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 of the 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
1
(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 was one 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 forces and 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)
2
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 the centre of the flexor surface of the
distal Sesamoid bone created by tensile forces of the DDFT.
The moments acting 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
3
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 hoof became 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 more
base wide compensatory 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)
4
Figure 2. Wilsons PZM (CoP)(Modified after)
Wilson states that since the CoP lies dorsal to the centre of rotation of the DIP joint (Schryver
et 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.
5
Therefore the sound well conformed horse trotting in a straight line should appear to land
level (M.C.V. Van Heel et al 2004) see figure3.1
Landing Phase of Stride 0% - 20% Limb
extended forward underneath head away
from centre of mass (CoM) Lateral heel first
point of impact. CoP moves rapidly forward
towards mid location in foot
Initiation
of
Breakover
80%
Mid Stance 50%
0% - 20%
Impact
(origin of
CoP)
Fetlock descends under peak load and limb becomes vertical as
CoM advances towards it. The CoP remains in a mid location
from 20% onwards to 80% of stance phase.
Heels begin to unload as tensile forces
in DDFT overwhelm the extending
moment of the CoP. CoM passes over
foot as limb is retracted under body.
Heels lift of at 80% of stance.
Toe Off 100%
From 80% onwards the CoP travels rapidly forward
as heels lift and rotate around the toe. The CoM is
now well advanced of the foot. The recoiling DDFT
flexes the fetlock and coffin joint causing toe off
and the start of the swing phase
100%during stance phase of temporal stride
Figure 3 Lateral view showing trajectory of CoP
pattern. The Percentage timings are after Wilson et al (2001).
6
1.3 Van Heel’s pressure force analysis work
80% - 100%
Unloading and
Breakover
(Rapid Movement)
20% - 80%
Loading
0% - 20% Landing
(Rapid Movement)
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).
7
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.
8
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
9
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 ground bearing 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 5 Duckett’s dot (CoP) hoof mapping method (Duckett 1990)
10
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 be small 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 pressure and
determine whether 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 sex were 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
11
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.
For the lateral view the beam was centred 2cm vertically below the most distal hair follicle at
the mid-point between the last weight bearing point of the foot and the proximo-dorsal aspect
of the hoof wall (after Kummer et al 2006). This was achieved by using digital callipers to
determine the length of the hoof capsule. The measurement was then halved to find the midpoint. A Wire marker of known length was placed at the dorsal wall from the distal row of
hair follicles at the coronary band. A pin is placed 9.5mm back from the true apex of the frog
(Duckett’s dot) Fig 5. The focal distance for all radiographs is 70cms.
2.4 Foot trim protocol
The trim relies heavily on the initial assessment and identification of anatomical landmarks
and proportions as such it is essential that the co-lateral sulci are clearly visible to their full
depth and that the true apex of frog is identified.
12
1. Exfoliating sole is removed to live sole between the 10 and 2 o’clock position.
2. The white line is then exfoliated to reveal the sole and horny wall interface.
3. Removal of the remaining exfoliating solar horn reveals the true solar plane.
4. The bars are trimmed to normal proportions, removing only damaged or week horn.
5. The frog is trimmed back to live frog and proportionate to the foot.
6. Excess hoof wall is removed parallel to the live sole (care is taken not to trim down so
far as to invade the live sole).
7. The heels are trimmed approximately to the widest part of the trimmed frog or the
palmar/plantar aspect of the exfoliated central sulci.
2.5 Data Collection
Using digital measuring software on lateral radiographs that have had the ground bearing
surface of the foot accurately plotted on them vertical lines were used to plot the following
locations and points to be measured to see how their position and proximity at theoretical
mid-stance was related to the ground bearing surface of the foot. As the weight sex and exact
breed of the animals was unknown the limbs could not be loaded to exact forces relevant to
individual body mass.
As a result the limbs were loaded and compressed in the pneumatic press until full contact of
the ground bearing surface was made and the metacarpus was in a vertical position. The hoof
pastern axis was determined by the preserved attachment of the suspensory. Due to the
unknown variables stated above a second position for the CoR MP was mathematically
plotted onto the lateral radiographs to simulate its loaded position, an arbitrary angle of 25
degrees was chosen and applied to all x-rays. See Fig.6
13
Figure 6 Shows how CoR MP 25 was plotted.
This produced CoR MP25 which was a constant for all x-rays and was a product of adding
the linear lengths between CoR MP to CoR PiP and CoR PiP to CoR DiP. This combined
linear length was plotted in a straight line from CoR DiP at 25 degrees making the
assumption that both P1 and P2 would be in alignment at this angle, this would give a better
more comparable inter limb position for the fetlock joint relative to the ground bearing
surface.
14
2.6 Biomechanical points being measured. (See Figure7)
All measurements were taken from H- The last weight bearing point of the heel and datum
point for all measurements on the ground bearing surface of the foot. The points to be
measured were as follows:1. CoR MP- Centre of rotation of metacarpophalangeal joint.
2. CoR MP 25 - Position of CoR MP when P1 has been aligned with P2 and orientated at 25
degrees to the ground bearing surface.
3. CoR PIP- Centre of rotation of proximal inter phalangeal joint
4. CoR DiP – Centre of rotation of distal inter phalangeal joint
5. CoP DA – Duckett’s anatomical centre of pressure
6. CoP AW – Centre of pressure based on Wilsons diagram
7. CoP Roo – Rooney’s centre of pressure
8. GBS- Ground bearing surface length of the foot
9. CoP -CoR D- Distance between CoP DA and CoR DiP.
10. CoP - CoR W- Distance between CoP AW and CoR DiP
11. CoP- CoR R- Distance between CoP Roo and CoR DiP
12. HW- Heel Width
13.TA- Toe angle
14. HA- Heel angle
15. P3E - Palmar angle of distal phalanx relative to ground bearing surface.
All data will be entered into a data spreadsheet for statistical analysis purposes..
15
Figure 7.The biomechanical points of interest of the distal limb and foot that have been
measured.
All measurements except those for the three CoP-CoR measurements are taken from the last
weight bearing point of the heel on the ground bearing surface of the foot. All angles are
measured relative to the ground bearing surface.
