Using Science to Guide Farriery (Manipulating Force for a Mechanical Advantage) Part 2
Hoof balance – Trimming protocol theories
Farriery technique have been shown to influence skeletal alignment within the foot (Kummer et al 2006; 2009), and the biomechanical hoof mechanisms involved in shock attenuation and as such presumably of consequence to the orthopedic health of the horse. Several farriery texts (Emery. et al; 1977; Hickman & Humphrey. 1987; Stashak. 1990; 2002; and Butler 2005) focus on specific aspects of the current foot balance model whilst offering contradictory advice on trimming methodology, most notably with regards to trimming of the heels, frog and sole. This advice, based on assimilated custom and practice, is presumably formulated on local environmental considerations. A number of authors (Duckett 1990; 2008, Ovnicek et al, 2003a and Savoldi 2007) support an approach that uses specific external reference points that they suggest assists in determining the position of the distal tip of the third phalanx (P3) and the centre of articulation of the distal interphalangeal joint (DIP). These authors also suggest that the morphological appearance of the sole indicates the orientation of the solar margin of P3 within the hoof suggesting that the heels are trimmed back so that the heel buttress is adjacent to the widest and highest aspect of the trimmed frog, thus maximizing the base of support. A recent study using a standardised trimming protocol (Figure 4 & 5) and hoof mapping protocol (Caldwell et al 2015) the external reference points of Duckett’s Dot (COP) and the widest point of the foot (COR) the authors also mapped to the extensor process of P3 and the centre of rotation of the distal interphalangeal joint (DIPJ) radiographically. In this study the external reference point COR was related to the position of the internal location of centre of rotation of the DIPJ, whereas COP did not accurately represent the location of the extensor process/semilunar line through the distal phalanx. Caldwell et al (2015) demonstrated that after trimming that variation of the hoof balance indicators occurred between individual feet, and concluded that this was likely to reflect variation in horses and that trying to trim each foot to an ideal shape may not hold true.
Relevance to farriery – A trimming Protocol Based on Anatomy
Accurate hoof trimming relies on the initial assessment and identification of anatomical landmarks such as the true apex and the widest highest point of the trimmed frog. Furthermore, the collateral sulci and the true apex of frog need to be clearly identified.
Foot mapping protocol – The bearing border reference points of CoR and BO can be marked out along the sagittal axis of the frog using the following grid mapping system (Figure 6). Two parallel lines are projected dorsally along the bearing border from the centre of the coronary margin of each heel buttress at the widest part of the frog to the toe area at the solar white line interface. Two additional lines are then projected diagonally from the heel buttress intersecting with the previous parallel lines terminating in the toe area. A horizontal line perpendicular to the sagittal axis of the frog is then drawn through the intersection of the diagonal lines which will correspond to the widest part of the ground bearing hoof. The intersection of the diagonal lines has been shown to be representative of true vertical location of CoR
The dorsodistal tip of P3 (BO) can be identified with an additional line perpendicular to the sagittal axis of the frog through the intersections of the previous parallel and diagonal lines terminating at the solar white line junction in the toe area (Figure 6).
Figure 4 After Savoldi (2007) foot trimming protocol. With frog trimmed and sole and white line exfoliated in preparation for trim (Savoldi 2007). The white line is then exfoliated to reveal the sole and horny wall interface. Removal of the remaining exfoliating solar horn reveals the true solar plane. The bars are trimmed to normal proportions, removing only damaged or the weak horn. The frog is trim back to live frog and proportionate to the foot.
Figure 5 Trimming protocol with the bearing border trimmed to a horizontal solar plain (Savoldi 2007). The excess wall at the bearing border is removed to a horizontal plane at the level of the sole plane. The heels are trimmed approximately to the widest part of the trimmed frog or the palmar / plantar aspect of the exfoliated central sulci.
Figure 6 the digital foot mapping of the external reference points along bearing border in relationship to internal anatomical points. Specific external reference points along the sagittal axis of the solar margin of the foot are measured from the heel-bulb to the dorsodistal margin of the bearing border. External reference points of the centre of rotation (CoR), the centre of pressure (CoP), the frog apex (FRA) and the point of breakover (BO). (Modified after, and reproduced with kind permission of, A. Parks).
The trim should be divided to address the various structures of the hoof; the frog, sole, the sole-wall junction or white line, the bearing border of the hoof capsule and dorsal hoof wall.
In our experience it is essential that the collateral sulci are clearly visible to their full depth and that the true apex of frog is identified (where the frog horn blends into the solar horn) when starting the trim. Perioplic horn that envelops the heel buttresses is removed to expose the collateral sulci to their full depth at the origin of the heel. The collateral margins of the frog are trimmed along its entire length forming an angle of about 55-70° with the bars (Figure 4). The ground bearing surface of the frog is trimmed, removing only damaged and diseased tissue. The trimmed frog should be proportionate to the foot with the caudal aspect of the bearing border of the hoof wall level with the horizontal plane of the wall and sole and able to allow ground contact during the contact and loading periods of the stance phase (Figure 4).The white line is then trimmed (Figure 4) by removing flaky solar horn and by trimming out the area to reveal yellow flexible horn at the true interface with the sole (Figure 4). The exfoliating solar horn is removed, exposing confluent solar horn, identifiable by the waxy horn at the sole – white line interface at the soles leading edge. This does not include the sole callus, a flat area of sole approximately 8mm wide and found at the toe area dorsal to the dorsodistal margin of P3. The bars are trimmed removing only damaged or weak horn (Figure 4). The excess wall at the bearing border is removed to the level of the sole producing a horizontal plane with the trimmed sole. This is accomplished by removing excess hoof wall parallel to the trimmed sole. Care must be taken not to trim the bearing border of the hoof wall below the level of the previously trimmed sole. The heels are trimmed (reduced in height) to extend the bearing border to approximately the widest / highest aspect of the trimmed frog or the palmar / plantar aspect of the trimmed central sulci (Figure 5). The hoof is then rasped from heel to toe, maintaining even pressure over the rasp to create a flat level surface on the ground surface of the foot. Flares are removed from the dorsal hoof wall DHW from the farrier position in order to maintain an equal amount of hoof wall around the bearing border of the hoof wall from quarter to quarter. It should be noted that the thickness of the DHW is said to be parallel proximodistal with the coronary band. The dorsal hoof wall was subject to both pure bending and compression during the stance phase.
