Introduction
The navicular apparatus is designed to dissipate force between the Deep Digital Flexor Tendon (DDFT) and the third phalanx and as such is under a huge amount of pressure, commonly, chronic overloading of these apparatus caused by foot imbalances predispose to injury (Ruff et al 2016).
The term “Navicular syndrome/Disease” has changed its meaning with the introduction of more specific diagnostic techniques. Horses that block out to palmer digital nerve blocks (PDNB) are now described as having “palmer foot pain” (PFP) until further investigation can more accurately diagnose specific structures affected (Smith 2015).
The complexed structures of the “podotrochlea apparatus” (fig.1) share a close working relationship and often multiple structures will be affected in navicular pathologies (Smith 2015).
Although horses can appear lame unilaterally, commonly horses will show changes of the navicular region bilaterally, certain conformations predispose to navicular pathology, low heels increase the load on the DDFT, and the surrounding structures and high heels can affect the vascular mechanisms of the hoof contributing to degenerative changes (Mieszkowska et al 2016).
Mieszkowska et al (2016) discussed the findings of 20 horses with palmer foot pain, it discovered that 75% of the horses had changes associated with the area of the DDFT, 45% had changes associated with the collateral sesamoiden ligaments and 25% had an enlarged navicular bursa (bursitis), also discussed in detail were the histopathological findings.
Uhl et al (2018) discussed the mechanical predispositions of navicular, stating that areas of increased DDFT load directly correlated to areas of DDFT core lesions, confirming the theory that conformations which increase dorsiflexion predispose to navicular pathology.
When assessing hoof pastern axis (HPA) radiographically the phalangeal alignment should be parallel with the dorsal surface of the distal phalanx (Logie 2017). The angle can differ greatly, less than 47 degrees for a sloping HPA, between 48 and 55 degrees for an ideal HPA or over 56 degrees for an upright HPA, but the phalangeal alignment will still be parallel, when this angle changes a BBHPA or broken forward HPA is created, in the case of a BBHPA this often changes the stance of the animal as it tries to alleviate the pressure on the navicular by standing out in front, this can then cause crushing of the heels and creates a perpetual cycle (Logie 2017).
Diagnosis
Diagnostic analgesia is the common place technique for assessing equine lameness, systematic nerve blocking helps to isolate regions of pain, navicular syndrome or PFP is diagnosed by a positive response to a PDNB, further diagnostics modalities are then utilised to acquire a more definitive diagnosis (Dyson 2013).
The introduction of Magnetic resonance imaging (MRI) has enabled a far more detailed diagnosis of PFP, it enables assessment of both osseous and intricate soft tissue structures of the region (Parkes et al 2015). Before MRI, the broad diagnosis of navicular disease was reached through positive response to PDNB and assessment of radiographic changes to the navicular bone itself, scintigraphy could also indicate the navicular region but non-specifically (Stewart 2017).
Diagnostic analgesia can isolate to the palmer digit, radiography can be used to asses osseous changes and ultrasonographic findings can assess certain soft tissue structures, but these techniques are non-specific and are generally now prerequisites to a definitive MRI diagnosis (Parkes et al 2015).
It was with the ability of more accurate diagnosis that navicular disease became navicular syndrome, although MRI has enabled this comprehensive level of diagnostics, due to the huge range of pathologies detectable it has become subjective as to which pathology is of most clinical significance (Stewart 2017).
Computed tomography similarly to MRI gives a clearer image of pathological changes, many osseous changes seen by these modalities are not evident radiographically and changes in the distal impar ligament, marrow, DDFT and other soft tissue structures are also not detectable. This advancement in diagnostics facilitates more accurate and earlier diagnosis which can result in better prognosis for the animal (Widmer et al 2000).
