Questions surrounding lameness test and why “bipartite” navicular bones shouldn’t be ignored *H&H VIP*

  • Peter Green MRCVS presents the latest scientific research — from lameness on the lunge to new findings about navicular bone deformities

    Is he really lame?

    Lungeing on a 10m or 20m circle is a regular part of a routine lameness work-up and of a pre-purchase examination for soundness.

    Many subtle lamenesses become more obvious at trot on a circle. Horses that appear to move perfectly in a straight line may become notably lame when lunged, especially if the surface is firm and the circle small.

    Vets in Sweden and the USA screened more than 200 riding horses in regular work that the riders believed to be sound. They measured the up and down movement of the head and pelvis using extremely sensitive accelerometers fixed to each horse’s poll and croup.

    From this group they chose 94 horses that had symmetrical head and pelvic movement when each of the pairs of limbs was bearing weight at trot. In other words, there was no nodding of the head or dipping of the pelvis — critical elements in the detection of lameness. These horses were proven by very accurate technology to be completely sound.

    The vets then repeated the measurements when the horses were lunged. More than 90% of them showed some degree of asymmetric movement: their head or pelvic movement — up or down — differed depending on which leg was bearing weight.

    These subtle differences were often not perceptible to the naked eye, but were consistent. The most common difference between the results on a straight line and on the lunge was a head nod that would be interpreted as an outside forelimb lameness on the lunge.

    These results are important for several reasons. First, the scientists found that more than half the horses that no one thought were lame were in fact nodding or dipping their pelvis very slightly. This raises the question of whether a slight nod or dip is normal in many sound horses, or whether more than 50% of “sound” horses are actually lame.

    It also questions the diagnosis of very subtle lameness that only shows on the lunge, especially if ultra-sensitive equipment is used.

    Maybe lungeing is just not a natural exercise for many horses?

    Navicular bones — new evidence

    The navicular bone is a small, thin, boat-shaped bone about the size of your little finger. It sits horizontally at the back of the foot, inside the heels, where it is squeezed between the deep flexor tendon and the back of the coffin joint.

    An occasional finding in sound horses is a navicular bone that consists of two halves, with a fibrous joint down the middle. Such “bipartite” navicular bones are congenital, arising because the two halves of bone ossify separately and never join together during foetal development and foalhood. Vets have to be careful to distinguish these from fractures.

    Until now, congenital bipartite or divided navicular bones have been regarded as an incidental finding in sound horses. When they are seen on pre-purchase X-rays they are noted, but owners are told not to worry about them because they’ve been there from birth and are not a problem.

    However, vets in Utrecht and Ghent have just published a report that may alter our thinking.

    They describe a series of horses with congenitally divided navicular bones that became diseased and caused chronic lameness. It seems that a bone consisting of two parts may flex when the tendon bears weight, with the fibrous joint acting like a hinge.

    The action of this repeated bending and straightening eventually leads to bone degeneration either side of the hinge and to incurable lameness.

    Not all horses with congenital division of the bone will eventually go lame, but in young horses it is impossible to predict which ones will stay sound. Vets will now have to exercise caution when dismissing this birth defect as an incidental and benign finding.

    Ref: Horse & Hound; 26 May 2016