Pathology associated with the navicular bone remains a common cause of forelimb lameness in horses. Since the introduction of advanced imaging modalities (particularly magnetic resonance imaging) it is now recognised that pathology of the navicular bone rarely occurs in isolation; concurrent abnormalities of the structures of the podotrochlear apparatus are often present. Different pathological processes of the navicular bone can have the same radiological appearance. Identifying the underlying disease process, as well as concurrent injuries, is essential for implementing the most appropriate management strategies in horses with navicular bone pathology. Horses with acute onset lameness or soft tissue injuries will often require a period of rest initially, whereas horses with chronic insidious lameness and no soft tissue pathology may be able to continue in work following treatment. Pathology associated with the navicular bone and structures of the podotrochlear apparatus are a common cause of forelimb lameness in horses, with abnormalities identified on magnetic resonance imaging examination in 72–91% of horses with lameness localised to the foot (Dyson et al, 2005; Murray et al, 2006). Since the introduction of advanced imaging modalities, particularly magnetic resonance imaging, veterinary understanding of navicular syndrome has improved sub-stantially. Navicular syndrome is a chronic, progressive condition affecting structures of the podotrochlear apparatus including the navicular bone, navicular bursa, deep digital flexor tendon, collateral sesamoidean ligaments and distal sesamoidean impar ligament that is often associated with lameness. Radiography remains the most commonly used diagnostic imaging modality in the initial investigation of horses with suspected navicular syndrome. The limitations of radiography as a sole diagnostic tool in horses with lameness associated with the podotrochlear apparatus are well recognised. It is rare to have abnormalities of the navicular bone in isolation. There are a variety of forms of navicular bone abnormalities that can be identified on radiography, including sclerosis of the spongiosa (medullary sclerosis), flexor cortex erosions, remodelling of the proximal border and extremities of the navicular bone, distal border fragmentation and cyst-like lesions within the navicular bone. In horses with suspected navicular syndrome, correct identification of the underlying disease process in addition to identification of concurrent pathology of the adjacent soft tissue structures is key to selection of the most appropriate management protocol for individual cases. This requires the use of advanced imaging techniques such as magnetic resonance imaging. Another abnormality of the navicular bone which can be identified using magnetic resonance imaging is the presence of bone oedema-like signal pattern. These radiological abnormalities of the navicular bone can represent different disease processes. The clinician must be cognisant of this and that both reversible and irreversible lesions of the navicular bone have been associated with the same signal patterns on magnetic resonance imaging.
Current management strategies in horses with navicular syndrome are focused on reduction of pain and alterations of the loading patterns of the podotrochlear apparatus. The multitude of pathologies that can be present in addition to navicular bone pathology mean that management and treatment must be tailored to the individual case. This article reviews the pathophysiology of bone oedema-like signal pattern, sclerosis of the navicular bone and the current concepts on management of these conditions.
Definition
In order to understand the concepts surrounding the management of horses with pathology of the navicular bone, veterinarians must first understand the pathology that these radiological abnormalities represent.
Navicular bone sclerosis
Navicular bone sclerosis, or increased radiopacity of the navicular bone, can be identified on both radiographic and magnetic resonance imaging. Radiographically, a normal navicular bone has a regular trabecular architecture in the spongiosa that is clearly defined from the dorsal and palmar cortices (Figure 1; 2). This definition is best assessed in both lateromedial and navicular skyline radiographic views (Dyson, 2011). On magnetic resonance imaging examination, sclerosis of the navicular bone appears as reduced signal intensity in all sequences. Navicular bone sclerosis can be centrally located if the sclerosis occurs following historical trauma to the bone, or toward the palmar aspect of the bone, affecting the endosteal aspect of the flexor cortex (Dyson and Murray, 2007). This pattern of sclerosis adjacent to the palmar cortex is more typical of the classical form of navicular syndrome. Magnetic resonance imaging has demonstrated that this sclerosis can also be associated with damage to the fibrocartilage or flexor cortex that is not visible radiographically, particularly if it is associated with alterations in the thickness of the flexor cortex (Blunden et al, 2006; Dyson et al, 2012).
