Equine Tendonitis and Desmitis Study Guide

Overview and Clinical Importance

Tendonitis (tendon inflammation) and desmitis (ligament inflammation) represent some of the most common and career-limiting soft tissue injuries in performance horses. These conditions account for approximately 43-54% of all musculoskeletal injuries in equine athletes and are a leading cause of early retirement and wastage in the racing and sport horse industries. Understanding the anatomy, pathophysiology, diagnosis, and treatment of these conditions is essential for the NAVLE examination.

The most commonly affected structures include the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT), suspensory ligament (SL), and accessory ligaments. Forelimbs are more frequently affected than hindlimbs due to the greater weight-bearing load they experience during locomotion.

Functional Anatomy

The palmar/plantar aspect of the equine distal limb contains the key structures involved in weight-bearing and locomotion. Understanding their anatomy is crucial for diagnosis and treatment.

Superficial Digital Flexor Tendon (SDFT)

The SDFT is the most superficial structure on the palmar aspect of the cannon bone. In the forelimb, it originates from the superficial digital flexor muscle at the level of the carpus and is reinforced by the superior check ligament (accessory ligament of the SDFT) just distal to the carpus. The tendon courses distally, passes over the palmar sesamoid bones through the digital flexor tendon sheath (DFTS), splits into two branches at the level of the proximal phalanx, and inserts on P1 and P2.

Key Function: The SDFT is an energy-storing (elastic) tendon that flexes the fetlock, pastern, and coffin joints while acting as a spring mechanism during locomotion. It operates near its functional limits during maximal exercise, making it highly susceptible to injury.

Deep Digital Flexor Tendon (DDFT)

The DDFT lies deep to the SDFT and originates from three muscle heads in the forelimb. The tendon is reinforced by the inferior check ligament (accessory ligament of the DDFT) at the mid-metacarpal level. It passes through the carpal canal, continues within the DFTS, passes through the manica flexoria at the fetlock, courses over the navicular bone (separated by the navicular bursa), and inserts on the palmar surface of P3 (coffin bone).

Key Function: The DDFT flexes all joints of the digit and stabilizes the distal interphalangeal joint. It experiences both tensile and compressive forces, particularly where it passes over the navicular bone.

Suspensory Ligament (SL)

The suspensory ligament (interosseous muscle) is technically a modified muscle that has become almost entirely tendinous. It originates from the proximal palmar aspect of the third metacarpal/metatarsal bone and the distal row of carpal/tarsal bones. It is divided anatomically into three regions: the proximal origin, body, and branches. The SL bifurcates in the distal metacarpus into medial and lateral branches that insert on the abaxial surfaces of the proximal sesamoid bones, with extensor branches joining the common digital extensor tendon.

Key Function: The SL is the primary supporter of the metacarpophalangeal (fetlock) joint, preventing hyperextension during weight-bearing. It is a critical component of the suspensory apparatus.

Summary of Key Anatomical Structures

Superficial Digital Flexor Tendonitis (Bowed Tendon)

SDFT tendonitis is the most common tendon injury in horses, accounting for 75-93% of all tendon injuries in equine athletes. The mid-metacarpal region (Zone 2B/3A) is affected in 97-99% of cases. The term bowed tendon refers to the characteristic palmar convexity of the metacarpal region caused by tendon swelling.

Etiology and Risk Factors

• Overstrain injury: Acute mechanical overload during high-speed exercise, jumping, or sudden deceleration
• Age-related degeneration: Cumulative microdamage and matrix degradation with repeated loading cycles
• Exercise on fatigued tendons: Training beyond the tendon's ability to remodel and repair
• Poor conformation: Long, sloping pasterns; hyperextension of fetlock
• Footing conditions: Deep, irregular, or hard surfaces
• Breed predisposition: Thoroughbreds (prevalence 11-30%), Standardbreds

Clinical Signs

Acute Phase: Heat, swelling, pain on palpation over the palmar metacarpus. Lameness varies from mild to severe. Cardinal signs of inflammation present. The tendon may feel soft or fluctuant due to intratendinous hemorrhage and edema.

Chronic Phase: Persistent enlargement (bowing) of the tendon. Firmness on palpation due to fibrosis. Lameness may resolve, but structural changes remain. Risk of re-injury is high.

Ultrasonographic Findings

Ultrasonography is the gold standard for diagnosis and monitoring of SDFT injuries. The entire palmar metacarpus should be examined in both transverse and longitudinal planes, comparing with the contralateral limb.

