Equine Flexural Deformities Study Guide

Overview and Clinical Importance

Flexural limb deformities (FLDs) are deviations in the sagittal plane characterized by abnormal flexion or extension of joints. These conditions are frequently encountered in equine practice, particularly in neonatal foals and rapidly growing young horses. FLDs represent a significant category of developmental orthopedic disease on the NAVLE and require thorough understanding of anatomy, pathophysiology, and treatment options.

FLDs are divided into two main categories: contractural deformities (hyperflexion, commonly called "contracted tendons") and hyperextension deformities (flexor tendon laxity). Both types can be congenital (present at birth) or acquired (developing later in life).

Anatomy Review

Understanding the anatomy of the equine distal limb is essential for diagnosing and treating flexural deformities. The key structures include:

Deep Digital Flexor Tendon (DDFT)

Originates from three muscle bellies on the forearm, courses down the palmar aspect of the limb around the navicular bone, and inserts on the palmar surface of the third phalanx (P3/coffin bone). The accessory ligament of the DDFT (inferior check ligament) originates from the palmar carpal ligament and joins the DDFT at the mid-cannon bone level. This ligament limits the stretch of the DDFT.

Superficial Digital Flexor Tendon (SDFT)

Originates from the medial epicondyle of the humerus, courses down the limb, and inserts on P1 and P2. The accessory ligament of the SDFT (superior check ligament) originates from the radius and joins the SDFT proximally.

Classification of Flexural Deformities

Etiology and Pathophysiology

Congenital Flexural Deformities

Congenital FLDs are present at birth and may result from:

• Uterine malpositioning: Most commonly cited cause; theorized that abnormal positioning restricts limb movement in utero
• Teratogenic insults: Exposure to locoweed, Sudan grass, or certain viral infections (e.g., equine influenza)
• Genetic factors: Arthrogryposis (multiple congenital joint contractures) may have hereditary components
• Nutritional factors: Iodine deficiency in the mare can cause goiter and flexural deformities in foals

Acquired Flexural Deformities

Acquired FLDs develop after birth and are more common than congenital forms. Key contributing factors include:

• Nutritional imbalance: High-energy diets causing rapid growth create a mismatch between bone growth and tendon elongation
• Pain-induced flexion reflex: Any source of chronic limb pain (physitis, osteochondrosis, P3 fractures, abscesses) causes protective flexion
• Developmental orthopedic disease (DOD): Physitis and OCD are frequently associated with acquired FLDs
• Mineral imbalances: Copper deficiency and calcium-phosphorus imbalances

Clinical Signs and Diagnosis

DIP Joint Contracture (Club Foot)

Club foot is the most common acquired flexural deformity. Clinical signs include:

• Upright hoof with steep dorsal hoof wall angle
• Long, contracted heels with short toe
• Prominent or bulging coronary band
• Concave (dished) dorsal hoof wall in chronic cases
• Walking on toes with heels off the ground (severe cases)
• Often unilateral, forelimbs more commonly affected

Club Foot Grading System

Fetlock Contracture

Clinical signs include:

• Knuckling over at the fetlock joint
• Inability to fully extend the limb
• Often bilateral, forelimbs more common
• May walk on dorsum of fetlock in severe cases
• Most common acquired contracture in older foals (9-18 months)

Carpal Contracture

Clinical signs include:

• Forward buckling at the carpus ("over at the knee")
• Unable to fully extend carpus
• Often associated with joint capsule contracture
• Severe cases (greater than or equal to 90 degrees flexion) have poor prognosis

Flexor Tendon Laxity (Hyperextension)

Clinical signs include:

• Hyperextension of fetlock (fetlock dropping toward ground)
• Toes elevated off ground, walking on heels
• Back-at-the-knee appearance
• Most common in hindlimbs
• Associated with prematurity, dysmaturity, or weakness
• Risk of skin abrasions on palmar/plantar aspect of pastern and fetlock

Diagnostic Workup

Diagnosis is primarily clinical. The following should be performed:

• Physical examination: Observe foal standing and walking; palpate tendons and limbs
• Manual manipulation: Attempt to manually correct the deformity; if manually reducible, prognosis is better
• Palpation of tendons: Palpate DDFT and SDFT while extending the limb to determine which is taut
• Radiography: Evaluate for P3 changes, physitis, OCD, fractures
• Nutritional history: Document diet, feeding regimen, growth rate

Treatment

Treatment of Congenital Contractural Deformities

Oxytetracycline

Dosing: 30-44 mg/kg IV (commonly 3g total dose), diluted in 500-1000 mL saline or lactated Ringer's solution, given as a slow IV infusion. May repeat once in 24-48 hours if needed. Maximum of 2-3 doses.

