268 EQUINE VETERINARY EDUCATION / AE / may 2008
Original Article Radiological interpretation of the navicular bone S. DYSON Centre for Equine Studies, Animal Health Trust, Lanwades Park, Kentford, Newmarket, Suffolk CB8 7UU, UK. Keywords: horse; foot; lameness; podotrochlear apparatus
Summary than previously. Nonetheless it is important to recognise that the absence of significant radiological abnormalities of the navicular Acquisition of a sufficient number of high quality bone does not preclude the existence of significant navicular radiographic views is an essential prerequisite to bone pathology. The purpose of this paper is to review accurate radiological interpretation of the navicular radiological interpretation of the navicular bone, based on bone. This requires appropriate preparation of the foot, previous publications, and clinical comparison with MR images careful attention to limb position and to both centring and/or post mortem specimens. and direction of the x-ray beam, according to hoof capsule conformation. Artefacts are easily created. a) Potentially significant radiological abnormalities include: entheseiophytes at the proximomedial and proximolateral aspect of the bone; proximal or distal extension of the flexor border of the bone, distal border fragments, 8 or more large and variably shaped distal border radiolucent zones; discrete radiolucent areas in the spongiosa with or without detectable communication with the flexor cortex; new bone at the sagittal ridge; increased thickness of the flexor cortex; sclerosis of the spongiosa; and a bipartite bone.
Introduction ai)
Radiography has long been the imaging modality of choice for the diagnosis of navicular disease. However, with the introduction of magnetic resonance imaging (MRI) into the diagnostic armamentarium, the limitations of radiography have been recognised (Dyson et al. 2006). It has also become apparent that there are probably a variety of different pathological processes that can affect the navicular bone, a structure which cannot be considered in isolation (Dyson et al. 2006; Dyson and Murray 2007). The navicular bone has a close relationship with both the other structures of the podotrochlear apparatus (the collateral sesamoidean ligaments [CSL] and the distal sesamoidean impar ligament [DSIL]), and with the navicular Figs 1a and ai: Lateromedial (LM) radiographic view of a foot. This is not a true LM view, but an oblique projection of the navicular bursa and deep digital flexor tendon (DDFT). Comparison of bone resulting in several superimposed lines representing the magnetic resonance (MR) images and radiographs has improved flexor cortex of the bone. This prohibits evaluation of the our interpretation and understanding of radiographs. Moreover, thickness of the flexor cortex and its demarcation from the the advent of computed and digital radiography has potentially spongiosa. The sagittal ridge cannot be assessed. An ossified enhanced our radiographic diagnostic capabilities, by improved cartilage of the foot is partially superimposed over the navicular bone. There is a small spur on the proximodorsal aspect of the image quality. As a result it should now be possible to identify navicular bone. There is also some radiopaque debris on the foot more radiographic abnormalities of potential clinical significance palmar to the navicular bone. EQUINE VETERINARY EDUCATION / AE / may 2008 269
b) (Fig 3). The foot must be positioned caudal to the contralateral foot with the fetlock joint extended, to avoid superimposition of the fetlock over the navicular bone. The x-ray beam should be tangential to the flexor aspect of the navicular bone. The precise angle depends on the conformation of the foot. With low collapsed heels a more shallow angle with the ground is appropriate, whereas with an upright foot the angle should be steeper. Inappropriate angulation results in artefactual increased opacity of the spongiosa and lack of definition between the trabecular and cortical bone (Fig 3b).
