Navicular degeneration Flashcards
What is the navicular bone?
- the distal sesamoidean bone
What is navicular degeneration?
▪Focal loss of the medullary architecture with medullary sclerosis
▪Fibrocartilagenous change of the flexor surface of the bone
▪Traumatic fibrillation of the deep digital flexor tendon which may lead to adhesion formation between the tendon and bone
▪Enthesiophyte formation on the proximal and distal borders of the bone
Aetiopathogenesis
▪Unknown but most likely biomechanical
▪Possibly some vascular component
Risk factors
▪Genetic component
– ↑ WBs, TBs, Quarterhorses
–↓Arabians, Friesians
▪Not seen in donkeys or ponies
– Possibly due to differences in conformation and biomechanics, as well as reduced forces
▪Typically older horses >10yo
▪Conformation very important
Clinical signs of navicular degeneration
▪Insidious onset bilateral forelimb lameness
▪Initially intermittent
▪Change in performance
▪Sound in straight line
– But maybe with a reduced stride length as the condition is bilateral affecting both limbs
▪Lame on lunge
– On the inside leg
– But isn’t always apparent until nerve blocks are done, at which point the contralateral limb lameness becomes apparent
▪Rarely positive to hoof testers
– But if present: when applying pressure between the frogs and the hoof wall or across the heel bulbs
Diagnosis of navicular degeneration
Positive response to PDNB
▪Typically complete resolution
▪Lameness switches to other leg
– Due to the pain of the contralateral limb
– Higher grade of lameness visible on the lung
▪Not definitive to navicular degeneration
Anaesthesia of navicular bursa more specific
▪Not frequently performed in primary practice
▪↑ complexity
▪Requires radiographic guidance +
contrast
Sometimes a abaxial sesamoid nerve block is needed
▪ If there’s radiation of pain proximally perhaps due to pathology within the DDFT
Radiographic imaging
3 orthogonal views of the navicular bone should be taken:
- Lateromedial
– Changes evident on LM view are most commonly associated with an enthesiophyte
- Flexor/skyline (Pa45Pr-PaDi obl)
- 85 degrees upright navicular (DPr-PaDi oblique)
– Will show presence of synovial invaginations in the distal border
-> Assessment of these is subjective as they’re apparent in all horses, but the size, shape and position is important
-> Larger invaginations are likely to be indicative of degeneration and also the presence of excessive number, esp when maximally positioned towards the sloping borders at the edge of bone
– Also helps identify cysts within the bone itself
MRI - pros & cons
Pros
▪Diagnosis of multiple pathologies
▪Early diagnosis of bone pathology
Cons
▪ Cost
Conservative management options
▪ Analgesics
– Most commonly bute
– Paracetamol if concerned re long term Bute use
▪Corrective farriery
– Wedges
– Rolled toe
– Bar shoes
▪Corticosteroids (triamcinolone, methylprednisolone)
– Show improvement when inject into the DIP joint (although logically have greatest effect when injected directly into the bursa)
▪Bisphoshonates (tiludronate, clodronate)
– Prevents bone resorption by osteoclasts, but evidence of their effect is limited
▪(Vasodilators e.g. isoxuprine)
– Historically used. Now rarely used.
▪Most horses eventually reach non-
responsiveness to treatment
Surgical treatment
▪Palmar digital neurectomy
–Provides pain relief but:
-> Nerve sensation will eventually restore
-> Complications include neuroma and catastrophic DDFT breakdown, pedal osteitis/foot penetrations (due to lack of sensation, and dz may progress to the point of being untreatable)
– Would result in complete soundness in horses where lameness is abolished by digital nerve blocks
▪Other surgical procedures unproven
Why is a broken back hoof pastern axis so bad?
- I increases the pressure on the distal sesamoid bone by the deep digital flexor tendon
- It also causes increased extension of the distal interphalangeal joint, this combined with the collapsed heels will increase the angle through which the DDFT turns
- Therefore, this increases the forces applied to the individual bone
Why is corrective farriery with wedges useful?
- they reduce the extension of the DIP joint esp for those with broken back feet
Why is corrective farriery with rolled toes useful?
- Ease breakover, allowing the foot to lift of the ground more quickly at the end of the stance phase and reduce leverage of the dorsal toe
Why is corrective farriery with bar shoes useful?
- Straight bars, heart bars, egg bars
- Often shoe of choice
- Whilst positioning the shoe over the navicular bone is thought to increase the pressure on the bone, this doesn’t occur
- They stabilise the foot and help further collapse of the heel
- Good on softer ground as increase the area over which the foot is supported
