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Flexural Deformities

Gayle W. Trotter DVM, MS
Diplomate ACVS
Associate Professor of Surgery
 
These deformities are commonly referred to as contracted tendons, even though true tendon contracture is unlikely the cause of the problem. However, with flexural deformities, the soft tissue structures on the palmar (forelimb) and less commonly plantar (hindlimb) are affected such that the bones align in varying degrees of flexion. The true cause and method of development of flexural deformities remains unknown, although horses with acquired deformities (those that develop after birth) often share similar factors to those associated with the developmental orthopedic disease complex.
 
Generally, flexural deformities are divided into congenital and acquired types. Congenital deformities are present at birth, and acquired deformities develop at some stage of the growing period. Acquired deformities manifest differently depending on the age of the horse.
 

Congenital Flexural Deformities

 
Many factors can be involved in congenital deformities. These include intrauterine positioning, ingestion of certain toxins by the mare such as locoweed or hybrid Sudan grass, collagen cross-linking defects, equine hypothyroidism/goiter, and unidentified predisposing genetic factors. Multiple factors may be involved in a given case. Congenital flexural deformities can be due to hyperflexion, or less commonly hyperextension due to flaccid or relaxed flexor muscles and tendons. Some of these deformities can cause dystocia or difficult foaling.
 
Digital Hyperextension Many foals are born with flaccid flexor tendons, such that their toes elevate off the ground during weightbearing. Most of these deformities are minor and self-limiting, and correction occurs during the first two weeks of life as muscle and tendon tone improves. In more severe cases, the foal may walk on the bulbs of the heel or pastern, such that soft tissue abrasion injuries develop. Temporary heel extensions (small door hinge taped to the foot, glue on shoes with a heel extension) may need to be applied until muscle and tendon tone improves.
 
Ruptured Common Digital Extensor Tendon This condition may be congenital or acquired, and the cause is unknown. Affected foals have characteristic swellings over the tendon sheath at the dorsolateral aspect of the carpus. Foals also appear slightly bowlegged, and they tend to `knuckle over' at the fetlock when walking, and appear `over at the knee' when standing.
 
Temporary splint application to prevent knuckling over is the most effective treatment, and surgery is not required. Presumably, the tendon ends fibrose or scar down, because functionally these foals become normal.
 

Congenital Carpal Deformities

 
These deformities are often severe in extent, and often involve one limb only. Manual correction is often impossible. Careful evaluation of these cases should be undertaken to determine whether surgical correction is even feasible. Many cases have sufficient contracture of the carpal joint capsule and ligaments to prevent correction, even after transection of all carpal flexor tendons. For less severe forms, temporary splinting is effective.
 

Congenital Flexural Deformities of the Foot and Fetlock

 
Affected animals either stand on the toe, or knuckle forward at the level of the fetlock. Careful palpation of the flexor tendons should be completed in both the weightbearing and non-weightbearing positions, to determine which of the flexor tendons is most involved. Involvement of both superficial and deep digital flexor tendons to a similar degree is sometimes seen.
 
If the foal can stand and the limbs can be manually extended into a normal position, the prognosis is favorable for resolution with temporary splinting. More severe forms may require check ligament desmotomy for correction.
 
More recently, the use of oxytetracycline has been advocated for some of these deformities. The initial description was for correction of deformities involving the foot only, although some success has been realized for fetlock deformities as well. The dose recommended was 3 gm given IV, followed by a second injection if correction was not observed. Success has also been seen using half this dose, and one case of renal toxicity has been reported in a foal after the use of this drug. The mechanism of action is unknown, but is thought to relate to calcium chelating properties of the antibiotic.
 

Acquired Flexural Deformities

 
There are many potential causes for acquired flexural deformities, and those factors important in the development of osteochondrosis are also the same factors that are important here. Flexural deformities seem to occur most commonly in fast growing individuals, and often those that are on a high plane of nutrition. Other factors that can be involved include pain, which results in the flexion withdrawal reflex and an altered stance. The source of pain could be OCD, joint infection, physitis, or some other form of acute trauma. If the altered stance is maintained, a flexural deformity will result. It has also been noticed that foals on a poor plane of nutrition that are then introduced to good quality feed often develop flexural deformities. The cause(s) of these deformities seem to be multifactorial, with nutrition and rapid growth rate being factors that are commonly present. If the problem occurs frequently on a given premise, careful evaluation of the feeding program needs to be done.

