Effect of toe and heel elevation on calculated tendon strains in the horse and the influence of the proximal interphalangeal joint

Authors

  • Siân E. M. Lawson,

    1. Centre for Rehabilitation and Engineering Studies, Newcastle University, UK
    2. UMR INRA/ ENVA Biomécanique et Pathologie Locomotrice du Cheval, Ecole Nationale Vétérinaire d’Alfort, France
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  • Henry Chateau,

    1. UMR INRA/ ENVA Biomécanique et Pathologie Locomotrice du Cheval, Ecole Nationale Vétérinaire d’Alfort, France
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  • Philippe Pourcelot,

    1. UMR INRA/ ENVA Biomécanique et Pathologie Locomotrice du Cheval, Ecole Nationale Vétérinaire d’Alfort, France
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  • Jean-Marie Denoix,

    1. UMR INRA/ ENVA Biomécanique et Pathologie Locomotrice du Cheval, Ecole Nationale Vétérinaire d’Alfort, France
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  • Nathalie Crevier-Denoix

    1. UMR INRA/ ENVA Biomécanique et Pathologie Locomotrice du Cheval, Ecole Nationale Vétérinaire d’Alfort, France
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Dr Sian E. M. Lawson, Centre of Rehabilitation and Engineering Studies, School of Mechanical and Systems Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, UK. T: +44 191222 8224; F: +44 191222 8600; E: sian.lawson@ncl.ac.uk

Abstract

The sagittal alteration of hoof balance is a common intervention in horses, with corrective shoeing being one of the most frequently applied methods of managing tendonitis. However, the effect of toe or heel elevation on tendon strains is poorly understood. This study aimed to examine the effect of toe and heel wedges on the superficial digital flexor tendon, deep digital flexor tendon, and the third interosseous muscle or suspensory ligament strains using in vivo data and an accurate subject-specific model. Kinematic data were recorded using invasive markers at the walk and trot. Computerized tomography was then used to create a subject-specific model of an equine distal forelimb and strains were calculated for the superficial digital flexor tendon, the deep digital flexor tendon accessory ligament and the suspensory ligament for seven trials each of normal shoes, and toe and heel elevation. As the proximal interphalangeal joint is often ignored in strain calculations, its influence on the strain calculations was also tested. The deep ligament showed the same results for walk and trot with the heel wedge decreasing peak strain and the toe wedge increasing it. The opposite results were seen in the suspensory ligament and the superficial digital flexor tendon with the heel wedge increasing peak strain and the toe wedge decreasing it. The proximal interphalangeal joint was shown to be influential on the strains calculated with normal shoes and the calculated effect of the wedges. Our results imply that corrective shoeing appears to decrease strain in the tendon being targeted; the possibility of increases in strain in other structures should also be considered.

Introduction

The use of corrective or orthotic shoes is a common conservative intervention in humans (Goodman, 2004) and horses (Moyer, 1980; Stashak, 1987) and is one of the most frequently applied methods of managing tendonitis. In humans the use of orthotics to treat tendonitis by reducing tendon strain includes the management of Achilles tendonitis (Genova & Gross, 2000; Wallace et al. 2004) and tibial tendon dysfunction, where orthotics have also been shown to support the arch and improve the alignment of the foot (e.g. Chao et al. 1996; Wapner & Chao, 1999).

The use of orthotics to relieve tendonitis in horses is less well understood. The introduction of heel elevation is believed to reduce the strain on the deep digital flexor tendon (DDFT) (Riemersma et al. 1996; Willemen et al. 1999) and facilitate rolling over the toe (Back, 2000). However, these therapies are mainly empirically based and lack conclusive scientific evidence. Six degrees of elevation has been shown to have a substantial effect on the kinematics of the distal forelimb (Chateau et al. 2004b, 2006) but the effect of such shoes on tendon strains remains controversial. Previous studies have examined the effect of heel elevation on measured strains of the superficial digital flexor tendon (SDFT), DDFT and the third interosseous muscle or suspensory ligament (SL) in vivo and in vitro. The addition of heel wedges has been seen to decrease strain in the DDFT in force plate studies of artificially induced tendonitis (Meershoek et al. 2002).

