9-Year-Old Quarterhorse Mare

Signalment & History

• This patient presents for a 2 month history of progressive bilateral thoracic limb lameness.
• On physical examination, her vitals are normal. On lameness examination there are mildly increased digital pulses in the right thoracic limb. There is mild distal interphalangeal joint effusion. Hoof testers are used on the right foot, and sensitivity was located in the toe region and medial and lateral bars bilaterally.

Findings

There is a large, triangular defect within the dorsal hoof wall, in the region of the toe, associated with a thin linear lucency characterized by a dorsocaudal direction within the mid aspect of the dorsal hoof wall. There is round lucent focus in the central flexor eminence of the navicular bone. Multiple, enlarged synovial invaginations are identified within the navicular medulla. The round lucent focus in the central flexor eminence of the navicular bone can be seen in the lateral and the dorsoproximal-palmarodistal views. There are severe enthesophyte formation along the palmaromedial and palmarolateral aspects of the navicular bone. Additionally, there is an ill-defined osseous fragment located on the lateral aspect of the distal margin of the navicular bone (these osseous bodies are more easily seen on the 60°  dorsoproximal-palmarodistal projection). The severe enthesophyte formation along the palmaromedial and palmarolateral aspects of the navicular bone (pink arrows) can also be seen in the dorsopalmar view.

Radiographic measurements of the normal foot

Figure A.

• Line a: parallel to hoof wall
• Line b: parallel to dorsal cortex of distal phalanx
• Line c: parallel to weight bearing surface of hoof wall.
• Angle ac: hoof axis
• Angle bc: foot axis
• Distance p: thickness of dorsal soft tissues measured 5mm distal to the junction of the extensor process with the dorsal cortex
• Distance d: thickness of dorsal soft tissues measured 6mm proximal to the most distal point of the dorsal cortex
  • The average thickness of d and p should be no greater than 18mm

Figure B.

• Line 2: length of the palmar cortex measured from the solar margin at the distal toe to the palmar articular edge between P3 and the distal sesamoid bone
• Distance f: vertical distance measured between coronary band and proximal edge of the extensor process
  • It should measure no greater than 7.9 mm

Figure C.

• P3 rotation
• Lines a and b are no longer parallel and converge proximally forming angle ab (rotation angle)
• Distance d is greater than distance p

• Line a signifies the border of the dorsal hoof wall.
• Line b signifies the border of the dorsal aspect of the distal phalanx.
• Lines a and b are no longer parallel and converge proximally forming a rotation angle of approximately 3 degrees
• Distance d is greater than distance p.
  • Distance d = 21.3 mm
  • Distance p = 15.4 mm

Figure D.

• P3 sinking
• Founder distance f’ is increased
• Distance p = distance d
  • Both are greater than normal

• Percentage that d is of line 2 will be greater than normal
  • No greater than 28.1%

• The founder distance f’ is increased in height, measuring 20.1mm
• Distance d is greater than distance p, measuring 21.3 mm and 15.4 mm, respectively.
  • Only distance p is greater than the reference value (16.3 mm).

• Percentage that d is of line 2 is greater than normal, measuring 33.8%.

Diagnosis

• Dorsal hoof wall defect (toe)
• Laminitis with mild rotation and remodeling of the distal phalanx
• Navicular bone degenerative changes, associated bone remodeling, and severe enthesopathy of the medial and lateral suspensory ligaments
• Osseous body associated with the impar ligament is likely a chip fracture, however ossification secondary to ligamentous damage, separate centers of ossification within the impar ligament, and ossification of cartilage particles in the adjacent synovial tissue cannot be ruled out.

Conclusions

Navicular Disease

• Anatomy of the navicular bone
  • Two surfaces: flexor and articular
  • Two borders: proximal and distal
  • Two extremities: medial and lateral
• Navicular disease: a chronic, progressive syndrome that involves
  • The navicular bone
  • Its fibrocartilaginous flexor surface
  • Its ligaments and capsular attachments
  • The deep digital flexor tendon
  • The navicular bursa
• It is a slowly developing, intermittent, bilateral forelimb lameness
• Pathogenesis: multifactorial
  • Navicular fibrocartilaginous degeneration with secondary tendon fibrillation
  • Palmar cortex bone erosions
  • Distal border synovial invaginations
  • Dilated vessels, vascular thrombosis, granulation tissue, and empty synovium-lined invaginations histologically
  • Enthesopathy involving ligaments of the proximal and distal borders
  • Chronic passive venous congestion of the foot (subchondral bone pressure and arterial hyperemia)
• Roentgen signs of navicular degeneration
  • Proximal border and extremities
    • Enthesophytes on the extremities
    • Remodeling
  • Distal border changes
    • Synovial invaginations
    • Small osseous fragments\
  • Flexor cortex changes
    • Cortical erosions
    • Mineralization of deep digital flexor tendon
  • Medullary cavity changes
    • Radiolucent cysts
    • Sclerosis

Laminitis

• A peripheral vascular disease manifested by decreased capillary perfusion within the foot
  • Significant amounts of arteriovenous shunting, ischemic necrosis of the laminae, and pain
• Different phases
  • Developmental phase: initiated when horse comes in contact with factors that trigger pathophysiologic mechanisms that cause laminitis
  • Acute phase: onset of lameness and lasts for variable periods of time or if rotation of distal phalanx occurs
  • Chronic phase: when signs of lameness are continual for longer than 48 hours or there is evidence of rotation of the distal phalanx

• Etiology:
  • Ingestion of toxic amounts of grain
  • Ingestion of large amounts of cold water
  • Concussion
  • Endometritis or severe systemic infections
  • Obesity and ingestion of lush grass pasture
  • Miscellaneous
    • Respiratory disease (viral)
    • Overeating beet tops
    • Hormonal
    • High doses of steroids
• Radiographic evidence of rotation of the distal phalanx is identified by divergence of the bone in relationship to the hoof wall

References

• Lindford RL, O’Brien TR, Trout DR: Qualitative and morphometric radiographic findings in the distal phalanx and digital soft tissues of sound Thoroughbred racehorses, Am J Vet Res 54:38, 1993
• Cripps PJ, Eustace RA: Radiological measurements from the feet  of normal horses with relevance to laminitis, Equine Vet J 31:427, 1999.
• Poulos PW, Brown A, Brown E, Gamboa L. On navicular disease in the horse.1989.Vet Rad;30(2):54-58.
• Morandi F, Watrous BJ, Toal RL. The navicular bone. In: Thrall DE (ed): Textbook of veterinary diagnostic radiology, 5th ed. St Louis: Elsevier Saunders, 2012;446-459
• Riedesel EA. The phalanges. In: Thrall DE (ed): Textbook of veterinary diagnostic radiology, 5th ed. St Louis: Elsevier Saunders, 2012;421-445.
• Stashak TS. Lameness. In: Stashak TS: Adam’s lameness in horses. Philadelphia: Lea & Febiger, 1987; 486-785