The word ‘Navicular’ often sends shudders down owners’ spines when mentioned. But is this reaction still warranted? Does this ‘disease’ or ‘syndrome’ still exist? Or should ‘navicular disease’ as a phrase be left in the veterinary medicine archives?
Firstly, a summary of the anatomy of the horses’ foot (Fig 1). There are 4 bones in the horse’s lower limb: P1 (long pastern), P2 (short pastern), pedal/coffin bone and then the navicular bone. The navicular bone, also known as the distal sesamoidean bone, is the small boat shaped bone which sits deep in the heel region behind the pedal bone and deep flexor tendon (DDFT), which extends down the back of the limb and attaches at the bottom of the pedal bone. As well as the DDFT, the navicular bone is also held in place by the impar ligament and the navicular suspensory ligaments. The navicular bursa is a shock-absorbing sac of fluid that shields the navicular bone from the DDFT as it slides back and forth as the limb flexes. The extensor tendon attaches to the front aspect of the pedal bone and aids in straightening the limb. And the collateral ligaments stabilize the coffin joint.
Fig 1: Anatomy of horses’ foot
Fig 2: Palmar Digital Nerve Block
Navicular Disease or Syndrome by the old definition, encapsulated any cause of lameness isolated to the foot of the horse. Horses present either unilaterally or bilaterally lame, predominantly in front. A standard approach to a lame horse involves assessment in a straight line and on the lunge on a hard and soft surface. Nerve blocks are subsequently performed with a positive response to a palmar digital nerve block, the lowest block which can be performed on the limb. This will eliminate any lameness originating from the back 2/3 of the hoof (Fig 2). Therefore, this condition was also referred to as Palmar Foot Pain. Following this, a series of radiographs would be performed. These would either reveal no bony abnormalities to explain the lameness or, would show changes to the navicular bone itself. The conclusion from either of these situations would be, ‘Your horse has Navicular Disease or Syndrome’. This condition was attempted to be managed with a period of rest and controlled rehabilitation, remedial farriery to improve any issues with foot balance or shape, sometimes along with other medications. Some horses improved, others did not. Veterinarians struggled to give owners definitive answers or a prognosis for their horse in the long term.
In recent years however, there has been rapid advancement in diagnostic techniques to image inside a horse’s hoof. This is mainly due to the development of MRI scanners. The first MRI performed on a horse was in the late 1990’s and this imaging modality has seen a steep increase in clinical use in the past 10 years. The most significant development is the change from high magnetic field imaging requiring the horse to be under general anaesthesia, to the use of low magnetic field imaging performed under standing sedation. Although the image quality and detail are superior with high field MRI’s, good quality diagnostic images can be achieved of the navicular bone, coffin joint, pastern, fetlock and associated ligaments of the lower limb with standing MRI.
PICTURE STANDING MRI
MRI imaging has massively expanded our knowledge and understanding of the anatomy and association between structures within the horses’ foot, and now enables veterinarians to give more accurate diagnosis and therefore prognosis on equine lameness.
Horses whose lameness is abolished after a palmar digital nerve block with no or minimal radiographic changes would now subsequently have an MRI performed. There are a large number of injuries or changes which can occur within the hoof capsule, including a lesion of the DDFT, inflammation of the impar ligament, degeneration of the navicular bone, inflammation within the navicular bursa and bone bruising of the pedal bone. Even within this short example list there are changes in tendon tissue, ligaments and bone all of which will modification of treatment regimens but more importantly significantly differ in timescales for rehabilitation and carry different prognosis for the horse.
Treatment in the past for these Palmar Foot Pain horses involved a combination of remedial farriery to aid breakover and improve heel support, bisphosphonates (Tildren/Equidronate) to modify bone turnover, time, rest and controlled exercise on a trial and error basis. Now that an accurate diagnosis can be achieved, treatment can be more accurately targeted to the injury and realistic time scales given to owners. Below are a few examples of lesions which can now be identified.
Deep Flexor Tendon Injury
Remedial shoeing – good heel support, +/- wedged heals (short term)
Bursoscopy dependent on location (look inside navicular bursa with arthroscope)
Minimum 6 months up to 12 months
Prognosis depends on severity and position of lesion
Coffin joint distension
(no other findings)
Improve foot balance
Medication into coffin joint (steroid, IRAP)
Distolateral fragmentation of navicular bone
Remedial shoeing – short toes, aid breakover, good heel support
Good (once stablisised if no associated pathology)
Significant degenerative pathology of navicular bone
(True Navicular Disease)
+/- medication of navicular bursa
Pedal bone oedema (arrow – should be all black)
Promote heal growth/improve foot support if flat foot conformation
Good once settled
Farriery is still a vital part of managing horses with lameness isolated to within the hoof capsule. The old saying of ‘no foot no horse’ is very true in these cases. Providing adequate heel support, ensuring a well-trimmed foot to aid breakover and addressing medio-lateral imbalances are all the mainstay of keeping the foot working properly. Additional treatments of bisphosphonates are still used on occasion, along with medication of coffin joints and/or navicular bursa as required. If all treatment options have been exhausted and the MRI does not so an active lesion which may continue to deteriorate if the horse is worked, having a neurectomy performed is a final option. This is a salvage procedure involves removing a small section of the nerves which go into the hoof to remove sensation and pain originating in the foot. There are complications associated with this option, including the risk of solar penetrations going unnoticed which could prove fatal for the horse.
In conclusion, the word ‘navicular’ should not fill owners with fear or depression in the 21st century because the term should not be used. MRI provides vets with the ability to accurately diagnose lameness isolated to the horses’ foot and target a precise treatment plan. Granted, there are some diagnoses which carry a guarded prognosis, but there are also many others which will result in the return of a sound horse after an appropriate period of rehabilitation and treatment. Navicular disease does exist, but only when the navicular bone itself is damaged or degenerative, which is only a small subset of the horses which in the past have been written off. Advances in veterinary medicine are happening every day and when we have the ability to use these advancements to give more accurate answers to owners, we need to update our terminology and leave old phases in the archives.