Ligaments are short fibrous connective tissues which connect bone to bone or cartilage, farriery can have huge potential influences on these structures in cause, prevention and treatment of their injury and as such understanding their anatomy and the pathogenesis of their injury is important in farriery practice.
The digit of the horse has no muscles, only ligaments and tendons and these structures are under a huge amount of strain during equine locomotion, the hoof is the first point of contact with the ground and the forces transmitted through that travel up the limb into all the musculoskeletal structures including the ligaments, farriery will have a direct effect on this energy transfer!
It is now becoming widely accepted that working on surfaces plays a significant role in the onset of many equine pathologies (Murray et al 2010, Hobbs et al 2014) and ligament damage is one of them. Conformation, work and work load also play large roles, farriery can play an integral part in each of these predispositions.
Two ligament pathologies stand out as causes of equine lameness most relevant to farriery practice, proximal suspensory desmitis and collateral ligament desmitis.
Recently the term desmitis in relation to the pathology of these structures has been changed to desmopathy. Desmitis by definition is the inflammation of a structure, however recent studies have attributed the break down of these structures in certain instances with degeneration, collagen fibre disruption, and necrosis without inflammation (GUTIERREZ-NIBEYRO et al 2005), for the purposes of this article we will use desmitis.
Proximal suspensory Desmitis
Proximal suspensory desmitis (PSD) is a common cause of lameness and drop in performance in race and sport horses. In race horses it is pre-dominantly a fore limb pathology, in sport horses primarily hind limb (Dyson 2018), and it can present both bi-laterally and uni-laterally. Often the rider will experience loss of hindlimb impulsion; unwillingness to go forward freely; stiffness; resistant behaviour; lack of power when jumping; refusing jumps uncharacteristically; difficulties in performing specific dressage movements, eg, canter pirouette, a reduced foot flight and shortened cranial phase of the stride may also be present. The lameness is often worse on the soft (Dyson 2007), and in circles, often with the effected limb on the outside (Conroy 2019), however with hind limbs this is not always perceivable (Dyson 2007). Often forced protraction or extension of the effected limb will illicit a pain response, and hind limbs will often displace their hocks laterally in locomotion to unload the structure (Conroy 2019).
Fig1. Anatomy of suspensory showing the 3 sections.
The suspensory ligament (SL) can be divided into three different sections, the proximal part, the body and the branches, it originates at the proximal extremity of the 3rd metacarpal/tarsal with 2 heads that quickly fuse, it is formed of ligamentous fibres but also contains muscle and fat. PSD is recognised as a lesion to the proximal aspect of the SL, to include avulsion fractures and the tearing of Sharpey’s fibres, often in chronic cases there can be ossification of ligamentous structures (Fig.2) and osseous changes (Fig.3)
Fig.2 Dorsoplantar radiograph of a proximal metatarsal region of an 8-year-old Warmblood dressage horse with proximal suspensory desmopathy. Lateral is to the right. There are several variably shaped radiopacities superimposed over the proximolateral aspect of the third metatarsal bone (black arrows) consistent with entheseous new bone. Dyson and Biggi 2019.
Fig.3 Osseous changes associated with PSD. Courtesy of Conroy (2019).
PSD usually presents with sudden onset of lameness that seems to improve within a few days (Brokken 2019), Acute symptoms can include localised swelling, heat and pain. Chronic cases can show no detectable external symptoms so require diagnostic analgesia to localise the region of pain (Dyson 1991), analgesia of the palmer metacarpal nerves should show lameness improvement (Dyson 2007), ultrasonography is considered the standard screening modality for the pathology, the abnormalities seen include an enlargement of the cross section, poor definition of the margins and disruption of the internal architecture (fig.5), however Dyson (2007) stated that accurate diagnosis should come from a combination of this with radiography (Fig.2-3) and scintigraphy (Fig.4) and in some cases MRI.
Fig.4 Nuclear scintigraphy showing an uptake in the proximo-plantar aspect of the third metatarsal. Courtesy of Conroy (2019)
Fig.5 The proximal suspensory ligament of the affected limb is grossly enlarged (yellow circle) and the fibre pattern is a mixed pattern with significant oedema and evidence of active inflammation! There is a black and grey swirl pattern noted in the proximal suspensory ligament (tissue inside the yellow circle) of the affected limb which is indicative of severe changes. (Porter 2019)
The causes of PSD are prolonged work on soft surfaces, dressage and racing being good examples, repeated unlevel landings and de-rotation of the foot in mid-stance are predisposing factors (Conroy 2019), extravagantly moving young dressage horses and older higher level competition horses are prone to the injury (Dyson 2007), the un-natural gaits, of particularly dressage horses (Fig.6), the sudden turning and stopping of polo ponies and sliders, hoof imbalances and poor conformation such as post legged behind (Dyson 2007) (Fig.7), and angular limb deformities (Conroy 2019) (Fig.8) strongly predispose to PSD as they all create an uneven strain on the structure and of course direct trauma can initiate the pathology. Routh et al (2019) studied the correlation between hock and metatarsophalangeal angles and PSD (Fig.9), questioning whether post legged conformation was a cause or effect of PSD, it confirmed the earlier findings of Dyson (2007) that there was a correlation between large tarsal angles and PSD, however there didn’t seem to be a relationship with metatarsophalangeal angles and the study concluded that to establish causality a long term longitudinal study would have to be performed.