Which views are required? bi) A comprehensive radiographic evaluation of the navicular bone requires LM, DPr-PaDiO and PaPr-PaDiO views. A recent publication suggested that PaPr-PaDiO views were not essential, because abnormalities were always detectable on conventional views (De Clerq et al. 2000). However, this study
Figs 1b and bi: The same foot as Figure 1a, after better cleaning. This is a true lateromedial view. The flexor cortex, which is thicker proximally than distally, is clearly demarcated from the spongiosa. The slightly undulating contour of the sagittal ridge of the navicular bone can be seen. The spur on the proximodorsal aspect of the navicular bone is less obvious. The ossified cartilage of the foot is also less obvious. Fig 2a: A poorly positioned dorsoproximal-palmarodistal oblique view of a navicular bone. The fetlock was too flexed during image acquisition resulting in the distal border of the Is the radiograph of diagnostic quality? bone being superimposed over the distal interphalangeal joint (black arrows). The black arrows highlight the distal aspect of the flexor cortex. The more proximal radiopaque line (white Correct positioning during image acquisition is of crucial arrowheads) represents the distal aspect of the dorsal cortex. importance for image interpretation. A true lateromedial (LM) The image has been windowed to demonstrate the lateral projection is required to assess the sagittal ridge of the flexor entheseophyte (white arrow) on the navicular bone. There is a aspect of the navicular bone; the thickness of the flexor cortex parasagittal vertical radiolucent line (black arrowheads) traversing the navicular bone mimicking a fracture. This is an from proximal to distal; the uniformity of opacity of the flexor artefact due to poor packing of the frog. cortex; the demarcation between the endosteal surface of the flexor cortex of the navicular bone and the trabecular bone of the spongiosa; the shape of the navicular bone to identify proximal or distal extension of the flexor aspect of the bone; to identify the presence of dorsoproximal periarticular osteophytes (Fig 1). It is important that the navicular bone is correctly positioned in dorsoproximal-palmarodistal oblique (DPr-PaDiO) views, so that the distal border of the bone is not superimposed over the distal interphalangeal (DIP) joint (Fig 2). Over or under flexion of the fetlock are the most common causes of superimposition of the distal border of the Fig 2b: A well positioned dorsoproximal-palmarodistal oblique navicular bone over the DIP joint. Two views at slightly view of a navicular bone. Medial is to the left. The distal border different angles may be required to evaluate the dorsal and of the navicular bone is well separated from the distal palmar aspects of the proximal and distal borders of the bone. interphalangeal joint. However there is a large area of Correct position of the limb and angulation of the x-ray apparent increased opacity in the centre of the navicular bone (arrows), the result of inadequate trimming of an overgrown beam are crucial to obtain a diagnostic palmaroproximal- frog. There is some debris on the hoof capsule superimposed palmarodistal oblique (PaPr-PaDiO) view of the navicular bone over the distolateral aspect of the navicular bone (arrowheads). 270 EQUINE VETERINARY EDUCATION / AE / may 2008
Fig 4: Palmaro 45° proximal-palmarodistal oblique radiographic Fig 3a: Palmaro 45° proximal-palmarodistal oblique radiographic view of a navicular bone. Medial is to the left. There is a large view of a navicular bone. The x-ray beam was slightly oblique radiolucent defect in the flexor cortex of the navicular bone from left to right and the foot was not positioned sufficiently medial to the sagittal ridge (arrow). There is mild sclerosis of far caudal. This results in the palmar aspect of the fetlock region the spongiosa dorsal to the sagittal ridge. Such radiolucent being partially superimposed over the dorsal articulation of the defects in the flexor cortex are likely to be associated with navicular bone with the middle phalanx. adhesions of the deep digital flexor tendon. No radiological abnormality was seen in lateromedial and dorsoproximal- palmarodistal oblique views.
focused on bones with advanced pathological change. There are many examples of either lucent zones in the flexor cortex of the navicular bone, or less commonly new bone on the flexor cortex that are only detectable in PaPr-PaDiO views (Fig 4). A weightbearing dorsopalmar view is occasionally useful to confirm the presence of a parasagittal fracture or a bipartite navicular bone. It may be the view that is most Fig 3ai: The same foot as Figure 3a, better positioned. The dorsal sensitive for detection of entheseophytes on the cortex of the navicular bone can be clearly evaluated. The proximomedial and lateral aspects of the navicular bone spongiosa appears less opaque than in Figure 3a, although (Fig 5). Appropriate preparation of the foot is crucial for exposure factors and cassette focus distance were identical. The diagnostic images. The shoe should be removed to enable flexor cortex of the navicular bone has uniform thickness and opacity. There are several small lucent areas within the spongiosa. proper trimming of the foot and to avoid superimposition of the shoe over the medial and lateral aspects of the navicular bone. The frog clefts should be packed with appropriate material to eliminate radiolucent lines superimposed over the navicular bone (Fig 2).
Development and shape of the navicular bone
The shape of the navicular bone is considered to be heritable in Dutch Warmblood horses. The proximal articular border Fig 3b: Palmaro 45° proximal-palmarodistal oblique may be convex, horizontal or undulating or concave as seen in radiographic view of a navicular bone. The foot had low a DPr-PaDiO view (Fig 6). In Dutch Warmblood horses it has collapsed heels and was not positioned sufficiently far caudally, been suggested that horses with a convex or horizontal resulting in superimposition of the fetlock over a large proximal articular border are at less risk of navicular disease proportion of the navicular bone. compared with those with an undulating or concave articular margin (Dik and van den Broek 1995; Dik et al. 2001a,b). Similar studies have not been performed in other breeds to verify these observations. A smoothly outlined depression in the sagittal ridge of the navicular bone is a normal variant seen in a LM view (Fig 7). The fossa along the distal border of the navicular bone varies in its depth in a LM view (Figs 1b, 7, 19b and 20c).
