Equine Suspensory Ligament Disruption – NAVLE Study Guide

Overview and Clinical Importance

Suspensory ligament desmitis represents one of the most common causes of lameness in athletic horses across all disciplines. The suspensory ligament (also known as the interosseous muscle or musculus interosseus medius) is a critical component of the equine suspensory apparatus, serving to prevent hyperextension of the metacarpophalangeal (fetlock) joint during weight bearing. Understanding the anatomy, pathophysiology, diagnosis, and treatment of suspensory ligament injuries is essential for NAVLE success and clinical practice.

Injuries to this structure can affect any region of the ligament and occur in both forelimbs and hindlimbs. The clinical presentation, treatment approach, and prognosis differ significantly based on lesion location and severity, making accurate diagnosis critical for appropriate case management.

Functional Anatomy of the Suspensory Ligament

The suspensory ligament is an evolutionary derivative of the interosseous muscle found in other species. In horses, this structure has become predominantly tendinous (ligamentous), though it retains some muscle fibers, particularly in the proximal region. This mixed composition is clinically important and can complicate ultrasonographic interpretation.

Origin and Course

Forelimb: The suspensory ligament originates from the proximal palmar aspect of the third metacarpal bone (MCIII) and the distal row of carpal bones. It lies within the groove formed between the second and fourth metacarpal (splint) bones, deep to the superficial and deep digital flexor tendons.

Hindlimb: Origin occurs at the proximal plantar aspect of the third metatarsal bone (MTIII) and the plantar aspect of the distal tarsal bones. The hindlimb proximal suspensory region is contained within a more restricted canal formed by the large heads of the second and fourth metatarsal (splint) bones and overlying fascia, which has important clinical implications.

Regional Division

The suspensory ligament is clinically divided into three regions based on anatomical features and injury patterns:

Insertion and Continuation

Each branch inserts on the abaxial surface of the ipsilateral proximal sesamoid bone. Small extensor branches continue distally to join the common/long digital extensor tendon on the dorsal aspect of the proximal phalanx. The suspensory apparatus continues distally as the distal sesamoidean ligaments (straight, oblique, cruciate, and short), which originate from the base of the proximal sesamoid bones and insert in the palmar/plantar pastern region.

Etiology and Risk Factors

Suspensory ligament injuries typically result from acute overload, chronic repetitive microtrauma, or a combination of both. Understanding risk factors is essential for prevention and case management.

Clinical Presentation

Clinical signs vary significantly based on lesion location, severity, and chronicity. A thorough understanding of presentation patterns aids in clinical suspicion and targeted diagnostic workup.

Proximal Suspensory Desmitis (PSD)

Forelimb PSD

• Lameness: Mild to moderate; often more evident on soft ground and when lame leg is on outside of circle
• Gait pattern: Advancing leg lameness (maximal head excursion during swing phase)
• Palpation: Pain on direct pressure to proximal palmar metacarpus; may detect heat or swelling in acute cases
• Flexion tests: Variable positive response to distal limb and carpal flexion
• Bilateral involvement: Common; may present as gait quality change rather than distinct lameness

Hindlimb PSD

• Lameness: Shortened cranial phase of stride; often worse with leg on outside of circle
• Palpation: Usually NO detectable swelling or sensitivity due to large splint bone heads obscuring the region
• Flexion tests: Often equally positive to all three hindlimb flexions
• Bilateral involvement: Very common; may cause reluctance to back
• Ridden signs: Loss of impulsion, resistant behavior, decreased hindlimb engagement

Suspensory Body Desmitis

• Most common in Thoroughbred and Standardbred racehorses
• Lameness: Variable; may be absent in chronic cases
• Palpation: Enlargement of ligament body; local heat, swelling, and pain in acute cases
• Caution: Pain on palpation of forelimb suspensory is common in horses with other primary problems and is NOT diagnostic in isolation

