The effect of orthopaedic shoeing on the force exerted by the deep digital flexor tendon on the navicular bone in horses.

This study quantifies both the intended effect of orthopaedic shoeing to decrease the load on the navicular bone and the eventual undesired effects on gait performance. The compressive force exerted by the deep digital flexor tendon on the navicular bone and on the quality of the trot and redistribution of forces over the flexor tendons and the suspensory ligament were studied as a function of orthopaedic shoeing in 12 sound Dutch Warmblood horses. A modified CODA-3 gait analysis system and a force plate were used to quantify objectively the load on the lower limb. The quality of the trot was assessed using the same gait analysis system while the horses were trotting on the treadmill. The effects of shoes with heel wedges and egg-bar shoes were compared to flat shoes and unshod feet. When heel wedges were applied, the maximal force on the navicular bone was reduced by 24% (P<0.05) in comparison with flat shoes. Egg-bar shoes did not reduce the force on the navicular bone, but in unshod feet this force appeared to be 14% lower (P<0.05) compared to flat shoes. Egg-bar shoes cause the horse's trot to be slightly less animated (P<0.05), compared to flat shoes and shoes with heel wedges. It is concluded that shoes with heel wedges reduce the force on the navicular bone as a result of a decreased moment of force at the distal interphalangeal joint in combination with a decreased angle between the deep digital flexor tendon distally and proximally of the navicular bone. Therefore it can be expected that in horses suffering from navicular disease, heel wedges will have the expected beneficial effect on the pressure on the navicular bone, while the effect of egg-bar shoes remains doubtful.