416


EQUINE VETERINARY EDUCATION / AE / AUGUST 2020


a)


b)


Fig 1: a) Ground surface and b) side view of the wooden shoe. Note the bevel around the perimeter of the shoe and the flat platform on the ground surface.


(COP). The position of the centre of pressure varies depending on foot conformation and trimming but will be located dorsal to the centre of rotation (COR) of the distal interphalangeal joint (DIPJ). The GRF being dorsal to the COR creates a moment about the distal interphalangeal joint termed the extensor moment. The extensor moment must be opposed by an equal and opposite moment, which is termed the flexor moment. A moment is the product of a force and its distance from a reference point to cause a body to rotate about an axis which in this case, is the centre of rotation of the distal interphalangeal joint. At rest, the extensor moment is the product of the weight borne by the limb (a force) and the horizontal distance from the point at which the ground reaction force acts on the foot (COP) and


the centre of rotation of the distal interphalangeal joint. The flexor moment opposes the extensor moment and is the product of the force (tension) in the tendon and the shortest distance of the DDFT from the centre of rotation (Fig 2a). The GRF determines the subsequent compressive and tensile stresses that are placed on the dorsal section of the foot. If a horse is standing on its limb, the weight of the horse acts on the foot through the COR and is relatively constant, but the position of the GRF where it acts on the ground surface of the foot can be shifted away from the affected area or redistributed. At breakover, the flexor moment begins to exceed the extensor moment such that the GRF moves dorsally to the toe at which point the heels lift off the ground (Fig 2b). As the COP moves dorsally, it places greater force at the toe; a force that can potentially be disruptive. The thickness of the wooden shoe allows breakover to be placed in the shoe further palmarly than conventional shoes. This


a)


reduces the extensor moment arm, brings the GRF closer to the centre of rotation and reduces tension in the DDFT (Fig 3a). This concept of changing the point of breakover can be clearly demonstrated by creating a bevel in two pieces of wood that are of different thickness (Fig 3b). Furthermore, the bevel created around the perimeter of the wooden shoe will reduce the moment which decreases the force required to breakover in a medial or lateral direction with less stress on the tissues (Fig 4). Finally, combined with the appropriate trim and impression material placed in the frog sulci, one firm flat surface (the foot) is created and now placed against the wooden shoe; the distribution of force becomes larger and more uniform because weight bearing is widely distributed across the palmar section of the foot.


Construction of the shoe


The author prefers wood due to its accessibility, light weight, the ease with which it can be constructed/shaped (both before and after application), malleability and its ability to dissipate energy at impact while remaining rigid (Reid 1994). There are shoes available commercially that have an ethyl vinyl acetate (EVA) pad substituted for the thicker ¾ inch section of plywood that is beveled. EVA is an extremely elastic material that can be sintered to form a porous material similar to rubber yet has resilience. The compressibility of this material distributes the load across the surface of the foot, however, it does wear and compress unevenly relative to the load placed on the limb causing the angle and the forces on the foot to change. Although more time consuming, the author prefers to use wood which can


b)


Extensor Moment


Flexor Moment


Fig 2: a) The biomechanical forces exerted on the foot and the moments about the DIPJ. b) The GRF moving dorsally in the toe during mid-stance and breakover (Image courtesy of Andrew Parks).


© 2019 EVJ Ltd


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92