Suggested pathogenetic pathways for acquired flexure deformities.

There tends to be two age groups that develop acquired flexural deformities. Deformities of the distal interphalangeal (DIP) joint are usually 1-4 months of age, whereas deformities of metacarpophalangeal (MCP) or fetlock joint are usually 12 -14 months of age. This age relationship is thought to be related to the effects of the accessory or check ligaments of the deep digital and superficial flexor tendons respectively, and the fact that limb growth at the distal limb is largely completed by three months of age, but continues at the level of the carpus or knee for a much longer period of time. There is no evidence to suggest that true contracture or shortening of the tendons occurs, but the check ligaments may restrict passive elongation of their respective flexor tendons during periods of rapid bone growth at the more distal or proximal locations. In the case of the deep digital flexor tendon, a flexural deformity affecting the foot will be the end result, as this tendon attaches to the distal phalanx or coffin bone. With involvement of the superficial flexor tendon, the deformity will appear at the level of the fetlock joint.

Acquired Flexural Deformity of the Distal Interphalangeal (Coffin) Joint

Diagnosis Affected foals have varying degrees of a more upright or clubbed foot appearance. In less severely affected foals, a dished appearance to the dorsal hoof wall will appear over time due to the mechanical influences placed on the foot. In more severely affected foals, the heel will not contact the ground. Very severely affected foals will walk on their dorsal hoof wall. If lameness is present, the foot should be radiographed to evaluate the distal phalanx or coffin bone, as bone reabsorption or even infection at the toe portion of the foot can occasionally develop.
 
Treatment Appropriate dietary management should be undertaken if necessary. With young foals, this management needs to address the mare, so that milk production can be limited. If the foal is already being creep fed, this should be curtailed. Exercise should also be controlled so that some exercise is given, but excessive exercise at pasture is eliminated. With less severely affected cases, a toe extension may be taped or glued to the foot, or a special glue-on shoe with a toe extension may be applied.
 
For more severe deformities, or deformities not responding to conservative treatment, a distal (carpal or inferior) check ligament desmotomy should be considered. The prognosis is highly favorable with this surgery, although some soft tissue blemish often develops at the surgery site. The treatment works by relaxing the restraint normally placed on the deep digital flexor tendon by this ligament. Correction is usually observed immediately after surgery. Postoperative bandaging care is important to limit the degree of the blemish. In very severe cases, transection or cutting of the deep flexor tendon proper may be required. Such cases are salvage cases not intended for athletic function.

Acquired Flexural Deformity of the Fetlock Joint

Diagnosis This deformity results in an upright or ‘posty’ conformation to the front legs, or with more severe involvement, ‘knuckling’ forward at the level of the fetlock joint. Some animals will also have a ‘pop’ or catch in their gait. Affected horses are usually in the 10 - 18 month old age group. This deformity is thought to be related to rapid bone growth originating at the growth plate of the distal radius, and restriction to passive elongation of the superficial digital flexor tendon by the proximal (radial or superior) check ligament. However, involvement of the deep digital flexor tendon can also occasionally result in this type of deformity.
 
Palpation of both flexor tendons should again be completed with the limb in both weightbearing and non-weightbearing positions to help determine which individual tendon, or whether both tendons, are involved.
 
Treatment Providing a properly balanced diet is of major importance in resolving this deformity. However, many cases will need more aggressive intervention for correction to occur. Some horses will respond to elevation of the heel of the foot, with or without a toe extension. Although an initial tendency may be to try to lower the heel of the foot, this will make the condition worse. More aggressive types of handcrafted shoes and splints designed to maintain the fetlock in a more normal position may be useful in selected cases. Exercise should be restricted and controlled but should not be eliminated.
 
If conservative treatment fails, transection of the proximal check ligament may be indicated. This procedure is designed to remove the restrictive effect of this ligament on elongation of the superficial digital flexor tendon. Careful preoperative palpation is mandatory to ensure that this is the most involved tendon. If this evaluation suggests that the deep digital flexor tendon is more involved, distal check ligament desmotomy would be indicated.
 
Proximal check ligament desmotomy is technically more difficult to complete than distal check ligament desmotomy, and postoperative seroma formation at the surgery site is also a common complication. However, the procedure can be very useful in selected cases. Some cases will still need to have temporary splinting to maintain proper extension of the fetlock.
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