title: Clinicopathological findings in horses with a bi- or tripartite navicular bone.
abstract: BACKGROUND
  Navicular bone partition is a rare condition reported in horses, which is during the evaluation of a lameness or prepurchase examination often misinterpreted for a parasagittal fracture. In this report, the clinicopathological findings of three cases of navicular bone partition are evaluated. The possible pathomechanisms underlying the condition are hypothesised, focusing on a potential origin of foetal vascular disturbance. This study is furthermore aiming at a clearer and earlier recognition of navicular bone partition, since this condition would finally predispose for a clinical lameness with a poor prognosis.
  
  
  METHODS
  Case 1 was a 10-year-old Belgian Warmblood gelding with a Grade 3/5 chronic, recurrent left-forelimb lameness that had persisted for 4 months. Perineural palmar digital nerve block of the distal foot abolished the lameness. Radiographic examination revealed a bipartite navicular bone in the left forelimb. Unfortunately, the animal was lost to follow-up. Case 2 was a 7-year-old Quarter Horse stallion with a Grade 3/5 recurrent right forelimb lameness that had persisted for 2 years. The lameness switched to the contralateral left forelimb with a palmar digital nerve block. Radiographic examination identified a tripartite navicular bone in both forelimbs. Pathological examination additionally revealed chronic degenerative changes of the cartilage and subchondral bone with marked cystic changes. Case 3 was a 5-year-old Dutch Warmblood gelding with a Grade 3/5 recurrent left hindlimb lameness that had persisted for 6 months. Owing to the uncooperative behaviour of the horse, only a combined peroneal and tibial nerve block could be performed, which abolished the lameness. Radiographic examination revealed a bipartite navicular bone in the left hindlimb. Pathological examination showed a navicular bipartition in the left hindlimb, with microscopic changes comparable to those evident in Case 2; additionally, cartilage indentations were also found in the navicular bones of the right front- and hindlimb at a similar location as the partition site in the left hindlimb.
  
  
  CONCLUSIONS
  It is speculated that a navicular bone partition has a congenital origin and is caused by vascular disturbance during foetal development. This may lead to aberrant endochondral ossification or the formation of multiple ossification centres resulting in navicular bone partitioning. In the adult horse, chronic repetitive biomechanical challenges at the partition sites may induce local degenerative changes with subchondral cyst formation and thus would cause a gradually developing chronic lameness with a poor prognosis.
authors.affiliation: Veterinary Clinic De Delta, Foppenpolder 1, NL-3155 EA, Maasland, The Netherlands.
authors.forename: Ellen J
authors.initials: EJ
authors.lastname: van der Zaag
authors.affiliation: Department of Pathobiology, Division Pathology, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, NL-3584 CL, Utrecht, The Netherlands.
authors.forename: Erik A W S
authors.initials: EA
authors.lastname: Weerts
authors.affiliation: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 108, NL-3584 CM, Utrecht, The Netherlands.
authors.forename: Antoon J M
authors.initials: AJ
authors.lastname: van den Belt
authors.affiliation: Department of Equine Sciences, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 112-114, NL-3584 CM, Utrecht, The Netherlands. W.Back@uu.nl.|Department of Surgery and Anaesthesia of Domestic Animals, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, B-9820, Merelbeke, Belgium. W.Back@uu.nl.
authors.forename: Willem
authors.initials: W
authors.lastname: Back
mesh_terms: D000818:Animals; D001847:Bone Diseases / Q000000981:diagnostic imaging / Q000473:pathology / Q000662:veterinary*; D005552:Forelimb / Q000473:pathology; D006614:Hindlimb / Q000473:pathology; D006734:Horse Diseases / Q000000981:diagnostic imaging / Q000473:pathology*; D006736:Horses; D007794:Lameness, Animal / Q000000981:diagnostic imaging / Q000473:pathology*; D008297:Male; D011859:Radiography; D013639:Tarsal Bones / Q000000981:diagnostic imaging / Q000473:pathology*
keywords: Bi-tripartite navicular bone; Bi-tripartite sesamoids; Navicular fracture; Navicular pathology; Partitioning