As well as the raw data analysis the linear length measurements will be converted into
percentage measurements of the GBS to remove the foot size variable and see whether any
proportional correlations exist between the anatomical points of interest and the GBS.
16
3.0 Results
Of the three reference points considered Duckett’s CoP is significantly palmar to the other
two. Wilson and Rooney’s CoP are similar in location with Rooney’s being more dorsal of
the two. This pattern was consistent in all feet regardless of the conformation of the hoof
(figure 8)
3.1 Bar graphs of CoPs and CoP CoR scatter graph
Duckett's Wilson's and Rooney's CoPs.
CoPs arranged by ascending CoP AW
140.00
120.00
100.00
CoP AW
CoP DA
CoP Roo
Linear (CoP AW)
Linear (CoP DA)
Linear (CoP Roo)
MM
80.00
60.00
40.00
20.00
0.00
E Q B I P J R MN H L O C G V T F D X S U A y
CoP AW 70 71 74 74 75 76 76 76 78 78 79 83 85 86 87 87 89 90 90 96 99 10 10
CoP DA 59 56 62 59 58 57 61 58 63 62 62 67 69 69 64 69 67 73 74 79 75 83 84
CoP Roo 76 80 77 87 88 83 92 88 87 89 91 93 93 94 96 97 95 91 10 10 11 10 11
Feet
Figure 8 A comparison of the three theoretical CoPs’
The middle value (CoP AW) was arranged into ascending order to give a better visualisation
of the relationship between the three values. As can be seen from the fit of CoP DA’s trend
line it has a slightly lower gradient (s.d7.56) the relationship between CoP AW (s.d.9.78) and
CoP Roo (s.d.10.34) is more uniform with both trend lines near parallel.
17
CoP-CoR D vs CoP-CoR W vs CoP-CoR R.
(Arranged by ascending CoP-CoR W)
70.00
60.00
y = 0.8271x + 37.476
R² = 0.6037
Distance mm
50.00
y = 0.7212x + 29.413
R² = 0.8765
C0P-CoR R
40.00
CoP-CoRW
CoP-CoRD
30.00
Linear (C0P-CoR R)
y = 0.2956x + 17.076
R² = 0.58
20.00
Linear (CoP-CoRW)
Linear (CoP-CoRD)
10.00
0.00
0
5
10
15
20
25
Feet
Figure 9 A comparison of the three different CoP CoR measurements arranged by ascending
CoP CoR W.
As can be seen in figure 9 from the steeper trend line gradients Wilson and Rooney’s CoPCoR measurements are far more variable than Duckett’s. Wilsons CoP-CoR s.d 5.24 and
Rooney’s s.d 7.22 compared to Duckett’s s.d of 2.69. The manipulation of the CoP CoR W
values has given a closer fitting trend line, had CoP CoR R been arranged in ascending order
the effect would be to give a close fitting trend line and a more random appearance to CoP
CoR W. Due to its low s.d of 2.69 CoP CoR D remains close fitting to its trend line no matter
which of the above is manipulated.
18
3.2 Table 1 Shows Pearson’s correlations and P values of raw data, correlations
calculated using Excel data analysis program. Negative correlations are shown in red.
Measurements
CoR MP 25
CoR MP
CoR PiP
CoR DiP
CoP DA
CoP AW
CoP Roo
GBS
CoP-CoR D
CoP CoR W
CoP-CoR R
Stronger
CORRELATIONS
CoP-CoR R
CoR PiP
CoR MP
CoP-CoR D
CoP-CoR W
HA
0.812
-0.700
0.671
0.664
0.545
-0.476
P < 0.05
P< 0.05
P < 0.001
P < 0.001
P < 0.01
P < 0.01
CoR PiP
HA
CoP CoR R
C0P CoR D
-0.733
-0.541
0.486
0.466
P< 0.001
P < 0.05
P < 0.05
P < 0.05
CoR DiP
CoP DA
CoP AW
HA
0.744
0.603
0.503
0.458
P < 0.001
P < 0.01
P < 0.05
P < 0.05
CoP DA
CoP AW
GBS
CoP Roo
CoP CoR W
0.933
0.875
0.766
0.717
0.478
P < 0.001
P < 0.001
P < 0.001
P < 0.001
P < 0.05
CoP AW
GBS
CoP Roo
CoP CoR W
CoP-CoR D
0.960
0.897
0.875
0.679
0.634
P < 0.001
P < 0.001
P < 0.001
P < 0.001
P < 0.01
GBS
CoP Roo
Cop-CoR W
CoP CoR D
TA
CoP CoR R
0.978
0.934
0.830
0.650
-0.525
0.491
P < 0.001
P < 0.001
P < 0.001
P < 0.01
P < 0.05
P < 0.05
GBS
CoP-CoR W
CoP-CoR D
CoP CoR R
TA
P3PE
0.963
0.905
0.800
0.739
-0.648
-0.433
P < 0.001
P < 0.001
P < 0.001
P < 0.001
P < 0.01
P < 0.05
CoP-CoR W
CoP-CoR D
CoP CoR R
TA
0.922
0.724
0.638
-0.636
P < 0.001
P < 0.001
P < 0.01
P < 0.01
CoP-CoR R
CoP CoR W
HA
TA
0.807
0.793
-0.492
-0.470
P < 0.001
P < 0.001
P < 0.05
P < 0.05
CoP-CoR R
TA
HA
0.834
-0.732
-0.535
P < 0.001
P < 0.001
P < 0.05
TA
HA
P3PE
-0.715
-0.610
-0.526
P < 0.001
P < 0.01
P < 0.05
P3PE
HA
0.583
0.535
P < 0.01
P < 0.05
HW
TA
HA
Weaker
P3PE
0.577
P < 0.01
P3PE
19
A table of critical values for Pearson correlation coefficients (Jeremy Miles download 2011)
was used to calculate the P values for the null hypothesis. As two of the radiographs did not
depict the fetlock joint, CoR MP 25 and CoR MP m were calculated at 19 degrees of freedom
on the critical values table. All other values were calculated at 21 degrees of freedom.