Treatment
Navicular bursoscopy is a commonly utilised surgery associated with DDFT pathology, it allows histological confirmation of diagnostics, it enables visualisation of detected lesions and allows for minimally invasive surgical procedures (Smith et al 2007). Post-surgery, regenerative medicine can be utilised in the form of stem cell injection, however this can lead to complications when administered to scar tissue (Mieszkowska et al 2016).
Treatment of desmopathy of the collateral ligaments with swelling of the navicular bone can include a full hoof cast (Mieszkowska et al 2016). Bisphosphonates such at tiludronate are often utilised to normalise osseous metabolism, injected into the navicular they can help treat osseous changes associated with navicular disease, although the results were questionable with chronically lame horses (Denoix et al 2003), this may be due to the complexity of the syndrome and damage to other non-osseous structures as bisphosphonates job is almost solely to inhibit osteoclast activation (Marcella 2015).
Injections into the region generally don’t look to cure the horse but rather render them serviceable for an amount of time, corticoids had differing effects from no effect to pain abolishment for many months, non-steroidal anti- inflammatory drugs are also effective, in allowing horses to “work through” the problem (Verschooten et al 1990, Curtis 2002).
Recent studies (Schoonover et al 2018) have bought into question the usefulness of bisphosphonates in decreasing lameness over and above remedial shoeing and anti-inflammatories which did prove effective.
Farriery treatment can have a substantial effect on the prognosis of the disease and aims to reduce the forces acting on the navicular area (Uhl et al 2018, Schoonover et al 2018), farriery interventions have changed over the years and traditional shoeing methods have come in to question, such as egg bars “supporting the heels” (Curtis 2002). Decreasing the work load of the DDFT by “bringing the breakover back” has become emphasised, a broken back hoof pastern axis (BBHPA) increases load on the structures associated with the DDFT, so correction facilitates depressurisation (Curtis 2002), however attaining this with wedges should be done cautiously as although raising the heels relieves the DDFT it moves the centre of pressure caudally potentially crushing the heels (Wilson et al 1998, Curtis 2002).
Hypothesis
Given the above information the author’s main hypothesis is that there will be a strong positive relationship between a BBHPA and navicular pathology, which will be diagnosed through MRI and histologically confirmed through surgery.
Case 1- Bellini
Age: 13
Breed: Hanoverian
Work: Unknown
Related clinical history
Bellini had previously undergone a bursoscopy in Nov 2017, followed by 4 months rehabilitation, his lameness deteriorated, he was sent for an MRI and subsequently bursoscopic surgery.
The previous MRI (2016) had shown: Right fore suprasesamoidean DDFT tearing with intra-bursal soft tissue and likely adhesions Bilateral navicular bursitis Bilateral navicular disease (more severe in right fore) Right fore possible mild / early osteoarthritis of the DIP joint Bilateral enthesopathy of the DSIL.
Diagnostic analgesia positive to PDNB.
Radiographs
MRI
Diagnoses of likely / probable clinical significance, suprasesamoidean lateral lobe DDFT tearing with intra-bursal soft tissue and likely adhesions (right fore). Marked navicular bursitis (right fore). Navicular disease (right fore). Mild / early osteoarthritis of the Distal interphalangeal joint (DIP) (right fore). Bilateral desmitis of the distal sesamoidean impar ligament (DSIL) with mild active distal enthesopathy of the DSIL (right fore).
Diagnoses of unlikely clinical significance, mild navicular remodelling (left fore). Mild dorsal fibrillation of the dorsal aspect of the infrasesamoidean and suprasesamoidean (both lobes left fore, medial lobe right fore). DDFT Mild periarticular remodelling/arthropathy of the left fore DIP joint. Mild chronic enthesopathy of the DSIL insertion (left fore).
Surgery and treatment
The bursoscopic histological findings were a 2 cm tear of the lateral lobe of the DDFT with protrusion of tendon fibres into the bursa. Surface fibrillation of the medial lobe of the DDFT with granuloma and mild surface fibrillation of the navicular cartilage.