Relevance to farriery – A Shoeing Protocol Based on Physiology
In order to discuss the design of a comprehensive shoeing plan, it is necessary to assesses criteria for shoeing. It is essential that a common interpretation of the anticipated use of the horse is agreed and understood by all those involved in the management, welfare and training of the horse. Terminology commonly used in farriery can often be interpreted differently, for example use the description hunter in the UK and a shoeing style based on a peripheral outline fit designed to minimize premature shoe loss and injury will be adopted by the farrier with the recommendation for a much reduced shoeing cycle. The act of farriery has been described as both art and science which has a direct influence on function of the structures within the foot. It is the author’s belief that a sound trimming technique based on anatomy and physiology of the foot is the mainstay of farriery intervention. The foot’s ability to maintain its integrity and health to from trimming alone will be dependent on the integrity of those structures and the mass or density of the foot present. Various shoes are then applied to protect or compliment what has been trimmed and to manipulate the basic biomechanical forces acting on the foot.
The authors approach is to employ a range of shoeing styles and techniques designed to increase the ground surface of the foot, unload areas of the foot, effect or reposition breakover, and dampen concussion to some degree. Biomechanically, these techniques can: A) change dorsopalmar or mediolateral orientation of the foot thus changing forces by moving CoP (Hagen 2016), B) reduce the forces associated with breakover (Moleman et al 2006), and C) decrease the stresses associated with the soft tissue pathologies by transferring load (Denoix et al 2001).
Design criteria of the shoeing plan – the basics of any shoeing plan, irrespective of fitting style, must have at its core the principle of minimizing the unintended consequences on the normal anatomical and physiological function of the lower limb and foot. Within the limits of the activity that the horse is to undertake the application of shoes should as far as practically possible maintain maximum biomechanical efficiency whilst enhancing performance and grip and preventing strain related injuries. Science has shown that physiological health of the foot is best served where all the epidermal structures are engaged in appropriate weight sharing during the stance phase (Hood et al 1997; Clayton 2011). Once attached, and by whatever means, the shoe becomes an integral part of the limbs propulsive and load bearing mechanism and as such careful consideration needs to be paid to the material selection, method of attachment and the position and nature of performance enhancing ancillary features. All will directly affect the mechanical efficiency of the whole horse. Every shoe that is applied will have unintended consequences. Careful consideration to the most effective way of mitigating the unintended consequence is a vital aspect of the shoeing plan in order to maintain the general health and function of the foot.
Selection of a broad light weight material as possible, that will last the duration of the expected shoeing cycle, normally between 35 & 42 days, will minimize the energy requirements musculoskeletal system and effect a smoother transition of the individual limbs into the swing phase of the stride. Whilst the careful selection of the materials profile suitable for the intended terrain or surface will enhance the safety of both horses and rider (Figure 6).
Figure 6 this is a good example of a typical English hunter front shoe fit style. This style of shoeing is designed for horses working at high speed over mixed terrain. The heels of the foot are reduced in height so as to extend the bearing border palmar/plantar with the heels of the shoe dressed and fitted to correspond to the shape and angle of the heel buttress so as to minimize premature shoe loss and injury. The overall fit is described as a peripheral outline fit and would normally be employed in conjunction with a reduced shoeing cycle of between 21 – 28 days.
Dorsopalmar placement of the shoe is considered to be of vital importance in the prevention of foot and lower limb related pathologies (O’Grady 2003; Eliashar 2004 & 2011). Current thinking suggests that the shoe is fitted and placed on the foot with the line or widest part of the foot located in the middle of the shoe and that the distance from this line to the perimeter of the toe and to the end of the heel of the shoe will generally be approximately equal with the objective of maintaining the point of breakover palmar / plantar to the dorsodistal tip of the DHW (O’Grady and Poupard 2003). Feet which do not match this “ideal” recommended undergoing corrective farriery with the aim to achieving the ideal shape (Colles 1983). Research has shown that post trimming variation of this important hoof balance indicator frequently occurs between individual feet, most likely reflecting the individual variation in horses. Trying to trim and shoe each foot to this ideal shape may well be contraindicated (Wright and Douglas, 1993; Caldwell et al 2015). Particularly in front feet, these proportions are more accurately described as being equal including the heel bulbs. Biomechanically the hoof rotates about its centre with the palmar / plantar aspects absorbing impact and dorsodistal aspects providing traction during the propulsive stage of the stance. As the hoof mapping system identifies the vertical position of the dorsodistal margin of P3 the author believes that the distance from this line to the CoR and to the widest point of the frog will generally be approximately equal and subsequently the inner margin of the shoe is fitted to this line (Figure 7). Viewed from a frontal and mediolateral plane the placement of the shoe then accurately reflects the ideal proportions previously described for the geometric foot balance model with coronary circumference was of equal height at any two opposing medial or lateral points and perpendicular to the sagittal axis of the limb (Figure 8a) and that the ideal foot should exhibit heel / toe angle parallelism with the phalangeal axis (Figure 8b).