The P values for 19 degrees of freedom; P < 0.05 = Critical value 0.456
P < 0.01 = Critical value 0.575
P < 0.001 = Critical value 0.693
Values higher than 0.456 were significant for a positive correlation, values lower than -0.456
were significant for a negative correlation
The P values for 21 degrees of freedom; P< 0.05 = Critical value 0.433
P< 0.01 = Critical value 0.549
P< 0.001 = Critical value 0.665
Values higher than 0.433 were significant positive correlations and values lower than – 0.433
were significant negative correlations.
Certain correlations were to be expected and unsurprisingly the CoR MP correlates strongly
to the CoR PiP (courtesy of the proximal phalanx) as does the CoR PiP to the CoR DiP
(courtesy of the middle phalanx).
CoR MP 25 Had a total of six correlations the strongest correlation was a positive one with
the CoP CoR R (0.812) this had a P < 0.001. The next strongest correlation was a negative
one with CoR PiP (-0.700) this was still a P < 0.001. In order of strength the next two
correlations were CoR MP (0.671) followed by CoP CoR D (0.664) both had a higher P value
at P< 0.01. The two weakest correlations were a positive one with CoP CoR W (0.545) and a
negative one with HA ( -0.476) these both had the highest P value of P< 0.05.
20
CoR MP, Had a total of five correlations and had a strong negative correlation to CoR PiP (0.733) with a P value of P < 0.001. The next strongest was HA but this was a negative
correlation (-0.541) with a higher P <0.05. In descending order the two weakest correlations
were CoP CoR R (0.486) and CoP CoR D (0.466) both with a P < 0.05
CoR PiP, This correlated to CoR DiP (0.744) with a P < 0.001. The next strongest correlation
was CoP DA at (0.603) with a P < 0.01. CoP AW at (0.503) and HA at (0.458) were the
weakest correlations both with a P < 0.05.Overall had a total of six correlations.
CoR DiP, The strongest correlation was CoP DA (0.933) with a P < 0.001 this would be as
expected as Duckett’s centre of pressure is an anatomical location using the extensor process
of P3 as a reference point which is an integral part of the DiP joint. The second strongest
correlation was CoP AW (0.875) with a P < 0.001. The GBS was the next strongest
correlation with (0.766) and P < 0.001 CoP Roo correlated at (0.717) and P < 0.001. The
weakest correlation was CoP CoR W at (0.478) with P < 0.05.Had a total of six correlations
overall.
CoP DA, Has a very strong correlation to CoP AW (0.960), and GBS (0.897) as well as a
strong correlation to CoP Roo (0.875) and CoP CoR W (0.679) these all had a P < 0.001, the
weakest correlation was CoP CoR D at (0.634) with a P < 0.01 Overall had a total of seven
correlations.
. CoP AW, GBS (0.978) and CoP Roo(0.934) were very strongly correlated. CoP CoR W
(0.830) was also strongly correlated and all three had a P < 0.001. CoP CoR D (0.650) had a
P < 0.01. TA had a negative correlation (-0.525) and a P < 0.05. The weakest correlation was
CoP CoR R (0.491) P > 0.05 Overall had a total of nine correlations.
CoP Roo, The strongest correlation was to GBS (0.963). It was correlated strongly to all
three CoP CoR measurements; CoP CoR W (0.905), CoP CoR D (0.800) and CoP CoR R
(0.739) all had a P < 0.001. There was a significant negative correlation to TA (-0.648) P <
0.01. The weakest correlation was a negative P3PE (-0.433) with P < 0.05 Overall had a total
of nine correlations.
21
GBS, CoP CoR W (0.922) and CoP CoR D (0.724) correlated with a P < 0.001. CoP CoR R
(0.638) correlated with a P < 0.01. A significant negative correlation with TA (-0.636) was
found with a P < 0.01.Overall had eight correlations the strongest being with CoP AW (see
above)
CoP CoR D, This correlated with the other two CoP CoR measurements CoP CoR R (0.807)
and CoP CoR W (0.793) both with a P < 0.001. The weakest two correlations were negative
HA (-0.492) and TA (-0.470) both had P < 0.05.Overall had ten correlations.
CoP CoR W, Strong correlation with CoP CoR R (0.834) P < 0.001, this was CoP CoR R’s
strongest overall correlation. There was a strong negative correlation to TA (-0.732) with a P
< 0.001. The weakest correlation is also negative HA (-0.535) with a P < 0.05. Overall had
ten correlations (see GBS above for strongest).
CoP CoR R, There is a strong negative correlation with TA (-0.715) with a P < 0.001, and a
significant negative correlation with HA (-0.610) with a P < 0.01. The weakest correlation
was a negative one to P3PE (-0.526) with a P < 0.05. Ten correlations overall (see CoP CoR
W above for strongest.
HW, No correlations were found.
TA, Positive correlations found to P3PE (0.583) giving a P < 0.01 and HA (0.535) giving a P
< 0.05. Eight correlations overall (see CoP CoR W above for strongest). .
HA, A positive correlation was found to P3PE (0.577) giving a P < 0.01.Seven correlations
overall (see CoP CoR R above for strongest).
P3PE, Correlated where mentioned above. Four correlations overall (see TA for strongest).
AS can be seen from table 1 above there are 53 correlations 19 of which involve the plotted
positions of the joints four of these correlations are purely joint related. Seventeen of the
correlations involve angle measurements 8 of which involve TA and 7 which involve HA and
4 which involve P3PE. Three of the correlations are angle to angle. All three CoP CoR
measurements correlate 10 times each which is the highest number of correlations achieved
three of these are with each other. CoP CoR R has a strong negative correlation to all three
angular measurements. CoP CoR D and CoP CoR W both negatively correlate to TA and HA.
22
3.3 Table 2 Pearson correlations and P values of percentage data. Linear measurements
are converted into percentages of GBS to remove foot size variable.