The torn tendon areas were resected, the granuloma was resected and the navicular cartilage was lightly debrided.
Aftercare
The horse was recommended box rest with in hand grazing for 5 min, twice a day for 2 weeks, after which sutures can be removed. Following that 4 months in hand grazing twice per day, if trotting sound after this time, 2 months small paddock turn out.
The horse is to be shod in 5 degree wedges.
Case 2- Coco
Age: 10
Breed: Warmblood
Work: Show jumping
Related clinical history
Intermittent right forelimb lameness of 2 years’ duration. Previous improvement to intra-articular medication of the distal interphalangeal joint. Currently 3/10 lame left fore on the lunge on a firm surface. Diagnostic analgesia positive to PDNB.
Radiographs
MRI
Diagnoses of likely / probable clinical significance Full thickness oblique tear of the medial lobe of the infrasesamoidean DDFT (right fore) Focal, mild DDFT tendinopathy of the medial lobe within the proximal recess of the navicular bursa (right fore) Moderate navicular bursitis with intrathecal tissue/adhesion (right fore) Bilateral mild navicular disease (right fore>left fore) Bilateral active distal DSIL enthesopathy (moderate-marked right fore, mild left fore) (Possible focal articular cartilage damage of the medial aspect of the DIP joint (right fore))
Diagnoses of unlikely clinical significance Focal narrow chronic/inactive tear in the lateral lobe of the infrasesamoidean DDFT (right fore) Bilateral mild entheseous remodelling of the insertions of the collateral ligaments of the DIP joint Bilateral sclerosis of the medial palmar process of the distal phalanx Mild synovitis of the DIP joint (left fore) Bilateral mild desmopathy of the collateral sesamoidean ligaments.
Surgery and treatment
Bursoscopic histological findings were a dorsal tear of the DDFT medial lobe with a granuloma and protruding tendon fibres. The granuloma was resected and the tendon was debrided.
Aftercare
Coco was recommended 14 days of 15min in hand grazing until the sutures were removed, once sutures removed 6 weeks of small paddock turnout followed by a re-examination at 2 months to decide on increased turnout. At 5 moths post-surgery ridden work can start limited to 20min per day. It was recommended coco was shod in 5 degree heel wedges.
Case 3- Collect
Age: 10
Breed: KWPN Warmblood
Work: Show Jumping
Related clinical history
Left fore lameness of several months’ duration. Improved to navicular bursa medication. Palmar digital nerve block positive left fore. Medial palmar digital nerve block 80% positive on right rein on soft lunge Distal interphalangeal joint block positive left fore. On clinical observation the horse was 1/10 Left fore (LF) lame in trot in a straight line. Positive LF flexion test. 4/10 LF lame circled on hard surface 1/10 RF. 2/10 LF lameness circled on the soft. Diagnostic analgesia positive to PDNB.
Radiographs
MRI
Imaging findings of possible/ probable clinical significance, Mild bilateral navicular changes (more severe in right fore). Bilateral mild distal interphalangeal joint arthropathy. Broken back hoof pastern axes (worse left fore). Space-occupying lesion of the medial hoof wall (left fore). Focal, likely chronic, parasagittal split in the insertional DDFT left fore.
Imaging findings of no/unlikely clinical significance, Mild bilateral dorsal fibrillation of the infrasesamoidean DDFT. Bilateral biaxial enthesopathy of the collateral ligaments of the DIP joint. Chronic desmitis of the collateral ligaments of the DIP joint left fore.
Surgery and treatment
Bursoscopic histological findings were, mild dorsal fibrillation of the DDFT lateral lobe, with cartilage fibrillation of the navicular bone. Mild dorsal fibrillation of the DDFT medial lobe, with a small surface tear of the of the DDFT. The DDFT surface was debrided and fibrillated tissue removed and the cartilage defect was resected. The distal para-sagittal split was inaccessible by surgery.
