Navicular syndrome (or navicular disease) is a degenerative syndrome involving the osseous and soft tissue structures of the podotrochlear apparatus (Wright et al, 1998). The degeneration of the podotrochlear apparatus is thought to be because of biomechanical overloading from a combination of workload and conformation (Wright and Douglas, 1993). Pain originating from this region is estimated to be responsible for at least one-third of chronic forelimb lameness in Quarter Horses, Thoroughbreds and Warmbloods (Stashak, 1998; Dyson et al, 2011). Because radiography is commonly the first imaging modality performed following lameness evaluation, accurate radiographic interpretation requires well-positioned radiographs with a thorough understanding of normal variants, artefacts and the spectrum of radiological lesions that can affect the appearance of the navicular bone. This article will review methodology for the evaluation and interpretation of the most frequently encountered radiographic lesions of the navicular bone based on a review of the literature and the authors' experiences. Note that although navicular lesions occur in both the fore and hind limbs, the forelimb terminology will be used for ease of discussion.
Normal radiographic appearance
The navicular bone, or distal sesamoid bone, is located on the palmar aspect of the distal interphalangeal joint suspended by the collateral sesamoidean ligament, which attaches to the distal abaxial aspects of the proximal phalanx and tethered to the distal phalanx by the distal sesamoidean impar ligament. The palmar border (flexor cortex) of the navicular bone is covered with a layer of fibrocartilage, which, along with the navicular bursa, helps create a smooth gliding surface for the deep digital flexor tendon. These structures are collectively named the navicular or podotrochlear apparatus. The navicular bone is made of compact cortical bone and internal spongiosa, which are radiographically distinguishable. The conical smooth central defects of the distal aspect of the bone are synovial invaginations that communicate with the distal interphalangeal joint (Poulos and Smith, 1988; Olive and Videau, 2017).
The radiographic projections that best highlight the navicular bone include:
- Lateromedial, dorso-55° proximal-palmarodistal oblique (upright pedal or D55°Pr-PaDiO)
- Palmaro-45-55° proximal-palmarodistal oblique (navicular skyline) (Figure 1).
Dorso 60° proximal 45° lateral-palmarodistomedial and dorso 60° proximal 45° medial-palmarodistolateral oblique views are for identification of distal border fragments. While it is an important component of routine foot radiographs, the horizontal beam dor-sal-palmar view typically provides the least information regarding the navicular bone.
Precise radiographic acquisition is important because obliquity will result in artefacts that interfere with accurate radiographic evaluation of the navicular bone. Whenever possible, removal of horseshoes and pads will improve the quality of radiographs acquired. The foot should be well cleaned, and debris removed from the sole. Packing the sulci of the frog has debatable benefit. Packing usually decreases artefacts in this area but will occasionally result in artefacts such as accentuating lucent gas lines or increasing the opacity when superimposed with bones which can be mistaken for fractures or sclerosis. Packing was found to aid only in approximately 10.8% of cases when assessing anatomic landmarks (Rowan et al, 2019). Repeating radiographs without packing, repacking to improve packing quality and/or obtaining obliqued images should be considered if there is a clinical concern.
One of the more challenging radiographs to acquire, the navicular skyline, can provide crucial information. A recent study found that caudal limb placement without toe elevation and a beam angle of 45º +/‒ 5º resulted in the best quality navicular skyline image when compared to neutral limb placement and toe elevation (Peeters et al, 2023). Another study suggested that additional views with varied beam angles of the navicular skyline could aid in detecting flexor abnormalities (Johnson et al, 2018). Poor positioning of the limb and/or insufficient beam angle of the navicular skyline view will result in superimposition of the proximopalmar border of the navicular bone and will artefactually increase the opacity of the spongiosa (Dyson, 2011). Likewise, during acquisition of the upright pedal view, insufficient beam angle will not provide adequate separation of the distal border of the navicular bone from the distal interphalangeal joint, limiting evaluation of the distal margin.
