Introduction

1.0 The hoof capsule is a three dimensional multi-directional reinforced composite structure (Reilly 1996) and its shape and morphology are influenced by the opposing forces of ground reaction and descending body weight acting on it during static stance and locomotion. The external shape of the hoof capsule reflects the distribution and magnitude of the stresses and strains that occur in the tissues and structures of the hoof during weight bearing and locomotion (McClinchy et al 2003) (Thomason et al 2004).

Conformation will influence the orientation ofthe distal limb segments and as such is fundamentally related to the bio-mechanics of the hoof and its ability to distribute and dissipate and absorb forces.Overload of or trauma to the hoof will cause the horse to adapt its posture. This adaption will alter joint angles at the pastern, fetlock, elbow and shoulder (Ridgeway 2003).

1.1 Rooney’s theoretical centre of pressure (CoP)

According to Rooney(Down loaded 2007) the vertical force of the ground reaction force(GRF) is exerted all over the bearing surfaces of the hoof in contact with the ground. In mechanics one considers that “spread-out” force to be concentrated at a single point called the centre of pressure (CoP). That is done in order to simplify the calculations. It does not mean that “all” the force is concentrated at that point; it means that one can account for the mechanics of the foot if one considers that the dispersed forces are all concentrated at that one point.Rooney gives good mathematical and diagrammatic explanations of the linear forces acting on the hoof that allow them to be plotted accurately on radiographs (Fig 1). Rooney also gives a simple definition of CoP as follows:that if a triangular support were to be placed under the horse’s foot at the centre of pressure, then at mid stance the foot would not tip forwards or backwards but balance.

Rooney explains how if the forces in the foot are not in equilibrium then the effects will be substantial. One example is the newborn foal with flaccid flexor tendons. In this scenario the extensor moments of GRF and the common digital extensor tendon are greater than the opposing pull of the deep digital flexor tendon(DDFT) and so the toe of the foot extends raising it off the ground. With the application of a plantar extension to the foal’s foot, the CoP moves towards the extension and behind the centre of rotation distal interphalangeal joint (CoR Dip) where it then becomes a flexing moment opposing the extensor apparatus and bringing the toe of the foot back down on to the ground.

Rooney’s work wasone of the first papers to consider biomechanics (1969). His work on the centre of pressure of the equine foot is based on the application of mathematics and Newton’s laws of physics. However,it is highly theoretical as no actual force plate data was used or available at that time.

Figure 1. Rooney’s Basic mechanics of the foot showing linear forcesand moments (Modified After Rooney 2007 download)

As can be seen from Figure 1,at mid stance the linear forces of the distal limb and foot are in equilibrium,

GRF (Ground reaction force) – Body weight = 0

Forward influence of body weight – surface friction = 0

Body weight + Forward influence = - R (Downward resultant vector)

CoP (centre of pressure of GRF) + surface friction = R (upward resultant vector)

Resultant vectors R – R = 0

In addition to forces and vectors of forces acting on the foot, moments must be considered. The definition of a moment is the perpendicular distance from the force (CoP) to the pivot point (CoR DiP). These moments have also been considered by Wilson et al (1998)The moments are:

CE moment - extending moment from CoR DiP to the extensor process of P3 created by tensile forces of the common digital extensor tendon and conjoining branches of the suspensory ligament.

CoP moment – extending moment from CoR Dip to the vertical component of CoP.

DDFT moment – flexing moment from CoR DiP to thecentre of the flexor surface of the distal Sesamoid bone created by tensile forces of the DDFT.

The momentsacting on the foot in static stance are also in equilibrium as well as the forces acting on the foot and leg. The moment of the common digital extensor tendon and branches of the suspensory ligament (CE)combine with the moment of the CoP (centre of pressure) to balance out the moment of the DDFT.

DDFT moment – (CE moment + CoP moment)= 0

1.2 Wilsons mathematical CoP

Forces and moments have also been considered by Wilson et al (1998), in agreement with Rooney (2007download) Wilson states that when considering foot ground interaction it is useful to imagine that all the force transferred by the foot is applied at a single theoretical point on the ground surface.Wilson however, used force plate studies to investigate the effect of imposed imbalance on the CoP by the application of toe and heel wedges to create palmar dorsal imbalance and medial and lateral wedging of the foot to create medio lateral imbalance.

Wilson et al (1998) concluded that the application of a standard steel horse shoe had a minimal effect on the point of force application of stance. The application of heel wedges delayed the unloading of the heels, while toe wedges advanced the unloading of the heels. The position of the CoP at mid stance was unaffected by the heel wedges suggesting that they do not unload the heels as is so often claimed. This finding explains the author’s own personal experience of palmar elevation of the foot where even with frog support to correct a broken back hoof pastern axis (HPA) the heels of the hoofbecame crushed. This effect is counterproductive to the rehabilitation of the long toe low heeled horses hoof capsule as the treatment exacerbates the condition. If there is involvement of strain or rupture of the DDFT then elevation of the heels has been shown to reduce the tensile forces within the DDFT(Riemersma et al 1996; Willemen et al 1999). This is due to the heel elevation inducing an increase in the extension of the metacarpophalangeal joint and an increase in the flexion of the proximal and distal interphalangeal joints (Bushe et al. 1987; Crevier-Denoix et al 2001; Rooney, 1984). Load is then transferred to the superficial digital flexor tendon and suspensory ligament (Lawson et al 2004).