Aftercare
The horse was recommended to be shod in a 3-degree wedge. A 3 month rehabilitation plan was put into place, which was to be followed by medication of the tendon sheath. The horses work was reduced to 20min per day ridden work, walk and trot only for the first month.
Case 4- Daisy
Age: 9
Breed: Unknown
Work: Eventing
Related clinical history
Left fore lameness for a duration of two weeks. Currently 3/10 lame. Diagnostic analgesia positive to PDNB.
Radiographs
MRI
Left fore, there is moderate dorsal border abrasion/fibrillation of both lobes of the DDFT in the proximal navicular bursa and through the navicular bone region. The lateral lobe is more affected proximally, but the medial lobe has a well-defined dorsal cleft in the distal navicular bone region. The navicular bursa is small and moderately thickened. There is a distolateral navicular bone fragment of ~0.2cm and the distal flexor cortex of this bone is mildly irregular. The collateral sesamoidean ligament is moderately thickened diffusely. Moderate distal interphalangeal joint effusion is present. The collateral ligaments of this joint are symmetrically mildly enlarged with patchy faint hyperintensity, more pronounced proximally. Motion artefact results in hyperintensity of the DDFT in the pastern region on sagittal T1 3D and transverse T2 FSE sequences.
Right fore, there is mild to moderate dorsal border irregularity in the lateral lobe DDFT in the distal navicular bone region and there is an oblique split lesion in this lobe at the insertion. There is moderate navicular bursal effusion but no synovial thickening. Mild to moderate distal interphalangeal joint effusion is present. The distal and palmar medial collateral ligament of this joint is mildly enlarged and hyperintense.
Surgery and treatment
The horse underwent navicular bursoscopy, exact structures treated are unknown.
Aftercare
Daisy was recommended Equipalazone for 3 days. Medication of both fore coffin joints was prescribed on removal of sutures. Exercise was reduced to 5 minutes of in hand walking 3-5 times a day for the first week, increasing by 5 minutes each week for 8 weeks, at which point turnout could be performed.
Case 5- Fina
Age: 7
Breed: Unknown
Work: Jumping
Related clinical history
Right forelimb lameness. 2/10 right fore lame on hard lunge. Minimal response to DIP joint medications. Positive response to PDNB although lameness switched to the left fore.
Radiographs
MRI
Imaging findings of likely/ probably clinical significance are mild bilateral navicular disease, right fore sesamoidean DDFT tendinopathy.
Imaging findings of unlikely clinical significance are mild bilateral dorsal border fibrillation of the infrasesamoidean and suprasesamoidean DDFT, mild bilateral synovitis of the DIP joints, mild bilateral DSIL desmitis, mild enthesopathy of the DSIL left fore.
Surgery and treatment
Bursoscopic histological findings were, medial dorsal fibrillation of the DDFT, with surface abrasions and an irregular appearance. Moderate fibrocartilaginous on the surface of the navicular bone. The fibrillated areas of the DDFT were debrided and the fibrocartilaginous layers lightly curetted.
Aftercare
Fina was recommended to stay in hospital for 48 hours on antibiotics and painkillers. Box rest was prescribed with in hand grazing for 2 weeks, a 5-degree wedge was recommended for shoeing after the 2 weeks followed by 6 weeks of small pen rest and then another month of small paddock turnout. Medication of the right fore digital flexor tendon sheath was recommended prior to ridden exercise.
Case 6- Macbrian
Age: 14
Breed: Warmblood
Work: Dressage
Related clinical history
The horse went acutely lame on left fore at a competition, veterinary examination confirmed he was 2/5 lame in a straight line and was recommended being shod in magic cushion. 6 weeks later Macbrian still presented 1/5 lame and was sensitive to hoof testers, he positively responded to PDNB and was recommended for MRI.
Radiographs
MRI
Diagnosis was DDFT tendonitis (parasagittal tear) (left fore). Bilateral mild navicular disease. Bilateral desmitis of the collateral ligaments of the distal interphalangeal joint (DIPJ) (more severe in left fore). Chronic synovitis of the DIPJ (most severe in left fore.