A grading scale of the radiographic appearance of the navicular bone in the upright pedal view has been previously established based on presence and severity of navicular bone sclerosis, conical channels (synovial invaginations), shape and enthesophytes (Dik and van den Broek, 1995). While a grading scale provides a useful template for consistent evaluation, classification on a grading scale and potential indications of certain grades should be interpreted with caution. In one study, a group of sound horses with mild radiographic changes of the navicular bone (grade 2) showed significant early histopathologic lesions, including fibrillation and disruption of the deep digital flexor tendon and thinning of the fibrocartilage and subchondral bone (Komosa et al, 2014). Clinical lameness, pattern of diagnostic analgesia, radiographic lesions and response to therapeutics should be evaluated together for a more individualised prognosis.
Radiographic evaluation should include the following regions:
- Spongiosa:
- Sclerosis of the spongiosa (decreased corticomedullary definition)
- Increased lucent regions (cyst-like changes, dilated synovial invaginations).
- Dorsal cortex:
- Distal interphalangeal osteophytes.
- Proximal border:
- Enthesopathy of the collateral sesamoidean ligament
- Dilated vascular channels.
- Distal border:
- Synovial invagination number and size
- Cyst-like lesions
- Impar ligament enthesopathy
- Distal border fragments.
- Palmar border (flexor cortex):
- Flexor cortex lysis or demineralisation
- Cortical thickening.
Normal anatomical variations of the navicular bone
Normal anatomical variants occur in many locations in the horse and knowledge of these variants will help prevent misdiagnosis or negative advice in pre-purchase exams (Becht et al, 2001; Hinkle et al, 2020). The shape of the navicular bone, specifically the proximal border, is considered heritable in Dutch and Hanoverian Warmblood horses (Dik and van den Broek, 1995; Dik et al, 2001). Horses with a convex or horizontal proximal border are reportedly at lower risk of navicular syndrome compared to those with an undulating or concave margin (Dik and van den Broek, 1995). The navicular bone shapes considered to be at the highest risk for development of navicular syndrome (the undulating or concave articular margins) were not present in the older horse population, indicating these shapes likely resulted in either early retirement and/or a shorter lifespan. Horses with navicular bones considered to be a lower risk (a convex or horizontal border) were more prevalent in the older population, but still developed navicular syndrome. This indicates that accumulated chronic biomechanical overload is harmful to all navicular bones regardless of shape (Dik et al, 2001).
Elongation of the navicular bone flexor cortex is a frequent finding identified on the lateral view in sound horses, occurring in over half of sampled sound horses at the distal margin and in a third of sound horses at the proximal margin (Kaser-Hotz and Ueltschi, 1992) (Figure 2a). Elongation of the navicular bone flexor cortex is not considered a valuable criterion to support the diagnosis of navicular syndrome because of its high incidence in sound horses (Kaser-Hotz and Ueltschi, 1992). The articulation between the distal phalanx and the navicular bone was more frequently parallel, although a convergent joint space was also found in sound horses (Kaser-Hotz and Ueltschi, 1992) (Figure 2b).
The morphology of the sagittal ridge (central eminence) of the flexor cortex of the navicular bone is best outlined on the navicular skyline radiograph. It can be variably shaped, including the typically smooth rounded shape, flattened, pointed or raised with a central plateau (Kaser-Hotz and Ueltschi, 1992; Becht et al, 2001). The variable presence of an internal crescent-shaped lucency in the sagittal ridge is frequently identified on the navicular skyline radiograph and represents noncompact bone between a reinforcement line of compacted cancellous bone and the flexor cortex (Berry et al, 1992) (Figure 3). Horses with a more prominent central lucency on the skyline image tend to have a more prominent concave curvature or smoothly marginated dimple of the flexor cortex on the lateral image (Dyson, 2011). Unlike a true flexor cortical erosion, the crescent shaped lucency on the navicular skyline radiograph is well defined, of consistent shape and bordered by smooth compact bone. Additionally, the appearance of the normal lucent region is generally symmetrical between limbs on the same horse.
