Case reveal

Distal interphalangeal joint, middle phalanx and distal phalanx (dorsal aspect and DDFT/impar ligament insertions):

• The distal interphalangeal joint joitn capsule is moderately thickened with proliferation and contains mild to moderate effusion. Osteophytes are identified along the distal interphalangeal joint margins and associated mild joint capsule enthesopathy is identified. There is diffuse articular cartilage loss accompanied by multiple intrasubstance abnormalities.

• The impar ligament demonstrates moderate to marked enthesopathy, characterized by midline cystic resorption surrounded by a sclerotic rim and associated with moderate fluid accumulation.
• Additionally, the distal phalanx shows mild dorsal distal fluid accumulation.

Navicular bone, navicular bursa, deep digital flexor tendon and collateral sesamoidean ligament:

• There is a shallow erosion of the flexor surface of the navicular bone with associated compact bone loss. This lesion is located distally and laterally to the sagittal ridge. Additional navicular bone changes include moderate palmar fluid accumulation, mild enlargement of synovial invaginations, distal margin remodeling, and irregularities along the flexor surface.

• The navicular bursa has increased fluid and severe synovial membrane thickening with extensive synovial proliferation. This proliferation extends to the dorsal margin of the deep digital flexor tendon (DDFT) and the palmar margin of the collateral sesamoidean ligament, raising concern for adhesion formation.

• There is extensive injury to the DDFT throughout the foot and pastern. At the level of the proximal interphalangeal joint, moderate fiber disruption is observed, with surrounding mild to moderate fiber abnormalities affecting both medial and lateral lobes. These changes progress distally, with tendon lobe enlargement, severe dorsal margin fraying, and fibrillation at the middle phalanx level. At the navicular bone level, mild linear fiber abnormalities and/or parasagittal splits are noted.
• A focal extension of the navicular bursa is seen through a medial lobe parasagittal split, with localized fluid accumulation at the palmar tendon margin. At the distal phalanx level, fiber abnormalities continue as core lesions, most prominently affecting the medial lobe with fiber disruption.
• Additionally, there is moderate enlargement and scarring of the collateral sesamoidean ligament at the middle phalanx level.

Collateral ligaments of the distal interphalangeal joint, distal phalanx collateral fossae and palmar processes:

• The distal phalanx collateral cartilages are mild to moderately ossified.

Proximal interphalangeal joint and remaining structures:

• The proximal interphalangeal joint has mild arthrosis.

Metacarpophalangeal joint (limited examination):

• The metacarpophalangeal joint has mild synovitis. The distal third metacarpal bone has mild dorsal sclerosis and fluid.

Palmar soft tissues and digital sheath:

• The previously described fiber abnormalities of the deep digital flexor tendon extend proximally into the pastern, most prominently affecting the medial tendon lobe. Findings include fiber disruption and a dorsal margin tear, along with mild to moderate fiber abnormalities in the lateral lobe. At this level, the tendon lobes are markedly enlarged, with the medial lobe more affected than the lateral. These abnormalities progressively diminish in severity proximally and resolve by the mid to proximal pastern, although the medial lobe remains enlarged up to the proximal pastern.
• The lateral oblique sesamoidean ligament shows mild to moderate focal dorsal fiber abnormalities with dorsal enlargement proximally, and mild to moderate palmar fiber abnormalities in the mid-pastern region.

• There is also mild effusion within the digital sheath.

Conclusions:

• Shallow flexor surface erosion with additional flexor surface irregularity, moderate palmar fluid, mild to moderate synovial invagination enlargement and distal margin remodeling, navicular bone
• Extensive, marked deep digital flexor tendinopathy with extension of the navicular bursa through the tendon and possible adhesion formation (foot and pastern)
• Severe proliferative navicular bursitis
• Mild to moderate proliferative synovitis and mild arthrosis, mild joint capsule enthesopathy and diffuse cartilage loss, distal interphalangeal joint
• Moderate enlargement and scarring, collateral sesamoidean ligament
• Moderate to marked impar ligament enthesopathy with osseous cyst-like lesion, sclerosis, and moderate fluid and mild dorsal distal fluid, distal phalanx
• Multifocal moderate desmopathy, lateral oblique sesamoidean ligament
• Mild arthrosis, proximal interphalangeal joint
• Mild synovitis, metacarpophalangeal joint
• Mild dorsal sclerosis and fluid, distal third metacarpal bone

Follow-up:

• The imaging findings in the left forelimb are consistent with a source of chronic lameness. The presence of osseous fluid may reflect either a contusion or a degenerative process. Typically, osseous fluid resulting from contusion resolves with rest, whereas fluid associated with degenerative changes may persist and contribute to ongoing lameness.
• Notably, the deep digital flexor tendinopathy at the distal phalanx level is of particular concern, as lesions in this region often require a longer duration to heal compared to more proximal injuries.
• A follow-up MRI examination is recommended in approximately six months, or sooner if dictated by clinical progression or treatment response.