Suspensory Branch Desmitis

• Often only single branch in single limb affected; both branches can be involved, especially in hindlimbs
• Lameness: Varies from mild to moderate; may improve within days
• Local signs: Heat, swelling on one side of fetlock; thickening over affected branch (best detected with leg elevated)
• Palpation: Pain with direct pressure or fetlock flexion
• Associated findings: Fetlock joint effusion, digital flexor tendon sheath effusion, sesamoiditis, avulsion fractures
• Flexion test: Strong positive distal limb flexion

Diagnostic Approach

Diagnostic Anesthesia

Regional anesthesia is essential for localizing pain to the suspensory ligament, particularly for proximal lesions and hindlimb pathology.

Ultrasonography

Ultrasound is the primary imaging modality for suspensory ligament evaluation. A systematic approach examining the entire ligament (origin, body, and branches) in both transverse and longitudinal orientations is essential.

Ultrasonographic Findings in Suspensory Desmitis

Hindlimb Imaging Considerations

The hindlimb proximal suspensory region presents unique imaging challenges. The best acoustic window is plantar-medial. A small stand-off pad improves contact. Perform ultrasound BEFORE diagnostic blocks to avoid creating sonographic artifacts from injected fluid.

Radiography

Radiographs should always accompany ultrasound in suspected suspensory desmitis to evaluate bony involvement:

• Proximal region: Enthesophyte formation, sclerosis, cortical irregularity at suspensory origin
• Branches: Sesamoiditis, avulsion fractures at proximal sesamoid apices, osteolysis
• Splint bones: Concurrent fractures, especially with branch injuries

Advanced Imaging

MRI: Gold standard for proximal suspensory ligament evaluation. Detects subtle changes not visible on ultrasound and allows concurrent assessment of bony structures. Particularly valuable for hindlimb PSD where ultrasound may be inconclusive.

Nuclear Scintigraphy: Useful for detecting concurrent or primary osseous injury at the suspensory attachment, especially when clinical presentation suggests bone involvement or when localizing lesions in complex cases.

Treatment Approaches

Treatment selection depends on lesion location, severity, ultrasonographic findings, chronicity, and intended use of the horse. A comprehensive approach typically combines multiple modalities.

Conservative Management

Regenerative and Advanced Therapies

Surgical Intervention

Plantar Fasciotomy and Neurectomy (Hindlimb PSD)

This combined procedure has become standard treatment for chronic hindlimb PSD that fails conservative management. The fasciotomy involves cutting the deep plantar metatarsal fascia to relieve compartment syndrome around the proximal suspensory. The neurectomy of the deep branch of the lateral plantar nerve (DBLPN) relieves pain from the proximal suspensory region.

• Success rate: Approximately 77-85% for horses with PSD alone
• Poor candidates: Horses with straight hocks or fetlock hyperextension (increased risk of further injury post-surgery)
• Complications: Iatrogenic suspensory damage, seroma formation, residual swelling, white hair development at incision site
• Post-operative care: Controlled exercise program similar to conservative management; serial ultrasound monitoring

Other Surgical Options

• Ligament splitting/desmoplasty: Creates channels in damaged tissue to promote revascularization; used for body and branch lesions with core lesions
• Arthroscopic debridement: For branch injuries with fetlock joint involvement; removes damaged tissue from insertional region
• Sesamoid fracture removal: Larger avulsion fractures at proximal sesamoid apices may require surgical removal

Prognosis

Memory Aid - Hindlimb vs Forelimb PSD Prognosis: FORE = FAVORABLE (90% with rest). HIND = HARDER (less than 20% without surgery, 85% with surgery). Think: 'The HIND end needs a HAND (surgical).'

Degenerative Suspensory Ligament Desmitis (DSLD)

Degenerative Suspensory Ligament Desmitis (DSLD), also known as Equine Systemic Proteoglycan Accumulation (ESPA), is a distinct condition from traumatic suspensory injuries. It is a systemic connective tissue disorder characterized by abnormal accumulation of proteoglycans between collagen fibers.

Key Distinguishing Features