THE BLOOD SUPPLY DIGITAL ARTERIES The medial and lateral digital arteries arise by division of the medial palmar artery (common digital artery) between the suspensory ligament and the deep digital flexor tendon and enter the digit on the abaxial surfaces of the proximal sesamoid bones of the fetlock (Fig 3).
Opposite the proximal phalanx each digital artery gives rise to a branch which forms, with the artery of the opposite side, an arterial circle around the bone.
At the level of the proximal interphalangeal joint, the digital arteries send major branches to the heels which supply the digital cushion, frog, lamellar corium of the heels and bars and the palmar perioplic and coronary coria.
Opposite the middle of the second phalanx, each digital artery again branches and forms an artery which runs deep to the cartilages and the extensor tendon, and connects with the artery of the opposite side, to form an arterial circle around both the second phalanx and coronary band.
This coronary circumflex artery supplies the digital extensor tendon, distal interphalangeal joint and supplies numerous branches to the coronary corium and proximal lamellae of the toe and dorsal quarters.
Proximal to the navicular bone each digital artery gives off a dorsal branch which passes through the notch or foramen in the palmar process of the distal phalanx and, running in the parietal groove on the dorsal surface of the distal phalanx, supplies the lamellar coria of the quarters and heels and anastomoses with the palmar part of the circumflex artery of the sole.
Each medial and lateral digital artery sends branches to both the proximal and distal borders of the distal sesamoid (navicular) bone.
The branches anastomose with each other and form direct cross connections between medial and lateral digital arteries above and below the distal sesamoid bone.
The proximal artery runs in the suspensory ligament of the distal sesamoid bone and its branches enter the proximal edge of the bone through fine vascular foramina.
Along the distal border a similar anastomotic arterial network runs in the distal interosseous (impar) ligament and its branches bifurcate and enter the distal sesamoid bone through fine vascular foramina or adjacent to (but separate from) the 3-5 synovial fossae which characterise the distal border.
The synovial fossae appear as small radiolucent inverted bottle- or flask-shaped areas along the distal border of the bone.
The careful and elegant work of Hertsch and Dammer ( 1988) using arterial injections of contrast medium and `fine focus’ radiography has unequivocably shown that each synovial fossa is lined with synovial membrane and connects directly with the distal interphalangeal (coffin) joint.
Indeed, contrast medium injected into the coffin joint will not only outline the capsule of the joint but the synovial fossa of the distal sesamoid bone as well.
Histological sections through the region show branches of the distal artery running, through connective tissue, palmar to the synovial membrane of the fossa (Poulos and Smith 1988).
Several branches of the distal artery enter the bone independently of the synovial fossae.
The terminal part of the digital artery enters the solar canal of the distal phalanx via the paired sole foramina and unites with the artery of the opposite side to form the terminal arch deep within the bone.
Branches of the terminal arch (4-5 mid-dorsally and 8-10 distally, near the solar border) radiate outwards through foramina in the dorsal surface of the distal phalanx and supply the lamellar corium and, after forming the circumflex artery, the corium of the sole.
In addition to the 12-15 main foramina, the dorsal surface of the distal third of the distal phalanx is perforated by numerous finer foramina (the bone in this region is very porous) and recent evidence (C.C.
Pollitt and G.S.
Molyneux, unpublished data) shows that many of the vessels within these foramina are arranged anatomically to perform counter-current heat exchange, ie a central artery surrounded by a sheath of capillaries and venules (similar to the pampiniform plexus of the mammalian testis).
This implies that the equine digit is an efficient thermoregulatory organ which is not surprising when the range of equine habitats, from the sub-arctic to the equator, is taken into consideration.
The lamellar corium derives most of its blood supply from the branches of the terminal arch which perforate the distal phalanx.
Numerous anastomoses form an arterial lattice beneath and between the epidermal lamellae and blood can flow proximally to the coronary circumflex artery and to the solar circumflex artery.
The circumflex artery of the sole is an anastomosis of all the distal branches of the terminal arch and the dorsal arteries of the distal phalanx and forms a complete arterial loop supplying the corium at the junction of the distal lamellae and peripheral sole close to the sharp solar margin of the bone.
All of the arterial blod supply of the sole (except for the angle between the bars and the heels) comes from axially directed arteries branching inwards from the circumftex artery.
There are no vascular foramina perforating the solar surface of the distal phalanx (except at the angle).
This means that almost the entire corium of the sole is dependent upon a blood supply which arises first on the dorsal surface of the distal phalanx and then curls under the margin of the distal phalanx.
The solar corium is sandwiched between the epidermal sole and the unyielding solar surface of the distal phalanx and is therefore prone to damage from compressive forces.
If a horse is deliberately forced to stand or walk on the soles of its feet (by overzealous trimming of the ground surface wall) the sharp distal rim of the distal phalanx effectively cuts off the blood circulation to the central solar corium and results in severe lameness and, in some cases, necrosis of the sole. DIGITAL VEINS There are three interconnected valveless venous plexuses in the foot.