Invasive studies have provided some evidence that toe and heel wedges have only a limited effect on tendon strains at the slower gaits (Riemersma et al. 1996) and it has been hypothesized that this may be due to changes in the length of the superficial and deep digital flexor muscle bellies.

It is widely believed that heel elevation reduces strain in the DDFT due to alterations in the digital joint angles. In vivo radiographic studies on standing horses (Bushe et al. 1987; Crevier-Denoix et al. 2001) and in vitro studies (Rooney, 1984) have demonstrated that elevation of the heels induces an increase in extension of the metacarpophalangeal joint and an increase of flexion of the proximal and distal interphalangeal joints. In moving horses, Willemen et al. (1999) and Scheffer & Back (2001) observed a decrease in the maximal extension of the metacarpo-phalangeal joint (MPJ) during the stance phase of horses trotting with 6° heel wedges. However, these studies were performed with markers stuck on the skin that may not represent the true movement of the underlying bones. Furthermore, the proximal interphalangeal joint (PIPJ) was not taken into account and this assumption may have affected the accuracy of the measurements. The use of toe elevation is not as common; however, recently the application of toe wedges to increase SDFT strain and therefore length in toe walking horses has also been attempted clinically in young animals.

Chateau et al. (2004b) observed, using an invasive kinematic approach, that 6° elevation of the heels slightly but significantly increased maximal extension of the MPJ at the walk as well as maximal flexion of the proximal and distal interphalangeal joints. Chateau and colleagues went on to demonstrate the effect of 6° of toe or heel elevation on the three digital joints using a similar invasive technique at the trot (Chateau et al. 2006). The use of heel wedges significantly increased maximal flexion and decreased maximal extension at heel off of the proximal and distal interphalangeal joints; and inverse effects (except for PIPJ maximal extension) were observed with the toe wedges. At trot the MPJ was not significantly affected by toe or heel wedges. An examination of the implications of these data on tendon strains is obviously needed if the effect of using wedges is to be properly understood.

As invasive markers were used it was possible for us to include the movement at the PIPJ. However, previous studies using surface markers have been unable to track accurately the movement of the phalangeal bones, due to skin movement (van Weeren et al. 1990) and marker placement limitations and so have been forced to treat the proximal and middle phalanx as a rigid body. Rigidly fixing the PIPJ can also be used as a surgical intervention (arthrodesis). There is therefore also a need to examine the implications of ignoring this joint on the strains calculated and the conclusions drawn.

The current study aims to demonstrate the effects of kinematic changes due to the use of heel or toe wedges on the tendon strains calculated at walk and trot using a subject-specific kinematic model of the distal limb. We hypothesize that if the alterations in tendon strain are primarily due to changes in the digital joint angles then these alterations should be detectable using an accurate model of the distal limb.

Methods

A subject-specific three-dimensional equine distal limb model of a 6-year-old French Trotter horse was created, using computerized tomography scans of the subject's bones, which was able to output strain for the SDFT, DDFT and SL for a given kinematic input (Fig. 1; Lawson et al. 2005, 2006). The sizes and shapes of the distal, middle and proximal phalanges, proximal and distal sesamoid bones and the metacarpal bones were extracted from the computerized tomography scans and rendered as three-dimensional objects using Matlab™ (Mathworks Inc., USA). The SDFT, DDFT and SL were added to the model using anatomical landmarks to locate their insertion points and were represented up to the height of the proximal metacarpus. The paths and insertions used were based on previous literature (Denoix, 1994, 2000), dissection and MRI studies. The SDFT muscle belly was ignored and the proximal part of the DDFT represented by its accessory ligament only. Their paths were wrapped around the bones at the proximal and distal sesamoid bones and at the proximal palmar extremity of the middle phalanx when appropriate. The paths of the tendons and the SL therefore depended on the positions of the bones and, in the case of the SDFT, the amount of strain in the DDFT.