Treatment

Treatment is based on the phase of healing and aims to reduce inflammation acutely, promote quality tissue repair, and prevent re-injury.

Prognosis

Prognosis for SDFT tendonitis is generally guarded to poor for return to high-level athletic performance. Re-injury rates are high (43-67% in racehorses). Factors affecting prognosis include severity and location of lesion, duration of injury, compliance with rehabilitation protocol, and intended use. Horses with lesions affecting less than 10% of the CSA have a better prognosis. Use of mesenchymal stem cells may reduce re-injury risk by approximately 50%.

Suspensory Ligament Desmitis

Suspensory ligament injuries are classified by location: proximal suspensory desmitis (PSD), body desmitis, and branch desmitis. Each presents with different clinical features and prognosis. The suspensory ligament is the second most frequently injured structure in the distal limb.

Proximal Suspensory Desmitis (PSD)

PSD is a common cause of both forelimb and hindlimb lameness. In the hindlimb, the proximal suspensory ligament is contained within a restricted canal formed by the splint bones and overlying fascia. This anatomy means that pain may arise from compression of adjacent nerves rather than solely from the ligament itself.

Clinical Signs

Forelimb PSD: Variable lameness (mild to moderate), worse on soft ground and with affected leg on outside of circle. Pain on palpation of proximal palmar metacarpus. Positive response to distal limb and carpal flexion tests. Over-flexion of fetlock indicates severe injury (poor prognostic sign).

Hindlimb PSD: Shortened cranial phase of stride; appears worse with affected leg on outside of circle. Palpable sensitivity is uncommon due to large splint bones covering the ligament. Bilateral involvement is common. Positive response to all three hindlimb flexion tests. Horses may be reluctant to back up.

Diagnosis

• Diagnostic analgesia: Low volar/plantar blocks alleviate most pain; infiltration of proximal SL may be needed for origin lesions
• Ultrasonography: Increased CSA, hypoechoic lesions, irregular fiber pattern, irregular margins. Compare with contralateral limb. Mineralization in chronic cases.
• MRI: Extremely useful for detecting subtle changes not visible on ultrasound; allows evaluation of adjacent bony structures
• Nuclear scintigraphy: Helps detect osseous injury at proximal attachment; negative results do not exclude PSD

Treatment and Prognosis

Forelimb PSD: Good prognosis with approximately 90% return to function with rest and controlled exercise for 3-6 months. Premature return to work results in recurrence.

Hindlimb PSD: More variable response; may require longer rehabilitation (6-12 months) or adjunct therapy including shock wave therapy (41-53% return to full work), platelet-rich plasma, or surgical neurectomy/fasciotomy for chronic cases. Prognosis significantly affected by ultrasonographic grade at diagnosis.

Suspensory Body Desmitis

Body desmitis is primarily an injury of racehorses, affecting forelimbs in Thoroughbreds and both fore and hindlimbs in Standardbreds. Clinical signs include enlargement of the ligament, local heat, swelling, and pain. The ligament is more superficial and easier to palpate in this region.

Treatment: Systemic NSAIDs, hydrotherapy, controlled exercise, shock wave therapy, regenerative therapies. Prognosis is generally good depending on lesion severity.

Suspensory Branch Desmitis

Branch injuries are common in sport horses, particularly jumpers. Swelling is visible on the affected side of the fetlock (medial or lateral). Concurrent pathology may include sesamoid bone avulsion fractures or splint bone fractures. Diagnosis requires radiography to evaluate sesamoid bone attachment.

Treatment: Rest, controlled exercise, shock wave therapy, local anti-inflammatories, regenerative therapies. Attention to foot balance is critical. Clinical signs may take 6+ months to improve and condition may recur.

Memory Aid - 'SL INJURIES' Mnemonic: Standardbreds and sport horses = Suspensory Ligament problems. Proximal = Poor hindlimb prognosis. Body = Basically racehorses. Branches = Better in jumpers with sesamoid checks!

Deep Digital Flexor Tendon Injuries

DDFT injuries are most commonly located within the digital flexor tendon sheath (DFTS) at the fetlock level or within the hoof capsule (associated with navicular syndrome). DDFT injury in the mid-metacarpal region is extremely rare.

DDFT Injury within the Digital Flexor Tendon Sheath

These injuries are more common in the hindlimb and typically present with moderate lameness that worsens with distal limb flexion. Digital sheath distension (windgalls) is a common finding. Longitudinal tears of the DDFT are the most common non-septic cause of tenosynovitis within the DFTS.

Clinical Signs: Sudden onset moderate to severe lameness, digital sheath effusion (asymmetric windgalls suggest injury), pain on palpation, increased digital pulse, heat.

