Evidence-based shoeing for orthopaedic pathologies: biomechanical basics of therapeutic  farriery

M. Oosterlinck*, 1, M. Dumoulin1 , M.A. Weishaupt2 1 Department of Surgery and Anaesthesiology of Domestic Animals, Faculty of Veterinary Medicine, Ghent University 2 Equine Department, Vetsuisse Faculty, University of Zurich

Introduction

Given the link between the external shape of the hoof capsule and its internal function, trimming and shoeing should optimise functionality and ultimately reduce stress, in the prevention of injury as well as in the treatment of established pathology. For centuries, farriery has been a craft relying merely on tradition, personal experience and empirical evidence.¹ Over the last decades, an increasing number of studies has provided insight in key concepts of hoof biomechanics. Unfortunately, there is still a lack of high-quality evidence-based research on trimming and shoeing for specific orthopaedic pathologies. However, the technological evolution may ultimately provide objective and quantitative tools to employ routinely in the assessment of equine locomotion and farriery. ² In this regard, pressure plates offer unique opportunities for evaluating limb loading symmetry³ , hoof contact area⁴ , the pressure distribution underneath the hoof⁵ , the toe-heel and medio-lateral hoof balance of the vertical ground reaction force on hard and soft surfaces^6-7 and in horses with conformational deficits.⁸ Building further on the preceding text by Weishaupt et al., this paper will summarise relevant biomechanical principles of therapeutic farriery and potential clinical applications.

Biomechanical key-points for farriery

1. Optimising hoof balance
Mediolateral hoof imbalance results in excessive compression on one side of the joint⁹ , which may cause progressive cartilage degradation. Moreover, imbalance may lead to progressive distortion of the hoof capsule, including flares, sheared heels, quarter cracks, and to increased stress on the soft tissue structures of the distal limb. In the treatment of these pathologies, correction of hoof imbalance is crucial. ¹⁰ If a hoof is visually observed to be landing on the lateral wall first, a careful assessment of hoof wall heights is a prerequisite. Usually, the side assumed to be longer should be trimmed shorter to allow a visually flat landing. Major imbalances should be corrected gradually over time. In some angular limb deformities, medial or lateral extensions (in case of valgus or varus deformity, respectively) may augment corrective trimming, although excessive modifications should be avoided. There is a significant increase in the moment about the distal interphalangeal joint even during one shoeing interval.¹¹ Corrective trimming (reducing leverage and restoring optimal mediolateral and craniocaudal balance) is critical before any shoeing technique is applied; shoeing without proper attention to trimming is a missed opportunity.10 The widest part of the solar surface of the hoof approximates the centre of rotation of the coffin joint, and guidelines for trimming and shoeing have been formulated based on this external landmark. ¹²

2. Shock dampening during initial impact phase
The hoof has inherent dampening characteristics, resulting in little or no vibrations extending above the fetlock. Shoeing decreases this natural dampening effect. However, shoes of different materials present differences in shock absorption (steel < synthetic), while viscoelastic pads with or without sole filling may further enhance this effect.^13,14 This may be particularly relevant in the treatment of subchondral bone injury or other forms of osteo-arthritis.

3. Appropriate slip/braking during the secondary impact phase
Hoof slip is a normal phenomenon during the secondary impact phase, and is heavily dependent on the track surface, speed and many other variables. The limited amount of horizontal movement during braking allows some energy dissipation and modulation of the magnitude and rate of loading. However, there is a narrow safety margin: too much slip resultsin instability and risk of falling; too little slip increases the risk of catastrophic injuries. Therefore, toe-grabs or studs/heel calks (and their number and exact localization) must only be used judiciously, and should be avoided when dealing for example with collateral ligament injuries. It may be worth considering other modifications like tungsten pins, a fullered and/or concave shoe, etc

