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.
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
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.
Fig.4 This marked radiograph expresses how heel elevation can help to address the lever arm from the COR, when P3 is “rotated” around the COR creating a BBHPA the length of the toe is increased as a fact of mathematics, heel elevation re-orientates P3 and simultaneously shortens the lever arm at the toe.
Heel elevation can be done in different ways, wedges are debated as a suitable method due to the increased load on the heels (Wilson et al 1998), however in the authors opinion if elevating to ideal HPA with the consequential loads mitigated, the slight risk of heel crushing is far outweighed by the reduction in risk of catastrophic and/or accumulative injury of the DDFT and NB, as well as the evident increase in comfort for those horses post in jury. A recent article by Church (2019) highlighted some points from Poss (2019) that expressed that using wedges on already weak structures can exacerbate the negative morphology, the constant loading causing the displacement of caudal hoof structures, interestingly Poss (2019) questioned whether the running forward of the heels was actually the collapsing back of the caudal hoof structures. Further reading on wedges can be read in a previous article of the authors. The research on wedges suggests that they be used cautiously, the author has had good results using frog support wedges for short periods to provide immediate phalangeal alignment and address prolapse. The fitting and type of wedges is important in their efficacy.
Heel elevation (even with frog support) however becomes null and void if every effort is not made to optimise balance around the COR with trim and shoe fit. The lever arm at the toe acts exactly as its title suggests, a lever, the longer the toe the more it will rotate P3 around its axis, so farriery intervention should always look to control the length of this lever. Toe length equates to breakover, the last point of the foot in contact with the ground at the end of the terminal phase of the stride, Duberstein et al (2013) found some benefits to bringing the breakover back and reducing the lever arm at the toe, it found a reduction in fetlock drop during the trot, possibly backing up the theory of increased support or reduced load on the Navicular apparatus, the study did elude to an optimal point of breakover past which there was reduced retraction of the limb. Van heel et al (2010) found a smoother hoof unrollment associated with the fitting of rolled toes, subsequently there was a reduced peak force on the distal limb. Oosterlink et al (2017) briefly reviewed the theories on breakover, it discussed that wedges decreased breakover duration and a shorter breakover reduced the moment arm of the vertical force on the coffin joint at heel-off but not the maximum load on the DDFT, however the author suggest the maximum load is subject to phalangeal alignment.
The author suggests that toe length, or rather breakover distance, be dictated by distance from COR to base of frog and that length to toe ideally be no greater than equal to distance from COR to heel. Extrapolating from these studies, creating balance around the COR on every axis is the most important principle in fending off the onset of NS by creating optimal load on the DDFT. The ideal HPA, is the ideal, because as Viitanen et al (2003) expressed is creates optimal loading of every structure. For more information on creating the ideal HPA follow this link.
There are other methods of heel elevation that may be preferred to, or interchangeable with wedges in addressing a BBHPA, further research is needed to quantify whether the disadvantages are negated but anecdotally there is evidence of their efficacy.
Fig.5 A graduated duo ellipse, note how it has been fitted under the toe to establish dorso-palmer balance. Anecdotally this shoe allows for elevation while channelling the load away from the heels. More research is needed to quantify this, however the author has found it to be an effective method of addressing conformations predisposing to NS.
Fig. 6-7 Show a before and after of a horse presenting with palmer foot pain, possibly undiagnosed NS, the BBHPA and its comorbids are contributing to excess loads on the DDFT, NB and all of the caudal structures of the hoof. Shoeing around the COR on every axis creates reduced forces and the Navicular apparatus is unloaded.
A product emerging in the farriery world that can address all the issues associated with a BBHPA, some of the comorbids such as thin soles and could prove to play a role in the prevention and treatment of NS is Formahoof.
Fig. 8 Shows a before and after application of Formahoof. Pre intervention the hoof has a distinct BBHPA and is highly predisposed to NS. Post intervention a more ideal conformation has been created, relieving the DDFT and NB.
Having said all the above, other factors play a role in some horses being destined to get NS, Poss (2019) eluded to it with the different internal makeup of hooves, stating poorly developed caudal structures are more likely suffer negative morphology. This comes down to micro conformation, animals with the same hoof pastern axis may not experience the same forces, some may “collapse” and some may keep their form, depending on their elastic modulus. Their microstructures may be differently configured and weak structures will inevitably fail quicker. For more reading on micro conformation.
Another factor to appreciate is fatigue life, Weller (2019) described how the tendons of horses have a certain amount of cycles that they can cope with before breaking down, it also discussed how the forces transferred through the limb grew exponentially through the different gaits, so an ex race horse that has galloped in 50 races is already far more predisposed to NS then a cob who has lived in a field wondering, possibly even if it had better hoof conformation. This coupled with the fact that gallop work reduces the angle of the hoof shows why NS is so common in thoroughbred ex race-horses.
In conclusion, although some horses may be predestined to develop NS, potentially it is unnecessarily ubiquitous, it is most often the culmination of accumulative, unfavourable, biomechanical forces that come from recognised predispositions. A BBHPA is not difficult to recognise, although it doesn’t always correspond to phalangeal alignment it is usually a good indication, radiographs can confirm suspicions and be valuable tools to the farrier in creating a shoeing plan, but in essence with the knowledge of the predisposition of a BBHPA we must take a pro-active approach to the conformation, whether this is conservatively through trimming, shoe placement and frog support or through more involved intervention, depending on severity and other individual factors, we should have the goal of an ideal HPA in mind. To some this may seem idealistic but the methods exist to achieve this balance around the COR and all the research suggests that the further away from ideal the stronger the link to pathology. Often artificially creating an ideal HPA raises a question, “What effect are you having on the anatomy?”, perhaps the question we should be asking is “what effects are we allowing if we don’t?”. We can and should reduce the frequency of pathology by recognising and addressing the beginning of the N.
Waguespack and Hanson, 2010, Navicular Syndrome in Equine Patients: Anatomy, Causes and Diagnosis, Surgical Views, https://www.vetmed.auburn.edu/wp-content/uploads/2015/01/PV1110_waguespack_Surgical.pdf
Ruff. K.C, Osborn. M.L, Uhl. E.W, 2016, Analysis of Forces Acting on the Equine Navicular Bone in Normal and Dorsiflexed Positions
Uhl. E.W, Blas-Machado. U, Kirejczyk. S.G.M, Osborn. M.L, 2018, Correlating Increased Mechanical Forces with Tissue Lesions in Equine Navicular Disease
M. Savoldi, 2019, Theory of Navicular, Eponamind Educational clinic, Paso Robles
The effect of gallop training on hoof angle in Thoroughbred racehorses