Lameness is the most common reason for enforced time out of work for horses. Of the numerous causes of lameness, foot pain – especially in the front feet – is the most frequently encountered problem.
There are, of course, numerous causes of foot pain, including bruises and corns, subsolar abscesses (pus in the foot), laminitis, pedal osteitis, fractures of the pedal bone and navicular disease. All of these conditions have been recognised for hundreds of years, but our understanding of many of them (especially navicular disease and “heel pain”) is very poor.
One of the reasons for the lack of understanding of such diseases is the difficulty in imaging the bony and soft tissue structures inside the foot.
The anatomy of the equine foot is highly complex, and the presence of the hoof capsule means that direct examination of the structures inside the foot is very difficult. Palpation is impossible, and techniques such as diagnostic ultrasound are very limited.
Although radiography is routinely used to image the foot, this only shows the bones. It provides no information about the numerous important soft tissue structures, such as the deep digital flexor tendon, the impar ligament, the suspensory ligament of the navicular bone, the collateral ligaments of the coffin joint and the digital cushion.
As newer imaging techniques, such as magnetic resonance imaging (MRI), become more widely available, so new information about many of these structures and their diseases will be learnt.
Although some diseases of the foot, such as laminitis, corns and subsolar abscesses, can usually be easily diagnosed through clinical signs and results of physical examinations, other conditions present more of a problem.
Many of the more chronic diseases of the foot present similar, non-specific clinical signs. Thus, the clinical signs of coffin joint arthritis, navicular disease, pedal osteitis and deep digital flexor tendonitis are frequently similar, and it is generally impossible to differentiate one from the other.
In the absence of specific clinical signs other than lameness, it may even be impossible to know that the site of pain is in the foot. Nerve blocks and joint blocks are frequently used in such situations, first to prove or disprove that the site of pain in a lame horse is the foot (or feet), and second to try to discover which area of the foot is painful.
Although the results can be extremely useful, recent research suggests that nerve and joint blocks may not be as useful or accurate as we once believed.
What are nerve and joint blocks?
Nerve and joint blocks involve the injection of a local anaesthetic either close to a nerve or directly into a joint or into another synovial cavity, such as the navicular bursa or a tendon sheath.
The local anaesthetic temporarily disrupts the function of any nerves or nerve endings it contacts. This results in areas of desensitisation (numbing). The desensitised area depends on which nerves or nerve endings are affected.
Thus, if the local anaesthetic is placed around a nerve, such as the palmar digital nerve that runs down on both sides of the back of the pastern before entering the foot, the areas supplied by that nerve will become desensitised. If the horse has pain in this area, the nerve block will temporarily numb it and the lameness may disappear.
If local anaesthetic is placed directly into the coffin joint, it will desensitise all the nerve endings in the joint. If the horse has pain inside the joint, this will temporarily relieve that pain.
However, placing local anaesthetic drugs into joints or other synovial cavities may affect nerves that touch or cross the lining of the cavity; thus, the joint block may desensitise other areas of the foot in addition to the joint itself.
These blocks are used as one part of an examination of the lame horse. Usually, the horse is trotted up (either in a straight line on a hard, level surface and/or on the lunge in both directions), and the degree of lameness is noted (using some sort of grading system). The block is then applied, and the horse trotted up again to see whether there has been any improvement in the degree of lameness.
If the lameness has improved, it is likely that the site of pain causing it is situated in the area of the foot that has been desensitised by the block.
What nerve and joint blocks are performed in the foot, and what do they mean?
There are four different nerve and joint blocks routinely performed in the horse’s foot.
- The palmar digital nerve block is performed by injecting a small volume of local anaesthetic drug (usually 1.5 ml) over the palmar digital nerves on both sides of the pastern, just above the bulbs of the heel and adjacent to the deep digital flexor tendon.Until recently, vets believed that a positive response to this nerve block indicated pain in the back third of the foot (including the back of the coffin joint and the navicular bone).This block used to be considered as one of the most reliable indicators of navicular disease. However, recent research has shown that it does in fact desensitise the entire coffin joint and most of the foot apart from the laminae and coronary band at the toe region. The block is therefore relatively non-specific, and really only indicates foot pain.
- The abaxial sesamoid nerve block is performed at the level of the bottom of the sesamoid bones at the back of the fetlock. This desensitises the entire foot, the pastern joint, the short pastern bone and their associated soft tissues. Depending on exactly where the block is performed, the back part of the fetlock joint may also be affected by this block.
- The coffin joint block is usually performed by injecting local anaesthetic solution through a needle placed into the joint just above the coronet at the toe. This not only desensitises the coffin joint, but also the navicular bursa, the navicular bone and the toe region of the sole. If a large volume of anaesthetic solution (greater than 6 ml) is injected, the heel region of the sole is also desensitised.Although this block affects the navicular area, a negative response cannot be relied upon to rule out navicular disease as the cause of lameness. Also, lameness due to deep digital flexor tendonitis in the region of the navicular bone may or may not be affected by this block.
- The navicular bursa block is usually performed by placing a 3.5in spinal needle between the bulbs of the heel just above the coronary band, and advancing it into the foot until it touches the navicular bone. A small volume (2-3 ml) of local anaesthetic solution is injected (sometimes with 0.5-1ml of an X-ray contrast agent so that correct placement of the block can be confirmed by subsequent radiographic examination).A positive response to this block indicates pain in the navicular bursa, the navicular bone, the supporting ligaments of the navicular bone, the sole at the toe or the deep digital flexor tendon inside the foot.
Interpretation of the results of nerve and joint blocks
As can be recognised from the above, the interpretation of the results of nerve and joint blocks is often not straightforward. Whereas it used to be believed that the different blocks were very specific and limited to the areas of the foot they desensitise, it is now recognised that this is not the case. Most of the blocks will desensitise more than one specific area, and there is much crossover between the different blocks.
In addition, it is now recognised that many diseases of the foot involve damage to multiple structures. For example, recent MRI studies have shown that horses with navicular disease commonly have variable involvement of many soft tissue structures in addition to the navicular bone (including the navicular bursa, the deep digital flexor tendon, the impar ligament, the suspensory ligament of the navicular bone and the coffin joint).
It is therefore not surprising that various horses with navicular disease will respond differently to the various nerve and joint blocks, depending on what specific structures are damaged.
Like most other parts of the lameness evaluation, nerve and joint blocks are only one part of the overall examination.
- This article first appeared in Horse & Hound (3 June, 2004)