Fig 3bi: The same foot as Figure 3b, better positioned, acquired Radiolucent zones along the distal border of the with a more shallow x-ray beam (palmaro 40° proximal- navicular bone palmarodistal oblique), but with the same exposure factors and cassette focal distance. There is much clearer demarcation between the flexor cortex and the trabecular bone of the The interpretation of distal border radiolucent zones, which spongiosa. represent synovial invaginations from the DIP joint (Poulos 1983, EQUINE VETERINARY EDUCATION / AE / may 2008 271
a) abnormal stress at the origin of the DSIL. Comparison between DPr-PaDiO and PaPr-PaDiO views helps to determine the dorsopalmar extent of the radiolucent zones (Fig 9). Large radiolucent zones close to, but apparently discrete from the distal border of the navicular bone may occur alone, or in association with distal border synovial invaginations (Fig 10). Such radiolucent zones usually extend from the palmar to dorsal cortices when assessed in a PaPr-PaDiO view and are not normally seen in clinically sound horses.
ai)
Fig 6a: Well-positioned dorsoproximal-palmarodistal oblique radiographic view of a navicular bone with a convex proximal border. There is a slightly bulbous appearance of the proximomedial and proximolateral aspects of the bone. There is some sclerosis along the distal aspect of the bone and several ill-defined radiolucent zones.
Figs 5a and ai: Dorsopalmar radiographic view of a foot. Medial is to the left. There is a rather irregularly outlined entheseophyte on the proximomedial aspect of the navicular bone superimposed over the middle phalanx (arrow). There is mild ossification of the lateral cartilage of the foot and moderate ossification of the medial cartilage of the foot.
Fig 6b: Well-positioned dorsoproximal-palmarodistal oblique radiographic view of a navicular bone with a horizontal proximal border. There are 2 radiopaque lines representing the dorsal (more distal) and palmar (more proximal) cortices of the bone. There are several variable shaped and sized lucent zones along the distal border of the bone with mild sclerosis.
Fig 5b: Dorsopalmar radiographic view of a foot. Medial is to the left. There is a large entheseophyte on the proximolateral aspect of the navicular bone (arrow) projecting beyond the middle phalanx.
1988), has long been subject to controversy. It is generally accepted that the larger the number of radiolucent zones (>7), of variable size and shape, the more likely they are to be of Fig 6c: Well-positioned dorsoproximal-palmarodistal oblique clinical significance (Fig 8) (Table 1). Such lucent zones may be radiographic view of a navicular bone with an undulating seen in association with localised sclerosis. Radiolucent zones proximal dorsal border (black arrows). Medial is to the left. positioned on the medial and lateral sloping borders of the bone There are several irregularly shaped lucent zones along the distal aspect of the bone and a large lucent zone at the lateral are considered more likely to be of clinical significance (Fig 8d). angle, distal to which is a large osseous fragment (white Clusters of axially positioned lucent zones, combined with distal arrows). This was associated with an adhesion to the deep elongation of the flexor border of the navicular bone may reflect digital flexor tendon. 272 EQUINE VETERINARY EDUCATION / AE / may 2008
Distal border fragments
Osseous fragments on the distal border of the navicular bone usually occur at the distal medial and lateral angles of the navicular bone (Figs 6c, 11b and c), although less commonly involve the medial or lateral sloping border (Fig 11a). Several studies have shown that such fragments occur more commonly in lame horses with other abnormalities of the navicular bone than in clinically normal horses (Wright 1993; Wright et al. 1998; Schramme et al. 2005; Blunden et al. 2006). Prevalence was low in a group of clinically normal horses undergoing a prepurchase examination (Kaser-Hotz Fig 7: Lateromedial view of a normal navicular bone. There is a and Ueltschi 1992). Such fragments may reflect ectopic smoothly outlined depression in the sagittal ridge of the mineralisation in the DSIL, a fracture of the distal border of the navicular bone (white arrows). Note the concave outline of the distal aspect of the bone (black arrow) representing the fossa navicular bone or a fracture of an entheseophyte at the origin between the dorsal and palmar cortices. of the DSIL. The presence of a radiolucent area at the medial
TABLE 1: Radiographic findings of the navicular bone in normal and diseased horses
Grade Condition Radiographic findings
0 Excellent Good corticomedullary demarcation; fine trabecular pattern. Flexor cortex of uniform thickness and opacity. No lucent zones along the distal border of the bone, or several (<6) narrow conical lucent zones along the horizontal distal border. Right and left navicular bones symmetrical in shape.
1 Good As above, but lucent zones on the distal border of the navicular bone more variable in shape.
2 Fair Slightly poor definition between the palmar cortex and the medulla due to subcortical sclerosis. Crescent-shaped lucent zone in the central eminence of the flexor cortex of the bone. Several (<8) lucent zones of variable shape along the distal horizontal border of the navicular bone. Mild entheseophyte formation on the proximal border of the navicular bone. Navicular bones asymmetrical in shape. Proximal or distal extension of the flexor border of the navicular bone.
