Comparative foot morphology

Comparative foot morphology
Skeletons of a human and an elephant.

Comparative foot morphology is exemplified through study of the form of distal limb structures of a variety of terrestrial vertebrates. A challenge to understanding the role of the feet of a variety of different organisms is the wide range of body types, foot shapes, arrangement of structures, loading conditions and other variables. However, there are also facets of similarities in feet among a variety of terrestrial vertebrates. The paw of the dog, the hoof of the horse, the manus (foot) and pes (foot) of the elephant, and the foot of the human all share some common features of structure, organization and function. Each organism's foot structures function as the load-transmission platform which is essential to balance, standing and locomotion strategies (such as walking, trotting, galloping and running).

Comparing foot morphology across a variety of terrestrial vertebrates has relevance to human-engineering problems through the philosophy of biomimetics. For instance, what is learned about comparative foot morphology across a variety of organisms may allow to effectively alter the load transmission of the foot of persons wearing an external orthosis due to paralysis from spinal-cord injury, or a prosthesis due to limb amputation from diabetes. This understanding transferred into technology may enhance the person's ability to stand with improved balance, to walk more efficiently, to participate in exercise or otherwise improve the quality of their life through movement.

Contents

Structure

Limb and foot structure of representative terrestrial vertebrates:

Variability in scaling and limb coordination

Elephant skeleton

Certainly there is a wide variety in the scaling of body and limb dimension, as well as the nature of loading, during legged standing and locomotion among and between quadrupeds and bipeds.[1] The anterior-posterior body-mass distribution varies considerably among mammalian quadrupeds, which influences limb loading. During standing, many terrestrial quadrupeds support a greater percentage of body mass in the forelimbs compared to the hindlimbs;[2][3] however, the distribution of body mass and limb-loading changes as the organisms undergo locomotion.[4][5][6] Scaling of body size and limb dimension in humans is exemplified by larger lower-limb mass compared to upper-limb mass. The hindlimbs of the dog and horse are slightly greater in mass compared to the forelimbs, whereas the elephant displays longer limbs relative to the scale of its body size. The elephant's forelimbs are longer than the hindlimbs.[7]

In the horse[8] and dog, the hindlimbs play a notable role in primary propulsion. In the human there is generally an equal distribution of loading upon each lower limb during legged locomotion.[9] The elephant, as the largest terrestrial vertebrate, displays its own unique range of locomotion that engages the hindlimbs and forelimbs similarly.[10] Quadrupeds and bipeds display differences in the relative phase of limb movements during walking and running gaits between forelimbs and hindlimbs, as well as right-sided limbs and left-sided limbs.[5][11] Many of the aforementioned variables address differences of the scaling of body and limb dimension as well as limb coordination and movement patterns. However there is little information on the role of the foot and its structures in contributing to function during weight-bearing. An examination of the distal limb and foot structure of some representative terrestrial vertebrates provides interesting similarities in comparative foot morphology.

Column organization of limb structures

Limb skeleton of a lion, an example of an angulated bony column

Even though differences in the scaling of limb dimension, limb coordination and magnitude of loading upon the limbs exists between the forelimbs and hindlimbs of many terrestrial vertebrates, the structure of the distal forelimb is similar to that of the distal hindlimb in the dog, horse and elephant.[7][8][12] In the human, the structures of the hand shares similarities in shape and arrangement to those of the foot. Terrestrial vertebrate legged quadrupeds and bipeds generally possess distal limb and foot endoskeleton structures that are aligned in series, stacked in a relatively vertical orientation and arranged in quasi column-like fashion in the extended limb.[1][13][14] In the dog and horse, the bones of the proximal limbs are oriented vertically whereas the distal limb structures of the ankle and foot assume an angulated orientation. In humans and elephants, vertical column orientation of the bones in the limbs and feet is also evident for associated skeletal muscle-tendon units.[6] The horse’s foot contains an external nail (hoof) oriented about the perimeter in the shape of a semicircle. The underlying bones are arranged in a semi-vertical orientation.[15][16] The dog’s paw similarly contains bones arranged in a semi-vertical orientation.