Measurements
Stronger
CoR MP 25
CoP
CoR MP
CoR PiP
CoR DiP
CoR
CORRELATIONS
CoR PiP
CoP CoR
CoP CoR
CoR MP
CoR DiP
CoP AW
CoP Roo
HW
R
-0.799
W
D
0.693
-0.681
-0.642
0.577
0.500
0.844
P < 0.001
P < 0.001
P < 0.01
P < 0.01
P < 0.05
P < 0.001
0.718
0.711
P < 0.001
CoR PiP
CoP
P < 0.001
CoP CoR
P < 0.001
CoP CoR
-0.782
R
D
W
P < 0.001
0.548
0.534
0.471
CoP AW
P < 0.05CoR
CoP
P < 0.05
CoP
CoR
P < 0.05
CoR
DiP
CoP DA
HA
CoP CoR
TA
0.832
W
R
0.802
0.706
0.624
D
0.452
P < 0.001
-0.821
-0.806
P < 0.001
P < 0.001
P < 0.01
-0.500
P < 0.05
CoR
P < 0.001
P < 0.001
CoP AW
CoP
CoP DA
CoP CoR
HA
TA
0.940
W
0.872
R
0.641
0.510
P < 0.001
-0.915
P < 0.001
-0.825
P < 0.01
P < 0.05
CoR
P < 0.001
CoP DA
CoP AW
CoP
0.899
P < 0.001
CoR
CoP
CoR
P < 0.05
P < 0.001
TA
CoP CoR
HA
W
0.652
R
0.580
-0.839
P < 0.01
-0.586
P < 0.01
P < 0.001
CoP AW
P < 0.01
CoP CoR R
TA
HA
W
-0.764
0.593
0.589
-0.862
P < 0.001
P < 0.01
P < 0.01
P < 0.001
CoP Roo
CoP-CoR D
CoP CoR D
CoP CoR R
0.619
0.615
P < 0.01
P < 0.01
CoP CoR R
CoP
0.678
W
P < 0.001
0.480
CoR
P
CoP-CoR W
CoP CoR R
HA
0.822
-0.563
P < 0.001
P < 0.01
CoP-CoR R
HW
TA
0.435
P < 0.05
TA
HA
0.535
P < 0.05
HA
Weaker
P3PE
23
As can be seen from table 2 there are fewer correlations overall when the foot size variable is
removed by converting the raw data into percentages of the GBS, although some
measurements have had significant increases to their number of correlations.
CoR MP25, CoP CoR R correlated the strongest (0.844) this was stronger than its linear
correlation. CoR PiP once again had a negative correlation but was also stronger than its
linear value (-0.799). CoP CoR W was much stronger than it’s linear value (0.718). CoP CoR
D was stronger than linear value (0.711) as was CoR MP (0.693). CoR DiP had not
previously correlated but had a strong negative correlation (-0.681), all these correlations had
a P < 0.001. CoP AW was also a new and negative correlation (-0.642), a new but positive
correlation was CoP Roo (0.577) these both had a P < 0.01. Finally HW (0.500) which did not
correlate at all in the raw data set became significant with a P < 0.05. CoR MP25 had nine
correlations overall (an increase of three)
CoR MP, CoR PiP was the strongest with a stronger negative correlation than in table 1 (0.782) and a P < 0.001. CoP CoR R (0.548) was slightly stronger than its linear value, as was
CoP CoR D (0.534) a corresponding increase now made CoP CoR W significant (0.471). All
three had a P < 0.05. CoR MP had five correlations overall.
CoR PiP, This correlated strongest with CoP AW (0.832) this was much stronger than its
linear value. CoP CoR W (-0.821) and CoP CoR R (-0.806) were both negatively correlated
and hadn’t been significant in the raw data. CoR DiP (0.802) correlated stronger as a
percentage and had been the strongest correlation in the raw data. CoP DA (0.706) was also
stronger as a percentage. All the above correlations had a P < 0.001. HA correlated stronger
than previously in raw data (0.624) with a P < 0.01. CoP CoR D (-0.500) had become
significant with a negative correlation and TA had a positive correlation (0.452), both had a P
< 0.05. CoR PiP had ten correlations overall (an increase of four).
24
CoR DiP, Strongest correlation was CoP AW (0.940) this was stronger than its previous
value in table 1. CoP CoR W (-0.915) had changed from a positive weak correlation in table 1
to a strong negative one here. CoP DA (0.872) was slightly weaker than in table 1 where it
had been the strongest correlation. CoP CoR R (-0.825) had become significant with a strong
negative correlation. All the above correlations had a P < 0.001. HA (0.641) had become
significant with a positive correlation P < 0.01. TA (0.510) had a positive correlation with a P
< 0.05. Overall CoR dip had eight correlations (an increase of two).
CoP DA, The strongest correlation was with CoP AW (0.899) but this was not as strong as its
correlation in table 1. CoP CoR W (-0.839) had been transformed from a positive correlation
in table 1 to a strong negative correlation here. These first two correlations had a P < 0.001.
TA (0.652) had become a significant correlation. CoP CoR R (-0.586) had become a
significant negative correlation, HA (0.580) had become a significant correlation. These last
three correlations had a P < 0.01. Overall CoP DA had six correlations (a decrease of one).
CoP AW, The strongest correlation was with CoP CoR W (-0.862) this had transformed from
a positive correlation in table 1 to a slightly stronger negative one here. CoP CoR R (-0.764)
had also transformed from a positive value in table 1 to a negative stronger value here, both
had a P < 0.001. TA (0.593) was slightly stronger but now a positive correlation, HA (0.589)
had become significant, both had a P < 0.01. Overall CoP AW had eight correlations (a
decrease of one)
CoP Roo, Strongest correlation was with CoP CoR D (0.619) this was weaker than its value
in table 1. CoP CoR R (0.615) was also weaker than its raw data correlation, both had a P <
0.01. Overall CoP Roo had three correlations (a decrease of six).
CoP CoR D, Correlated strongest with CoR MP 25 (see above). CoP CoR R (0.678) positive
correlation that was not as strong as its value in table 1 with a P < 0.001. CoP CoR W (0.480)
was also weaker, P < 0.05. Overall CoP CoR D had six correlations (a decrease of four).
25
CoP CoR W, A very strong correlation with CoP CoR R (0.822) not as strong as previous
value in table 1. P < 0.001.HA (-0.563) had a negative correlation that was stronger than its
value in table 1. Overall CoP CoR W had seven correlations (a decrease of three)
CoP CoR R, All correlations listed above. Overall CoP CoR R had 9 correlations (a decrease
of one)
HW, Strongest correlation was with CoR MP 25 (see above). TA (0.435) with a P < 0.05.
