Distal Limb Lameness in the Sport Horse: A Clinical Approach to Diagnosis

Diagnosis of lameness caused by lesions of the distal limb requires a thorough clinical examination, including careful palpation, manipulative tests, hoof tester evaluation, examination of the horse in hand and under saddle, and regional anesthesia. Specific causative lesions may require technical imaging modalities to fully evaluate. Author’s address: Fairfield Equine Associates, PC, 32 Barnabas Road, Newtown, CT 06470; e-mail: [email protected]

1. Introduction

Today’s high-level sport horse represents a significant investment of time and money. High-level show jumpers, dressage horses, and Western performance horses often take several years to train to their upper levels, and injuries or illness can substantially affect the time to produce a winning horse. Owners and trainers are concerned that these horses receive the best possible care without excess expenses and down time. The veterinary care of such horses should take a more aggressive approach to lameness diagnostics of the horse in training, not simply one of attending to lameness after the horse is no longer able to train. Recognition of potentially serious distal limb lameness early on may prevent significant loss of training and competition time as well as extending the horse’s career.

2. Anatomy and Risk Factors

For the purposes of this discussion, the author will address the anatomical region from the fetlock distally. The bony structures involve the three metacarpals, (MC2, MC3, MC4 or medial splint bone, cannon bone, and lateral splint bone, respectively), the proximal medial and lateral sesamoids, the proximal phalanx (P1, long pastern bone), the middle phalanx (P2, short pastern bone), the distal phalanx (P3, coffin bone), and the distal sesamoid (navicular bone). Each of the articulations is supported by collateral ligamentous structures and fibrous joint capsules. The distal sesamoidean ligaments (as the extension of the suspensory apparatus) and the flexor tendons attach along the palmar aspect of the pastern and foot and provide support. The dorsal digital extensor attaches along the dorsal aspect, ultimately inserting on the extensor process of the distal phalanx. 

The distal limb plays a significant role in adaptation to footing and shock absorption during locomotion. The corium parietis, composed of the epidermal and dermal lamellae, function as a ligament suspending P3 within the hoof wall. There is considerable movement of P3 within the hoof wall on landing and propulsion because of this function. The digital cushion also functions in shock absoprtion.^1-3 Foot balance, both medial/lateral and dorsal/palmar, can have a profound effect on the excursion of joints and the stress on soft tissues of the distal limb.^1,4 Footing surfaces that are very hard, excessively soft, or unstable can result in aberrant motion and stress that may result in injury. The demands of high-level sports result in more concussion as well as greater extension and flexion than casual exercise. Fatigue may not be as significant a factor in injuries of the sport horse compared with racing horses, but chronic repetitive trauma is thought to be a factor. The types of injuries seen in the sport horse, racing horse, or pleasure horse may vary considerably as to intensity and potential chronicity.^5,6

3. History and Clinical Signs

Distal limb lameness may present in a peracute fashion with extreme discomfort such with a P3 fracture or foot abscess, but more often, signs may be more subtle with the horse having a subtle lameness that changes somewhat with work. “Gee doc, he feels tight in his shoulders,” is a frequent comment for more insidious forms of early front distal limb lameness. Distal hind limb issues may mimic more commonly thought-of conditions such as distal tarsitis or proximal suspensory desmopathy in that the horse is “weak” behind and may “warm out” of the lameness. Some heat or swelling may be perceived in the distal limb, but often this is not the case. The current competitive environment in North America often allows for nonsteroidal anti-inflammatory drug (NSAID) use in competing horses; horses with mild distal limb pain are able to compete comfortably. However, when Monday morning rolls around and the effects of the NSAIDS are gone, the horse is suddenly more uncomfortable than before the event. This presents a good opportunity to investigate the case.

Horses may present with a unilateral lameness of varying degrees (0–5/5) or appear bilaterally lame, especially if circled or ridden, which will be discussed later. Some lameness may be very subtle and will require special efforts to make it more apparent for an objective diagnosis. Many horses vary in their degree of lameness, improving profoundly with a few days of rest. Such mild lameness issues may be best re-examined after a day or two of exercise.

4. Physical Examination

This author is of the opinion that it is essential to have periodic veterinary inspections of the training sport horse to catch subtle lameness and performance problems before they become serious clinical issues. Although many trainers and owners are very adept at catching subtle soundness and health related issues, many things may be overlooked by the individual who sees the horse every day. Periodic veterinary inspection should include a general health check and a thorough lameness evaluation. The frequency of these exams will be somewhat dependent on the age and activity of the horse, but two to four times yearly should be a minimum. Horses with successfully managed orthopedic disease may need to be checked more often.

A typical lameness examination should first involve a thorough visual exam of the horse, taking notice of body symmetry and muscle structure. Observing the horse in its stall may give the examiner some clues regarding the horse’s level of comfort. Watching the horse step out of its stall may give big clues about chronic lameness issues. After the visual inspection, a palpation examination should be performed. The author uses a modification of an “acupuncture” examination that allows for complete palpation of the horse while eliciting responses from potentially painful areas. Careful inspection of the feet for balance and symmetry should be performed. Flexion manipulations can then be performed in a “passive” sense (not asking the horse to walk or jog away) while noting any resistance or painful responses. It may be appropriate to use hoof testers at this point before starting any exercise. Next, the horse can be moved in hand at a walk and trot, taking note of any obvious lameness or unusual foot flight or limb motion. This is best performed on hard footing if available. It is useful to see the horse walk in circles as well as on a straight line. Likewise, jogging the horse in circles as well as in a straight line may provide much more insight related to the horse’s level of comfort. If the horse is too fractious to lunge in circles, jogging in hand on circles may provide an acceptable alternative. The author frequently gives a small dose of sedative (detomidine hydrochloride, 1.5 mg total dose)^a to evaluate lameness if the horse is not well-behaved. Watching the horse move in straight lines and circles on both hard and soft footing can be immensely valuable in detecting lameness.

The next step is to perform flexion tests of all limbs with the horse then walking or trotting away. These may give clues to soreness that is not otherwise evident. It may be worth pursuing some positive flexion responses or readily apparent lameness at this point while simply recording some others for future reference. Keep in mind that a positive distal limb flexion may indicate a problem in any number of places, from the distal interphalangeal joint (DIPJ) to even a suspensory ligament issue. It is a tool for regionalization and augmentation for observation purposes.

“Wedge” tests are sometimes useful to aid in localization of the source of lameness. A reverse wedge that stretches the deep digital flexor tendon (DDFT) may increase a lameness related to that structure; a wedge placed laterally under the foot may stretch the medial collateral ligament of the DIPJ and increase lameness if that structure is injured. The latter effect is because of compression on the same side as the wedge and rotation of the DIPJ on the contralateral side that stretches the collateral ligament on that side.¹ [...]