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As attentive equestrians, it is our duty to make sure that we don’t overlook any intrinsic excuses for intermittent lameness.

VL 12a

VL 12a

12.1 Intrinsic Causes of Tripping in the Horse

Instigator: Premature breakover of the limb—the horse picks the limb up before the amount of leverage required to break over naturally is realized.Possible Underlying Circumstance(s)Comments Excessive toe length of the foot. The horse’s toe(s) may be so long that he/she is unable to acquire enough mechanical leverage to break the foot over naturally. In this scenario, the horse simply “runs out of leg” and has to pick the foot up before the natural breakover process can be initiated. A person can relate to this dilemma by attempting to walk or run while wearing swim fins on his/her feet. The amount of leverage afforded by the fins is too much for the length of the human leg to accommodate. Hence the person is forced to pick his/her feet up prematurely, thereby increasing the risk for tripping. Excessive tension of the deep digital flexor tendon (DDFT). The process of foot breakover is initiated by tension along 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 (ssss1, ssss1). Excessive tension and/or pain associated with one or both of these structures can prompt the horse to pick the foot up prematurely, thereby increasing the risk for tripping. Excessive tension of the distal accessory (or check) ligament (DAL). Pain associated with the deep digital flexor tendon (DDFT). Pain associated with the distal accessory (or check) ligament (DAL). Pain associated with structures that the DDFT influences, such as the navicular and coffin bones. The events that precipitate breakover ultimately involve anatomic structures that reside within the foot. The horse might choose to pick up the foot prematurely in an attempt to avoid pain if it’s generated when increased tension and/or pressure is applied to one or more of these structures. Instigator: Decreased proprioception—a lack awareness pertaining to the spatial orientation and/or movement of one or more limbs.Possible Underlying Circumstance(s)Comments Decreased sensory nerve input that occurs in some cases of neurologic disease. As you might imagine, a horse that cannot accurately perceive the location or movement of one or more limbs is destined to trip on occasion. Instigator: Unwillingness to achieve maximum stride height—the horse voluntarily avoids normal flexion of the limb.Possible Underlying Circumstance(s)Comments Joint pain, especially when associated with the carpus, elbow, shoulder, stifle, and/or fetlocks. Some joints undergo considerable flexion during protraction of the limb. Since joint pain is usually exacerbated upon increased flexion, most horses will resist bending the respective limb(s) as much as possible when one or more of these joints is affected. This, in turn, makes them much less likely to clear the ground surface and more likely to trip. Muscle pain, especially when associated with the brachiocephalicus, biceps brachii, quadriceps femoris, and extensor musculature. Certain muscles function to raise and/or protract the horse’s limb, either through direct or indirect means. Most animals will choose to disengage these muscles when they’re sore, thus inhibiting their ability to clear the ground surface and/or complete a full stride with the affected limb(s). Instigator: Inability to achieve maximum stride height—the horse is not able to achieve normal flexion of the limb.Possible Underlying Circumstance(s)Comments Muscle fibrosis (scarring), especially when associated with the brachioce-phalicus, biceps brachii, quadriceps femoris, and extensor musculature. When scar tissue replaces normal muscle fibers, some functionality of the structure is relinquished. Horses will be more likely to trip if certain muscles cannot be adequately employed to raise or protract the horse’s limb. Inadequate muscle strength, especially when associated with the brachiocephalicus, biceps brachii, quadriceps femoris, and extensor musculature. Horses will sometimes encounter a loss of muscle strength and function when they are debilitated due to metabolic issues such as chronic infection, pneumonia, myopathy, and malnutrition. These horses may be forced to exert increased effort in order to avoid stumbling. Biomechanical interference involving muscles, ligaments, and/or tendons. Physical interference associated with structures that comprise the horse’s stay mechanism(s) can sometimes precipitate stumbling. This is more prevalent in the hind limb (see ssss1, ssss1). Referred gait abnormalities. An obligation to maintain diagonal synchrony and/or accommodate a dramatic shift in weight can cause the horse’s secondary limb to trip on occasion.

The act of faltering usually occurs due to an event that transpires at the time of foot breakover (fig. 12.2) and/or during limb flight. Horses that experience excessive tension or pain during maximum limb extension (just before breakover) and/or pain-mediated or biomechanical restriction during flexion (limb protraction) will be predisposed to stumbling. Both of these issues are accentuated by circumstances that delay breakover of the foot and/or increase the length of the stride (an excessively long toe, for example).