Figure 1.

The model during stance in the toe wedge condition, displaying the third metacarpal (MC3) and first second and third phalanges (P1, P2 and P3), and paths of the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT) and the third interosseous muscle or suspensory ligament (SL).

Tendon and ligament modelling

The proximal origins of the muscle bellies of the deep digital flexor muscle were not included in the model. A virtual origin was used for the DDFT at the level of the proximal limit of the third metacarpus, and following the orientation of the accessory ligament. The insertion of the DDFT was taken as the centroid of its large attachment site on the distal phalanx.

The path of the DDFT wrapped around a surface which is a fixed distance palmar to the proximal sesamoid bones, shaped to represent its path over the sesamoidean ligaments and taking into account the movement of the proximal sesamoid bones. It then wrapped around a surface matching the shape of the distal sesamoid bone. A third, intermediate wrapping location was intermittently used when the constraint of not penetrating the bones caused the DDFT to wrap around the proximal palmar extremity of the middle phalanx. The tendons began to wrap around these wrapping surfaces at the point at which they would be most asymptotic to them, as indicated by the dot product. The DDFT was modelled as changing its width proportionally with the amount of strain present.

Similarly to the DDFT, the SDFT took a virtual origin at the level of the proximal limit of the third metacarpus, whilst maintaining an anatomically correct orientation. The insertion of the SDFT was taken as a virtual central point between its medial and lateral insertions on the middle phalanx, ignoring its distal bifurcation. Between origin and insertion it passed over a wrapping surface which was a variable distance palmar to the proximal sesamoid bones to represent its path over the DDFT. The shape of the wrapping surface was similar to that of the DDFT. The radius of its distance from the edge of the proximal sesamoid bones varied proportionally to the changing width of the DDFT.

The SL origin was modelled as a point taken at the centre of the broad attachment to the proximal metacarpus. The SL attachment to the distal row of carpal bones was ignored in this model. It runs distally to a virtual insertion taken as the midpoint of the insertions of its medial and lateral divisions on the corresponding sesamoid bones. As the majority of the fibres terminate here, the strain calculations for the SL were restricted to the proximal part of the ligament running from the carpal bones to the proximal sesamoid bones.

Previous work (Lawson et al. 2005, 2006) has found the influence of variations in the origin and insertion sites used for the SDFT, DDFT and SL in this model to be minor and consistent and the influence of wrapping surfaces near the joints to be slightly larger but still consistent.

Kinematics

The movement of the left forelimb of the subject was recorded using a Zebris CMS-HS (Zebris Medical GmbH, Germany) system as described in Chateau et al. (2004a). The positions of invasive marker clusters in the subject's phalanges and third metacarpus were collected at walk (1.18 ± 0.06 m s−1) on a hard surface and at trot (3.9 ± 0.02 m s−1) on a treadmill (Chateau et al. 2004a, 2006). Data were recorded for normal shoes, shoes with a 6° elevation at the toe and shoes with a 6° elevation at the heel. The procedures used were reviewed and approved as being within the animal welfare guidelines of the Direction des Services Vétérinaires, Val de Marne, France.

The movement of the proximal and distal sesamoid bones was simulated using the constraints of keeping the articular surfaces of the metacarpo-sesamoidean and phalangeal-sesamoidean articulations in contact and maintaining an isometric virtual distal ligament. The palmar (intersesamoidean) ligament uniting the proximal sesamoid bones was included as a rigid structure, effectively tying the proximal sesamoid bones into a single, solid structure. Stiff (isometric) virtual ligaments were included, tying the proximal and distal sesamoid bones to the proximal and distal phalanges, respectively. The effect of the attachment site used for these ligaments on model outputs was shown to be negligible (Lawson et al. 2007). In order to calculate sesamoid bone position and orientation a further constraint was used of maintaining an optimal contact between the articular surfaces of the sesamoid bones and that of the third metacarpal in the case of the proximal sesamoid bones or the distal pahalanx in the case of the distal sesamoid bone (Fig. 1). An optimization routine was used to find the positions and orientation of the sesamoid bones which gave the most contact between the three-dimensional articular surfaces. This minimized the distances between the meshes of the articular surfaces using a least-squares fitting routine but did not include any solutions which required bone penetration. The simulation of the sesamoid bones was validated in the sagittal plane (Lawson et al. 2007).