Surgery and treatment
Bursoscopic histological findings were, an acute haemorrhagic tear in the lateral body of the DDFT. The torn tendon was debrided.
Aftercare
Unknown.
Discussion
Fig 66 shows a tight correlation between the cases, all cases were positive to PDNB, 83% had a BBHPA, all had osseous changes to the navicular, all had changes in the non-lame foot, a surprisingly small percentage (60%) were diagnosed with bursitis, all had either arthritis or synovitis, 83% had pathology associated with the attachment of tendon or ligament (Enthesopathy), 83% had a DDFT tear or split, all had ligament pathology (Desmopathy), 83% had DDFT fibrillation, all had the same surgery performed (Bursoscopy), all had the same structure treated and 83% had the same shoeing prescribed post-surgery.
Although we have a very small sample selection, we can see some clear relationships from the data when compared to larger studies. 100% of the cases positively responded to PDNB, although lameness was not completely abolished in most, this shows that PDNB is a reliable diagnostic tool in isolating palmer hoof pain, this corresponds with other studies (Smith 2015, Dyson et al 2010). Dyson (2010) suggested that different nerve blocks could help to isolate further the region of pain stating that horses with osseous changes responded better than horses with DDFT pathology to intra-articular analgesia of the distal interphalangeal joint (DIPJ), in this study 100% of the horses had DDFT pathologies and navicular changes, with 83% having DDFT lesions or splits, this also corresponds with Dyson (2010) and Mieszkowska et al (2016) who found the majority of positive PDNB’s, 75% and 61% respectively, to have DDFT pathology as the most likely cause of lameness, this also highlights the importance of an MRI in navicular diagnosis as other modalities showed a poorer reliability in DDFT lesion detection in our cases and in other studies (Dyson 2010, Parkes et al 2015).
100% of the cases had multiple pathologies and changes in the non-lame foot, this corresponds well with studies that define navicular syndrome as bi-laterally dominant (Sampson et al 2009, Mieszkowska et al 2016), however 83% of cases in this study presented as uni-laterally lame, with 1 horse shifting lameness after PDNB, this can perhaps be answered by the difficulty in recognising bi-lateral lameness and owners only being aware of pathology when one foot becomes more severe (Sampson et al 2009), this study possibly shows that severe DDFT injury could be the contributing factor to our horses presenting uni-laterally lame (table 2). In 83% of the cases MRI and in 100% of the cases, histological findings and surgery attributed lameness to DDFT pathology, found in, and corresponding to, the limb signalled as lame, further research could be done to determine the significant factors in horses with bi-lateral navicular pathology presenting as uni-laterally lame.
83% of the cases had a digital conformation that exaggerated dorsiflexion, increasing tension on the DDFT, showing a mechanical pathogenesis, a BBHPA is already widely known as predisposing to navicular pathology (Mieszkowska et al 2016, Ruff et al 2016, Uhl et al 2018) and contributes to the breakdown of these apparatus cumulatively over time through chronic overloading (Ruff et al 2016, Uhl et al 2018), this questions whether, had the conformation been recognised and addressed at the earliest, the pathology could have been slowed down or even halted. In the Authors opinion this finding could point to new practice of routine HPA radiographs of sound working horses to aid in maintaining as close to ideal HPA as possible, potentially increasing the longevity of the horses working life. Of the known shoeing plan post-surgery, 83% of the horses where prescribed being shod in wedges, we know that changing the angle of the hoof with heel wedges directly effects and relieves the DIPJ (Moleman et at 2006) reducing the load on the DDFT and navicular structures, Willemen et al (1999) showed a 24% reduction in force on the navicular for horses shod in wedges. We also know that heel wedges increase load on the heels (Wilson et al 1998), although this study would suggest heel wedges could have possibly prevented the injury to 83% of these cases, more research is needed to establish whether routinely wedging a BBHPA is the best course of action in preventing Navicular pathology taking into consideration the transference of load onto other structures such as the Suspensory ligament and superficial digital flexor tendon (Riemersma et al 1996).