3 Poor Poor corticomedullary definition due to medullary sclerosis. Thickening of the dorsal and flexor cortices. Poorly defined lucent areas in the flexor cortex of the bone. Many (>7) radiolucent zones along the distal horizontal or sloping borders of the navicular bone. Lucent zones along the proximal border of the bone. Large entheseophyte formation on the proximal border of the bone. Discrete mineralisation within a collateral ligament of the navicular bone. Radiopaque fragment on the distal border of the navicular bone.
4 Bad Large cyst-like lesion within the medulla of the navicular bone. Lucent region in the flexor cortex of the navicular bone. New bone on the flexor cortex of the navicular bone.
a) c) e)
b) d) f)
Figs 8a–f: Dorsoproximal-palmarodistal oblique views of 6 navicular bones showing various patterns of radiolucent zones (synovial invaginations) along the distal border of the bone. Figures 8a and b are considered within the normal range, but Figures 8c–f are abnormal. EQUINE VETERINARY EDUCATION / AE / may 2008 273
Fig 9a: Dorsoproximal-palmarodistal oblique radiographic view Fig 11a: Dorsoproximal-palmarodistal oblique radiographic of a navicular bone. Medial is to the left. There are several view of a navicular bone. Medial is to the left. There is a large variably shaped and sized lucent zones along the distal border osseous fragment (arrows) distal to the medial sloping border of the navicular bone, especially axially. of the bone. There is no reaction in the adjacent navicular bone. However there are several axial lucent zones along the distal border of the bone.
Fig 9b: Palmaroproximal-palmarodistal oblique radiographic view of the same foot as Figure 9a. Medial is to the left. The lucent zones within the spongiosa are very variable in size; there is some sclerosis axially. Fig 11ai: Palmaroproximal-palmarodistal oblique radiographic view of the same foot as Fig 11a. Medial is to the left. The medial and lateral aspects of the navicular bone and the palmar processes of the distal phalanx are superimposed. This obscures the fragment, which is partially superimposed over the flexor cortex. There is some sclerosis of the endosteal bone of the flexor cortex of the navicular bone.
or lateral angle of the distal border of the navicular bone is suggestive of the presence of a fragment, which can be present uniaxially or biaxially. Fragments can be seen as an incidental abnormality, especially if the opacity of the adjacent navicular bone is normal (Fig 11b), but if seen in association Fig 10a: Dorsoproximal-plantarodistal oblique radiographic with a radiolucent area are more likely to be associated with view of a hindlimb navicular bone. There are 2 large axial lameness (Fig 11c). Such fragments have been associated with lucent zones proximal to, and separate from, the distal border marked reactions in the adjacent navicular bone seen in MR of the navicular bone. There are multiple less well-defined small lucent zones along the distal border of the bone. images, presumably the result of chronic movement, or associated with adhesions to the DDFT and/or associated lesions of the DSIL. Fragments are most easily detected in a DPr-PaDiO view, but the ease with which they can be seen is somewhat position dependent. In some horse the presence of a fragment can be verified in a PaPr-PaDiO view (Fig 11b) in which an opacity representing the fragment is usually seen superimposed over the spongiosa of the navicular bone.
Radiolucent zones in the proximal aspect of the navicular bone
Fig 10b: Plantaroproximal-plantarodistal oblique radiographic view of the same foot as Figure 10a. The 2 large lucent zones Proximal border radiolucent zones are occasionally seen and (arrows) extend from the dorsal to plantar cortices and are are not normal. They have been seen in association with separated from each other by a sclerotic band. lesions of the CSL. 274 EQUINE VETERINARY EDUCATION / AE / may 2008
Fig 13: Palmaroproximal-palmarodistal oblique radiographic Fig 11b: Dorsoproximal-palmarodistal oblique radiographic view of a navicular bone. There is a diffuse area of reduced view of a navicular bone. Medial is to the left. There is an oval- opacity of the palmar aspect of the sagittal ridge. This is not shaped osseous fragment (arrows) distal to the lateral angle of considered normal, but was not associated with any other the navicular bone. There is no reaction in the adjacent radiographic abnormality. There was an identical area of navicular bone. However, there are several axial lucent zones slightly increased signal intensity on magnetic resonance along the distal border of the bone. images, without any other reaction. Other lesions were identified that were considered to be causing lameness.