Overall HW had two correlations (an increase of two).
TA, A significant but not strong correlation to HA (0.535) this gave a P < 0.05. TA had five
correlations (a decrease of three).
HA, Is correlated to P3PE (0.577) P < 0.02 .Overall HA had six correlations (a decrease of
one).
P3PE Correlated to TA and HA and as the angles cannot be transformed into a percentage the
correlation values are the same as for table 1.
26
4.0 Discussion
4.1 The effects of transforming raw data into percentages.
As can be seen from data Tables one and two transforming raw data into percentages can
cause correlations to change from positive to negative and an example of how this can happen
is given below. In the raw data, CoP AW has strong positive correlations with CoR DiP and
CoP CoR W see 3.2 Table 1. This is because CoP AW is the linear distance from the heel to
the midpoint between CoR DiP and the toe and CoP CoR W is the linear measurement from
CoR DiP to CoP AW. It is easy to see how an increase in size in one will increase the size of
the others. The bigger the foot the bigger the measurements and the strength of the correlation
will be down to how proportional the measurements stay to each other across the range of
foot sizes. In the raw data all foot A measurements are greater than foot T measurements.
Raw data
CoR DiP
Foot A= 62.01mm
Foot T=40.48mm
CoP AW
Foot A= 104.76mm
Foot T=87.93mm
CoP CoR W Foot A= 42.74mm
Foot T=41.45mm
GBS
Foot T=129.38mm
Foot A= 145.16mm
When transformed into percentages;
CoR DiP
Foot A = 42%
Foot T = 32%
CoP AW
Foot A = 71%
Foot T = 66%
CoP CoR W Foot A = 29%
Foot T = 34%
GBS
Foot T = 100%
Foot A = 100%
As can be seen because CoR DiP in foot A is in a more dorsal location relative to the GBS
there is only 58% of the ground bearing surface left to bifurcate to give the CoP CoR W and
CoP AW measurements (CoR DiP + CoP CoR W = CoP AW) the CoP CoR W measurement
for foot A is 5% smaller than foot T whose more palmar location of CoR DiP left 68% of the
GBS to bifurcate giving the higher percentage value of 34%. To put another way CoP AW +
CoP CoR W must = 100% so CoP AW can only increase at the expense of CoP CoR W. The
27
percentage data is useful in comparing the proportional positions of the anatomical structures
such as the centres of rotation relative to the ground bearing surface of the foot, but due to the
situation highlighted above the raw data is more significant for the moment arms and their
comparison to the angle measurements.
4.2 Heel angle correlations
HA vs CoR MP
60.00
HA degrees
50.00
40.00
30.00
HA
20.00
Linear (HA)
10.00
0.00
0.00
20.00
40.00
60.00
80.00
COR MP mm
Figure 10 Negative correlation between HA and CoR MP
The above correlation (figure 10) shows the raw data negative correlation between heel
angle (HA) and the fetlock joint (CoR MP) P< 0.05. It would appear that the relative position
of the fetlock joint to the ground bearing surface has a significant effect on heel angle. The
further the fetlock is behind the last weight bearing point of the heels then the lower the heel
angle.
28
Ha vs CoP CoR R
60.00
HA degrees
50.00
40.00
30.00
HA
20.00
Linear (HA)
10.00
0.00
0.00
20.00
40.00
60.00
80.00
CoP CoR R mm
Figure 11 Shows negative correlation between HA and CoP CoR R.
Not only does heel angle seem to be influenced by the remote force of the fetlock joint but
also by the extending moment arm of the distal interphalangeal joint. All three CoP CoR
measurements (moment arms) were negatively correlated to heel angle. CoP CoR R (Figure
11) was the strongest raw data correlation of HA (negative), P< 0.01. CoP CoR D the weakest
of the three. This was consistent with the pattern that the moment arms exhibited on all feet,
this being that Duckett’s moment arm was always the shortest and Rooney’s always the
longest. HA was positively correlated to TA so this would explain its strong correlation to
CoP CoR R as this was plotted by taking a line from the centre of rotation of the distal
interphalangeal joint and running it down to the ground parallel to the dorsal wall.
However this was not the case with the CoP CoR measurements and heel angle when the raw
data was transformed into percentages of the ground bearing surface, and shows the
importance of working with the raw data before it is transformed as information can be lost.
CoP CoR D was weakened to become insignificant (-0.492 to -0.206) as was CoP CoR R (0.610 to -0.425). Interestingly CoP CoR W became stronger (-0.535 to -0.563 and P< 0.05 to
P < 0.01). When the CoP CoR percentage data was plotted on scatter graphs with HA data
organised in ascending order, the graph with the best fit trend lines was the CoP CoR W
graph.(see Figure 12).
29
CoP CoR's % vs HA degrees (arranged by
ascending HA)
CoP CoR's % / HA degrees
60.00
50.00
CoP-CoRD
40.00
CoP-CoRW
C0P-CoR R
30.00
HA
20.00
Linear (CoP-CoRD)
10.00
Linear (CoP-CoRW)
Linear (C0P-CoR R)
0.00
0
5
10
15
20
25
Linear (HA)
Feet
Figure 12 CoP CoR percentages of GBS compared to HA.
As can be seen from Figure 12 above as a percentage the CoP CoR W value is always
approximately 30% hence the smooth fit trend line ,this appears to be the reason for the slight
increase in correlation, the CoP CoR R percentage data is more randomly arranged around its
trend line. As the foot size variable is not as significant when comparing lever arm
measurements to angle measurements as was shown by the other two CoP CoR measurements
correlations becoming insignificant and the fact that an angle cannot be transformed into a
percentage, the author does not regard this percentage CoP CoR W (see Figure 13) correlation
as significant as the raw data CoP CoR correlations.
