Regional variation in digital cushion pressure in the forefeet of horses and elephants.
- Published
- Accepted
- Subject Areas
- Veterinary Medicine, Zoology, Anatomy and Physiology
- Keywords
- biomechanics, foot, heel pad, locomotion, biomaterials.
- Copyright
- © 2014 Warner et al.
- Licence
- This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ PrePrints) and either DOI or URL of the article must be cited.
- Cite this article
- 2014. Regional variation in digital cushion pressure in the forefeet of horses and elephants. PeerJ PrePrints 2:e231v1 https://doi.org/10.7287/peerj.preprints.231v1
Abstract
Horses and elephants have extreme foot designs; horses have an unguligrade foot posture with small, single-toed, rigid hooves housing a small fibrous digital cushion, whereas elephants have large, multi-toed, functionally plantigrade, compliant feet with large adipose filled digital cushions. The morphology of the digital cushion is divergent in these species, in terms of its size, shape, volume, composition and organisation. In the context of foot-ground contact this is interesting, because feet nonetheless have to perform similar mechanical functions in all terrestrial species. How well the digital cushion functions under load may contribute to the aetiopathogenesis of foot disease; a sub-optimal digital cushion is less likely to distribute (and thus reduce) high pressures, moderate impact shock and vibration, or prevent unwarranted bone displacement.
In this study, we seek to understand how the digital cushion morphologies evident in horse and elephant feet influence internal and external foot pressures. Our novel use of invasive blood pressure monitoring equipment, combined with a pressure pad and force plate, enabled measurements of (ex vivo) digital cushion pressure under increasing axial loads in seven horse and six elephant forefeet. Linear mixed effects models (LMER) revealed that internal digital cushion pressures increase under load and differ depending on region; elephant feet experienced higher magnitudes of medial digital cushion pressure, whereas horse feet experienced higher magnitudes of centralised digital cushion pressure. Direct comparison of digital cushion pressure magnitudes in both species, at equivalent loads relative to body weight, revealed that medial and lateral pressures increased more rapidly with load in elephant limbs. Within the same approximate region, internal pressures exceeded external, palmar pressures (on the sole of the foot), supporting previous Finite Element (FE) predictions. High pressures and large variations in pressure may relate to the development of foot pathology, which is a major concern in horses and elephants in a captive/domestic environment.
We suggest that heterogeneity within the digital cushion allows it to perform conflicting mechanical functions during locomotion; different digital cushion regions may be responsible for specific functions and therefore feature appropriate properties to do so. Determining how internal structures such as the digital cushion respond to locomotor loading is essential to understanding foot health and pathology, as well as the functional consequences of evolutionary changes in foot morphology.
Supplemental Information
Elephants: Individual variation in mean internal pressure by region.
Coloured lines show individuals, black line shows fitted LMER used for analysis. Despite pathologies being present in some elephant limbs, the models for each individual follow a similar pattern, hence excluding pathological limbs from the analysis was not deemed necessary.
Elephants: Individual variation in maximum instantaneous internal pressure by region.
Coloured lines show individuals, black line shows fitted LMER used for analysis. Despite pathologies being present in some elephant limbs, the models for each individual follow a similar pattern, hence excluding pathological limbs from the analysis was not deemed necessary.
Horses: Individual variation in mean internal pressure by region.
Coloured lines show individuals, black line shows fitted LMER used for analysis.
Horses: Individual variation in maximum instantaneous internal pressure by region.
Coloured lines show individuals, black line shows fitted LMER used for analysis.
Experimental set up.
A) Horse forelimb, B) elephant forefoot.
Centre of pressure (CoP, black circles) in each horse foot, across all trials.
Arrows indicate the orientation of the hoof. Data for Horse 1 and 2 are missing due to technical difficulties with the pressure pad (see Data S9 for details). Warm colours denote higher pressures, whereas cool colours denote lower pressures, white areas show zero pressure. The purpose here was to show the CoP in relation to the hoof, rather than pressure magnitudes. Note the limited contact area of the foot, which is highly variable and likely to depend on hoof balance/trimming and concavity of the sole.
Centre of pressure (CoP, black circles) in each elephant foot, across all trials.
Arrows indicate the orientation of the 3rd digit. Data for Elephant 1 are missing due to technical difficulties with the pressure pad (see Data S9 for details). Warm colours denote higher pressures, whereas cool colours denote lower pressures, white areas show zero pressure. The purpose here was to show the CoP in relation to the foot, rather than pressure magnitudes. Note the variation in contact area between individuals, which is likely to vary due to toe nail length, and the presence of fissures in the sole (dependant on abrasion/foot trimming).