Evolution, Medicine, and Public Health [2017] pp. 48–49 doi:10.1093/emph/eox001
Evolutionarily medicine Why do humans get bunions?
clinical briefs
Pierre Tamer*,1 and Scott Simpson2 1
Department of Anatomy, School of Medicine, Case Western Reserve University, Cleveland, OH, USA, and 2Department of Anatomy, Case Western Reserve University,
Cleveland, OH, USA *Corresponding author. School of Medicine, Case Western Reserve University, Cleveland, OH, USA. Tel: 216-368-2000; E-mail:
[email protected]
HALLUX VALGUS
intrinsic and evolutionarily based aspects of the human foot which increase this risk.
Targeted pathology Hallux valgus is the most common forefoot problem in adults. It is characterized by an anatomical deformity involving lateral deviation of the hallucial phalanges and medial prominence of the first metatarsal head, commonly known as a bunion. This acquired deformity is often correlated with foot pain, imbalance and impaired gait, falls in older adults and decreased healthrelated quality of life. Epidemiological studies report a higher prevalence of hallux valgus with increasing age and in more females than males, consistent with potential causative factors such as osteoarthritis and narrowed footwear [1]. Although footwear can be a significant external contributing factor, we propose there are
Figure 1. Grasping primate foot with abducent hallux (left); hallux valgus in humans (right) Note the orientation of the plantar surface in the
EVOLUTIONARY PERSPECTIVES The human foot differs from that of all other primates in that we use our foot as a rigid lever during the propulsive phase of locomotion rather than as a grasping organ for arboreal climbing (Fig. 1). To facilitate pedal grasping in apes, the hallucial metatarsal exhibits extorsion along its long axis and the lateral metatarsals are internally rotated, which is contrasted with the realignment of the non-abducent human first metatarsal, allowing the plantar surface to be fully in contact with the ground (Fig. 2). The abducted hallux in primates permits an axis of flexion of the hallux orthogonal to the lateral phalanges, while in humans the adducted and rotated hallux creates an axis of flexion in the same plane as the lateral phalanges, facilitating toe-off at the metatarsophalangeal joint [2]. Primarily as a consequence of hallucial torsion, when the hallucial phalangeal flexors—Flexor Hallucis Longus and Flexor Hallucis Brevis—contract during propulsion, their course shears obliquely across the inferior aspect of the metatarsal head. Although this orientation is normally balanced by constitutive ligaments and muscles, under increasing stress the oblique orientation of the phalangeal flexors can laterally discplace the hallucial sesamoids and the intervening Flexor Hallucis Longus tendon, thereby deflecting the hallucial phalanges laterally.
Figure 2. Axial torsion of first metatarsal heads in humans, gorillas and chimpanzees normalized by the basal dorso-plantar axis. Medial is to the left in upper image
FUTURE IMPLICATIONS Further addressing the evolutionary based anatomy and mechanisms underlying the propensity of humans to develop hallux valgus promotes a better understanding of the frequency of its occurrence, behavioral changes to reduce risk and the development of future therapies. Currently, therapies vary by the severity of the deformity, ranging from non-operative medications and inlays under the forefoot to surgical procedures such as osteotomies or arthrodesis. Within all of these procedures, however, challenges remain, clinical indications vary and high-level evidence is scarce regarding when to use specific operative techniques over others [3].
supplementary data Supplementary data is available at EMPH online.
chimpanzee’s abducent hallux
ß The Author(s) 2017. Published by Oxford University Press on behalf of the Foundation for Evolution, Medicine, and Public Health. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
Why do humans get bunions?
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