The effect of bilateral palmar digital nerve analgesia on the compressive force experienced by the navicular bone in horses with navicular disease

Horses with navicular disease have an increased load on the navicular bone in early stance. This has been suggested to be a response to pain in the he el region. Seven horses with clinical, radiographic and scintigraphic signs of navicular disease underwent forceplate and kinematic analysis before an d after desensitisation of the heel region with a bilateral palmar digital nerve block. The compressive force exerted on the navicular bone during sta nce, and stride kinematics, were determined in each state. After regional a nalgesia of the palmar digital nerves (PDNB) the compressive force on the n avicular bone was lower throughout stance. The mean +/- s.d. peak force at the beginning of stance was 7.05 +/- 1.10 N/kg before, and 6.46 +/- 1.15 N/ kg after PDNB (P = 0.01) and at the end of stance the mean peak values were 5.00 +/- 2.05 N/kg before, and 4.39 +/- 1.65 N/kg after PDNB (P = 0.05), W e explained this finding as indicating that the horse responds to heel pain (including pain in the navicular region) by contracting the deep digital f lexor muscle to unload the heels. This increases the compressive load on th e navicular bone, which may cause remodelling and, in some horses, damage t o the overlying flexor cartilage, which is then painful and identified as n avicular disease. This mechanism identifies navicular disease as a possible end point for a variety of heel related conditions.