Diagnosis: Ultrasonography (75% specificity, 63% sensitivity for longitudinal tears), MRI provides superior detail, tenoscopy for definitive diagnosis and treatment.

Treatment: Tenoscopic debridement for DDFT tears and manica flexoria lesions, removal of adhesions, sheath lavage. Return to athletic function: 38-54%.

DDFT Injury within the Hoof Capsule

DDFT injuries within the hoof are more frequent in the forelimb and are often associated with navicular syndrome. The DDFT passes over the navicular bone, separated by the navicular bursa, making this area susceptible to compressive and tensile forces.

Lesion Types: Dorsal border lesions (best prognosis), core lesions, and parasagittal splits (worst prognosis). Overall, 25% of horses return to previous athletic level within 18 months of MRI diagnosis.

Treatment: MRI-guided treatment; navicular bursa injection with corticosteroids and hyaluronan, rest and rehabilitation, bursoscopic debridement of accessible lesions. Prognosis is guarded to fair.

Exam Focus: Sudden appearance of a windgall (DFTS distension) with acute lameness = suspect DDFT injury. MRI is the gold standard for DDFT lesions within the hoof capsule. Dorsal border DDFT lesions have better prognosis than core lesions or parasagittal splits.

Accessory Ligament Desmitis

Inferior Check Ligament (AL-DDFT) Desmitis

The inferior check ligament is a strong fibrous band that continues from the palmar carpal ligaments to join the DDFT at mid-metacarpus. It prevents overstretching of the DDFT during maximal digital extension. Injury is relatively uncommon in racehorses but is common in ponies, middle-aged horses, and warmblood breeds. The forelimb is almost exclusively affected; hindlimb injury is rare as the structure is vestigial.

Clinical Signs: Acute: local heat, pain, swelling in proximal one-third of metacarpus dorsal to DDFT. Sudden onset moderate to severe lameness. Chronic: persistent swelling, possible adhesion formation leading to flexural deformity.

Diagnosis: Ultrasonography showing enlargement, fiber pattern disruption, hypoechoic lesions, loss of normal ligament borders. Compare to contralateral limb.

Treatment and Prognosis: Rest (6-12 months), controlled exercise, NSAIDs, shock wave therapy, regenerative therapies. Return to work: 57-75% in uncomplicated cases. Adhesion formation significantly worsens prognosis. Concurrent SDFT injury lowers prognosis. Desmotomy may be indicated for chronic cases or flexural deformities.

Degenerative Suspensory Ligament Desmitis (DSLD)

DSLD, also known as Equine Systemic Proteoglycan Accumulation (ESPA), is a chronic, progressive, systemic connective tissue disorder. It is characterized by abnormal accumulation of proteoglycans between collagen fibers, leading to collagen fiber disorganization and ligament weakening. The condition affects not only suspensory ligaments but also other tendons, ligaments, blood vessels, and sclera.

Breed Predisposition

Peruvian Pasos (most commonly affected), Arabians, American Saddlebreds, Quarter Horses, Thoroughbreds, and some European breeds. Estimated heritability of 0.22, indicating a genetic component.

Clinical Signs

• Bilateral or quadrilateral lameness that does not improve with rest
• Progressive hyperextension (dropping) of the fetlock joints
• Palpable enlargement of suspensory ligament body and/or branches
• Hindlimbs commonly affected; straight hock conformation often associated
• Poor hindlimb action rather than overt lameness in early stages
• Progressive and ultimately debilitating

Diagnosis

Ultrasound shows suspensory ligament enlargement with changes in echogenicity, hypoechoic irregular fiber disruption, and possible intralesional calcification. Nuchal ligament biopsy may support diagnosis. Definitive diagnosis often requires post-mortem examination demonstrating proteoglycan accumulation in affected tissues.

Treatment and Prognosis

Prognosis is poor. No effective treatment exists to halt progression. Management is supportive and includes corrective shoeing, pain management, and activity restriction. Most horses eventually require euthanasia due to severe lameness and poor quality of life.

Rehabilitation Principles

Controlled exercise rehabilitation is essential for all tendon and ligament injuries. The goal is to progressively load healing tissue to promote appropriate fiber alignment and strength while avoiding re-injury.

Key Points: Ultrasound monitoring every 2-3 months or before changes in exercise level. Look for improved echogenicity, fiber alignment, and stable/decreased CSA. Premature return to work is the most common cause of re-injury. Total rehabilitation time is typically 9-12 months for moderate to severe injuries.