Pathology in the distal limb is one of the most common etiologies of lameness and poor performance in the equine athlete.
This is particularly true in the case of forelimb lameness, where problems in the distal limb or foot account for greater
than 50% of the lameness cases. However, distal limb problems are also common in the rear leg, and should not be overlooked.
Disorders of the distal limb is defined as problems occurring from structures distal to the fetlock. This is a complicated
anatomic area involving numerous soft tissue and bony structures that when damaged can each be a source of pain and lameness.
The basic lameness exam is employed as for other causes of lameness. The nature of the lameness is highly variable, due to
the large variety of disorders that can occur. The typical lameness caused by problems in the distal forelimb is a weight
bearing lameness, and is worse on the hard surface. This means that the maximal head excursion (head up) is when the lame
leg is on the ground. This is particularly true of lameness caused by problems in the joint and navicular bone. Disorders
of the soft tissues within the distal limb and foot may also present with a weight bearing lameness, however some will alternatively
present as a swinging leg lameness that can mimic lameness from problems higher in the leg. It is important to evaluate these
horses on different surfaces and also with a rider, as the lameness may be variable under these different circumstances.
Physical exam and palpation of the distal limb is extremely important to isolate areas of palpable pain and sensitivity, as
well as any joint effusion or swelling. A thorough examination of the hoof and hoof conformation should be done on both the
sound and lame limb. Horses that have had a chronic, low grade lameness will often have a smaller, more upright hoof. When
this is present, it can be determined that the horse has been under-loading the hoof for a long time. Hoof testers should
be applied and can be a sensitive way to detect subsolar bruising or a subsolar abscess. However, I find hoof testers to be
insensitive for detecting other causes of lameness related to the distal limb. Distal limb flexion is almost always positive
when problems are present in the distal limb, but is by no means a specific test.
Once the basic lameness examination is complete, I generally proceed with diagnostic anesthesia of the palmar or plantar digital
nerves. This block will desensitize the heel area, entire sole, coffin joint, and a variable amount of the pastern joint,
so is not sensitive for any one problem. I prefer to do this block first because it is relatively non-invasive and is usually
well tolerated by the animal. With additional time and volume of the medication, significant proximal diffusion can occur
which can alleviate pain arising from the fetlock joint or either higher. Therefore only a small volume of local anesthetic
(2-3 mL/nerve) should be used and the horse evaluated at 5-7 minutes post blocking. Efficacy of the block can be assessed
by checking skin sensitivity at the heel areas with the horses ipsilateral eye covered. Proximal diffusion can be assessed
by checking skin sensitivity higher up the leg.
Lameness caused by disorders of the distal limb are commonly bilateral, with one limb being more affected than the other.
Often times when the most affected leg is blocked, the animal will begin displaying a lameness in the contralateral limb.
When this happens, you can be assured that you have identified the region of pain in the blocked limb. The other limb is then
similarly blocked to ensure that the problem is confined to the distal limb as well. In cases where routine diagnostic imaging
have failed to identify the cause of the lameness or treatment has been unsuccessful, I may proceed with intra-articular anesthesia
once the peri-neural block has worn off. Intra-articular anesthesia of the distal interphalangeal joint is performed first,
followed by intra-articular anesthesia of the proximal interphalangeal joint if there has been no improvement. Even intra-articular
anesthesia is not entirely sensitive, as the palmar digital nerves will course adjacent to the palmar and plantar pouch of
the distal interphalangeal joint, so that the local anesthetic will diffuse to this area and anesthetize structures distal
to the joint (navicular bone and all soft tissues of the heel). Intra-thecal anesthesia of the digital flexor tendon sheath
can be performed if there is a suspicion of a distal deep digital flexor tendonitis. Occasionally diagnostic anesthesia of
the navicular bursa is performed, however this technique is difficult in the unsedated animal. Aseptic preparation of the
area is absolutely essential for these intra-synovial blocks to minimize any potential for contamination, which may be life