Spongiosa: sclerosis and corticomedullary definition
The spongiosa is frequently called the medulla or medullary cavity of the navicular bone. However, this is a misnomer because sesamoid bones lack a true medullary cavity, where yellow marrow resides and no spongy bone is present in long bones. Spongiosa, also known as spongy bone or cancellous bone, has a lattice-like network of trabeculae of which some spaces contain red bone marrow (Gabriel et al, 1998; 1999; Houssaye et al, 2022). The opacity of the normal spongiosa is always less than that of the flexor cortex in the normal horse, with a sharp distinctive margin between the flexor cortex and spongiosa. Sclerosis of the spongiosa and loss of the distinction between the flexor cortex and spongiosa are frequently referred to as ‘loss of corticomedullary definition’, which is generally accepted terminology despite the lack of a true medullary cavity.
Most frequently, sclerosis is identified as a focal or diffuse region of increased opacity, often extending from the endosteal surface of the flexor cortex (Figure 4). A diffuse haze across the entire spongiosa or more linear increased opacity bisecting the long axis of the bone should flag the interpreter to consider superimposition as the source of this appearance. If the navicular skyline image is not obtained at the appropriate angle, which is horse and hoof dependent, then the borders of the proximal or distal margin become superimposed with the spongiosa. To prevent this error, the examiner must ensure that the joint space between the navicular bone and the middle phalanx should be sharp and clear; if this interface is ill-defined on the skyline view, there is a greater likelihood of artefactual increased opacity of the navicular bone. Sclerosis of the spongiosa is associated with increased forces exerted by the deep digital flexor tendon on the flexor cortex (Pool et al, 1989). Sclerosis is an early and consistent finding in navicular syndrome, although it has reportedly been found in 16% of sound horses (Kaser-Hotz and Ueltschi, 1992). Focal round regions of sclerosis on the abaxial aspects of the spongiosa on the navicular skyline image may represent superimposition of a distal border fragment.
Cyst-like lesions in the spongiosa separate from synovial invaginations or flexor cortex erosions have been identified in some navicular bones but have not been investigated with histology. In one study, the presence of cyst-like lesions was positively associated with the severity of distal sesamoid impar ligament injury based on magnetic resonance imaging and histopathology (Dyson et al, 2010). The majority of round lucent defects found in the spongiosa of the navicular bone represent flexor cortical erosions superimposed with the spongiosa on the D55°Pr-PaDiO oblique view or abnormal dilation of the synovial invaginations rather than true cystic lesions.
Proximal border: enthesopathy of the collateral sesamoidean ligament and vascular channels
Osseous proliferation of the lateral and medial proximal margins of the navicular bone is best identified on the upright pedal view and represents enthesophyte formation at the attachment of the collateral sesamoidean ligament of the navicular bone (Dyson, 1988; Pool et al, 1989). This change has been reported to be found in 5% of sound horses and horses affected by navicular syndrome, so the clinical significance remains unclear (Kaser-Hotz and Ueltschi, 1992) (Figure 5). In the authors' opinion, this finding may occur with other significant navicular changes, but rarely represents a primary clinically significant lesion when found alone.
As navicular syndrome develops, progressive enlargement of the arterial supply of the proximal border can be detected radiographically as a change in shape on the proximal border on the lateral view, with increased concavity to the proximal cortex. In more advanced cases, lucent regions may be viewed on the proximal aspect of the navicular bone on the upright pedal view. The lucent vascular channels in the proximal border of the navicular bone are superimposed with the distal border synovial invaginations on the navicular skyline image, limiting the value of the skyline view for assessing this change. These changes indicate a shift in arterial supply to the navicular bone: decreased distal arterial supply to an increased proximal arterial supply to the point that distal arteries may become absent in horses with navicular syndrome (Rijkenhuizen et al, 1989).
