Polyurethane casts have the advantage of being clean, dry and odourless and therefore welcome in an office, class-room or beside a computer monitor for study sessions. It is uncanny how much detail is preserved in each cast; in the distal transverse replica one could actually count the number of epidermal lamellae (TNH0000).
A replica can be defined as ‘a copying closely resembling the original concerning its shape and appearance – any close or exact copy or reproduction’. Viewing a replica one should experience a real feeling or the ‘aura’ of the original based on its previous history and experience. Comparing a normal foot with a severe chronic laminitis replica certainly conveys the sorry history and suffering experienced by horses and ponies affected by this disease. This is what makes replicas such valuable teaching aids; they invoke the real thing. To share this experience archived material has been retrieved to produce, four hand-painted rigid polyurethane replicas ready for purchase from the website www.3dvetanatomize.com. Each replica is boxed with an accompanying captioned diagram showing key anatomical and pathological features. They are painted with long-lasting enamel pigments and finished with a matt top-coat. For clarity, the colour scheme is diagrammatic and is the same as in the sagittal diagram Figure 5.4 in ‘The Illustrated Horse’s Foot’.*
The first replica, from the foot of a normal horse (NH0000), is a sagittal section. The accompanying diagram has 46 arrowed labels. It shows the classic anatomical structures and relationships of the horse’s foot. The dorsal cortex of the distal phalanx (DP) is parallel to the dorsal hoof wall and a constant distance from it. The parietal cortex of the DP is suspended from the stratum lamellatum of the inner hoof wall by the suspensory apparatus of the distal phalanx. The palmar digital arteries unite in the solear canal of the DP as the terminal arch. The proximal and distal parietal arteries branch from the terminal arch and via foramen in the DP supply arterial blood to the stratum lamellatum. The distal scutum covers the flexor surface of the distal (navicular) sesamoid bone opposite the deep digital flexor tendon before the latter inserts on the palmar surface of the DP. Likewise the distal sesamoidean impar ligament attaches the distal border of the navicular bone to the palmar DP. Opposite the middle scutum on the proximal palmar aspect of the middle phalanx is the fibrocartilaginous pad of the deep digital flexor tendon. Companion guides for the study of this replica are figures 5.4 and 5.10 in the ‘Illustrated Horse’s Foot’.*
The second replica from the foot of a normal horse (TNH0000) is a transverse section cut just above the bearing border of the hoof sectioning the distal margin of the distal phalanx. It shows parietal cortex of the distal phalanx perforated by numerous vascular channels; these are absent in the dense, solear cortex. The sublamellar and solear dermis are on both sides of the distal margin of the distal phalanx. There are numerous veins in the sublamellar dermis (the sublamellar venous plexus). This plane of section can be used to count the number of epidermal lamellae of the hoof wall and bars (approximately 560). The dermal and epidermal lamellae (stratum lamellatum) are more numerous dorsally and decrease in number towards the quarters and heels. Each palmar process of the distal phalanx is attached to hoof lamellae at the heel on the outside (abaxially), the bar on the inside (axially) and the hoof angle palmarly. A companion guide for the study of this replica is Figure 5.19 in the ‘Illustrated Horse’s Foot’.*
The third replica from the foot of a normal horse (FNH0000), is a frontal section cut just palmar to the apex of the frog. The distal interphalangeal (DIP) joint is sectioned at the level of the strong collateral ligaments of the DIP joint. It is noteworthy how deeply the collateral ligament attachment zones are recessed into the bone. This is commensurate with their importance in mediolateral stabilisation of the DIP joint. The palmar digital arteries are sectioned just palmar to their entrance into the solear canal of the distal phalanx. The apex of the frog and the most dorsal part of the digital cushion are aligned proximodistally with the insertion of the deep digital flexor tendon. The venous plexi of the solear and sublamellar dermis and the coronary cushion are prominent. A companion guide for the study of this replica is Figure 5.14 in the ‘Illustrated Horse’s Foot’.*
The fourth replica (CLH0000) is a sagittal section from the foot of a Thoroughbred horse with severe chronic laminitis. It shows the distal phalanx sunk deep into the hoof capsule and its distal margin in contact with the now necrotic horny, convex sole. The founder distance is twice the normal and the coronary horn tubules are kinked and displaced distally explaining an apparent retardation of dorsal hoof growth. There is a large lamellar wedge between the dorsal hoof wall and the parietal surface of the distal phalanx. The distal part of the wedge is made up of tubular terminal horn and dorsal sole that is likely proliferating, expanding and growing palmarly in the direction of the distal margin of the distal phalanx. The bearing border of the dorsal hoof capsule is distracted dorsally from the distal phalanx making it impossible for it to bear weight and forcing the foot to be loaded on the dorsal sole directly beneath the dislocated distal phalanx. This explains the bone lysis and modelling, the contused solear dermis, the pressure necrosis and infection of the sole and the intractable pain. Companion guides for the study of this replica are figures 14.19, 14.32 and 14.39 in the ‘Illustrated Horse’s Foot.’*
More replicas are planned for the future. It is anticipated the current models will be useful for veterinarians, teachers and farriers to educate clients, students and horse owners about normal foot anatomy and function as well as the pathological processes occurring during laminitis. We welcome feedback especially ideas for new and different replicas. Contact us for any questions or suggestions.
* Reference: The Illustrated Horse’s Foot: A Comprehensive Guide. by Emeritus Professor Christopher C. Pollitt, Elsevier, St Louis. ISBN:978-0- 7020-4655- 1