Application of mediolateral wedges by Wilson et al (1998) resulted in the CoP moving towards the wedged side, this effect was more pronounced on the medial side. This Wilson et al (1998) cited was due to the possibility that with lateral wedges the horse can adopt a morebase widecompensatory stance which leads to increased medial loading. This response would be difficult for the horse with medial wedges as the contra lateral limbs would interfere with each other during locomotion.

Wilson et al (2001) uses a different method for calculating the theoretical centre of pressure or point of zero moment (PZM). The distance from the forward most ground bearing point of the toe to a vertical line dropped from the centre of rotation(CoR) of the distal interphalangeal joint (DiP) was used to calculate the moment arm of the PZM (CoP) see Figure 2.

As this study does not have any force plate data for the purposes of calculation, the plotting of Wilson’s CoP is based purely on his diagram (Fig 1 page161 in his 2001 study). The PZM bifurcated the ground bearing surface between the centre of rotation and the toe, and intersected the dorsal wall with the perpendicular moment arm. In the event of the bifurcated measurement not intersecting at the dorsal wall with the moment arm the bifurcated measurement will take priority. The author’s interpretation of Wilsons diagram is seen in(Figure2)

Figure2. Wilsons PZM(CoP)(Modified after)

Wilson states that since the CoP lies dorsal to the centre of rotation of the DIP joint (Schryveret al.1978; Wilson et al. 1998), the GRF acts to extend the DIP joint. This is balanced by the flexing moment of the DDFT. Fig. 2

Unlike the COR the COP (centre of pressure) is not a fixed anatomical point and will move or change during the landing, loading and breakover phases of the temporal stride pattern due to the changing position of the horses centre of mass see figure 3.The COP originates at the point of first impact, in the majority of horses this is the lateral heel, even though to the human eye they appear to land level,this is because the landing phase of the horses stride happens so fast that the human eye cannot detect the lateral landing pattern of the foot. Therefore the sound well conformed horse trotting in a straight line should appear to land level (M.C.V. Van Heelet al2004) see figure3.1

Figure 3Lateral view showing trajectory of CoP during stance phase of temporal stride pattern. The Percentage timings areafter Wilson et al (2001).

1.3 Van Heel’s pressure force analysis work

Figure 3.1 Trajectory of CoP during stance phase of stride (after Van Heel et al 2004) and percentage timings (after Wilson et al 2001).

The above diagram uses visual trajectory based on Van Heel et al (2004) pressure mat analysis which is sensitive enough to plot position of first strike relative to the foot, and the time dependant data based on Wilson’s (2001) force plate analysis.

From this first initial impact the COP advances rapidly to the middle of the hoof as loading begins with the increasing GRF (15-20% of stance). The theoretical centre of pressure is mid stance (maximum loading) and stays close to this mid location and doesn’t move rapidly forward again until 75-80% of stance when the heels start to unload prior to breakover (A.M. Wilson et al 2001).

Van Heel (2004) used her pressure force measuring system to study the effect foot trimming had on balance. In this study she found the horses preferred way of landing was lateral asymmetrical in both front feet and hind feet. The duration of landing was greater in the hind limbs than the fore limbs and trimming reduced landing duration in both front and hind feet. The horses had a fixed unrollment pattern with a maximum lateral displacement before returning to the sagittal axis of the hoof. Trimming was found to decrease the individual left right difference in maximum lateral displacement (Van Heel 2004)

Van Heel (2005)studied the same population of horses to see how the location of the centre of pressure changed over an eight week shoeing cycle and how a rolled toe optimised hoof unrollment.(2005) The results indicated that the measured shift in CoP was less than calculated and the differences were largest in the hind feet. The hoof unrollment pattern in the front feet stayed basically the same over the eight week cycle, but a substantial lateral shift of the lateral trajectory of the CoP was found in the hind feet. This she concluded was due to the horses having a limited ability to compensate for changes in hoof capsule conformation over time, but this capacity for compensation was less in the forelimbs than in the hinds. Therefore the relative increase in the loading of these limbs during the shoeing cycle is greater than the hind limbs.

