Navicular-A Retrospective Study
Updated: Feb 7
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).
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).
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).
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
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.