Mechanism of action: The exact mechanism is not fully understood. Proposed mechanisms include:

• Calcium chelation reducing muscle contraction
• Inhibition of matrix metalloproteinase (MMP) expression by myofibroblasts, decreasing collagen remodeling
• Decreased mechanical properties of tendons allowing elongation under weight-bearing

Efficacy: Most effective in foals less than 2 weeks old. Efficacy decreases rapidly with age; minimal effect after 14 days of age. Effect is temporary; must be combined with physical therapy (exercise, splinting) to maintain correction.

Splinting and Bandaging

Neonatal muscles, tendons, and ligaments exhibit a unique relaxation response when supported. The firmer the support, the more relaxation achieved. Splints can be repositioned every 24-48 hours. Casts are more rigid but not adjustable. Watch for pressure sores - foal skin is very delicate.

Controlled Exercise

Controlled turnout on even, flat ground encourages natural correction through weight-bearing and stretching. Avoid excessive exercise. Painful animals will contract the limb further due to the withdrawal reflex.

Analgesics

NSAIDs (flunixin meglumine, phenylbutazone) should be used judiciously to relieve pain and encourage weight-bearing. Foals must be comfortable to exercise.

Therapeutic Farriery - Toe Extensions

Glue-on toe extensions help stretch the DDFT with each step. Applied after initial correction to maintain gains.

Treatment of Acquired Contractural Deformities

Nutritional Management

Reduce energy intake to slow growth rate. Important: Do not starve, as this leads to compensatory growth spurt when refeeding. Reduce concentrate, maintain quality forage. Balance minerals (especially Cu, Zn, Ca, P).

Address Underlying Pain

Identify and treat any source of pain: hoof abscesses, P3 fractures, physitis, OCD. No treatment will succeed if the underlying cause persists.

Corrective Trimming

Lower heels gradually, apply toe extensions. Frequent trimming every 2-4 weeks is essential. Goal is to re-establish normal hoof-pastern axis.

Surgical Treatment

Treatment of Flexor Tendon Laxity

Most cases are self-limiting and resolve within days to 2 weeks as the foal strengthens. Treatment includes:

• Controlled exercise: Light walking 2-3 times daily on firm, flat ground
• Heel extension shoes: Glue-on shoes with caudal (heel) extensions force the toe down under the foal's weight
• Light bandaging: To protect skin from abrasions (do NOT bandage snugly to "support" - this is counterproductive)
• Swimming exercise: Occasionally used for hindlimb hyperextension

Prognosis

Exam Focus: Prognosis hierarchy: Distal deformities have BETTER prognosis than proximal. Club foot greater than fetlock greater than carpus. Younger animals have better prognosis than older animals.

Memory Aids and Board Tips

"CLUB" Mnemonic for Club Foot

C - Contracture of DDFT musculotendinous unit

L - Long heels, short toe

U - Upright hoof wall (dorsal)

B - Bulging coronary band

"OXY for BABIES" - Oxytetracycline Rule

Oxytetracycline is effective only in young neonates (less than 2 weeks old). After 14 days, the magical relaxation effect is lost. Remember: OXY = Only eXtremely Young!

Check Ligament Surgery Selection

Inferior check desmotomy = releases tension on DDFT = for DIP (coffin) joint/club foot

Superior check desmotomy = releases tension on SDFT = for fetlock contracture

Memory tip: "Inferior is for the foot (Inferior = lower = DIP), Superior is for higher (fetlock)"

Common Digital Extensor Tendon Rupture

Often seen with congenital FLDs as a nonpainful swelling on the dorsolateral carpus. No specific treatment required. Does not worsen prognosis but indicates longer treatment duration needed.