic disorders, is a reiteration of natural selection.

Of all of the extant primates, humans are the only plantar obligate bipeds [8]. With direct contact on the ground, the foot is highly selected to provide balance and propulse in a particularly efficient way [7,8]. Both maneuvers require a rigidly functioning lever arm [7]. In 1935, Elftman and Manter [9]. demonstrated that the axes of a normally functioning transverse midtarsal joint are aligned in pronation and become incongruent in supination. During supination, the midtarsal joint locks to create a rigid construct for propulsion [9–12]. During propulsion, the hindfoot inverts and the midtarsal joint locks, buttressing the calcaneus against the cuboid and providing a rigid lever for effective push-off [11–13]. The researchers, and others who have followed them, noted that the locking mechanism that occurs in the bipedal human was absent in the terrestrial nonhuman hominin [9,13–19]. Midtarsal joint axis congruity is demonstrated in pronation and supination in nonhuman hominins [20]. The result is continuous midtarsal joint mobility, creating a midfoot break during heel lift [7,21]. The “two-stage” heel lift interrupts motion fluidity proximally at the heel and distally at the midtarsal joint or tarsal metatarsal joint [9,17,18,22–25]. The mechanics of propulsion and functionality in the hominin foot are contingent on the structural orientation of the midfoot joint axes [7].

Midfoot joint hypermobility in the hominin foot mirrors the pathomechanics of flatfoot [7,21]. Both display a predominantly valgus hindfoot with a reduced supinatory phase in the normal gait cycle [11,21,26]. Consequently, midtarsal joint axes are in a constant state of parallelism [11,26]. These abnormal weightbearing conditions create persistent subluxation of the talocalcaneal joint, resulting in the foot’s inability to support propulsion in the normal gait cycle [27,28]. The additional failure of plantar medial soft-tissue support generates mid-foot loading with medialization of forefoot pressures [2,29,30]. Gastrocnemius and soleal contractures compound the flatfoot deformity and reinforce the two-stage heel lift [31]. The inability to reduce hindfoot valgus and midfoot abduction evidences a two-stage heel lift comparable with that of a terrestrial hominin [6,29,31].

The talus plays a pivotal role in the transition from facultative to obligate bipedal hominin [21,32,33]. It is a mosaic of apelike and humanlike features [32,34]. During ontogeny, the head, neck angle, and torsion angle begin distinctly apelike and evolve to display human properties [34]. The talar body and trochlear surface naturally exhibit evolved humanlike characteristics [14,34,35]. The apelike distal aspect of the talus correlates to the presence of opposable thumbs [14,35–37]. Bipedal hominin fossils demonstrate interdependence between talar neck angle and first-ray divergence [15,35–37]. During ontogeny, reduction of the talar neck angle is a mechanism for removing first-ray divergence [14,15,35–37]. Evolutionary recapitulation theory is evidenced by primitive hominin fossils with increased first-ray divergence and talar neck angle and the ontogenetic reduction of first-ray divergence and talar neck angle in humans [22,35]. Similarly, talar torsion begins modestly and increases during ontogeny [38]. With greater talar torsion comes the ability to maintain incongruent midtarsal joint axes, providing a supinatory locking mechanism [9]. Archaeological evidence demonstrating minimal talar torsion in primitive hominins supports the prehuman inability to lock the midtarsal joint [7,39]. An incomplete maturation of talar neck angles and talar torsion angles has been associated with a higher propensity for developing flatfoot in humans, although no fossil or ontogenetic evidence demonstrates graded correlation [40–42]. Ontogenetic human development and phylogenetic hominin evolution display talar traits of flatfoot.

Paleoanthropological studies of the talus demonstrate morphological features in hominin fossils similar to the robust structure of the human talus [14,32,43]. A functional convergence generated by Wolff’s law produces common features involving a longer and narrower trochlea, a plantarly angled and narrower talar head, and a lower talar body height [32,43–45]. Similarly, when examining the talar morphological features in the symptomatic flatfoot, Anderson et al [1] found an overall narrower and shorter talar presentation with heads elongated in the transverse plane compared with in tali in feet with a normal anatomy. Long-standing flatfoot is also associated with dorsal compression and flattening of the lateral portion of the head of the talus, analogous to paleoanthropological evidence [34,42]. These structural implications create agenesis of the sustentaculum tali and allow the talus to function in a plantarflexed attitude [42]. In addition, a long talus demonstrates an anteriorly displaced cyma line and further contributes to talonavicular instability [42]. Although the normal human talar morphological condition denotes a cylindrical talar body structure in a hinged ankle joint, terrestrial nonhuman hominins display more conical contours suggestive of a ball-and-socket joint, faulting under vertical superincumbent loads [33]. Consequently, current-day analysis of talonavicular and talocalcaneal joint arthrodeses have been met with positive reviews as a means to control pes planus [4,44].