The strains produced in the SDFT, DDFT and SL were calculated using the kinematic model and compared between the three conditions at both walk and trot during the stance phase of seven trials for each condition. The reference lengths were taken as the mean length of the structure when the hoof contacted the ground in the normal shoes. The same reference lengths were used for all conditions as only one horse was examined.

The strains were also recalculated for all conditions whilst ignoring the movement in the PIPJ. In order to recreate the effect of using surface markers the proximal and middle phalanges were treated as a straight rigid body with a compensating alteration to the movement at the metacarpophalangeal and distal interphalangeal joint angles to allow the positions of the distal phalanx and third metacarpus to remain unchanged. The strains calculated were then compared with the full results calculated to extract the influence of the PIPJ and the implied effect of ignoring or fixing this joint.

The peak strains were calculated for the SDFT, DDFT and SL for the seven trials for each condition, and compared using an anova.

Results

Peak strains and their standard deviations are given for the SDFT, DDFT and SL for all conditions at walk and trot in Table 1. In all three structures the differences seen between the conditions examined were statistically more significant at walk. All the alterations seen in peak strain were significant (P < 0.05) apart from the effect of the heel wedge on the SDFT at trot.

Table 1.  Peak strains [mean (SD)] for the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT) and suspensory ligament (SL) calculated with normal shoes, toe and heel wedges at walk and trot
 SDFT mean (SD)DDFT mean (SD)SL mean (SD)
Max. strain at walk (%)
 Toe6.65 (0.08)5.68 (0.11)4.74 (0.11)
 Normal6.71 (0.08)5.53 (0.07)4.99 (0.08)
 Heel6.82 (0.07)5.15 (0.11)5.17 (0.09)
Max. strain at trot (%)
 Toe8.36 (0.08)5.48 (0.06)6.63 (0.07)
 Normal8.49 (0.11)5.39 (0.15)6.78 (0.07)
 Heel8.41 (0.04)5.01 (0.11)6.89 (0.07)

SDFT

At trot the heel wedge caused no significant change in the maximal strain in the SDFT whereas at walk the peak strain was increased. Toe elevation decreased maximal strain in the SDFT at both walk and trot. These changes were small but statistically significant (Table 1). At both gaits, although the gross pattern of strain remained the same, differences were apparent in the timing and rate of strain increase between the three conditions (Figs 2 and 5). In particular, at walk the toe wedge induced a higher rate of increase of strain in the SDFT in early stance.

Figure 2.

Strain in the superficial digital flexor tendon (SDFT) with normal shoes, toe and heel wedges at walk. Seven stance phases are plotted for each condition.

Figure 5.

Strain in the superficial digital flexor tendon (SDFT) with normal shoes, toe and heel wedges at trot.

DDFT

In both walk and trot the heel wedge decreased maximal strain in the DDFT and, conversely, toe elevation increased maximal strain. However, overall maximal strain varied little and was lower in trot than in walk (Table 1). At both gaits the timing and rate of strain increase remained the same between the three conditions, although some differences were seen in the shock absorption phase of early stance (Figs 3 and 6).

Figure 3.

Strain in the deep digital flexor tendon (DDFT) with normal shoes, toe and heel wedges at walk.

Figure 6.

Strain in the deep digital flexor tendon (DDFT) with normal shoes, toe and heel wedges at trot.

Sl

In walk and trot the heel elevation increased maximal strain in the SL, and toe elevation decreased maximal strain (Figs 4 and 7). In trot the rate of strain increase was higher in the heel wedge condition and the maximal strain was earlier. As with the other two tendons the majority of the differences in strain pattern between the conditions were seen in early stance. The SL strains in normal shoes and in toe and heel wedges appeared to follow the SDF strains.