Fig.6 Many of the dressage moves including the half pass put un-natural stresses on the musculoskeletal system of the horse. Pinterest.com
Fig.7 Post legged conformation predisposes to proximal suspensory desmitis. (Dyson 2007).
Fig.8 Carpal Valgus is 1 example of angular limb deformities that can cause uneven stresses on the suspensory ligament. Adelaidehillsequine.com.au
Fig.9 Rouith et al (2019) angle measurements used to study the correlation between hock and metacarpophalangeal angles and PSD.
Dyson (2007) outlined how the treatment selected depends on external factors,
“Treatment depends on time constraints, athletic expectations, rules for medication control, degree of lameness and architectural disruption of the SL, conformation, chronicity of the lesion, age of the horse, and the number of limbs affected. Published data on the efficacy of different treatment modalities is somewhat limited, and most studies have lacked control groups.”
Veterinary treatments for chronic cases include shock wave or radial pressure wave therapy, proven to help return horses to work earlier (Boening et al 2000, Crowe et al 2004), however acute pathology responds well to conservative management with as much as 90% returning to athletic work in fore limb injuries with a rest period of 3 months, hind limb injuries however carry a more guarded prognosis (Dyson 2007) and returning to work sooner than the prescribed rest period can result in re-injury. Regular ultrasonographic assessment can aid in the treatment process and return to work shouldn’t happen until the results appear stable (Conroy 2019).
Dyson (2007) discussed the use of corticosteroids, their anti-inflammatory properties can aid in swelling reduction and expressed tibial neurectomy and fasciotomy as the preferred treatment showing a 79% success rate. Bute can be used to allow the horse to work without worsening of clinical signs (Conroy 2019) assuming stability of the structure.
As uneven strain on the SL predisposes to its injury, possibly the most important factor in farriery treatment of the pathology is creating static and dynamic balance. hoof growth effects physiology and maintaining optimal balance throughout the shoeing cycle is also critical, a regular, bespoke shoeing cycle that looks to provide stability should be adhered too.
When we bring soft surfaces and movement into the shoeing equation we are faced with a set of environmental variables we need to manage.
Surfaces are being studied and have been recognised as a predisposing factor in certain common injuries (Murray et al. 2010, Hobbs et al. 2014). Parkes and Witte (2015) discussed how the horse has a limited ability to alter its own limb compliance and gait in response to the external environment, and the unnatural movements we ask of them. So farriery can play a vital role in manipulating ground reaction forces.
The interaction of the hoof with the soft ground can create increased loads on certain soft tissues (Fig.10). This can be a causation but is also an important factor to mitigate in treatment and rehabilitation.
As increased toe sinking creates increased suspensory strain (Fig.10), farriery can manipulate shoe-ground surface and reduce this (fig.11)
However, on hard surfaces the treatment focuses on reducing fetlock descent by applying increased length to the shoe. If the horse works on both surfaces then multiple modifications can be made (Fig. 12).
Collateral ligament Desmitis
Collateral ligaments provide stability to the joints helping them to move in their intended direction only. The main ligaments of the digit are located at its joints, the metacarpo-phalangeal, proximal inter-phalangeal and distal inter-phalangeal.
Fig. 13 Anatomy showing the (collateral) ligaments and tendons of the digit.
Collateral ligament desmitis (CLD) is a common cause of equine lameness, Parker (2019) stated it made up 30% of diagnosed distal interphalangeal joint pain. Schramme and Martinelli (2016) mentioned one study claimed it to be the second highest cause of lameness, after deep digital flexor tendon injuries, in non-radiographic findings. Another study found it to be the most common injury in warmbloods.
Dyson (2006) found that the medial ligaments were affected in 73% of cases, but also found a significant amount of horses showing pathology in both lateral and medial sides. Unlike PSD the injury has a poor prognosis with rest alone, however farriery, medication and shock wave therapy combined showed a 60% return to athletic work (White and Barrett 2016).