Distal border: synovial invaginations and distal border fragments
A few conical lucent regions are normally present on the distal border of the navicular bone, highlighted by the upright pedal view and found in the centre of the spongiosa on the navicular skyline image. These lucent regions represent synovial invaginations from the distal interphalangeal joint and do not communicate with the navicular bursa, as confirmed by computed tomography comparison of bursography versus arthrography and gross dissection (Poulos and Smith, 1988; Olive and Videau, 2017). Previous reports of up to seven synovial invaginations have been reported in normal, sound horses with an average of four to five typically present (Colles, 1983; Dyson, 1988; Poulos and Smith, 1988; Park, 1989). Hind feet typically have fewer synovial invaginations than fore feet (Butler, 2016). Normal invaginations should have a greater height than width (Colles, 1983). Overall, radiographs underestimate the number and depth of synovial invaginations of the navicular bone with computed tomography identifying an average of two invaginations more than radiography (Claerhoudt et al, 2012). Computed tomography of 50 Warmblood horses identified an average total number of synovial invaginations of 5.9 +/‒ 1.6 (Claerhoudt et al, 2012).
In the authors' experience, the number and size of synovial invaginations are also breed-dependent. Warmblood horses typically have larger synovial invaginations than small breed horses (Figure 6). Given the variation in the normal shape and number, there can be a grey area between normal and abnormal on an individual horse. In those cases, comparison between limbs is of value, as asymmetry is more suggestive of abnormal changes. One report found abnormal invaginations in 11% of sound horses, which indicates that abnormal invaginations identified in isolation may not be conclusive for diagnosis of navicular syndrome (Kaser-Hotz and Ueltschi, 1992). In the authors' experience, it is not uncommon to find horses with enlarged synovial invaginations without other navicular changes that are unaffected clinically. In more advanced cases, enlarged synovial invaginations may be associated with localised osteonecrosis extending into the cortical and medullary bone (Blunden et al, 2006) (Figure 7). The way in which synovial invaginations enlarge can be of importance; enlargement of the synovial invaginations in a dorsalpalmar dimension can result in pressure resorption of the palmar endosteum of the navicular bone and, in more advanced cases, flexor cortical lysis.
Since synovial invaginations are extensions of the distal interphalangeal joint, it has been postulated that this abnormality is an indicator of joint disease rather than navicular syndrome (Olive and Videau, 2017). Recent investigation with magnetic resonance imaging has found associations between synovial invagination changes and both distal interphalangeal joint and navicular apparatus abnormalities (McParland et al, 2023). Given this, when observing synovial invagination changes, pathology of the distal interphalangeal joint and the navicular apparatus should both be considered and further evaluated (McParland et al, 2023).
Distal border fragments are osseous bodies of various size and shape which are found at the lateral or medial angles of the navicular bone. Distal border fragments are most easily identified on the upright pedal or oblique upright pedal radiograph (Figure 11). These fragments have been described as avulsion fracture of the navicular bone, fracture of an enthesophyte at the origin of the distal sesamoidean impar ligament, a separate centre of ossification, synovial osteoma or dystrophic mineralisation in the distal sesamoidean impar ligament (Poulos and Brown, 1989; Wright, 1993). Larger fragments can also be identified on the lateromedial view and as a rounded region of increased opacity superimposed with the spongiosa on the navicular skyline. One study found distal border fragments in 13.6% of horses and 9.8% of feet (Biggi and Dyson, 2011). Fragments were identified equally in a single foot or both front feet and were more commonly uniaxial and could be medial or lateral (Biggi and Dyson, 2011). Radiography has low sensitivity (37.8–42.8%) but high specificity (99.4–100%) for the detection of distal border fragments (Schramme et al, 2005; Biggi and Dyson, 2010). Fragments with more severe signal changes in the distal border (based on higher-grade magnetic resonance imaging) and medium- and large-sized fragments of the navicular bone were most likely to be identified radiographically (Biggi and Dyson, 2010). Fragments observed radiographically were likely to be associated with other pathological abnormalities within the navicular bone, but up to 45% of the higher-grade fragments and 43% of the large-sized fragments were not detected radiographically (Biggi and Dyson, 2010). Image quality has a substantial impact on the likelihood of detecting fragments. A poorly prepared hoof with solar debris, inadequate radiographic technique, insufficient beam angle of the upright pedal view and lack of oblique upright pedal views are all factors that decrease fragment identification.