Figure 4.

Strain in the suspensory ligament (SL) with normal shoes, toe and heel wedges at walk.

Figure 7.

Strain in the suspensory ligament (SL) with normal shoes, toe and heel wedges at trot.

Effect of PIPJ

The removal of the PIPJ had a strong effect on the strains calculated in the SDFT, DDFT and SL (Fig. 8a–c) and the effect of the wedges (Fig. 9a–c). The magnitude of peak strain was increased in the normal shoe condition in all cases (Fig. 8a–c) when the strains were calculated without allowing for interphalangeal joint movement. In the DDFT, when proximal interphalangeal movement was ignored the heel wedge was still seen to decrease strain and the toe wedge to increase it (Fig. 9a). In the SDFT and SL both the toe wedge and the heel wedge now seemed to increase peak strain (Fig. 9b,c). Changes to the timing were also seen, particularly in the SL.

Figure 8.

Strain in (a) the superficial digital flexor tendon (SDFT), (b) the deep digital flexor tendon (DDFT) and (c) the suspensory ligament (SL) with normal shoes at trot, with and without the proximal interphalangeal joint (PIPJ) included in the model.

Figure 9.

Strain in (a) the superficial digital flexor tendon (SDFT), (b) the deep digital flexor tendon (DDFT) and (c) the suspensory ligament (SL) with normal shoes, toe and heel wedges at trot calculated without the proximal interphalangeal joint (PIPJ) included in the model.

Discussion

This study reports significant differences to calculated strains in the SDFT, DDFT and SL due to the addition of toe and heel elevation to the horse's shoes, and the influence of the PIPJ on these calculated strains. The model used has been tested for sensitivity to alterations in the origins and insertions used for tendons and ligaments and these effects were shown to be minor (Lawson et al. 2005, 2006); however, the simplification of both origin and insertion sites as single point attachments is a limitation of the modelling approach used. The largest sensitivity demonstrated was to changes in the tendon paths to the wrapping near the joints; however, all the effects seen due to perturbations influenced the accuracy of the model and not the resolution of the model in intercondition comparisons. That is, the absolute values reported by a musculo-skeletal model will vary according to its design, but this should not affect its ability to compare between different conditions.

The exclusion of the muscle bellies of the SDFT and DDFT remains a limitation of this model, as it is possible that the influence of these muscles may change under the different conditions examined. There is evidence that these structures have only a minor influence on the strain in the tissues examined (Wilson et al. 2001; McGuigan & Wilson, 2003), and so it has been assumed here that if there are any changes in their influence, then these can be ignored. Similarly, the tendon bifurcations and the extensor branch of the interosseus ligament were assumed not to be influential, and particularly not to effects of the sagittal plane nature interventions considered here.

The results shown here for the trot are in accordance with those of Meershoek et al. (2002) who saw that at trot heel wedges decreased DDFT accessory ligament strain. At the trot we also found that the heel wedge increased SL strain and the use of a toe wedge increased DDFT strain and decreased SL and SDFT strain.

The results presented here for the walk correlated with those found at trot with the addition of a significant increase in SDFT strain during heel elevation. These results at walk agree with those of Riemersma et al. (1996), who showed that at walk a heel wedge decreased strain in the DDFT accessory ligament and increased strain in the SL, and a toe wedge increased strain in the DDFT accessory ligament.

The kinematic data used in this study showed no significant changes at the MPJ with the addition of wedges at trot. However, maximal flexion of both the proximal and distal interphalangeal joint were significantly increased with heel wedges and significantly decreased with toe wedges at both walk and trot. At walk maximal extension of the MPJ also showed a small but significant increase.

The DDFT did not follow the same pattern of alterations as the SDFT and SL, as it was strongly influenced by the distal interphalangeal joint. As would be expected, the increased flexion seen at this joint in the heel wedge condition resulted in a decrease of strain in the DDFT, and conversely the decreased flexion seen in the toe wedge condition significantly increased DDFT strain. Differences were also seen in the early shock absorption phase. This is due to differences in the landing of the hoof. With the heel wedge, impact was heel first and was followed by a sudden forward rotation of the hoof that preceded complete stabilization of the hoof. This forward rotation of the third phalanx, and consequently sudden flexion of the distal interphalangeal joint, probably explains the decrease of DDFT strain.