Column orientation of the foot complex can be altered in the human and elephant. Humans achieve a plantigrade orientation and elephants achieve a semi-plantigrade alignment of the hindlimb foot structure.[6] The change in column like orientation of bones and joints in the foot to achieve plantigrade or semi-plantigrade orientation allows the foot of the human and elephant to adapt to various terrain.[17]

Distal cushion

Distal cushion of a raccoon and feet of an elephant

Many representative terrestrial vertebrates possess a distal cushion at the under surface of the foot. The dog's paw contains a number of visco-elastic pads oriented along the middle and distal foot. The horse possesses a centralized digital pad known as the frog, located at the distal aspect of the foot, which is surrounded by the hoof.[12] Humans possess a tough fibroelastic fat pad anchored to the skin and bone of the rear portion of the foot.[18][19]

Perhaps one of the most unique distal cushions is exemplified in the elephant's foot. The forefoot (manus) and hindfoot (pes) contain massive fat pads similarly scaled to address the massive loads of the largest terrestrial vertebrate. In addition, a cartilage-like projection (prepollex in the forelimb and prehallux in the hindlimb) appears to anchor the distal cushion to the bones of the elephant's foot.[20]

The distal cushion in all of these organisms (dog, horse, human and elephant) appear to function as dynamic structures during locomotion, undergoing compression and expansion when loaded; it has been suggested that these structures reduce loads to the skeletal system.[18][19][20][21]

Organization

Arrangement of foot structures:

Due to the wide variety of body types, scaling and shapes of the distal limbs of terrestrial vertebrates, there is limited knowledge and controversy in understanding the organization of foot structures. One organizational approach to understanding foot structures is the division by regional anatomy. Foot structures are divided into segments from proximal to distal and are grouped according to similarity in shape, dimension and function. According to this approach, the foot may be described in three segments: as the hindfoot, midfoot and forefoot.

The hindfoot is the most proximal and posterior portion of the foot.[22] Functionally, the structures contained in this region are typically robust, possessing a larger size and girth than structures in other regions of the foot. Structures contained within the hindfoot are usually adapted for transmitting large loads between the proximal and distal aspects of the limb when the foot contacts the ground. This is evident in the human and elephant foot where the hindfoot undergoes higher magnitudes of loading during initial contact during many forms of locomotion.[23] The hindfoot structures of the dog and horse are located relatively proximal compared to elephant and human foot arrangement.

The midfoot is the intermediate portion of the foot that lies between the hindfoot and forefoot. The structures in this region are intermediate in size and typically function to tranasmit loads from the hindfoot to the forefoot. The human transverse tarsal joint of the midfoot transmits forces from the subtalar joint in the hindfoot to the forefoot joints (metatarsophalangeal and interphalangeal) and associated bones (metatarsals and phalanges).[24] The midfoot in the dog, horse and elephant contains similar intermediate structures with functions similar to that of the human foot.

The forefoot represents the most distal portion of the foot. In the human and elephant, the bone structures contained in this region are generally longer and narrower. The structures of the forefoot play a role in providing leverage for terminal stance propulsion and load transfer.[6][23]

Function

Load transmission of the foot in representative terrestrial vertebrates:

Dog paw

Dog paw

The dog's paw is in a digitigrade orientation. The vertical columnar orientation of the proximal bones of the limbs that articulate with distal foot structures arranged in quasi-vertical columnar orientation appears well-aligned to transmit loads of weight-bearing between the skeleton and ground. The angled orientation of the elongated metatarsal and the digits extends the area for storage and release of mechanical energy in muscle tendon units originating proximal to the ankle joint and inserting at the distal aspect of the foot bones.[6] When muscle tendon units undergo lengthening, then the load strain facilitates mechanical activity. These muscle tendon unit structures may be well-designed to aid in ground reaction force transmission essential for locomotion.[25] In addition, the pads of the distal paw appear to allow load attenuation, by enhancing shock absorption during paw contact with the ground.

Horse foot

Section of a horse foot

The horse's foot is in an unguligrade orientation. The columnar orientation of bones and connective tissue is similarly well-aligned to transmit loads of weight bearing. The thick keratinized and semicircular-shaped hoof undergoes a change in shape during loading and unloading. Similarly, the cushioned frog centrally located at the rear ends of the hoof undergoes compression during loading and expansion when unloaded. Together the hoof and cushioned frog structures may work in concert with hoof capsule to provide shock absorption.[21] The horse hoof also acts dynamically during loading, that may perhaps shield the endoskeleton of high loads that would otherwise create critical deformation.

