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TRIPPING in the Forelimbs

During ambulation, the horse's hoof works like a lever with the toe acting as the pivot point. The term BREAKOVER refers to the action of the hoof as it pivots over the toe to lift and move the limb forward.

Breakover

 

BREAKOVER

Technically speaking, breakover is the phase of the stride between the moment the horse's heel lifts off the ground and the moment the toe is lifted. During this phase, the toe acts as a pivot point (fulcrum) around which the heel rotates. HEEL-OFF is the moment that the heel leaves the ground and TOE-OFF is the instant at which the toe leaves the ground.

The process of breakover is initiated by tension in the deep digital flexor muscle and the distal accessory ligament (DAL or inferior "check" ligament), both of which act through the deep digital flexor tendon (DDFT) and suspensory ligaments of the navicular bone.

Sequence of Events During Breakover

  • Deep digital flexor muscle tightens.

  • Deep digital flexor tendon tightens.

  • Pressure is applied to the navicular bursa.

  • Hydrolic pressure is applied to the navicular bone through the navicular bursa.

  • Distal (impar) and proximal suspensory ligaments of navicular bone tighten.

  • Tension increases at the insertion of the DDFT.

  • The coffin bone rotates in response to these forces, thereby initiating the breakover process.

Breakover Anatomy

 

The timing of breakover (i.e. the point at which hoof tips forward to begin the cranial or flight phase of the stride) is dictated by DDFT/navicular ligament tension combined with the amount of force required to overcome the leverage that is intrinsic to the horse's foot and limb. Differences in limb length/ conformation, hoof wall length/ angle, hoof-pastern limb axis, footing type/consistency and trimming/ shoeing strategy(ies) can all affect the timing of breakover.

On a hard surface, the hoof remains flat on the ground until heel-off (the heel leaves the ground). On a softer surface, the toe rotates into the footing prior to heel-off, thereby attenuating tension of the DDFT and navicular ligaments. This, in turn, alleviates pressure along the navicular region. It is for this reason that softer/ deeper footing usually benefits horses with navicular inflammation. Concurrent application of bar shoes can prevent sinking of the heel(s) into the footing (sometime referred to as the "snow-shoe" effect), further alleviating navicular discomfort.

It should be noted that simply decreasing external leverage associated with the foot does not always speed the process of breakover. For example, wedged pads are commonly used to "quicken" breakover, especially in the thoracic (fore) limbs. Although application of a wedged pad increases hoof angle and decreases external hoof leverage, it also decreases tension along the deep digital flexor tendon and distal accessory ligament (structures which initiate the breakover process). The increase in hoof angle may decrease the amount of DDFT/DAL tension required to initiate breakover, but the actual timing of breakover might be delayed in this scenario.

In most cases, however, we can presume that breakover is delayed as a result of long-toe and low-hoof angles. The long toe acts as a long lever arm, requiring more time and effort to rotate the heel around the toe. Shortening of the lever arm facilitates breakover and eases challenge to the structures which initiate it.

Movement and stress on the limb's internal structures that occur just before and during external breakover are directly affected by forces applied to the outside of the hoof. These "internal breakover" structures must be managed properly in order to maximize a horse's soundness and performance.

 

Breakover Process

 

Why do Horses Trip?

Horses will usually trip on the front end (in the thoracic limbs) for one basic reason:
They pick the limb up before they acquire the amount of leverage required to breakover naturally.

This usually occurs in one of three situations:

1) The toe is so long that the horse is biomechanically unable to acquire enough mechanical leverage to breakover naturally. In this scenario, the horse simply "runs out of leg" and has to pick the limb up before the limb initiates natural breakover of the foot. Humans can relate to this dilemma while attempting to walk or run with swim fins on their feet. The amount of leverage afforded by the fins is too much to for the length of our leg to accommodate, so we are forced to pick our feet up prematurely, thereby increasing our risk for tripping.

2) There is pathology in the limb that encourages the horse to pick the limb up prematurely either through pain-mediated or biomechanical means. Pain directly associated with DDFT tension and/or indirectly associated with the navicular apparatus is the most common form of pathology causing horses to trip up front. The pain perceived as natural breakover is approached may overwhelm the animal and prompt premature lifting of the limb. It is for this reason that many horses with navicular inflammation will walk/ trot with their front feet out in front of them and avoid the caudal (posterior) phase of the stride. This is also why horses with navicular inflammation frequently trip.

3) There is pathology in the contralateral hind limb that prompts the horse to pick up the limb prematurely either through pain-mediated or biomechanical means. In an attempt to maintain diagonal synchrony, the front limb is also lifted early, precipitating the trip. Click HERE to see a detailed description of diagonal synchrony.

 

Manipulating Breakover to Improve Performance and Alleviate Tripping

The most common manipulation of breakover is moving the breakover point back from the toe in an attempt to reduce the force required for the foot to move over the point of the toe, particularly in horses with heel pain or flexor tendon problems.

According to Doug Butler, PhD, FWCF, CJF, there are 3 primary reasons alter a horse's breakover:

  • To change action (also called motion or animation) for a show horse (such as a Saddlebred)

  • To change the timing and/or pattern of the gait to influence gait deficits

  • To alleviate pathologic stress by reducing the leverage required to break over

 

Common techniques used to facilitate breakover of the limb include:

  • Corrective trimming to increase hoof angle and/or decrease toe length

  • Application of bar shoes

  • Application of wedged pads

  • Setting, squaring, rolling and/or rockering of the toe.

 

 

If you have any questions about Tripping in the Forelimbs please call our office at (678) 867-2577. We look forward to serving you!
 
THE ATLANTA EQUINE CLINIC: 1665 Ward Road, Hoschton, Georgia 30548 - ph. 678-867-2577

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