Aside from a morphological perspective, the role of the talus in bipedalism extends to the surrounding joints. Present-day studies demonstrate the intimate connection of the talocalcaneal and talonavicular joints secondary to the odd morphological condition of the talus [44]. Cadaveric studies show that a subtalar joint arthrodesis limits talonavicular joint motion to 26%; whereas a talonavicular joint fusion limits subtalar joint motion to 9% [46]. Some surgeons have suggested that the flatfoot deformity originates from primarily pathologic talonavicular or talocalcaneal joints [4,6,44,47,48]. The talocalcaneal joint axis in flatfoot is medially displaced and more aligned with the horizontal plane, allowing for greater sagittal and frontal plane motion and less transverse plane motion [49]. The resultant talocalcaneal hypermobility increases the talar pronatory effect, propagating flatfoot deformity [49]. In the hominin foot, the talocalcaneal joint has demonstrated a paramount role in the torque conversion supporting the prehuman upright transition [22,50,51]. Concomitant with a decreased talar declination angle, an elevated subtalar joint axis generates greater transverse plane motion with frontal plane motion, thereby allowing weightbearing compensation across uneven surfaces [39,50–52]. The talonavicular joint has also been shown to play a significant role in hominin and pathologic flatfoot biomechanics [22,53]. Examining rearfoot kinematics and joint morphological features in prehuman fossil evidence, Kidd [22] postulated that the talonavicular joint, determined by its concavity-convexity, is essentially apelike, whereas the calcaneocuboid joint, with its more linear joint surfaces, is distinctively humanlike. Kidd [22] proposed that the lateral side of the hominin foot evolved first to stabilize midtarsal flexibility as a terrestrial adaptation and that the medial side followed.

Before the advent of the pedal arch, the medial column bore weight, as illustrated by the two-stage heel lift [39]. Harcourt-Smith and Aiello [19] demonstrated that the navicular tuberosity transmitted loads from rearfoot to forefoot by correlating tuberosity size with hominin terrestriality. Enlarged tuberosities indicate increased medial-side weightbearing and the consequent lack of a pedal arch [9,53]. At the brink of bipedalism, the medial longitudinal arch provided effective weight transfer in stance through the windlass mechanism, creating the tripod effect [26,54]. Anatomical foot architecture promotes equal distribution over the load-bearing surfaces: first metatarsal head, fifth metatarsal head, and calcaneus [26,54]. Similar to the flatfooted hominins, the consequence of a midfoot sag in human flatfoot is a weightbearing medial column [6,30]. With a valgus heel, abducted midfoot, and incompetent soft tissues, the load is increased on the navicular tuberosity, breaking the tripod mold of normal human anatomy [5,31,55].

As Morton [56] hypothesized, in the “prehuman” foot, medial structures played a greater functional role, contrasting to the limited weightbearing role in humans. Fossil evidence demonstrates a saddle-shaped joint at the metatarsal-cuneiform interface, supporting a shortened diverging first ray, as in opposable thumbs [56]. Embryological studies demonstrate a proximal retraction of the great toe in hominins comparable with the proximal migration of the thumb in the human hand [7]. The web that binds the first and second digits persists in ontogenetic human pedal development [7]. The first interspace web indicates considerable phylogenetic mobility, with limited soft-tissue structures connecting the first and second metatarsals [7,56]. The advent of the bipedal foot arises with alignment of the hallux with the lesser digits [7]. Hominins lost the ability to grasp with the replacement of the digital web by the cleft [7,56]. The anatomical void of soft-tissue attachments connecting the first and second metatarsals persists in humans. The structural chasm concedes to first-ray deviation in humans as in hominins, permitting bunion deformity.

The notion that first-ray deformity is a subsidiary of pes planus pathologic disorder has been suggested by multiple authors [2,3,57–59]. Studies [3] show that the incidence of flat feet in the presence of bunions is significantly greater (8–24 times) than in the healthy population. A flatfoot deformity creates significant medialization of forefoot ground reaction forces [2,6,19,29]. As the tarsometatarsal joint collapses, the hypermobile metatarsal rises superiorly on the saddle-shaped medial cuneiform, upsetting the musculoskeletal balance circumferential to the first metatarsophalangeal joint [2,6,56]. The flexor hallucis brevis and adductor hallucis muscles further deform the first ray with the loss of retrograde force at the interposed joint [26,57,58]. The absence of soft-tissue attachments between the first and second metatarsal heads further propagates the medial direction of the deformity [57,60–61]. First-ray instability leading to bunion deformity parallels mobility in hominins with opposable thumbs.

In recent years, there have been renewed attempts to develop Lamarckian and Darwinian theories explaining the evolution of bipedalism [7,22,43,62]. Natural selection rules the forces at play recapitulating pedal pathology in evolution [62]. A cross-disciplinary approach profoundly increases the depth of understanding of flatfoot deformity.

This literature review sought to provide morphological and functional parallels across primitive and current timelines. The present evidence warrants additional investigation in the venues of terrestrial hominin biomechanics, anatomical study of fossils, and fetal evolution. Further research is necessary to establish with greater certainty a theory explaining the formation of key pedal structures and the ease of deviation toward pathologic deformity.