Navicular syndrome: causes, diagnosis and treatment - Vet Times

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Navicular syndrome: causes, diagnosis and treatment

Author : Carolin Gerdes Categories : Equine, Vets Date : July 20, 2015

ABSTRACT

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.

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Figure 1. Sagittal cut of a specimen of a foot showing the structures forming the podotrochlear apparatus.

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.

Causes and contributing factors

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 preexisting 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.

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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 undulatingshaped 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.

Diagnosis

Diagnosing navicular syndrome can be challenging and usually requires a combination of several diagnostic tests.

Clinical examination

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.

Diagnostic local anaesthesia

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Figure 2. Radiographic examples for a range of navicular bone changes. A: dorsoproximalpalmarodistal oblique view (upright navicular view) showing a core lesion in the navicular bone. B: palmar 45? proximal-palmarodistal oblique view (skyline navicular views) with a lucency extending into the cortex of the navicular bone and an increased number of synovial invaginations. C: lateromedial view demonstrating increased density of the navicular bone with an elongated shape in proximo-distal orientation.

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,

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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.

Diagnostic imaging

Radiography 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).

Figure 3. Image examples of advanced imaging techniques. The red arrows highlight the primary

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