20 Jul 2015
Carolin Gerdes
Job Title
Figure 1. Sagittal cut of a specimen of a foot showing the structures forming the podotrochlear apparatus.
Navicular syndrome is caused by pain arising from the navicular bone (distal sesamoid bone) and closely associated structures such as the deep digital flexor tendon, the navicular bursa, distal sesamoidean ligament or collateral sesamoidean ligaments (Figure 1). Collectively these structures are called the podotrochlear apparatus. Navicular disease is a common cause of forelimb lameness, but the hindlimbs can also be affected. This article discusses the pathophysiology and contributing factors for the disease as well as the diagnosis and treatment options for navicular syndrome.
Navicular syndrome results in chronic lameness caused by a variety of different conditions with a number of aetiologies. It is, therefore, not surprising the clinical presentation can vary significantly. Some horses present with acute, sudden onset and severe unilateral lameness while others show an insidious onset of mild, slowly progressing bilateral forelimb lameness.
Although mature riding horses are most commonly affected, navicular disease is also diagnosed in younger horses that have only recently been introduced to work. Navicular syndrome is seen in different breeds of horses and in horses with different foot conformations, such as “flat-footed” Thoroughbreds with a long toe/low heel conformation, as well as warmbloods with a more upright or narrow foot conformation.
The actual cause of pain and lameness in horses with navicular disease is poorly understood. Biomechanically, the navicular bone is subject to forces that are greatest at hyperextension of the distal interphalangeal joint during propulsion. Different foot conformations may alter the forces placed on the foot; for example, low heels lead to increased dorsiflexion and a higher pressure being placed on the palmar and distal aspects of the navicular bone.
More recently, lesions in the deep digital flexor tendon (DDFT) in the digit have been recognised clinically, but information available on the pathophysiology of these injuries is limited. A variety of lesions in the DDFT are recognised, such as sagittal tears, dorsal fibrillation or abrasions and core lesions. Primary lesions may be caused by repetitive stress or acute trauma superimposed on pre-existing degenerative changes, which can be seen without concurrent navicular pathology. More common are secondary lesions, such as dorsal fibrillation or incomplete parasagittal splits, together with navicular disease.
An increase of proteoglycans in the distal DDFT and distal sesamoidean impar ligament as a response to stress has been shown in older horses. Furthermore, it has been suggested, low or weak heels make horses more susceptible to developing DDFT lesions.
Horses with navicular disease load their feet differently to normal horses. A study by McGuigan et al in 2001 looked at the effect of palmar digital nerve anaesthesia on compressive forces applied to the navicular bone in horses with navicular disease.
In an attempt to unload the heel, diseased horses show a paradox reaction of contracting the deep digital flexor muscle, which actually leads to increased force on the navicular bone and the deep digital flexor tendon. This increased force was reduced when the horses underwent local anaesthesia of the palmar digital nerves. The study has also shown no difference in the actual force amount reached during stance phase between sound horses and horses with navicular disease. The main difference was the peak force, which was reached a lot earlier in the lame group, resulted in a higher loading rate in horses with navicular disease.
Radiological studies on the shape of navicular bones (Dik et al, 1995; 2001) have shown the shape of the proximal articular margin on a dorsoproximal-palmarodistal oblique view (upright navicular view) may affect the risk of development of navicular disease. Horses with a concave or undulating-shaped proximal margin, as it is commonly seen in Dutch warmbloods, show more radiological changes associated with navicular disease and are at a higher risk of developing it. This predisposition is seen in comparison to navicular bones with a straight proximal articular border, which are less likely to develop navicular disease.
Diagnosing navicular syndrome can be challenging and usually requires a combination of several diagnostic tests.
On clinical examination, horses with a proximal injury to the deep digital flexor tendon in the digit may be sensitive to palpation in the distal palmar pastern region. Some horses will resent static flexion of the distal limb. Dynamic lameness examination should be carried out as usual. Lunging on hard surfaces is useful to highlight subtle lameness. There is a suspicion of significant soft tissue involvement when horses present with lameness that is more or as equally obvious on soft surfaces.
Lameness caused by navicular syndrome is frequently improved significantly following anaesthesia of the palmar digital nerves at the level of the proximal margin of the ungual cartilages. Sometimes it is necessary to anaesthetise the palmar digital nerves at the abaxial sesamoid level. Due to the close proximity of structures potentially involved in causing lameness of the digit, great care should be taken when interpreting blocking results. Especially in the situation of a positive abaxial sesamoid nerve block it is important to rule out possible pathology in the pastern and fetlock regions.
Primary pastern or fetlock joint pain can be ruled out by intrasynovial anaesthesia of these joints on a separate occasion. In horses with bilateral lameness, a switch of lameness to the contralateral limb is observed following a positive block. Once the lameness has been localised to the foot it may be useful to wait until the previous nerve blocks have worn off before carrying out more detailed, intrasynovial diagnostic local anaesthesia.
Horses with navicular syndrome may respond completely or partially to anaesthesia of the distal interphalangeal joint and/or navicular bursa. In addition to providing diagnostic information, positive blocks can also help to determine a treatment plan; for example, horses with a positive response to diagnostic anaesthesia of the navicular bursa are more likely to improve with medication in the same area. A negative response to analgesia of the distal interphalangeal joint or the navicular bursa makes it less likely the horse has navicular disease.