Elephant foot

Leg skeleton of the modern elephant

The hindlimb and foot of the elephant is oriented semi-plantigrade and is closely associated with the structure and function of the human foot. The tarsals and metapodials are arranged and form an arch, similar to the human foot. The five toes of each elephant foot are enclosed in a flexible sheath of skin.[20][26] Similar to the dog's paw, the elephant's phalanges are oriented in a downward pointed alignment. The distal phalanges of the elephant do not touch the ground directly and are attached to the respective nail/hoof.[27] Distal cushions occupy the spaces between muscle tendon units and ligaments within the hindfoot, midfoot and forefoot bones on the plantar surface.[28] More interesting is that the distal cushion is highly innervated by sensory structures (Meissner's and Pacinian corpuscles), rendering the distal foot one of the most sensitive structures of the elephant (more so that its trunk).[20] The cushions of the elephant's foot match the demands for storing and absorbing mechanical loads when compressed and in distributing locomotor loads over large areas to maintain foot tissue stresses within acceptable levels.[20] Further, the musculoskeletal foot arch and sole cushion of the elephant act in concert similar to the horse's cushioned frog and hoof[6] and the human foot.[29] In the elephant the nearly half-cupula-shaped arrangement of bony elements of the metatarsals and toes displays interesting similarities to the structure of the arches of the foot in humans.[29][30]

Human foot

Skeleton of the human and gorilla (gorilla in non-natural posture)

The unique ability of the human foot to achieve plantigrade alignment creates a distal-limb structure that is adaptable to a variety of conditions. The less mobile and more-robust tarsal bones are uniquely shaped and aligned to accept and transmit large loads during the early phases of stance (initial contact and loading response phases of walking, and inadvertent heel strikes during running). The tarsals of the midfoot which are smaller and shorter than the hindfoot tarsals appear well oriented to transmit loads between the hindfoot and forefoot which is necessary for load transfer and locking of the foot complex into a rigid lever for late stance phase. Contrary to this, the midfoot bones and joints also allow for the transmission of loads and interjoint motion that unlocks the foot to create a loose packed structure that renders the foot highly compliant to a variety of surfaces. In this configuration, the foot is able to accept and dampen the large loads encumbered at heel strike and early weight acceptance.[17] The forefoot with its long metatarsal and relatively long phalanges allows for transmission of loads during the end-of-stance phase to allow push-off and transfer of forward momentum. The forefoot also serves as a lever to allow balance during standing and jumping. In addition, the arches of the foot that span the hindfoot, midfoot and forefoot play a critical role in the nature of transformation of the foot from a rigid lever to a flexible weight accepting structure.[23][24]

With a running gait, the foot-loading order is usually the reverse of walking. Foot strikes the ground with the ball of the foot and then the heel drops.[31] The heel drop extends the Achilles tendon elastically, which is recovered during the push-off.[32]

Clinical implications

A great deal of interest from veterinary and healthcare professionals emerges when the foot of a dog, horse, elephant or human develops an abnormality. In these instances, efforts to understand the nature of the pathology are typically undertaken in order to develop and implement a clinical treatment plan. The paws of the dog and the hindfoot work together to absorb the shock of jumping and running and to provide flexibility of movement. If the dog's skeletal structures in areas other than the foot are compromised, then the foot may bear the burden of compensatory loading. Structural faults such as straight or loose shoulders, straight stifles, loose hips, and lack of balance between the forefoot and hindfoot, can all cause gait abnormalities that, in turn, lead to damage to the hindfoot and paws as the foot structures compensate by bearing greater loading.

In the horse, lack of moisture of the hoof may stiffen the external foot structure. The stiffer hoof reduces the foot's load attenuation, rendering the horse unable to bear weight upon the distal limb. Similar characteristic features emerge in the human foot as pes-cavus alignment deformity created by tight connective tissue structures and joint congruency create a rigid foot complex. Persons with pes cavus display characteristic features of reduced load attenuation, and other structures proximal to the foot may compensate with increased load transfer (i.e., excessive loading to the knees, hips, lumbo-pelvic joints or lumbar vertebra).[24] Foot disorders are common in elephants in captivity. However, their cause is poorly understood.[33]

See also

                       

References

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