Author: Ashley Teasley, Certified Equine Thermal Imaging Specialist
Just how important is mediolateral balance of the palmar/plantar foot in the development of pathology to the navicular bone?
Did you know that there exists a direct connection between the ungular cartilages and the navicular bone? It's called the chondrosesamoidean ligament and there are two of them.
The chondrosesamoidean ligaments are a pair of ligaments connecting the lateral and medial borders of the navicular bone to the axial aspect of the ungular cartilages and the palmar processes of the distal phalanx.
Imagine a shunted heel, or a proximally displaced ungular cartilage. That imbalance causes the chondrosesamoidean ligament to experience a proximal strain and ultimately causes a mediolateral imbalance of the navicular bone itself. Essentially the navicular bone is "tipped up" on the affected side.
This is just one example but negative plantar angles could present different ChSL tension patterns.
In June of 2020, a retrospective study was published in the Equine Veterinary Journal that discussed a condition titled Chondrosesamoidean Ligament Enthesopathy: Prevalence and findings in a population of lame horses imaged with positron emission tomography. From here forward, chondrosesamoidean ligament will be abbreviated as ChSL.
Key points from this study:
-ChSL Enthesopathy was significantly associated with foot lameness.
-The prevalence and significance of this lesion had not been previously investigated meaning this in a way was a groundbreaking discovery
-ChSL was found in a large proportion of horses with foot pain.
-ChSL enthesopathy may be an important but previously not recognized component of foot pathology in horses.
-20 feet from 14 different horses were considered affected and used for intepretation
Some other interesting observations worth noting from this study:
-With the exception of the case with a large resorptive lesion, all of the ChSL enthesopathies in this study had been overlooked on CT or MRI when previously available.
-Abnormalities of the navicular bone and increased uptake in the ipsilateral (same side) palmar process of the distal phalanx were the lesions most commonly present with ChSL enthesopathy.
-Although the pathophysiology appears somewhat specific to the distal phalanx and remains poorly understood, it seems that increased stress on the ligament attachment leads to activation of osseous resorption resulting in the formation of a fibrous tissue-lined cavity at the ligament attachment, consistent with resorptive enthesopathy.
-The previously uncommon recognition of this lesion is likely due to the difficulty to clearly identify the ChSL due to its small size and close connections with other soft tissue structures.
How can we identify mediolateral imbalance as a contributing factor based on this study? Let's take a look at Figure 2. In the bottom right image, we can see that the spacing between the medial palmar processes of the distal phalanx and the navicular bone are smaller than that on the lateral side yet the navicular bone itself appears level or parallel with the landing surface and no observations were made in regard to the health of the navicular bone. We can also observe the differences in the collateral grooves both in height and width. Is this evidence of soft tissue pathology developing prior to boney changes to the navicular?
We can observe similar differences in Figure 3. In this set of images a more contracted frog can be seen but also the navicular bone itself appears to be proximally displaced on the medial side suggesting mediolateral imbalance. Note the diagnosis of the navicular bone condition. Does this provide evidence that long term soft tissue strain leads to degenerative pathology of the navicular bone?
Does this study provide enough information on the importance of mediolateral imbalance in the development of negative pathologies within the foot? Can we monitor or assess this balance in the palmar foot affordably and non-invasively? Yes, we can with Veterinary Standard Infrared Imaging. Whilst VSIR is not capable of identifying internal anatomical underlying structures, considering the anatomical relationship to external landmarks such as the ungular cartilages and furthermore their connection to deeper internal structures like the chondrosesamoidean ligaments and considering that VSIR has a high sensitivity for physiological changes including inflammation and blood flow it is reasonable to suggest that thermal patterns observed in the palmar/plantar hoof can be indicative of the state of pathology within the internal hoof.
This VSIR image is of the palmar aspect of the distal limb of a horse diagnosed with navicular syndrome with the left fore having a more prevalent lameness. The thermal asymmetry between the medial and lateral ungular cartilages is 1.2°C whereas the thermal asymmetry between the medial and lateral ungular cartilages in the right fore is only .8°C. Does this asymmetry match the diagnosis? Does this thermal asymmetry tell us the state of pathophysiology of the internal hoof? Is it reasonable to predict this thermal pattern to be indicative of inflammation of the chondrosesamoidean ligament and further the pathology of the navicular bone?
Navicular syndrome is a common diagnosis of lameness in the horse and one that keeps veterinarians, farriers, guardians and anyone else involved with the horse constantly trying to find answers. Oftentimes, we are being reactive rather than proactive in our hoof care. Can we be proactive by being more cognizant with our approach to the soft tissues of the equine hoof? Yes we can and utilizing Veterinary Standard Infrared Imaging to assess and monitor hoof balance is the first place to start!
Norvall A, Spriet M, Espinosa P, Ariño-Estrada G, Murphy BG, Katzman SA, Galuppo LD. Chondrosesamoidean ligament enthesopathy: Prevalence and findings in a population of lame horses imaged with positron emission tomography. Equine Vet J. 2021 May;53(3):451-459. doi: 10.1111/evj.13299. Epub 2020 Jun 29. PMID: 32491220.