• theequinedocumentalist

The Beginning of the N - Navicular

Updated: Jan 27, 2020

Navicular syndrome (NS) is all too common a pathology, one paper expressed it as appertaining to a third of all forelimb lameness’ and yet many papers express the mechanical theory of its development, which could theoretically be addressed by farriery at the first instance, meaning in many cases the onset of pathology being stunted or stopped. A recent retrospective study by the author briefly outlines the definition, diagnosis and treatments of the pathology.

https://www.theequinedocumentalist.com/post/navicular-a-retrospective-study

This article expands on the mechanical farriery considerations associated with the type of lower digit conformation found in the study and discusses both preventative and treatment measures.

Waguespack and Hanson (2010) outlined the biomechanical considerations and stated that the primary source of pressure on the navicular bone (NB) is compression from the deep digital flexor tendon (DDFT), Ruff et al (2016) expanded on this, expressing the increased compressive force on the NB from the DDFT in conformations exhibiting increased dorsiflexion, this was echoed by Uhl et al (2018) which stated conformations described by Ruff et al (2016) as being mechanically predisposed to navicular and that DDFT lesions corresponded with areas of increased load. The conformation these papers are referring to is a broken back hoof pastern axis (BBHPA), which in more severe cases leads to negative palmer angles (NPA).



Fig.1 Radiographs of 2 fore hooves from the same horse. The bottom radiograph exhibits a BBHPA bordering on NPA, although both feet undergo the same regime in work-load and farriery the increased dorsiflexion in the foot with a BBHPA has created increased load on the navicular apparatus and lead to the onset of pathology, this expresses the findings of the papers stated, showing that conformation was the contributary factor in the development of navicular. This finding correlated with the aforementioned study by the author which found the majority of randomly selected navicular cases as presenting with a BBHPA, with DDFT pathology as the defining factor in the presentation of unilateral lameness in bilateral navicular changes.


Negative palmer angles were outlined by Floyd (2010), the main defining point being that the solar surface of the distal phalanx (Pedal bone (P3)) has a negative angle in relation to the ground surface. Floyd (2010) expressed that there were different grades of NPA, lower grades can be addressed with trimming and routine shoeing while higher grades required mechanical intervention. The author has found that BBHPA’s exhibit the same difference in severities although not presenting with a negative angle, some can be addressed with trimming and shoe placement while others require more involved farriery. In both conformations it’s the difference in angle of orientation between the phalanges that creates the increased load on the DDFT, not the absolute angle of the pedal bone as this is individual, although with regards NPA this alignment would be impossible. The DDFT is stretched as it physically has a greater distance to travel, every degree that P3 is out of line with the middle and proximal phalanx means increased load on the DDFT and NB.



Fig.2 A completely straight HPA is rare but as the phalangeal alignment becomes more broken there is increased predisposition to navicular. The internal phalangeal alignment very often correlates to the external pastern and hoof angle. Often a mistake that is made when reading radiographs is that if the absolute angle of the pedal bone is within a normal range, the predisposition of a broken back phalangeal alignment can be overlooked because there is a lack of understanding that it is the relationship between the bones and not their individual orientation that creates the environment for pathology.

Caldwell (1987) discussed the predisposition of a long toe low heel conformation, calling it pre-navicular syndrome, outlining the abnormal and increased stresses on the caudal structures of the hoof. The study stated the importance of farriery intervention in restoring balance and that neglecting to do so perpetuates the negative morphology. A recent article (Ramsey 2019) summarised the findings of Savoldi (2019) and expressed that the cyclic loading of hoof structures, causing a build up of micro damage and leading to eventual collapse due to the demands of humans on the horse are a contributing factor to the onset of navicular. This study offered an equation, Pressure + Time = Pathology. This theory is backed up by Peele et al (2010) which showed a reduction of hoof angle as a result of gallop training, showing the cyclic overload leading to structural collapse. These “collapses” that often equate to a BBHPA, create a pre-navicular syndrome which left unchecked predisposes to the onset of the big N. frog and digital cushion function have been shown to be important in the morphology of the hoof, they play a vital role in force dissipation and support of the internal structures of the foot as well as playing a vital role in blood circulation and haemodynamic mechanisms. When these structures are loaded (Correctly) and complimented by weight sharing with all the solar structures, there is some mitigation of this collapse. There have been few studies to quantify the effects of padding on the hoof but Casserly (2018) showed an improvement in hoof morphology and HPA with frog support padding and studies at the Epona Institute correlated with Price (2019) in finding stabilisation of the internal structures possibly helping to reduce micro damage as expressed by Savoldi. The authors experiences correlate with this theory, in weaker feet the application of padding to load share the solar structures helps to negate collapse. Further reading

https://www.theequinedocumentalist.com/post/frog-pressure-is-the-heart-bar-obsolete

Horses presenting with a BBHPA and or NPA by any genesis will either have navicular, or will be strongly predisposed to it, this has implications for farriery practice, this conformation can not be ignored, a pro-active approach should be taken over and above a reactive approach post injury.

To understand how farriery can be pro-active we have to understand the forces in play and how these can be optimised.


Fig. 3 A typical presentation of a BBHPA predisposed to navicular. In the top left corner a more ideal HPA is presented. The BBHPA has certain features that directly affect biomechanics and pressure on the NB. A long lever arm from the centre of rotation (COR) (which is the centre of the distal condyle of the middle phalanx (P2)) to the toe means increased stress on the DDFT, it has to overcome this lever arm to initiate breakover and this is over and above the fact that it already has increased load statically due to being “stretched” over the fulcrum of the NB. These increased loads mean the DDFT is closer to its elastic limit and could possibly suffer elastic creep as even without movement it is in a stretched state.

Studies have shown that an increased angle of P3 reduces the load on the DDFT, Moleman et al (2006) showed that changes in the angle of the hoof (due to hoof growth) directly affected the moment arm around the distal interphalangeal joint (DIPJ) and Willeman et al (1999) showed a 24% reduction in pressure on the NB in wedges. These studies address the issue of phalangeal alignment, something farriery can directly affect. Schoonover et al (2005) found that heel elevation alone decreased lameness in horses presenting with NS, Viitanen et al (2003) expressed that there was an increase in DIPJ pressure with a 5 degree elevation although stated that ideal conformation created reduced pressure, perhaps pointing at elevation up to, and not beyond, a straight phalangeal alignment as being beneficial.