Crescent-shaped radiolucent zones in the flexor cortex
A crescent-shaped lucent zone in the flexor cortex of the navicular bone in the sagittal ridge seen in a PaPr-PaDiO view is considered to be a normal variant, representing a reinforcement line (Berry et al. 1992) (Figs 12, 18a). The crescent-shaped lucent zone in the sagittal ridge of the Fig 11bi: Palmaroproximal-palmarodistal oblique radiographic navicular bone is rarely seen in very young horses. It represents view of the same foot as Figure 11b. Medial is to the left. The early navicular bone modelling, in response to stress. A fragment is seen superimposed over the spongiosa of the relatively sclerotic reinforcement line develops in the navicular bone (arrows). subchondral bone parallel with the flexor cortex in the region
Fig 11c: Dorsoproximal-plantarodistal oblique radiographic view of a navicular bone. Medial is to the left. There are large Fig 14a: Dorsoproximal-palmarodistal oblique radiographic lucent zones at the lateral angle of the navicular bone (black view of a navicular bone. Medial is to the left. There is an oval arrow), distal to which is an osseous fragment (white arrows). shaped acentric radiolucent defect in the bone. There are There are several other large radiolucent zones along the distal multiple lucent zones along the distal border of the bone. border of the navicular bone.
Fig 14ai: Palmaroproximal-palmarodistal oblique radiographic Fig 12: Palmaroproximal-palmarodistal oblique radiographic view of the same foot as Fig 14a. Medial is to the left. There is view of a navicular bone. There is a well-defined crescent reduced opacity of the flexor cortex medial to the sagittal ridge shaped lucent area in the flexor cortex of the navicular bone at (white arrows) and endosteal sclerosis at and lateral to the the sagittal ridge. This is a normal variant. sagittal ridge (black arrowheads). EQUINE VETERINARY EDUCATION / AE / may 2008 275
only detectable radiographic abnormality. Such lesions usually occur at the sagittal ridge or abaxial to it (Figs 4 and 15). The lesion usually reflects erosion of the overlying fibrocartilage and adhesions of the DDFT. There may be associated sclerosis of the trabecular bone dorsal to the lesion.
New bone formation on the flexor cortex of the navicular bone
Fig 14b: Dorsoproximal-palmarodistal oblique radiographic New bone formation on the flexor border of the navicular view of a navicular bone. Medial is to the left. There is a very bone is usually only detectable in a PaPr-PaDiO view and is large central circular radiolucent area. There are several ill- defined lucent zones along the distal border of the bone.
Fig 14bi: Palmaroproximal-palmarodistal oblique radiographic Fig 15: Palmaroproximal-palmarodistal oblique radiographic view of the same foot as Fig 14b. Medial is to the left. There is view of a navicular bone. No abnormality was seen in a a large radiolucent defect in the flexor cortex of the navicular lateromedial projection. There were several variably shaped bone (arrows). The flexor cortex is thickened and there is lucent zones in the distal border of the bone in a extensive sclerosis of the spongiosa, with loss of demarcation dorsoproximal-palmarodistal oblique view. There is a large between the cortex and spongiosa. radiolucent defect in the flexor cortex at the sagittal ridge (arrows); in the spongiosa several large lucent zones are seen. of the sagittal ridge. The intervening bone is relatively There is mild sclerosis of the spongiosa. radiolucent and is projected in the palmaroproximal- palmarodistal oblique view as the crescent shaped lucent zone in the sagittal ridge. If the bone between the reinforcement line and the flexor cortex becomes compacted, then the lucent zone becomes less clear and may be obliterated. An ill-defined loss of opacity at the most palmar aspect of the sagittal ridge has been seen as an incidental finding (Fig 13), with no other pathological change, but may be a precursor to the development of significant degeneration (see ‘Lucent zones in the flexor cortex’). Fig 16: Palmaroproximal-palmarodistal oblique radiographic Central or acentric lucent zones view of a navicular bone. There is new bone on the palmar aspect of the flexor cortex of the navicular bone at the sagittal ridge. No significant radiological abnormality was seen in other views. Central or acentric radiolucent zones, cyst-like lesions, in the spongiosa of the navicular bone seen in DPr-PaDiO views are almost invariably of clinical significance (Fig 14). They usually involve the middle third of the bone from proximal to distal, but not invariably so. In some horses such lesions can be seen to involve the flexor cortex of the bone in PaPr-PaDiO views. However, not all such lesions involve the flexor cortex. Moreover some lesions do penetrate the flexor cortex, but this cannot be verified radiographically. This depends on which part of the flexor cortex is tangential to the x-ray beam, the location of the lesion and its size. Fig 17: Dorsoproximal-palmarodistal oblique view of a Lucent zones in the flexor cortex navicular bone. Medial is to the left. There is a proximal border fragment at the insertion of the lateral collateral sesamoidean ligament (arrows), which was not present on previous A large radiolucent area in the flexor cortex of the navicular radiographs obtained as part of a prepurchase examination bone is invariably clinically significant, and in some horses is the 3 months previously. 276 EQUINE VETERINARY EDUCATION / AE / may 2008
usually centred around the sagittal ridge (Fig 16). It has only been seen in association with lameness, and has not necessarily been seen in conjunction with other abnormalities of the navicular bone.
Proximal border fragments
Fragments displaced from the proximal border of the navicular bone are rare and in the author’s experience have been associated with lameness and presumably reflect an avulsion at the insertion of the CSL (Fig 17).