The SL and the SDFT showed very similar responses to toe and heel elevation at both walk and trot. The SDFT both crosses the MPJand PIPJ and so will have been affected by the changes in these joints. The part of the SL examined was modelled as only crossing the MPJ, as it inserts on the proximal sesamoid bones. Unfortunately, this means that any changes in length calculated for the SL are highly sensitive to the modelling of the movement of the proximal sesamoid bones, being a measure purely consisting of proximal sesamoid bone position relative to the third metacarpal bone. The movement of the proximal sesamoid bones is partly constrained by a virtual ligament attachment to the proximal phalanx and so is influenced heavily by proximal phalanx position. Previous studies (Lawson et al. 2007) have shown this technique, which also constrains the articular surfaces to remain in contact, to be a reliable method of simulation of the sesamoid bone in the sagittal plane. Extrasagittal motions of the digital joints have been shown to be minor (Chateau et al. 2001) and were not statistically altered by the wedges in walk (Chateau et al. 2004b) or trot (Chateau et al. 2006). At trot there were no significant changes in MPJ movement with the addition of wedges but significant alterations to SL strain were still seen. It is possible that alterations to the timing of the MPJ motions caused this effect. Significant alterations were also seen in the SDFT at trot in the toe and wedge conditions, and these may have been due to significant changes seen in PIPJ behaviour.

The effect of ignoring the PIPJ was also examined, as this is a common technique in non-invasive motion analysis. The PIPJ was shown here to have a strong influence on strain, particularly in the DDFT. The exclusion of the PIPJ from the calculations, and the calculated compensation in metacarpophalangeal and distal interphalangeal joint angles, increased the strains calculated in the SDFT, DDFT and SL with normal shoes. These changes also altered the effect of the heel and toe wedges on each strain. Previous studies of the effect of toe and heel elevation on tendon strain have ignored the effect of the PIPJ; however, these results would imply that changes to the PIPJ angles are influential in tendon strain. The examination of the PIPJ is presented here as of interest to the calculation technique. Caution should be employed when extracting any implications from this result for clinical treatments that involve leaving this joint in a rigid state as these would inevitably alter the subject's kinematics in a way that has not been considered here.

A consistent reference length was used to calculate the strains in this study as the same horse was used for all trials and all conditions. The reference length was taken as the average length of the structure when the hoof first contacts the ground with normal shoes. This length was used for all the conditions as it was considered that although the position of the foot and therefore tendon length changed slightly at first contact when using the different shoes the actual neutral length of the tendon remained unchanged and therefore the strains should be considered using the same scale. It is possible that with long-term use of elevated shoes the tendons may adaptively shorten or lengthen, and that this would therefore alter the strains produced. The results presented here are only for multiple trials with one subject and so require further confirmation; however, variability between subjects was shown to be low (Chateau et al. 2006).

This study aimed to examine the effect of toe and heel wedges on SDFT, DDFT and SL strains using in vivo data and an accurate subject-specific model at walk and trot. The effect on calculated DDFT strain was the same for walk and trot with the heel wedge decreasing peak strain and the toe wedge increasing it. The opposite results were seen in the SL. The SDFT showed similar results to the SL, with the heel wedge increasing peak strain at walk and toe wedge decreasing peak strain at walk and trot. Despite only having a small range of movement the PIPJ was shown to be influential, increasing the strains calculated with normal shoes and affecting the effect of the wedges on tendon and ligament strain.

Various types of corrective shoeing have become a popular form of intervention in horses. This work supports the assumptions made about the decrease in strain in the tendinous structures targeted, but it also implies that further consideration is needed to examine the effect of such interventions on the strain in other structures, and to avoid unintentional increases in strain in the SDFT and SL.

Ancillary