Gross, histological and histomorphometric features of the navicular bone and related structures in the horse

Abstract

Summary Forelimb navicular bones and associated soft tissues were collected from 3 groups of horses and subjected to pathological examinations. The groups consisted of 38 horses with clinical navicular disease (ND) and 2 control groups, with no history of forelimb lameness, consisting of 25 age‐matched mature horses (A‐MC) and 9 immature horses (IC). Histological and histomorphometric studies were performed on tissue samples from 10 ND, 10 A‐MC and 5 IC horses. Gross changes seen only in ND horses included: full thickness defects in the palmar surface fibrocartilage, palmar cortex erosion, medullary lysis, flexor digitorum profundus tendon (FDPT) surface fibrillation, FDPT core lesions and adhesions between the FDPT and navicular bone. Palmar surface partial thickness fibrocartilage loss and distal border fragmentation were seen with a significantly greater incidence in ND than in A‐MC and not observed in IC. Remodelling of the proximal border, FDPT surface colouration, palmar surface fibrocartilage colouration and proximal border entheseous bone were identified in ND and A‐MC but not in IC. Mid‐ridge synovial fossae and horizontal depressions in the palmar surface were identified in all groups. Histologically palmar fibrocartilage thinning and loss were associated with reduced palmar fibrocartilage cell density and chondrocyte cluster formation. Palmar fibrocartilage fibrillation, palmar cortical bone defects, fibromyxoid stromal change in the medulla, medullary pseudocyst formation and entheseous new bone formation were all seen in ND. The adjacent FDPT showed fibrillation, tag formation and degeneration of the dorsal surface. Necrotic foci were also present within the body of the tendon. Although not always present, medullary bone pseudocysts, separate mineralised foci and most changes on the dorsal surface of the FDPT were specific to ND. Bone histomorphometric parameters were compared among groups. Cross‐sectional area reduced from the sagittal ridge to the medial and lateral margins of each navicular bone. IC navicular bones had a smaller subchondral area, subchondral bone volume and a greater osteoid volume than in the AC, indicating that these differences were age‐related. In ND the medullary area was decreased but the trabecular bone volume increased. The palmar subchondral area was increased but contained bone with an increased porosity and osteoid volume. Changes occurred from the medial to the lateral margins of the bone in horses with ND indicating remodelling of the bony elements throughout the bone in ND. The histological and histomorphometric changes in the navicular bone and palmar fibrocartilage were considered similar of those found in articular hyaline cartilage and subchondral bone in osteoarthritis.

MRI and navicular disease – on the brink of success or another false dawn?

“…In 1998, Wright and others described core lesions of the deep digital flexor tendon, and fibrillation of the dorsal border of the tendon, in a postmortem study of horses with navicular syndrome (Wright and others 1998). Attempts to image these lesions were made ultrasonographically, and it proved technically possible to achieve images of the deep digital flexor tendon, both distal to the navicular bone, and proximal to it (Hauser and others 1982, Kristoffersen and Thoefner 2003, Sage 2002).…”

Relationship between histopathological lesions and magnetic resonance imaging (MRI) in diseases of the foot of the horse

“…Such lesions have not yet been characterised histologically and their etiology remains speculative, but recent evidence suggest that their presence is associated with degenerative changes in the impar ligament (Dyson et al 2010). In recent postmortem studies, osseous fragments associated with a defect in the distal margin of the navicular bone were more common in horses with navicular disease than in age-matched controls (figures 3a and 3b) (Busoni et al 2005;Schramme et al 2005;Blunden et al 2006;Wright et al 1998). Histologically, distal border fragments have variously been described as avulsion fractures, separate centers of ossification, osseous metaplasia of the impar ligament or synovial osteoma but pathological evidence elucidating their pathogenesis remains elusive.…”

“…They were initially described with equal frequency in horses with clinical navicular disease and agematched control horses (Wright et al 1998). Although it was suggested that these vascular changes could be age-related (Wright et al 1998), larger and later pathological studies have found that vascular changes in the DDFT were significantly more common in the DDFT of horses with foot pain (Busoni et al 2005;Blunden et al 2006 b;Blunden et al 2009). It was therefore proposed that vascular thrombosis and occlusion could result in matrix changes that predispose horses to injury of the distal portion of the DDFT (Blunden et al 2006 b).…”

“…Bundles of fibers torn away from the surface of the tendon have a tendency to curl up proximally in the navicular bursa. Crevices, splits and fibrillations of the dorsal border of the DDFT may be accompanied by either small, discrete or large, circumscribed foci of necrosis (Wright et al 1998). Chondrocyte clusters can be seen to produce chondroid matrix in areas of degenerative change (Blunden et al 2009).…”

“…Loss of fibrocartilage in this location is the most common lesion significantly associated with navicular disease and most likely represents the earliest pathology of classic navicular disease (Wright et al 1998). Fibrocartilage loss from this location remains difficult to identify in vivo, even with the use of MRI (Schramme et al 2009).…”