Previous studies have described distal border fragments in both lame and non-lame horses (Poulos and Brown, 1989; Kaser-Hotz and Ueltschi, 1992; Wright, 1993; Schramme et al, 2005; Dyson, 2011). The frequency of occurrence has been determined to be higher in lame horses (Wright et al, 1998; Blunden et al, 2006). Other research found no increased probability of lameness if a fragment was present, although there was a slight increased probability of lameness if both medial and lateral fragments were present (Yorke et al, 2014). In the authors' experience, smooth, small ovoid fragments of otherwise normal navicular bones are often found incidentally – both radiographically and with magnetic resonance imaging.
Examining the image for radiolucent defects on the distolateral or distomedial aspects of the bone can help improve identification of distal border fragmentations and their associated fragment beds. While these lesions can correspond with fragmentation, false positives have been recognised following magnetic resonance imaging evaluation (Biggi and Dyson, 2010). False positive radiolucencies at the angles of the navicular bone are typically as a result of artefacts from superimposition of solar gas or incidental contour variation in radiography.
Dorsal border: osteophytes
The dorsal cortex of the navicular bone articulates with the distal interphalangeal joint. Curved or triangular osseous proliferations occuring at the dorsal-proximal aspect of the navicular bone adjacent to the second phalanx represent osteophytes (Wright et al, 1998). Osteophytes at this site most likely represent osteoarthritis of the distal interphalangeal joint rather than a sign of navicular syndrome (Wintzer and Dämmrich, 1971; Hertsch and Zeller, 1976).
Palmar border (flexor cortex): erosive lesions
Flexor cortical erosions have been identified in fewer than 1% of sound horses, thus making them a strong indicator of clinically significant navicular syndrome (Kaser-Hotz and Ueltschi, 1992). This finding is considered highly relevant by the authors because of the likelihood of a career-limiting prognosis. Flexor cortical defects are most frequently accompanied by other radiographic signs of navicular syndrome, such as sclerosis of the spongiosa, thickening of the flexor cortex and loss of a distinct interface between cortex and spongiosa (Kaser-Hotz and Ueltschi, 1992) (Figure 9). However, it is possible to have small flexor cortex erosions identified on magnetic resonance imaging that do not have any identifiable bone change on the radiographs; this is most likely in the early phase of disease. Increased availability of magnetic resonance imaging and owner's willingness to have their horses undergo advanced imaging has revealed these smaller changes which are not evident radiographically but could be the source of lameness.
Flexor cortical thickness can vary with breed and individual horse (Gabriel et al, 1998). The mean width of the flexor cortex as measured on the navicular skyline radiograph was 3.6 mm in a large study of sound Warmblood horses and reported as 2 mm in Thoroughbred horses (O'Brien et al, 1975; Kaser-Hotz and Ueltschi, 1992). Flexor cortical widths greater than 4.3 mm were found in horses considered to have navicular syndrome (Ueltschi, 1983). Thickening of the flexor cortex is usually identified more prominently on the proximal aspect of the bone and often corresponds with endosteal sclerosis.
Central round lucent regions of the navicular bone identified on the upright pedal view can represent cyst-like lesions of the spongiosa or flexor cortex erosions. This must be correlated with the appearance of the flexor cortex on the lateral and navicular skyline images to differentiate the disease processes. Acquiring additional navicular skyline images of varying degrees has been proven to be helpful in the identification of flexor cortical erosions (Johnson et al, 2018). A shallower angle for the navicular skyline will better highlight the distal aspect of the navicular bone where lysis is known to occur (Figure 10). This can also be helpful to better differentiate true lesions from artefact created by gas within the sulci of the frog.