30
CoP CoR W %
CoP-CoRW% vs HA
35.00
34.00
33.00
32.00
31.00
30.00
29.00
28.00
27.00
26.00
25.00
20.00
CoP-CoRW
Linear (CoP-CoRW)
30.00
40.00
50.00
60.00
Heel Angle degrees
Figure 13 CoP CoR W % negatively correlated to HA, P< 0.01.
4.3 Mechanics of the fetlock joint
As the spatial positions of the metacarpophalangeal joint (CoRMP25 and CoR MP) relative to
the ground bearing surface are influential on both heel angle and moment arms of the distal
interphalangeal joint (CoP CoR), then the mechanical effects of the fetlock joint on the hoof
cannot be overlooked.
In the introduction it was explained how the basic mechanics of the foot worked and how the
linear forces were in equilibrium (Figure 1), and that when they were not in equilibrium the
effects would be substantial. The example given was the foal with flaccid tendons. The reason
why the extending moment, when greater than the flexing moment can actually lift the toe off
the ground is because of the oblique alignment of the phalangeal bones and the palmar
position of the fetlock joint relative to the ground bearing surface of the foot.This causes the
heels to become a fulcrum to the remote force of descending body weight. The author
theorises how the mechanics of the fetlock joint may account for the results found (see Figure
14).
31
Figure 14 Theoretical equilibrium diagram of metacarpophalangeal joint. (R.J. Mather 2011)
Descending body weight – GRF = 0
DDFT clockwise flexing moment – Anti-clockwise extending moment = 0
Unsupported component of CoR MP anti-clockwise extending moment (UCEM) +
Descending body weight = - R (Resultant remote compressive force on heels)
Unsupported component of CoR MP (fully loaded) anti-clockwise extending moment
(UCEM fully loaded) + Descending body weight = - R1 (Increased resultant compressive
force on heels).
As the CoR MP descends in an arc tension in the DDFT increases, at the same time a
corresponding and proportional increase occurs in the length of the unsupported component
of the anti-clockwise extending moment (UCEM). In this study the last weight bearing point
of the heel was the datum point for all measurements relative to the GBS, therefore CoR MP
32
and CoR MP25 are both linear measurements of UCEM. Given the negative correlations in
this data set between heel angle and CoR MP and CoR MP25 it would appear that UCEM
acts as a lever arm for the remote force of descending body weight travelling down the leg
that has a compressive or in extreme cases a crushing effect on the heels. If this theory is
correct then there would be a measureable and physical effect on the trajectory of the CoP.
4.4 Fetlock joint and CoP trajectory
In Figure 3.1 the trajectory of the CoP is plotted on the solar surface of the foot (after Van
Heel et al 2004) as can be seen the CoP advances forward rapidly to just dorsal of the apex of
the frog but then makes a palmar regression to a location at mid stance just palmar and medial
to the apex of the frog. Van Heel gave no explanation for this regression in the trajectory of
the CoP, and as the horses were trotted over the pressure force measuring system in a straight
line and constant velocity then the centre of mass of the horse would have been moving
smoothly forward in relation to the foot once it had landed and loaded, so a more linear
progression of CoP would be expected.
Tensile forces in the DDFT are trying to flex the foot against the GRF and are responsible for
the advancement of the CoP to its position just before mid stance in front of the frog apex. As
stated above the fetlock moves in a palmar distal arc as it loads with descending body weight
relative to the GBS and, because a greater lever arm can exert a greater load from an applied
force, as the UCEM increases to its maximum length (mid stance, fetlock fully descended) it
momentarily pulls the CoP rearward (Figure 3.1). These interacting and opposing
mechanisms of the fetlock and foot may also explain why after rapid advancement the CoP
occupies a mid location in the foot for so long (55% - 60% of stance phase), before rapidly
advancing again at the onset of breakover (Figure 3).
4.5 HA and CoR DiP
As can be seen from the negative correlation between HA and CoR MP (figure 10) indicates
higher loading forces or prolonged loading forces causing the heels to collapse and run
forwards. This was further backed up with data from this work by the percentage correlation
of HA against CoR DiP (figure 15) Although this is an angle being compared to a
transformed linear measurement the removal of the foot size variable is important in
establishing the position of CoR DiP in relation to the GBS. The CoR DiP when calculated as
a percentage of the GBS (figure 16) to remove the foot size variable was found to have a
33
mean of 38.12% and a median value of 38.67% the standard deviation was 3.77. This
aproximates with the 60:40 split found in previous study (Mather 2009)
HA vs CoR DiP%
60.00
HA degrees
50.00
40.00
30.00
HA
20.00
Linear (HA)
10.00
0.00
0.00
10.00
20.00
30.00
40.00
50.00
CoR DiP % of GBS
Figure 15 Positive correlation of HA against percentage CoR DiP
In the previous study using Duckett’s hoof mapping technique (Mather 2009) the CoR DiP
was found to divide the ground bearing surface of the foot in a 60:40 dorso/palmar split and
not bi-sect the ground bearing surface of the foot in a 50:50 split as is so commonly claimed
in many farriery and veterinary texts, one such example being No foot No Horse (Williams,
Deacon 1999). This finding did concur with another study (Craig et al 2005), in not
supporting this 50:50 split. Their study found an average of 67.06% of the ground bearing
surface in front of the COR but had a higher s.d of 5.41% although their study was, like this
one based on radiographic evidence and not external reference points. They did not state what
foot trimming protocol was used in their study.
34
CoR DiP as % of GBS
50.00
45.00
40.00
35.00
30.00
MM 25.00
CoR DiP
20.00
15.00
10.00
5.00
0.00
A B C D E F G H I J L MN O P Q R S T U V X y
FEET
Figure 16 Shows the variability of the CoR DiP as a percentage of the ground bearing surface
of the foot.
The above graph shows that as the heel angle reduces, the percentage distance from the last
weight bearing point of the heels to the CoR DiP also reduces indicating under run heels
causing the GBS to migrate forward relative to the CoR DiP. The CoR DiP distance varied
between 31.29 % - 45.15 % of the GBS (Figure 16).