This study could be showing a relevance in the work and work surface of the horses with the onset of navicular pathology, 1 horse’ work is unknown however 100% of the remainder of the group were performance horses working mainly on soft surfaces, Hernlund et al (2017) discussed how surfaces effect the performance and incidence of injury of performance horses, Murray et al (2010) echoed this, it stated that there were many factors possibly contributing to the risk of injury when working on arena surfaces, low elasticity of the working surface lead to increased muscle work being one of them. Hobbs et al (2014) discussed the horse-hoof-ground interaction influences, the backwards tilt of the foot into softer surfaces can be a contributing factor in DDFT over-extension, this can be counteracted by wider heels or “rolled reversal shoes”. It is unknown what shoes the case studies had, but potentially this “heel sinkage” in soft surfaces, coupled with their already existing conformational predisposition could have exasperated the pathogenesis. The peak loading of the hoof in midstance could have acute catastrophic effects and/or cumulative degenerative effects on the podotrochlea structures if the surface does not provide resistance enough to reduce the heel sinking therefore overloading an already over-extended DDFT (Hobbs et al 2014). More study could be done to establish a relationship between the backward rotation of the hoof in soft surfaces and the pathogenesis of navicular syndrome.
It is unknown whether the horses in this study returned to full athletic work, of the known aftercare programmes, all were prescribed a progressive rehabilitation programme, studies have shown that prognosis to return to athletic work for DDFT injuries depends on the site and severity of the injury and the time between injury and diagnosis, horses that were rested and gradually brought back into work had a considerably better prognosis than horses who returned to work immediately, remedial farriery also proved to be an integral part of return to work (Bolt and Dixon 2016).
Of the cases where surgical procedure was known, the DDFT was Debrided, the rationale for this procedure is the reduction in persistent inflammation caused by the fibrillated tissue and the facilitating of scar tissue formation (Bolt and Dixon 2016). Understanding the pathogenesis of Degenerative joint disease (DJD) helps us to understand this rationale and the degenerative nature of Navicular syndrome (Fig.67).
Fig. 67 Diagram showing the cyclic degeneration of a synovial joint (Conroy 2019).
Although this is not a Navicular joint this diagram can be used to express the same degenerative process that can occur in the pathogenesis of navicular syndrome. Fibrillation of the DDFT or any other process creating foreign bodies within the synovial fluid called “joint mice” stimulate macrophages in the synovium, they remove the “joint mice” out and into the subintima where more macrophages are recruited, this process initiates inflammation (synovitis). Cytokines are released by the macrophages which creates a recruitment/inflammation cycle, the macrophages and cytokines stimulate the synoviocytes to produce proteases, these will break down cartilage creating more joint mice, change the viscosity of the synovial fluid and stimulate the osteocytes/osteoblasts to create osseous changes, all of this creates perpetuation and pain as the cytokines stimulate the nervous system (Conroy 2019). Understanding this process highlights the importance of early detection of predispositions as the structures begin to break down over time, as stated by Bolt and Dixon (2016) the amount of DDFT fibrillation was directly proportional to the amount of synovitis.
Conclusion
Although this study sample is too small to draw any conclusive evidence from, it does hold true to the wider research available. Conformation proves to predispose to foot pathologies and farriery interventions can aid in the prevention, treatment and rehabilitation of navicular injury, environmental effects can exasperate pathogenesis, but some can be addressed through considered farriery methods. Navicular syndrome is a prevalent disease in the equestrian world however in the authors opinion current farriery emphasis is post diagnosis, this study could suggest that an increase in pre-lameness diagnostics and preventative farriery measures could aid in the reduction of cases.
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