Navicular bone fractures versus bipartite or tripartite navicular bone
Previous hypotheses for the underlying cause of bipartite or tripartite navicular bones include formation of multiple ossification centres and/or a disturbance in blood supply leading to incomplete ossification of the cartilaginous bone model or traumatic fracture during the neonatal or juvenile period (van der Zaag et al, 2016). Distinguishing the partite navicular bone from traumatic fractures depends on the degree and duration of lameness and radiographic features. Horses with bipartite or tripartite navicular bones usually present with chronic, persistent lameness with a gradual onset (van der Zaag et al, 2016). On radiographic evaluation, these navicular bones have a uniform distribution of partition sites at approximately one-third of the length of the navicular bone without substantial peri-articular remodelling (van der Zaag et al, 2016). The partitions usually have rounded bone edges, and can have lysis or cyst-like lesions develop at the partitions' interface depending on the stage of degeneration (Figure 11). Conversely, fractures in the acute phase will have sharp margins, whereas chronic fractures may have rounded margins – chronic fractures are unlikely to occur at such a distinct interval as a partite navicular bone (Figure 12). Bipartite navicular bones have been reported to occur both unilaterally and bilaterally (van der Zaag et al, 2016). Comparison to the contralateral limb can be helpful to distinguish between bipartite navicular bones and fractures, as fractures rarely occur bilaterally. A unilateral lesion can remain unclear at times as to the underlying cause. Both disease processes are often associated with progressive degenerative disease, although some horses with fractures will make a full recovery.
Distal phalanx angle and navicular bone relationship
Not only should the radiographic appearance of the navicular bone be examined, but the distal phalanx angle should also be assessed radiographically. The relationship between the distal phalanx angle and the biomechanical forces placed on the navicular bone are important considerations for making recommendations to owners and farriers for shoeing. A low palmar angle of the distal phalanx will result in biomechanical overload on the navicular bone by the deep digital flexor tendon (Wilson et al, 2001; Eliashar et al, 2004; Weaver et al, 2009) (Figure 13). Supporting the palmar angle with therapeutic shoeing, such as heel wedges, can decrease the force of the deep digital flexor tendon on the navicular bone by 4 times for every 1° the palmar angle is increased (Eliashar et al, 2004).
Conclusions
High quality radiographs are paramount to accurate interpretation of the navicular bone. Navicular syndrome will typically begin with sclerosis of the spongiosa, with multiple radiographic lesions appearing during progression. Enthesopathy of the proximal border at the attachments of the collateral sesamoidean ligament and distal border fragments may not indicate navicular syndrome. Changes such as enlarged synovial invaginations are of variable clinical significance and should be interpreted with caution. Flexor cortical erosion is a relevant finding that can result in career-limiting, progressive disease. Absence of radiographic abnormalities do not exclude navicular bone pathology; magnetic resonance imaging can help identify radiographic occult lesions as well as concurrent soft tissue injury and can, therefore, be used to augment the radiographic exam.
Key Points
- Patient positioning, radiographic beam angle and technique are key elements in producing high quality radiographs.
- The spectrum of radiographic navicular bone changes includes normal variants and clinically incidental findings to careerending lesions. Accurate interpretation of these findings is key to reaching a diagnosis.
- Abnormalities of the navicular synovial invaginations have recently been associated with both distal interphalangeal joint osteoarthritis and navicular apparatus pathology on magnetic resonance imaging. All radiographic changes of the foot should be considered during interpretation for the most accurate diagnosis.
- Decreased corticomedullary distinction (sclerosis) of the navicular bone is an important indicator of pathologic change. Artefactual increased opacity is common as a result of superimposition by the frog, sulcus packing material and inadequate beam angle on the navicular skyline.
- Flexor cortical erosions are the most significant radiographic lesion of the navicular bone. Additional radiographic beam angles, such as a shallower angle on the navicular skyline projection, have recently been found to aid in detection of the flexor cortical erosions.