Thickness of the cortices Fig 18b: Lateromedial radiographic view of a foot with a pathologically thick flexor cortex of the navicular bone, The thickness of the cortices of the navicular bone varies especially distally. There were no other significant radiological within and between horses. The thickness of the flexor cortex abnormalities of this navicular bone of a 4-year-old Warmblood. is easiest to evaluate in a well positioned LM view. Like any However, there was focal increased radiopharmaceutical uptake bone, shape is influenced by Wolff’s law. In a sound horse with in the navicular bone and more extensive pathological change one foot that is much more upright than the other, the flexor identified using magnetic resonance imaging. cortex of the more upright foot is often thinner than the contralateral foot (Fig 18a). In one form of navicular disease there is progressive thickening of the cortices of the bone and the trabeculae of the spongiosa (Wright et al. 1998). Abnormal thickness of the flexor cortex of the navicular bone probably reflects pathological change (Figs 18b and c).
Fig 18c: Lateromedial radiographic view of a foot with a pathologically thick flexor cortex of the navicular bone. There is proximal extension of the flexor cortex. This was associated with massive thickening of the collateral sesamoidean ligament.
Fig 18a: Lateromedial radiographic view of an upright foot with a thin flexor cortex of the navicular bone. The contralateral foot was more normally conformed and the flexor cortex of the navicular bone was thicker.
Fig 18ci: Palmaroproximal-palmarodistal oblique radiographic view of the same foot as Figure 18c. Medial is to the left. The dorsal and palmar cortices are very thick, resulting in a small area for the spongiosa, which is also somewhat sclerotic.
Sclerosis of the spongiosa Fig 18ai: Palmaroproximal-palmarodistal oblique radiographic view of the same foot as Figure 18a. Medial is to the left. The A normal navicular bone has a regular trabecular architecture flexor cortex is thin. The sagittal ridge is less prominent than in that is clearly defined from the dorsal and palmar cortices in Figures 3, 4 and 14a. This is a normal variant. There is a broad both LM and PaPr-PaDiO views. In one form of navicular crescent-shaped radiolucent zone in the flexor cortex at the sagittal ridge, another normal variant. disease there is generalised sclerosis of the spongiosa, EQUINE VETERINARY EDUCATION / AE / may 2008 277
involving particularly the middle third of the bone from affecting the endosteal aspect of the flexor cortex (Fig 19b). laterally to medially. This can also develop as a sequel to acute This can be seen in association with abnormal thickness of the trauma to the bone (Fig 19a). However, a thick overlying frog cortices of the bone, and may be associated with a defect in can create increased opacity in this region and this should be the flexor cortex not detectable radiographically. differentiated from genuine sclerosis. Alternatively sclerosis may be localised towards the palmar aspect of the bone, Corticotrabecular demarcation
Demarcation between the spongiosa and the flexor cortex of the navicular bone must be assessed from both LM and PaPr- PaDiO views. However, it must be borne in mind that
Fig 19a: Palmaroproximal-palmarodistal oblique radiographic view of a navicular bone. There is increased opacity of the middle third of the spongiosa of the bone. This was associated with focal intense increased radiopharmaceutical uptake. There had been acute onset severe lameness 8 weeks previously. No radiographic abnormality was present 4 days after the onset of lameness. This sclerotic reaction is thought to reflect the response to focal trauma to the bone. Care should be taken not to confuse increased radiopacity associated with the overlying frog with genuine sclerosis. Fig 20a: Lateromedial radiographic view of a navicular bone with a large proximal entheseophyte (arrow).
Fig 19b: Lateromedial radiographic view of a foot with a pathologically thick flexor cortex of the navicular bone. There is poor demarcation between the flexor cortex and the Fig 20b: Lateromedial radiographic view of a navicular bone trabecular bone of the spongiosa. Note the distal extension of with a small proximal entheseophyte (arrow). the flexor cortex of the bone and the large fossa on the distal aspect of the bone.