Navicular bone oedema-like signal pattern
In comparison to sclerosis, navicular bone oedema-like signal pattern is only detectable on magnetic resonance imaging. It is characterised by a decreased signal intensity in T1 weighted magnetic resonance images and an increased signal intensity in fluid-sensitive and fat-supressed magnetic resonance images (Figure 3; 4) such as T2 and short tau inversion recovery. The presence of a bone oedema-like signal pattern may be as a result of inflammation, necrosis, haemorrhage, fibrosis and cyst formation (Dyson et al, 2012). Bone oedema-like signal pattern has been reported in several locations within the navicular bone, including adjacent and parallel to the palmar cortex, at the origin of the distal sesamoidean impar ligament, as a focal or generalised pattern in the proximal or distal spongiosa or throughout the entire spongiosa (Busoni et al, 2005; Dyson et al, 2012). The intensity of bone oedema can vary from mild to marked. Oedema-like signal pattern in the navicular bone has been associated with both transient and irreversible lesions, confirming that different pathological processes can be associated with the same signal pattern on magnetic resonance imaging. In horses with transient lesions, the bone oedema-like signal pattern represents inflammation within the bone and can resolve with time. Transient lesions typically present in two forms:
- Associated with primary pathology of the deep digital flexor tendon, where the bone oedema-like signal pattern is located parallel and dorsal to the palmar cortex (Figure 5). This bone oedema-like signal pattern can resolve with time, although occasionally more severe pathology develops (Dyson et al, 2012)
- Following acute onset trauma to the navicular bone where the bone oedema-like signal pattern represents inflammation and can resolve with rest and time. Assessing the response to treatment and follow up magnetic resonance imaging remains the only reliable way to confirm this finding antemortem.
Irreversible lesions attributed to fibrosis and necrosis of the navicular bone have also been associated with oedema-like signal pattern including:
- Inflammation and bone oedema-like signal pattern in early-onset navicular disease. This always affects the distal third of the bone but can extend further proximally. Abnormal signal intensity in the navicular bone was the most frequent observation in horses with recent onset navicular syndrome and no radiographic findings (Sampson et al, 2008; 2009). This oedema-like signal pattern can persist in cases with chronic navicular syndrome. In Sampson et al's study (2008), sclerosis (reduced signal intensity in PD and T2 weighted images) was seen alongside bone oedema in 27% of recent onset navicular syndrome cases and 20% of horses with chronic navicular syndrome. This change in trabecular density may be an indicator of irreversible damage:
- In association with damage to the fibrocartilage and subchondral bone of the palmar cortex
- A subset of horses has been identified with diffuse increased signal intensity of the spongiosa associated with primary navicular bone pathology, without magnetic resonance imaging evidence of abnormalities of the flexor cortex (Dyson et al, 2012).
Clinical signs and diagnosis
Clinical signs
The clinical signs associated with pathological changes in the navicular bone are non-specific. Horses are typically presented because of lameness or poor performance. The onset of clinical signs can be acute or chronic. Conformation traits such as small narrow feet, upright limb conformation and large bodies have been associated with navicular disease (Wright, 1993). Poor foot conformation and improper trimming and shoeing are also thought to be predisposing factors (Wright, 1993). At a walk, horses with more advanced disease may adopt a toe-first landing gait, as opposed to the normal flat foot or heel-toe foot placement, to reduce load on the palmar foot structures. When trotting in a straight line, overt lameness may not be recognised as pathology of the navicular bone is often bilateral; however, lameness is exacerbated when lunging, particularly on a hard surface. In horses with traumatically induced pathology of the navicular bone, a unilateral lameness may be present.
Regional analgesia
The source of lameness is localised to the palmar foot using regional, intra-articular and intra-bursal blocks. A palmar digital nerve block is often the first block performed. This anaesthetises most of the foot, including the palmar structures of the foot, and can also anaesthetise the distal interphalangeal joint. In some cases, it can also anaesthatise the metacarpophalangeal joint (Schumacher et al, 2013). Intra-articular analgesia of the distal interphalangeal joint anaesthetises the joint, the sole and likely the palmar digital nerves (Schumacher et al, 2013). As a result of this, the clinician must bear in mind that analgesia of the distal interphalangeal joint will desensitise the palmar structures of the foot including the navicular bone. Analgesia of the navicular bursa should be considered the most specific block for structures of the podotrochlear apparatus. This block should not desensitise the distal interphalangeal joint (Schumacher et al, 2013), although diffusion of mepivacaine between the navicular bursa and distal interphalangeal joint has been shown to occur in cadaver limbs (Gough et al, 2002). As a result of the lack of specificity of the blocks of the digit, diagnostic imaging is essential to achieve a diagnosis once the lameness has been localised to the foot.