The three theoretical CoPs all correlate strongly with each other as can be seen in figure 8 this
is to be expected as they were all plotted or calculated using the distal interphalangeal joint as
a reference point although CoP Roo also used the dorsal wall angle as a reference point, and
as a result in the raw data all three correspondingly have a strong positive correlation with
CoR DiP. All three also correlate strongly with GBS, CoP AW being the strongest (Figure
17) this is because of their proportional consistency regardless of foot size. Surprisingly in the
percentage data CoP Roo no longer correlated with the other two CoPs or CoR DiP this is
believed to be because the CoP Roo position is mainly dictated by the dorsal toe angle.
35
CoP AW vs GBS
120.00
CoP AW mm
100.00
80.00
60.00
CoP AW
40.00
Linear (CoP AW)
20.00
0.00
0.00
50.00
100.00
150.00
200.00
GBS mm
Figure 17 Shows positive correlation of raw data CoP AW vs. GBS
The above graph shows how the CoP AW remains proportional to the GBS across the range
of foot sizes. The CoP CoR measurements are the moment arms of the distal interphalangeal
joint according to Duckett, Wilson and Rooney. In the raw data all three are negatively
correlated to toe and heel angles (figure 18), so as the CoP CoR measurements increase the
toe and heel angles decrease (long toe low heel) as these are extending moments then it can
be assumed that a corresponding increase in the tensile forces of the DDFT will also be
required to keep the corresponding flexor moment in bio mechanical equilibrium.
TA degrees
CoP CoR R vs TA
60.00
58.00
56.00
54.00
52.00
50.00
48.00
46.00
44.00
42.00
40.00
20.00
TA
Linear (TA)
30.00
40.00
50.00
60.00
70.00
CoP CoR R mm
4.6 Figure 18 CoP CoR R had the second strongest negative correlation to TA, P<0.001.
36
CoP CoR R was the only CoP CoR measurement to correlate with P3PE (Figure 19) as well
as TA, HA.
C0P-CoR R vs P3PE
70.00
60.00
Cop CoR mm
50.00
40.00
C0P-CoR R
30.00
Linear (C0P-CoR R)
20.00
10.00
0.00
-2.00
0.00
2.00
4.00
6.00
8.00
10.00
P3PE degrees
Figure 19 CoP CoR R negatively correlated to P3PE – 0.526 P<0.05
37
The three CoP CoRs are positively correlated to CoR MP 25 (unsupported component of
fetlock extending moment arm) so the greater the CoR Mp 25 the greater the CoP CoR
measurement (figure 20). The implication of this is that the direct and remote leverage forces
acting on the lower limb and foot are directly responsible for and reflected in the physiology
and morphology of the hoof capsule.
CoR MP25 mm
CoR MP25 vs CoP CoR
100.00
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
CoR MP25
Linear (CoR MP25)
0.00
20.00
40.00
60.00
80.00
CoP CoR R mm
Figure 20 The positive correlation between CoR MP25 and CoP CoR R.
4.7 Study limitations and future studies.
In this experiment the superficial and deep digital flexor tendons were not clamped when the
limbs were loaded into the press so the orientation of the pastern under load was maintained
purely by the attachment of the suspensory ligament. In the live horse overextension of the
proximal interphalangeal joint during loading is resisted by contraction of the superficial
digital flexor muscle, this increases the tension in the superficial digital flexor tendon which
tightens against its insertions on the distal end of the proximal phalanx and proximal end of
the middle phalanx, preventing the pastern joint from buckling (Stashak 2002).
This lack of tension from the superficial digital flexor tendon may have had an effect on the
pastern alignment of the feet when loaded in the press; this would influence their relative
position to the GBS. Furthermore the body weight of the horses was not known so the limbs
could not be loaded to 30% of body weight as would be the case when standing still and
square. As the assumption was made that P1 and P2 would be in alignment when CoR MP25
38
was plotted the position of the proximal interphalangeal joint at this angle (CoR PiP25) was
not plotted. The author feels that, with hindsight, CoR PiP25 should also have been plotted
given the variables mentioned above. As P1 and P2 were aligned at the same angle then CoR
PiP 25 and CoR MP25’s positions relative to the GBS would have been dictated purely by
their linear lengths, so any correlation between them and other anatomical features and
measurements would have been more relevant. The proportional lengths that P1 and P2 are,
of their own combined length, may be influential to the posture of the hoof pastern axis in the
live horse at mid stance. As has been shown by the results a longer moment arm (CoP CoR)
leads to a lower toe and heel angle and a larger unsupported component of the fetlock
extending moment, so it is safe to assume that a very short CoP CoR measurement would
give a very upright toe and heel angle and very short unsupported component of the fetlock
extending moment. It is widely regarded in both farriery and veterinary professions that
neither extreme is desirable, horses with long toes and sloping pasterns are more prone to
tendon injuries and short toed upright pastern horses are more prone to concussive injuries of
the hard tissue. Therefore for any given foot size there will be an optimum range of moment
arm lengths that will allow the lower limb and foot to function to their maximum potential
while remaining within their safe stress strain limit. With more research this could lead to the
possibility of a scientific method for scoring the conformation of the lower limb and foot
radiographically as opposed to the more subjective methods that are generally used. Clearly
more research is needed and an ideal future study on live horses using a pressure force
measuring system to measure the actual, as well as theoretical CoP on three groups with
alternative characteristics in foot conformation would be desirable. The first group being
horses that are long toe low heel and the second group being horses with “normal” feet, the
third group would have upright feet. This would help highlight the relationships between the
mechanics, ground reaction forces and posture of the horse.
4.8 Conclusion
Some of the correlations although strong are to be expected as they are between two
anatomical structures (joints) that are directly connected by a bone, even so these correlations
would be more relevant in a live horse study as they are a product of the angle of the bone
and its linear length and would give an indication of the posture of the horse and how the
distal limb orientation affects the hoof capsule. These spatial positions of the distal three
joints of the horse’s forelimb, relative to the ground bearing surface of the foot and its
39
associated ground reaction forces and resulting moment arms , given the results of this study,
may affect and influence the conformation of the hoof capsule.
In this study I have shown the strong correlations and theorized about possible relationships
between biomechanical lever arms, spatial positions of joints and angulations’ of toe and heel.