The dorsal venous plexus lies in the deep part of the lamellar corium.
The palmar/plantar venous plexus lies in the deep part of the sole corium and on the inner axial surfaces of the cartilages of the distal phalanx.
The coronary venous plexus lies in the coronary cushion covering the digital extensor tendon and the outer abaxial surfaces of the cartilages of the distal phalanx.
It anastomoses with the palmar/plantar venous plexus via foramina in the cartilages (note that both sides of the cartilages are covered by plexuses of veins).
The three plexuses are drained by the medial and lateral digital veins.
Most of the deep veins within the foot are valveless although valves occur in the more superficial coronary, subcoronary and heel veins. REACTIONS OF THE VENOUS BLOOD DURING CONCUSSION The hoof is subjected to a range of weight-bearing and locomotor forces.
These forces are believed to cause expansion of the frog and to deform all the soft tissue of the hoof, including the digital cushion, the cartilages and the vascular systems.
Because the soft tissues of the hoof are encased by the hard keratinised wall which cannot expand substantially (Fischerleitner 1974), the internal deformation of the hoof forces evacuation of the venous blood from the hoof quite quickly.
The multiple routes of drainage of the wall and sole venous plexuses, the absence of valves in most veins of the hoof, the presence of valves in the proper digital veins and caudal hoof veins, and the presence of a double layer of venous plexuses on either side of the flexible cartilages are all mechanisms to evacuate the venous blood quickly and to distribute the pressure evenly.
The absence of valves would help evacuation by allowing venous blood to take any convenient path.
The presence of the valves in the caudal hoof veins and proper digital veins prevents retrograde blood flow to the hoof and thereby ensures the efficient venous return of blood to the heart (Mishra and Leach 1983a, b). THE SKELETON The skeleton of the foot consists of the proximal, middle and distal phalanges, the cartilages of the distal phalanx, and the distal sesamoid bone.
In transverse section the distal phalanx in the forefoot is semicircular whereas in the hind foot it is oblong craniocaudally.
The distal sesamoid bone articulates with both the middle and distal phalanges.
In low ringbone, the phalanges are affected in the region of the distal interphalangeal joint and this may interfere with the movement of the joint.
Pyramidal disease is a form of ringbone affecting the extensor (pyramidal) process of the distal phalanx.
Navicular disease may involve the distal interphalangeal bone, its suspensory ligaments, the navicular bursa and the subjacent deep flexor tendon. REFERENCES AND FURTHER READING Coffman J. (1983) Digital blood flow; ;ts importance in shoeing and in acute and chronic laminitis.
Ln: Equine Incernal Medicine.
Proc. 6ch Bain-Fallun Memorial Lectures.
University, of Sydney.
Australian Eyuine Veterinary Associatiun, Artarmon.
Pp 64 ·67.
And Hickman, J. ( 1977) The arterial supply to the navicular bone and its variations in navicular disease.
J. 9, 150-154.
Fischerleitner, TE (1974) Rontgenographische untersuchungen uber den einfluss der lageveranderungen des hufserahlund kronbeines auf die mechanic der homkapsel des pferdes im belsastungsgerac.
Inaugural dissertation, Vienna.
And Dammer, H. (1988) The bluod supply cif normal and diseased navicular bones.
Radiol. 29, 276-281.
Hood, D.M., Amoss, M.S., Hightower, D., McDonald, D.R., McGrath, J.P, McMullan, WC.
And Scrutchfield, WL. (1978) Eyuine laminitis 1.
Radioisotopic analysis of the haemodynamics uf che fuot during the acute disease.
Surg. 2, 439-444.
Kainer, R.A. (1989) Clinical anatomy of the eyuine foot.
Clinics North Am. (Eyuine Pract.), pp 127.
Leach, D.H. (1980) The structure and function of the eyuine hoof wall.
Thesis, University of Saskatchewan.
And Oliphant, L.W (1983) Ultrastructure of the eyuine hoof wall secondary epidermal lamellae.
Res. 44, 1561-1570.
And Leach, D.H. (1983a) Extrinsic and intrinsic veins uf the eyuine hoof wall.
And Leach, D.H. (1983b) Electon microscopic study of the veins of che dermal lamella uf che eyuine hoof wall.
Molyneux, G.S., Haller, C.J., Mogg, K.C.
And Pollitt, C.C. (1993) The distribution, structure and innervation uf arteriovenous anastomoses in the eyuine foot.
Anat. (in press).
And Molyneux, G.S. (1990) A scanning electrun microscopical study of the dermal microcirculation of che eyuine foot.
J. 22, 79-87.
And Smith, M.E (1988) The nature uf enlarged ‘vascular channels’ in the navicular hone of the horse.
Radiol. 29, 60-64.
Smith, R.K.W and Schramme, M.C. (1992) The use of cuntrast agents in the diagnosis of penetrating wounds of che fuot in five cases.
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