Radiographs
Radiographic examination of the foot is often performed initially. The shoes should be removed, and the foot prepared appropriately to allow for comprehensive assessment of all structures. Standard projections should include lateromedial, dorsoproximal-palmarodistal oblique and palmaroproximal-palmarodistal oblique images (Dyson, 2011). Sclerosis, remodelling of the proximal and distal margins of the bone, the presence and number of synovial invaginations, distal border fragmentation, thickness of the cortices, corticomedullary definition and the presence of flexor cortex defects can be assessed radiographically. The clinician must be aware of the potential for artefacts related to the frog and sole packing to affect their interpretation of radiographic abnormalities.
Magnetic resonance imaging
Magnetic resonance imaging is commonly used in horses with lameness localised to the palmar foot region, and is used to identify bone oedema-like signal pattern within the navicular bone which cannot be detected using conventional imaging (Vanhoenacker and Snoeckx, 2007). Magnetic resonance imaging also allows for comprehensive evaluation of all structures of the podotrochlear apparatus including the deep digital flexor tendon, collateral sesamoidean ligaments, distal sesamoidean impar ligament and navicular bursa. Frequently, magnetic resonance imaging abnormalities of the soft tissue structures of the podotrochlear apparatus are seen in addition to abnormalities of the navicular bone. As previously discussed, the location and intensity of the bone oedema-like signal pattern can give further information regarding the nature of the disease process. In two studies, abnormalities of the navicular bone in isolation were seen in only 2.2% (6/263) and 3.5% (12/347) of horses (Murray et al, 2006; Dyson and Murray, 2007). In a US study of 151 horses, in which Quarter Horses predominated, there was a much higher incidence of primary navicular bone lesions (33%) (Sampson et al, 2008). In this study, horses with more chronic lameness (>6 months duration) were more likely to have concurrent soft tissue lesions, with only 16% of horses in the chronic group considered to have primary navicular bone issues. This suggests that bone oedema-like signal pattern or inflammation of the navicular bone may be a sign of early onset navicular disease.
Management and treatment
Before determining the best management strategy for individual cases, the clinician must fully understand the pathology and be aware of any concurrent lesions which may alter the therapeutic protocol. The approach to horses with traumatic and reversible lesions may differ from the approach to horses with classical navicular syndrome and lesions involving the fibrocartilage. The presence of additional soft tissue abnormalities must also be considered. Differences in foot conformation, shoeing, foot balance and the presence of concurrent lesions dictate the need for individually tailored recommendations.
Current concepts for management of navicular bone pathology focus on reducing pain. This can be achieved using rest or a reduction in workload, medications such as systemic non-steroidal anti-inflammatory drugs, intrathecal medications, bisphosphonates and synthetic venoactive drugs. These are often used in combination with changes in trimming and shoeing to alter the biomechanical loading of the structures of the podotrochlear apparatus.
Management changes
Exercise modifications
A period of box rest may be indicated in cases with acute trauma and associated inflammation of the navicular bone. The period of rest required will often be determined by the presence of concurrent soft tissue abnormalities. In horses with concurrent acute lesions of the deep digital flexor tendon, a longer period of rest and controlled exercise will be required when compared to horses with sole lesions of the navicular bone. Repeat magnetic resonance imaging examination is useful to assess for resolution of the lesion.
Farriery
Corrective trimming and shoeing are an important aspect in management of horses with navicular bone pathology. The horse should be encouraged to land normally; it is widely accepted that a well-balanced hoof should land flat on a hard surface (O'Grady and Poupard, 2003; Ross, 2003). Abnormal landing increases forces on the navicular bone and may potentiate the condition. As part of the initial investigation, careful attention must be paid to the balance radiographs and the landing pattern of the foot during walking and trotting. The degree to which lameness can be improved by trimming and shoeing depends on the existing foot conformation. If a horse already has good foot conformation and is well shod, it is possible that further improvement with shoeing alone will be minimal. The aim with corrective trimming is to restore correct dorsopalmar and mediolateral foot balance while maintaining heel mass and shortening the toe. In select cases, usually those with concurrent deep digital flexor tendon pathology, temporary heel elevation can be beneficial.
Medical treatment
Non-steroidal anti-inflammatory drugs
Non-steroidal anti-inflammatory drugs can be used following the initial diagnosis to provide analgesia. Non-steroidal anti-inflammatories such as phenylbutazone, suxibuzone and Metacam are unsuitable for long-term continual use as a result of unwanted side effects such as gastroduodenal ulceration, right dorsal colitis and renal papillary necrosis, but they may aid in the transition period while additional therapies are being implemented.