4. Optimal pressure distribution during the support phase
The application of a wedge creates a shift of the centre of pressure towards the side of the elevation¹⁵ , and similarly, an egg-bar shoe moves the centre of pressure palmarly.¹⁶ Moreover, hoof wedges and egg-bar shoes alter distal limb joint angles^17-20 and forces acting on the flexor tendons: a heel wedge and a toe wedge result in a decrease and an increase, respectively, of the strain in the deep digital flexor tendon, whereas opposite effects are seen in the superficial digital flexor tendon and the suspensory ligament.²¹ This may be useful in the treatment of tendinopathies as well as flexural deformities. Similar effects can be achieved by altering the width of the shoe at any point about its perimeter in relation to the rest of the shoe¹⁰, at least when the horse is moving on a deformable surface. For example, egg-bar shoes prevent sinking of the heels into the ground on a sand track¹⁹ and as such resemble the biomechanical effect of a heel wedge on a hard surface. Similarly, shoes with a wide toe prevent sinking of the toe in a soft surface and decrease strain in the superficial digital flexor tendon and the suspensory ligament. Optimal hoof balance results in the most even contact area between the joint surfaces of the coffin joint and, hence, the lowest pressure per unit area.²² Using dedicated diagnostic imaging, the specific sites of cartilage degeneration/subchondral bone injury and/or ligamentous injury may be revealed. In case of cartilage/bone injury, the compression on the affected side may be relieved by promoting sinking of that side of the shoe in a deformable surface. Anecdotally, a wider lateral branch or a lateral heel wedge combined with rolling or rocking the medial side of the toe may be beneficial as part of the treatment of bone spavin and upward fixation of the patella. In case of collateral ligament desmitis, providing a wider base of support at the affected side can decrease tension. In the treatment of laminitis, styrofoam pads increase the weight-bearing surface, decrease the overall pressure, and shift the pressure distribution palmarly.²³ Empirically, a wooden shoe with a bevelled perimeter has similar advantages while decreasing stress associated with breakover, allowing realignment of the distal phalanx and heel elevation as needed. ²⁴

5. Optimising breakover
Stress associated with breakover may be a therapeutic target for osteoarthritis, navicular syndrome, laminitis and a variety of soft tissue lesions. A wedge shoe decreases breakover duration compared with a normal, plain shoe and with an egg-bar shoe.¹⁶ Moreover, a wedge shoe causes a 24% reduction of the maximal force on the navicular bone.²⁵ The choice between a square toe, a set-back shoe, a reverse shoe, a rolled toe, a rocker-toed shoe, a natural balance shoe etc. is largely depending on personal preferences and characteristics of the individual case. Different opinions exist regarding the use of a rolled toe.²⁶ Overall, shoes aiming at promoting breakover reduce the moment arm of the vertical force on the coffin joint.²⁵ However, this occurs only at heel-off, when the total vertical force has already reduced considerably. As the maximal coffin joint moment is reached before heel-off occurs, these adaptations do not affect the maximal load on the deep digital flexor tendon^27-28 , although hoof-unrollment has been shown to be smoother, thereby lowering the stress on the distal limb during breakover, at least on a hard surface. ^29-30 Interestingly, unshod horses experience 14% lower forces on the navicular bone than horses shod with standard, flat shoes, which may have therapeutic implications. ²⁵

6. Optimising hoof mechanism
Shoeing restricts the horizontal movement of the heels³¹, which may affect shock absorption as well as the blood circulation in the hoof. However, glue-on shoes did not result in decreased expansion of the heels compared with nailed horseshoes, albeit only in the fore limbs. In contrast, contraction of the heels decreased significantly more with glue-on shoes compared with nailed horseshoes. ³² Recently, a horseshoe consisting of two independently moving halves (‘Moerman shoe’) allowed a similar amount of heel expansion as in a barefoot situation, whereas a conventional single-clipped shoe significantly restricted the hoof mechanism.³³ In some cases, (temporarily) restricting the hoof mechanism may be desirable (e.g. in the treatment of fractures of the distal phalanx), using a straight-bar shoe with additional clips beyond the third nailhole³⁴ , often combined with sole packing, or a rim shoe, or fiberglass cast applied directly around the hoof capsule. A 5° heel elevation has also been shown to significantly decrease hoof deformation. ³⁵

Conclusion

Biomechanical studies provide useful information for evidence-based application of farriery techniques. However, randomised controlled clinical trials are rarely available, and therefore, individual assessment and clinical judgment remain of fundamental importance. The effect of track composition and its maintenance on shock dampening and hoof slip³⁶ may be at least equally important in the treatment of orthopaedic pathologies, and affects the selection of horseshoe modifications (e.g. wedges vs. extensions). Any trimming or shoeing should be tailored to the individual case, and should be based on a thorough static and dynamic evaluation of hoof balance, a full diagnostic work-up of any pathology, and adapted to the specific requirements for the chosen sport discipline. Based on the biomechanical aspects of the hoofground interaction, farriery can be focused on: (1) Optimising hoof balance; (2) Shock dampening; (3) Appropriate hoof slip; (4) Optimal pressure distribution; (5) Optimising breakover; (6) Optimising the hoof mechanism. Ideally, several of these principles are already applied in the prevention of injury. The goal should always be to find the simplest solution that meets the objectives.¹⁰

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Biomechanical basics of therapeutic farriery - Oosterlinck et al. (2017)