A little bit more…

Deep digital flexor tendon lesions in the foot

• Deep digital flexor tendon (DDFT) injuries within the equine foot are a significant cause of lameness and poor performance, particularly in sport horses such as Warmbloods and Quarter Horses. These injuries are often multifocal and may involve the suprasesamoidean, sesamoidean, and infrasesamoidean regions of the tendon. Accurate diagnosis is critical for prognosis and management, and magnetic resonance imaging (MRI) has become a cornerstone in the evaluation of these lesions.

MRI Features and Lesion Distribution

• MRI provides superior soft tissue contrast and multiplanar imaging capabilities, allowing for detailed assessment of tendon morphology, signal characteristics, and associated pathology. Lesions of the DDFT typically appear as areas of increased signal intensity on T1-weighted gradient echo (GRE), T2-weighted fast spin echo (FSE), and short tau inversion recovery (STIR) sequences. These signal changes reflect alterations in tendon composition, including edema, fiber disruption, necrosis, and fibrocartilaginous metaplasia.
• In Dyson et al.’s study of 75 horses with foot pain:
• 46 horses (61%) had DDFT lesions considered a major contributor to lameness.
• Lesion location:
  • Proximal to the navicular bone only: 23 horses
  • Insertional region only: 3 horses
  • Both proximal and insertional regions: 20 horses
• This means that 43 out of 46 horses (93%) had lesions involving the region distal to the pastern (i.e., at or beyond the navicular bone), even when proximal lesions were present.

Common lesion types include:

• Core lesions: Central areas of fiber disruption, often longitudinally oriented.
• Sagittal splits: Linear tears in the sagittal plane, sometimes extending to the dorsal border.
• Dorsal border fibrillation or tears: Superficial fiber fraying or full-thickness defects, often associated with navicular bursitis.
• Insertional lesions: Tendinopathy at the attachment to the distal phalanx, frequently accompanied by enthesopathy or adhesions to the distal sesamoidean impar ligament (DSIL).

MRI also enables identification of concurrent pathology in the podotrochlear apparatus, including navicular bone edema, flexor surface erosions, and synovial proliferation, which may influence prognosis.

Low-Field vs. High-Field MRI: Diagnostic Performance

Low-field MRI (LF-MRI, ~0.27 T) is widely used in standing horses due to its accessibility and safety. High-field MRI (HF-MRI, 1.0–3.0 T) offers superior resolution and sensitivity, especially for detecting subtle lesions distal to the navicular bone.

• HF-MRI is better at identifying:
  • Small dorsal border tears
  • Adhesions to the navicular bone or DSIL
  • Insertional lesions with associated enthesopathy
• LF-MRI is effective for:
  • Diagnosing moderate to severe lesions
  • Serial monitoring of healing, especially via STIR and T2-weighted sequences

Prognosis by Lesion Type and Location

Prognosis varies significantly depending on lesion type, location, and chronicity:

• Core lesions and sagittal splits: Associated with slower healing and lower return to athletic activity. Only 41–50% of horses return to previous levels of work.
• Dorsal border lesions: Better prognosis, with up to 73% returning to some level of work. Small, shallow tears respond well to conservative management; extensive or chronic lesions have guarded outcomes.
• Insertional lesions: Guarded prognosis regardless of whether isolated or multifocal. Only 33% of horses return to previous levels of activity.
• Combined DDFT and navicular pathology: Poorer outcomes, especially when adhesions or flexor surface erosions are present.
• Longitudinal tears at the level of the digital flexor tendon sheath: Prognosis is poor, with only 27–41% returning to previous levels of work.

Serial MRI is valuable for monitoring healing. Reduction in STIR and T2-weighted signal intensity correlates with improved outcomes. Horses with persistent fluid signal or lesion progression may require extended rehabilitation or altered treatment strategies.

References:

• Scharf, Alexandra, et al. “Magnetic resonance Imaging for diagnosing and managing deep digital flexor tendinopathy in equine athletes: Insights, advances and future directions.” Equine Veterinary Journal (2025).
• Dyson, S., et al. “Lameness in 46 horses associated with deep digital flexor tendonitis in the digit: diagnosis confirmed with magnetic resonance imaging.” Equine veterinary journal 35.7 (2003): 681-690.
• Vanel, Maïa, et al. “Clinical significance and prognosis of deep digital flexor tendinopathy assessed over time using MRI.” Veterinary Radiology & Ultrasound 53.6 (2012): 621-627.
• Mair, Tim S., and J. E. N. Kinns. “Deep digital flexor tendonitis in the equine foot diagnosed by low‐field magnetic resonance imaging in the standing patient: 18 cases.” Veterinary Radiology & Ultrasound 46.6 (2005): 458-466.