Bisphosphonates
Bisphosphonates such as clodronate (Osphos) and tiludronate (Tildren) and are licensed for use in horses with navicular disease and are used in horses with oedema-like signal pattern in the navicular bone. The efficacy of the bisphosphonates is not well supported (Kamm et al, 2008), although Denoix et al (2003) have shown a positive effect of tiludronate in horses with recent onset lameness and osteolytic lesions of the navicular bone.
Intra-articular and intrabursal medication
Many practitioners prefer to treat horses with navicular syndrome by injection of the distal interphalangeal joint. Commonly used intra-articular treatments include a combination of corticosteroids and hyaluronic acid. It has been demonstrated that trimacinoloone acetonide diffuses directly between the distal interphalangeal joint and the navicular bursa in normal horses (Boyce et al, 2010). Numerous techniques have been described for injection of the navicular bursa, and radiographic or ultrasonographic guidance is required depending on the technique used (Schramme et al, 2000; Dabareiner et al, 2003; Spriet et al, 2004). Injection of the navicular bursa is considered technically more challenging and carries greater risk than injection of the distal interphalangeal joint. Intrabursal medication with corticosteroids with or without hyaluronic acid is often used in horses with osseous pathology of the navicular bone, although success with this treatment is variable. Slightly improved outcomes are reported in studies where horses with navicular syndrome were treated with a combonation of corticosteroids and hyaluronic acid; 75–80% of horses returned to work when a combination treatment was used (Dabareiner et al, 2003; Marsh et al, 2012) versus 3–66% of horses where corticosteroids alone were used (Verschooten et al, 1990; Gutierrez-Nibeyro et al, 2010). However, Bell et al (2009) found no significant effect in horses treated with a combination of hyaluronate and corticosteroids versus corticosteroids alone. These authors did find that outcomes were significantly improved in horses receiving greater than 10 mg of triamcinolone by intrabursal injection versus thoses receiving smaller doses. Intra-articular and intrabursal injection of other products including interleukin-1-receptor-antagonist-protein (Waguespack and Hanson, 2011), polyacrylamide hydrogel (Bathe, 2020), autologous protein solution (Prostride) and alpha-2-macroglobulins have also been reported, but large case series demonstrating the efficacy of these products is lacking.
Calcium dobesilate
Calcium dobesilate has also been used with good success in a small number of horses with increased signal intensity in the navicular bone on fat-supressed images (Janssen et al, 2011). Calcium dobesilate is a synthetic venoactive drug used to treat high protein oedema in humans. The proposed mechanism of action is macrophage-driven removal of osmotically active proteins. The use of this drug is based on the hypothesis that impaired venous drainage of the bone leads to venous hypertension in the bone marrow, resulting in intraosseous hypertension and bone oedema. This leads to a compartment syndrome in the subchondral bone and ultimately results in a vicious cycle of pain and progressive pathological changes. In Janssen et al's (2011) study, treatment consisted of 3 mg/kg of calcium dobesilate orally twice daily for the first 2 months, then reducing to 3 mg/kg orally once daily for a further 2 months. Inclusion criteria for this study were a lameness duration between 1 and 12 months, lameness localised to the foot, no radiographic abnormalities, magnetic resonance imaging identification of decreased T1 signal intensity and increased signal intensity in fluid sensitive sequences without other magnetic resonance imaging abnormalities. In this study, 11 of the 12 horses returned to soundness with a reduction in magnetic resonance imaging signal intensity in the navicular bone. However, the outcomes must be interpreted with caution – there was no control group, and it is possible that bone oedema would have improved over the course of the study period. A larger study with appropriate controls is required to completely assess the effects of calcium dobesilate in horses with bone oedema-like signal pattern of the navicular bone.
Extra-corporeal shockwave therapy
Shockwave therapy of the navicular bone has been used for its analgesic effects in horses with trabecular sclerosis and trabecular resorption (Bolt et al, 2004; Rijkenhuizen, 2006). However, it is no longer commonly used as a treatment for navicular syndrome in the United Kingdom.
Surgical treatment
Surgical treatment is an option in select cases with navicular bone abnormalities. The choice of surgical technique will depend on the presence of concurrent pathology and response to previous treatment. Surgery can be performed alongside medical treatment or following failed medical therapy. Surgical treatment options include navicular bursoscopy, osteostixis of the navicular bone and palmar digital neurectomy.
Navicular bursoscopy
Navicular bursoscopy can be considered in cases with concurrent intrabursal deep digital flexor tendon pathology and adhesion formation, although the prognosis for a return to work remains only fair with 28% and 44% of horses returning to their previous level of work following bursoscopy in two studies (Schramme et al, 2011; Smith and Wright, 2012). Debridement of damaged fibrocartilage and palmar cortical erosions of navicular bone has been reported to smooth the gliding surface for the deep digital flexor tendon (Nixon, 2009); however, the efficacy of performing this is unknown.