The symptoms are a reluctance to work hard, a short stride and reluctance to sharp changes in direction, in the acute phase there can be evidence of joint effusion.
Farriers can also notice uneven shoe wear, the horses reluctance to bear weight on a particular side can show with excessive wear to the opposing branch of the shoe.
Injury to these structures has similar causes to PSD, Hoof imbalance (Fig 14), working on unlevel ground/surfaces and conformation, all of which cause uneven strain. Schramme and Martinelli (2016) emphasised the effect of un-natural gaits stating “It has been determined that asymmetric foot placement, with the hoof quarters at different heights, results in rotation and sliding of the distal and middle phalanges in both the transverse and frontal planes. This type of collateral motion is encountered mainly during the stance phase of asymmetric propulsion, such as when the horse is moving in circles, and places particular stress on the collateral ligaments of the coffin joint.” This statement could also potentially apply to hoof imbalances and uneven footing.
Fig.14 Dorso-palmer radiograph showing hoof imbalance, the joint spaces on the left are compressed and the right are strained, stressing the collateral ligaments.
Turner and Sage (2002) questioned poor hoof balance as a predisposing factor in CLD, stating that compared to Navicular which showed a 90% correlation to poor hoof balance, CLD only had poor hoof balance in 45% of its cases, suggesting that the other predisposing factors mentioned above, specifically work surface, play a larger role in the pathogenesis. However in the authors opinion there are certain factors of footing that the farrier can influence, markedly the amount of sink into the surface (Fig. 12), so this finding does not lessen the farriery influence.
Diagnosis of CLD as with PSD is commonly a multi-modality process, Often there are no outward symptoms of CLD and palpation or manipulation to induce lameness are also non-effective but trotting on hard circles usually induces lameness (Dyson 2006) after location through diagnostic analgesia, ultrasound is the most common and useful diagnostic tool, as with PSD a disruption of echogenicity, showing thickening, mottling and in severe cases core lesions can be seen. Radiographs and MRI (Fig.16) are also utilised, Dyson (2006) found that MRI was the most reliable modality with 100% of cases being detected, however due to its cost ultrasound remains the commonly used tool. Radiographs can help to detect enthesophytes (Enthesophytes are calcification of ligamentous structures(Fig. 2)), and other osseous changes associated with the pathology (Parker 2019).
Fig. 16 MRI showing collateral ligament desmitis.
Treatment of CLD includes the use of NSAID’s, followed by a controlled return to work, monitored by regular ultrasound as with PSD, casting is often administered to immobilise the area and shock wave therapy has shown to be beneficial. Palmer digital neurectomy has shown to be effective in the abolishment of pain (Turner and Sage 2002).
Farriery considerations for the treatment of CLD are similar to PSD, providing stability and a balanced interaction with the ground being important. Hoof balance is, on every plane, as always, of primary importance.
Commonly a shoe with one branch wider than the other on the affected side are applied (Fig.17), however with the finding of a high amount of bi-lateral ligament damage within affected hooves, this method should be questioned on case by case basis to ensure worsening of the lesser affected side doesn’t occur. Shoes with a wide web on both branches can help to keep the hoof above ground on soft surfaces and half round/bevelled shoes can be applied on hard ground to facilitate sympathetic breakover easing tension on the effected structures.
Discussion
It is clear that how the hoof interacts with the ground plays an integral part in pathogenesis of ligamentous injury, many texts mention the farriery interventions post diagnosis in the treatment of these pathologies, however it is clear to see that pro-active farriery could go a very long way in the prevention of injury.
Some of the studies mentioned make it clear that the work asked of the horses and the surfaces that work takes place on can predispose to ligament injury.
Understanding the individual work asked of each horse should provide the farrier with information on which biomechanical factors to assess when shoeing that horse, for instance, If the horse is working mainly on soft surfaces, shoeing it in a narrow deep section may not be the best prescription.
These days, specifically in England, horses do not have a single use so shoeing them becomes more complexed as the different jobs require different biomechanical considerations, my previous article on evidence based farriery addresses the different considerations, however with the ever growing research into the effects of arena surfaces and their tendency for pathological initiation, coupled with the research mentioned previously in this article on CLD, PSD and navicular being prevalent causes of equine lameness and considering their causes, it would be logical that perhaps shoeing for surfaces be the first consideration. Keeping hooves above the ground and facilitating multi-directional sympathetic breakover, being penultimate to optimal static and dynamic hoof balance. In conclusion, farriery can play a vital role in the treatment of ligament pathology, but also with the understanding of its pathogenesis the farrier has the ability to increase the longevity of the equine working life by playing a pro-active role.
References
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