Fig 19bi: Palmaroproximal-palmarodistal oblique radiographic view of the same foot as Figure 19b, with a pathologically thick flexor cortex of the navicular bone and marked endosteal Fig 20c: Lateromedial radiographic view of a navicular bone with sclerosis. There was a partial thickness flexor cortex defect that proximal and distal extension of the flexor cortex of the navicular was not detectable radiographically. bone. Note the large fossa in the distal aspect of the bone. 278 EQUINE VETERINARY EDUCATION / AE / may 2008
Fig 20d: Lateromedial radiographic view of a navicular bone Fig 22: Lateromedial radiographic view of a foot. There is a with proximal and distal extension of the flexor cortex of the large osteophyte on the proximodorsal aspect of the navicular navicular bone. There were also biaxial distal border fragments bone (arrow), in association with periarticular modelling of the seen in a dorsoproximal-palmarodistal oblique view. distal dorsal aspect of the middle phalanx and the proximodorsal aspect of the distal phalanx (arrowheads). positioning of the foot during image acquisition may influence the appearance of the navicular bone in the skyline projection, Entheseophytes and give a false impression of trabecular sclerosis. The 2 views should be compared carefully (Figs 3 and 19). Entheseophytes at the proximal medial and lateral aspects of the navicular bone are best identified in DPr-PaDiO or DPa Distal or proximal extension of flexor border views. Small lateral entheseophytes are a common incidental abnormality and presumably reflect asymmetric stress at the The shape of the navicular bone should be assessed carefully insertion of the lateral and medial CSLs. However, the in a LM view (Verschooten et al. 1989). Distal extension of the presence of a large lateral entheseophyte, or a medial flexor border of the navicular bone may reflect chronic stress entheseophyte, is more likely to reflect abnormal stress on the at the origin of the DSIL (Figs 20a,c and d). Proximal extension podotrochlear apparatus, and potentially to be associated of the flexor border of the navicular bone may reflect chronic with lameness (Figs 5, 21a and b). stress at the insertion of the CSLs (Figs 20b,c and d). Periarticular osteophytes
The navicular bone is part of the distal interphalangeal (DIP) joint. An osteophyte on the dorsoproximal aspect of the
Fig 21a: Dorsopalmar radiographic view of a foot. Medial is to the left. There is a moderately sized entheseophyte on the proximomedial aspect of the navicular bone. Medial entheseophytes are not a normal finding. There is also moderate symmetrical ossification of the cartilages of the foot.
Fig 23: Lateromedial radiographic view of a right front foot of a 5-year-old Warmblood with severe bilateral forelimb lameness. The distal aspect of the navicular bone is unusually close to the distal phalanx. There is distal extension of the flexor cortex of the navicular bone, which is also thickened. There is a small periarticular osteophyte on the dorsoproximal aspect of the navicular bone. There is modelling of the extensor process of the distal phalanx. A large lateral entheseophyte, biaxial distal border fragments and sclerosis of the spongiosa Fig 21b: Dorsoproximal-palmarodistal oblique radiographic view of the navicular bone were seen in other radiographic views. of a navicular bone with a large entheseophyte proximolaterally. There was also modelling of the distal dorsal lateral aspect of This is not normal. There are also several variably shaped and the middle phalanx and narrowing of the DIP joint space sized lucent zones along the distal border of the bone. laterally, consistent with osteoarthritis of the DIP joint. EQUINE VETERINARY EDUCATION / AE / may 2008 279
navicular bone seen in a LM view is likely to reflect disease of the DIP joint, which can occur alone or in association with navicular disease (Fig 22).
Position of the navicular bone
The position of the navicular bone in a LM radiograph relative to the middle and distal phalanges is partly influenced by the position of the limb during image acquisition. However, occasionally the bone is abnormally close to the distal phalanx. Fig 25: Dorsoproximal-palmarodistal oblique radiographic view This has been seen in young horses with lameness recognised of the left (medial to the left) front foot of a yearling soon after the introduction of work and has generally been Thoroughbred with a recent onset of low grade, bilateral associated with advanced pathological changes of the shifting forelimb lameness. There had been no prior lameness. navicular bone and the DIP joint (Fig 23). This may reflect a There is a lateral parasagittal oblique radiolucent line in the navicular bone with large lucent areas adjacent to the congenital abnormality. radiolucent line. Similar radiological abnormalities were seen in the contralateral foot, in addition to large lucent zones along Fracture of the navicular bone the distal border in the right front foot. The proximodistal height of each navicular bone was greater laterally than medially. These probably represent congenital bipartite Fractures of the navicular bone are usually parasagittal and are navicular bones. seen in the acute phase as one or 2 radiolucent lines traversing the bone obliquely from proximal to distal (Fig 24a). In more chronic fractures radiolucent zones develop along the fracture line, which may be quite broad (Fig 24b). Less commonly other configurations of fracture occur.
Bipartite navicular bone
Rarely the navicular bone develops as 2 separate centres of ossification that never unite by osseous union. This may occur unilaterally or bilaterally. There is a broad, well-defined lucent line between the 2 pieces. If unstable, secondary lucent areas may develop along the borders of the lucent line. A bipartite navicular bone may be identified in a clinically normal horse, Fig 24a: Dorsoproximal-plantarodistal oblique view of a hind foot. Medial is to the left. There is a complete, recent, but such horses may experience episodic lameness if in full nondisplaced parasagittal fracture of the navicular bone athletic function (Fig 25). However, in contrast to a fracture, (arrows). The horse had an acute onset of severe lameness 2 there is not a history of acute onset severe lameness. days previously. Proximal displacement of the navicular bone secondary to rupture of the DSIL
A rare injury, to date recorded only in steeplechase horses, is proximal displacement of the navicular bone, secondary to rupture of the DSIL (Heitzmann and Denoix 2007). This occurs predominantly in hindlimbs.