Osteostixis of the navicular bone
Brock et al (2023) have had promising results following core osteostixis of the navicular bone in seven horses. In this case series, seven western performance quarter horses with bilateral forelimb lameness attributed to navicular syndrome underwent navicular bursoscopy and core osteosixis of the navicular bone in one limb, and navicular bursoscopy alone in the contralateral limb. In humans, core osteostixis has been used to treat osteonecrosis and bone oedema. Core osteostixis can transiently reduce intraosseous pressure and result in remodelling and neovascularisation of the bone. In experimental cases, these drill tracts cases are filled with woven bone at 12 weeks (Jenner and Kirker-Head, 2011). In the Brock et al (2023) case series, 5/7 limbs in the core osteostixis group were sound at 24 weeks, compared with 1/7 limbs treated with bursoscopy alone. Five of seven limbs treated with osteostixis had an osseous cyst-like lesion in the treated limb, and all cases had moderate to marked increased fluid signal in the navicular bone before surgery.
Following core osteostixis, the bone oedema decreased in 5/7 limbs in the core osteostixis group and in 3/7 limbs in the bursos-copy only group. The magnetic resosnance imagine appearance of osseous cyst-like lesions was unchanged after surgery. In 3/7 limbs that underwent core osteostixis, new deep digital flexor tendon lesions were identified post-operatively vs 1/7 limbs in the bursos-copy only group. Although the results of this small case series appear promising, care should be exercised before considering this procedure. Results from western performance horses may not be transferable to other breeds and uses, and the effect of osteostixis on the magnetic resonance imagine appearance of the navicular bone was comparable to those horses that underwent bursoscopy alone. In addition, the development of new deep digital flexor tendon lesions following this procedure must be further investigated. An approach via the palmar pouch of the distal interphalangeal joint may result in a reduced risk for iatrogenic damage to the deep digital flexor tendon, although this has not been evaluated in clinical cases.
Palmar digital neurectomy
Palmar digital neurectomy may be considered if there is a failure of other treatment methods. Cases with primary navicular bone pathology that have remained lame despite appropriate treatment and medication are good candidates for this procedure (Gutierrez-Nibeyro et al, 2015). This is not suitable for horses with concurrent pathology of the deep digital flexor tendon.
Conclusions
Magnetic resonance imaging and histopathological studies have demonstrated that radiological abnormalities of the navicular bone can represent different disease processes. In addition, it is uncommon to have pathology of the navicular bone in isolation. Understanding that pathological process is key to implementing the most appropriate treatment strategy. In horses with reversible lesions of the navicular bone associated with trauma or concurrent soft tissue injury, a period of rest, exercise modification and anti-inflammatory medication is indicated. In horses with irreversible changes, such as oedema-like signal pattern associated with flexor cortex erosions or persistent oedema-like signal pattern indicative of fibrosis and necrosis of the navicular bone, treatment strategies are similar; however, rest is unlikely to provide improvement either clinically or on magnetic resonance imaging examination. Management is focused on reducing pain, inflammation and the concussive forces acting on the navicular bone. Repeat magnetic resonance imaging examination to assess resolution of bone pathology may be necessary to differentiate between reversible and irreversible lesions of the navicular bone. New therapies are emerging for horses with pathology of the navicular bone such as calcium dobesilate and core osteostixis; however, current reports are limited to a small number of cases and the outcomes of these studies must be considered in light of this.
KEY POINTS
- Navicular bone oedema and navicular bone sclerosis can represent different pathological processes within the navicular bone.
- Pathology of the navicular bone rarely occurs in isolation. Concurrent abnormalities of the soft tissue structures of the podotrochlear apparatus (deep digital flexor tendon, collateral sesamoidean ligaments, digital sesamoidean impar ligament and navicular bursa) are often present.
- The cornerstone of management of horses with navicular bone pathology is reducing pain and inflammation. This is often achieved through systemic non-steroidal anti-inflammatory drugs, intra-articular or intrabursal medication, modifications in exercise and shoeing changes.
- Horses with acute onset lameness or concurrent soft tissue pathology will often require a period of rest.
- In horses with irreversible damage to the navicular bone, a period of rest is unlikely to improve the clinical or magnetic resonance imaging appearance of the bone.
- Repeat magnetic resonance examination may be required to differentiate between reversible and irreversible lesions of the navicular bone.