In van Heels 2005 study into the use of a rolled toe in the shoeing of sound warm bloods when compared to a flat shoe, the results showed that the kinematics of the limb and temporal stride pattern were unaffected, this was in agreement with another study (Eliashar et al 2002) that compared the kinetics of breakover of three horseshoeing styles. However Van Heel (2005) found the displacement, velocity and trajectory of the CoP were significantly affected from midstance to toe off (breakover).The flat shoe had a higher single peak in the velocity of the CoP at the initiation of heel lift whereas the rolled toe shoe had a lower velocity peak at the initiation of heel lift followed by a second smaller velocity peak prior to toe off. The higher peak reading with the flat shoe was interpreted to represent a more abrupt breakover process. After mid stance the flat shoe exhibited a greater lateral displacement than the rolled toe shoe which had a smoother more linear trajectory towards the toe and point of breakover. Van Heel (2005) cited that the smoother unrollment of the hoof would lead to less heavy and less abrupt changes in loading of the internal structures of the digit and therefore reduce the risk of injury.

The author of this paper did not have a reliable theoretical method of mapping van Heel’s CoP measurement onto the lateral radiographs, but van Heel’s extensive works on the subject cannot and should not be overlooked.

1.4 Duckett’s anatomical CoP

Figure 4 Duckett’s centre of pressure (CoP).

Duckett’s centre of pressure is based on anatomical reference points of the foot (Duckett 1990) and is represented by a vertical line dropped from the termination point of the common digital extensor tendon at the extensor process down through the semi lunar crest of the distal phalanx where the deep digital flexor tendon terminates and down to the ground surface of the foot (Fig 4)

Duckett believes that as the anatomical structures that are responsible for flexion and extension of the hoof are in vertical alignment to the groundbearing surface, then this must be the point at mid stance when the forces in the foot are in equilibrium.

Figure 5 shows where Duckett’s CoP which is often refered to as Duckett’s ‘dot’ appears on the bottom of the shod foot. Duckett states that in the average sized horse this dot is aproximately 3/8 inch(9.5mm) palmar to the true apex of the frog, and can be used as a guide for dorso/palmar balance in the trimming of feet by farriers. Duckett (1990) believed when the foot has been trimmed correctly the dorsal toe length will be equal to the distance from the toe to the mapped centre of rotation of the distal interphalangeal joint on the solar surface of the foot, and that this measrement would also equal the distance from the last weight bearing point of the heels forward to the ‘dot’.

Figure 5Duckett’s dot (CoP) hoof mapping method (Duckett 1990)

Rooney (1969)in agreement with Duckett (1990) acknowledges that the common extensor and extensor branches of the suspensory exert a moment on the DiP joint. It is interesting to note that anatomically the common digital extensor tendon is conjoined by the Abaxial branches of the suspensory ligament a little below the middle of the proximal phalanx which greatly increases its width. It then passes over the pastern joint and inserts on the proximal aspect of the middle phalanx and the extensor process of the distal phalanx (Hickman’s 1988).

Wilson however believes that the GRF acting at the CoP extends the DiP joint and cites that moments from the extensor tendons and navicular ligaments are assumed to besmall and are taken as zero for his calculation (Bartel et al. 1978; Willemen1997).

1.5 Aim

The aim of this study was to define and evaluate three theoretical centres of pressureand determinewhether they had any correlations with anatomical points of interest or angles of interest in the equine front foot.

2.0 Materials and methods

A population of 23 cadaver limbs cut off above the carpus to preserve suspensory attachment of varying size and unknown sexwere used for this study. The feet were chosen for not exhibiting any visible foot pathology.

The feet were then mapped and marked up to be radio graphed and photographed to the protocols as outlined below. Two markers were placed on the feet, the first was to locate Duckett’s dot, the theoretical centre of pressure (COP) and the second was a marker of known fixed length so as to be able to calibrate digital x-ray and photographic images using computerised measuring software (Ontrack). The COP marker would be used by a co worker (P.Conroy) in a parallel study to assess the accuracy of the external reference point hoof mapping technique used on the solar ground bearing surface of the foot.

2.1 Equipment used

Hoof trimming kit-Standard farriers’ tool kit

Pneumatic press

Digital cameras Fuji Finepix & Kodak C875 Zoom

Digital X-ray machine

Ontrack / digital software analysis system

Laptop computers

2.2 Setting up for radiographs

Prior to radiography, the room is prepared by making a cross on the floor using white sticky tape. Care is taken to ensure that the arms of the cross intersected at 90 degrees to each other. The press is centred over the middle of the cross and the X-ray machine was placed along the line of the cross. A laser pen will be rigidly attached to the x-ray machine to drop a beam distally to a marked point on the floor. This will allow the centre of the cross hairs in the beam window of the X-Ray machine to be synchronised and accurately aligned with the arm of the cross on the floor Having achieved correct alignment in this way the head of the X-Ray machine is locked on its stand and remained un-altered for the duration of the radiography session.

2.3 Radiographic method

The limbs were all loaded into the mechanical press to the same standard, this was with the cannon bone being vertical to the floor and the bearing border of the foot in full contact with the ground plate of the press.