In a study by Dyson et al in 2003 looking at 46 horses with deep digital flexor tendonitis in the digit, lameness was abolished following palmar digital analgesia in 24% of horses. A total of 68% of horses with primary tendonitis responded to intra-articular anaesthesia of the distal interphalangeal joint, whereas 92% of horses with concurrent abnormalities of the navicular bone improved significantly.
A report by Ross (1998) showed 164 horses that had responded positively to diagnostic local anaesthesia of the medial and lateral palmar digital nerves were found to have lameness caused by a number of conditions such as navicular disease, pathology in the distal and proximal interphalangeal joints, pedal bone fractures, laminitis and other diseases of the foot. This highlights the imprecision of the palmar digital nerve block in localising lameness to the foot.
An anatomical study by Bowker et al (1993) demonstrated the majority of sensory fibres of the navicular bone, and the collateral sesamoidean ligaments are located dorsally to the suspensory ligaments of the navicular bone and directly subsynovial to the distal interphalangeal joint. It is, therefore, important to consider the possibility of desensitisation of these nerves when administering local anaesthetic to the distal interphalangeal joint.
Radiography of the foot is the standard imaging modality of choice for navicular syndrome. To fully assess radiographically for navicular disease the shoes should be removed and the feet should be prepared thoroughly to avoid artefacts created by the frog. The sulci of the frog should be packed with mouldable packing material such as play dough. A full set of radiographs includes lateromedial and dorsopalmar views of the foot, as well as upright navicular views (dorsoproximal-palmarodistal oblique view) and skyline navicular views (palmar 45° proximal-palmarodistal oblique view).
Radiographic changes in the navicular bone range from radiolucent areas, such as an increased number and size of distal border invaginations, to core lesions or numerous cyst-like lesions in the central part of the bone (Figure 2). Increased density can be seen in sclerotic navicular bones, which appear radiographically with a loss of corticomedullary definition (Figure 2), loss of trabecular pattern or thickening of the flexor cortex. There may be distal or proximal border fragmentation. The shape of the navicular bone may be abnormal with enthesophyte formation on the lateral and medial wings, elongation with new bone formation predominantly on the distal aspect or spur formation (osteophytes) on the proximal and distal articular margins.
Interpreting the radiographic appearance of navicular bones is challenging. There is a range of radiographic findings seen both in clinically normal horses and horses with navicular disease.
MRI is frequently applied in cases presenting with navicular syndrome and is the ideal advanced imaging modality to assess the podotrochlear apparatus. MRI is particularly useful in cases with inconclusive radiographs, but it is also carried out to assess the full extent of pathology in cases with obvious radiographic findings. Standing low-field scanners allow the application of MRI in the sedated horse and the acquisition of high quality MR images.
An increased bone turnover is seen in the navicular bone of horses with navicular disease. This is shown by increased radionuclide uptake. Although gamma scintigraphy offers a sensitive method to detect increased bone turnover, this finding does not necessarily correlate with pathological change or current pain. A number of horses have a positive response to analgesia of the distal interphalangeal joint and/or navicular bursa without detectable radiological abnormalities, but with increased radiopharmaceutical uptake in the navicular bone. In cases with primary bone necrosis or end-stage navicular disease, bone scan images may be normal.
CT is used to assess lameness localised to the foot. In a study by Valance et al (2012), both the anatomical visualisation and lesion identification of horses with lameness localised to the foot was compared between standing low-field MRI and CT (plain images and contrast-enhanced images). Similar visualisation scores were given to most structures of the navicular apparatus for MRI and plain CT. There was an advantage of MRI in visualising distal aspects of the DDFT and the distal sesamoidean impar ligament. Conclusions drawn from comparing lesion identification between the modalities showed a weakness of CT recognising DDFT lesions distal to the proximal aspect of the navicular bone. MRI centred on the podotrochlear apparatus may fail to identify proximal soft tissue lesion such as mineralisation.
Ultrasonography of the foot is not routinely used to assess horses with navicular syndrome. A number of publications describe the anatomy and use of ultrasonography of the foot. Significant limitations of the method exist in visualising and identifying lesions affecting the podotrochlear apparatus.
Once navicular syndrome has been diagnosed as the cause of lameness a number of conventional and novel treatment options are available. The choice of treatment for each individual case will strongly depend on the exact diagnosis and the structures involved. The approach will depend on severity and chronicity of the injury
as well as other factors, such as the age, purpose, intended use
and current level of exercise of the horse. Other orthopaedic conditions, such as hindlimb lameness or back pain, should be taken into account.
The most common treatment options are listed.
Remedial farriery is an important part of managing horses with navicular syndrome. A good working relationship and teamwork between the farrier and the veterinary surgeon is important and both parties should be in agreement with the trimming and shoeing protocol for each individual case. Generally, specific principles should be applied considering the horse’s foot conformation and changes should be applied gradually over several trimming/shoeing periods. Sudden or radical changes risk increasing lameness. The positive effect of remedial farriery on the degree of lameness will be greater in horses with inappropriate shoeing or poor balance at the time of diagnosis. General principles to be applied in horses with navicular syndrome are correcting dorsopalmar and lateromedial foot balance, easing breakover by shortening the toe and maintaining the heel, heel support and protection from concussion of the palmar foot. The foot-pastern axis should be straight although this can not necessarily be achieved depending
on the conformation.
Heel elevation may improve comfort in the short term – particularly in horses with tendon injuries. However, a negative effect of heel elevation on the heel mass in the long term should be considered. Positive results have been achieved with egg bar shoes and variations such as sports-bar or straight-bar shoes. In some cases, natural balance shoes may be appropriate.