Hindlimb navicular bone lesions
Fig 24b: Dorsoproximal-palmarodistal oblique radiographic Navicular disease is not exclusive to forelimbs and is view of a navicular bone. Medial is to the left. There is a occasionally seen in hindlimbs either unilaterally or bilaterally. vertical, broad, lateral parasagittal radiolucent line, adjacent to Rarely this has been seen in association with unusual proximity which are some ill-defined lucent areas. The lateral wing of the navicular bone is modelled. This could be an old fracture or between the navicular bone and the distal phalanx, suggestive represent a bipartite navicular bone. The horse had been in the of a congenital abnormality. owner’s possession for 4 years, with no history of lameness, prior to sudden onset lameness 2 weeks previously. There was Conclusion normal radiopharmaceutical uptake in the navicular bone. Lameness was associated with a core lesion of the lateral lobe of the deep digital flexor tendon. The contralateral navicular There are a variety of radiographic abnormalities of the bone appeared normal. navicular bone that are potentially detectable, provided that 280 EQUINE VETERINARY EDUCATION / AE / may 2008
the radiographs are of high quality and are appropriately Dik, K., van den Belt, A. and van den Broek, J. (2001b) Relationships positioned. There is some degree of variation of the of age and shape of the navicular bone to the development of navicular disease: a radiological study. Equine vet. J. 33, 172-175. architecture of the navicular bone in clinically normal horses, but some abnormalities, even when subtle, can reflect severe Dyson, S. and Murray, R. (2007) Magnetic resonance imaging of the equine foot. Clin. tech. equine Pract. 6, 46-61. pathology. The absence of significant radiological changes Dyson, S., Murray, R., Blunden, T. and Schramme, M. (2006) Current does not mean that the navicular bone is structurally normal, concepts of navicular disease. Equine vet. Educ. 18, 45-56. or preclude the bone as a potential source of pain causing Heitzmann, A. and Denoix, J.-M. (2007) Rupture of the distal lameness. sesamoidean impar ligament with proximal displacement of the distal sesamoid bone in a steeplechaser. Equine vet. Educ. References 19, 117-120. Kaser-Hotz, B. and Ueltschi, G. (1992) Radiographic appearance of the Berry, C.R., Pool, R.R., Stover, S., O’Brien, T.R. and Koblik, P.D. (1992) navicular bone in sound horses. Vet. Radiol. Ultrasound 33, 9-17. Radiographic/morphologic investigation of a radiolucent crescent Poulos, P. (1983) Correlation of radiographic signs and histological within the flexor central eminence of the navicular bone in changes in the navicular bone. Proc. Am. Ass. equine Practnrs. Thoroughbreds. Am. J. vet. Res. 53, 1604-1611. 29, 241-255. Blunden, A., Dyson, S., Murray, R. and Schramme, M. (2006) Poulos, P. (1988) The nature of enlarged ‘vascular channels’ in the Histological findings in horses with chronic palmar foot pain and navicular bone of the horse. Vet. Radiol. 29, 60-64. age-matched control horses. Part 1: Navicular bone and related Schramme, M., Murray, R., Blunden, A. and Dyson, S. (2005) A structures. Equine vet. J. 38, 15-22. comparison between MRI, pathology and radiology in 34 limbs De Clerq, T., Verschooten, F. and Ysebaert, M. (2000) A comparison of with navicular syndrome and 25 control limbs. Proc. Am. Ass. the palmaroproximal-palmarodistal oblique view of the isolated equine Practnrs. 51, 348-358. navicular bone to other views. Vet. Radiol. Ultrasound 41, 525-533. Verschooten, F., Roels, J., Lampu, P., Desmet, P., De Moor, A. and Dik, K. and van den Broek, J. (1995) Role of navicular bone shape in Picavet, T. (1989) Radiographic measurements from the the pathogenesis of navicular disease: A radiological study. Equine lateromedial projection of the equine foot with navicular disease. vet. J. 27, 390-393. Res. vet. Sci. 46, 15-21. Dik, K., van den Belt, A., Enzerink, E. and van Weeren, P. (2001a) The Wright, I. (1993) A study of 118 cases of navicular disease: radiological radiographic development of the distal and proximal double features. Equine vet. J. 25, 493-500. contours of the equine navicular bone on dorsoproximal- Wright, I., Kidd, L. and Thorp, B. (1998) Gross, histological and palmarodistal oblique (upright pedal) radiographs from age 1 to histomorphometric features of the navicular bone and related 11 months. Equine vet. J. 33, 70-74. structures in the horse. Equine vet. J. 30, 220-234.