However I cannot determine the causal relationships between the lever arms and hoof capsule
morphology because of the experimental design, as all the data was collected from cadaver
limbs. If the structural integrity of a functional hoof is overwhelmed and fails (heels collapse)
the lever arms will be increased further exasperating the condition, or does the skeletal
architecture of the lower limb dictate the lever arms which then determines the mechanical
efficiency of the hoof and therefore its ability to resist stress and accommodate strain. How
these relationships exist can only be determined from a prospective live animal study. This
would need an immature population of horses to be followed over an extended period of time
to see how their hooves, conformation and posture developed.
5.0 Manufacturers’ addresses
Ontrack Equine Software, c/o Lameness Solutions LLC, PO Box, 152 Lake Elmo, Minnesota
55042, USA. www.ontrackequine.com
40
6.0 References
Bartel, D.L., Schryver,H.F, H.F,. Lowe,J.E and Parker,R.A.(1978) Locomotion in the horse: a
procedure for computing the internal forces in the digit. Am.J.vet Res.39,1721-1727.
Bushe, T., Turner T.A., Poulos, P.W., et al (1987) The effect of hoof angle on coffin, pastern
and fetlock joint angles. Proc Am Ass Equine Practnrs 33,729-738.
Caldwell, M.N., Reilly, J.D., Savoldi, M. Quantitative Horse Hoof Trimming Protocol for
Research Purposes.(2010) Forge Magazine p4-10.
Craig J. (2005) Hoof and Bone Morphology of the Equine Digit: Challenges to some
Common Beliefs. The European Farriers Journal Issue 114.
Crevier-Denoix., Roosen, C., Dardillat, C. et al(2001) Effects of heel and toe elevation upon
the digital joint angles in the standing horse. Equine Vet J 33 74-78.
Hickman J, Humphrey M, eds. Hickman’s farriery, 2nd ed. London: J.A.Allen,1988; 36-37.
Ducket, D (1990) The assessment of Hoof Symmetry and Applied Practical Shoeing by Use
of an External Reference Point. International: Farriery and Lameness Seminar. Newmarket
England 2(suppl.) 1-11.
Eliashar, E., McGuigan, M.P., Rogers, K.A., and Wilson, A. M,.(2002) A comparison of
three horseshoeing styles on the kinetics of breakover in sound horses. Equine vet.J 34 (2)
184-190
Williams G, Deacon M (1999) No Foot No Horse. Kenilworth Press Ltd., Addington,
Buckingham,MK18 2JR.
McClinchey, H.L., Thomason, J.J. and Jofriet, J.C. (2003) Isolating the effects ofequine hoof
shape measurements on capsule strain with finite element analysis.
Vet. comp. Orthop. Traumatol. 16, 67-75.
41
Miles.
J.
(2011
Download)
Table
of
critical
values
for
Pearson
correlation.
www.jeremymiles.co.uk/misc/tables/pearson.html
Riemersma D.J., Van den Bogert A.J., Jansen M.O., Shamhardt H.C. (1996) Influence of
shoeing on ground reaction forces and tendon strains in the forelimbs of ponies. Equine Vet J
28, 126-132.
Rooney.J.R. (1969) Biomechanics in lameness in Horses.: Williams & Wilkins Co.
Rooney.J.R. (1984) The angulation of the forefoot and pastern of the horse. Equine Vet Sci 4 ,
138-143.
Rooney J.R. (Download 2007) Basic mechanics of the foot and horseshoe. http://
www.horseshoes.com/advice/rooney1/mechanicsoffootandshoe.htm
Ridgeway (2003) “A Bridge to the future” Veterinary Conference 10/8/03 Low heel high
heel syndrome - Unrecognised problems. Dr Kerry.J. Ridgeway DVM.
Reilly, J.D., Cottrell, D.F., Martin, R.J. and Cuddeford, D. (1996)Tubule Density in Equine
Hoof Horn. Biomimetics 4, 23-36
Schryver,H.F, H.F., Bartel,D.L., Langrana,N. and Lowe,J.E (1978) Locomotion in the horse:
Kinematics and external and internal forces in the normal equine in the walk and
trot.Am.J.vet.Res.39.1728-1733
Stashak.T.S (2002) Adams’ Lameness in Horses 5th ED. Lippincott Williams & Wilkins
Philadelphia pp 22.
Thomason, J.J., Bignell, W., Batiste, D. and Sears, W. (2004) Effects of hoof shape,
body mass and velocity on surface strain in the wall of the unshod fore hoof of
Standardbreds trotting on a treadmill. Equine Comp. exerc. Physiol. 1, 87-97.
42
Van Heel, M.C.V., Barneveld,A., van Weeren,P.R. and Back,W,(2004) Dynamic pressure
measurements for the detailed study of hoof balance: the effect of trimming.Equine vet.
J.36.778-782.
Van Heel M.C.V. Moleman.M., Barneveld. A. Van Weeren, P.R., and Back. W.(2005)
Changes in the location of centre of pressure and hoof unrollement pattern in relation to an 8
week shoeing interval in the horse. Equine Vet.J.
Van Heel, M.C.V. (2005) Distal limb development and effects of shoeing techniques on limb
dynamics of today’s equine athlete. Chapter 8. Febodruk BV, Enschede.
Willemen,M.A.,(1997) Horseshoeing, a biomechanical analysis. PHD thesis, University of
Utrecht.
Willemen,M.A., Savelberg, H.H.C.M., Barneveld, A. (1999) The effect of orthopaedic
shoeing on the force exerted by the deep digital flexor tendon on the navicular bone in horses.
Equine Vet J 31, 25-30.
Wilson,A.M., Seelig,T.J., Shield,R.A. and Silverman,B.W.(1998) The effect of hoof
imbalance on point of force application in the horse. Equine vet.J.30.540-545.
Wilson,A.M., McGuigan,M.P., Fouracre, L. and MacMahon,L. (2001) The force and contact
stress on the navicular bone during trot locomotion in sound horses and horses with navicular
disease. Equine vet.J.33,159-165.
Mather, J., Caldwell. M ., Reilly,J.,(2009). Preliminary study of a geometric proportioned
foot trim on the bearing surface of the equine front foot. Forge Magazine (April) 8-13.
43
Word count 10065
44
45
46
47
48