A recent study suggests that the angle of the DDFT as it passes over the navicular bone might have a lot to do with it. Justin McCormick, MS, a senior veterinary student and assistant at the University of California, Davis, completed a retrospective study with colleagues comparing the DDFT angles of horses with confirmed DDFT lesions in the navicular area to those of sound horses. He presented the results at the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif.

McCormick noted that DDFT lesions in the navicular area can cause heel pain with variable lameness that resolves with a palmar digital nerve block. For this study, authors examined 20 horses with this presentation, good-quality radiographs  that offered no explanation for the lameness, and moderate DDFT lesions just proximal to (above) and at the level of the navicular bone (detected with computed tomography). They were compared with 20 sound horses of similar age and breed that were presented for purchase examinations during the same time.

Comments by Dr. A.P.Toledo :

1 - First of all, we have to discuss this problem understanding what the physicist calls a second-class lever. The Fig. 1 shows a comparison between a wheelbarrow and the digit of the horse as second-class lever.

The front leg is an intricate series of levers and pulleys. The lever with which we are most concerned is that created by the cannon bone and the digital bones during the third phase of movement. This lever develops its working form at the instant that the center of gravity passes over the foot and the efforts of break-over are about to begin. The lever is formed by the digital bones and the hoof. The fulcrum is the ground at the toe. The weight to be moved is between the fulcrum and the point where the lifting power is applied.  

Fig. 1 - Comparison between a wheelbarrow and the digit of the horse as second-class lever

In the horse the power (P) is applied primarily by the deep digital flexor tendon (DDFT). The weight (W) is transmitted down the cannon bone onto the fetlock.

If the muscles must apply more pressure at the tendons, their pulleys at sesamoide and navicular bones suffer great compression.

2 - The above mechanical description is closely dependent of the DDFT insertion angle and, consequently, depends on the hoof angle and hoof trimming. The smaller the hoof angle (long toe and low heel), the smaller the DDFT insertion angle and more effort and time required to the leg to reach break-over. 

The Fig. 2 shows the result of a research conducted by Dr. A.P.Toledo studying the inter-relation between a horse DDFT effort increase (DP-kg) and the hoof angle variation (°). 

3 - The angle of the DDFT over the navicular bone depends on the hoof angle and is related with the biomechanical implications which cause the lesions of navicular area. The same power over one smaller area causes a greater pressure, which is the primary cause of the lesion.

Fig. 2 - Hoof angle deviation versus DDFT effort ( Dp kg) - Ref. of a horse with 60 degrees of scapula angle and body weight 500 kg,

Conclusions: (1) one degree of deviation means 10,5 kg or 3% of the weight bearing supported by the forelegs(350kg).(2) a deviation of about 10 degrees means 25% or more than  87 kg of DDFT effort increase. (3) DDFT lesions of navicular area are strongly dependent of hoof angle and hoof trimming.