Page 479 - Adams and Stashak's Lameness in Horses, 7th Edition

Lameness of the Distal Limb 445

documented that the diffusion and distribution pat- Radiography
terns of anesthetics are affected by volume, injection Despite its limitations, radiography remains the ini-
VetBooks.ir abolished within 10 minutes following a nerve block, tial diagnostic tool to assess the navicular bone in most
Some advocate that if lameness is not
site, and time.
79,80
horses with navicular disease/syndrome. However,
the block should be deemed negative and to continue
blocking. 111 It is important to be mindful of technique, degenerative changes within the bone can be missed
because a 40% change in bone density is required before
volume of anesthetic used, and timing between nerve it can be identified with radiographs. Therefore, the
96
blocks. However, using good technique does not guar- lack of abnormalities in the navicular bone does not
antee that multiple structures have not been desensi- eliminate the bone as the site of the pain and does not
tized and that the block has extended beyond what is necessarily indicate a soft tissue problem in the foot.
to be expected. Several studies evaluating horses showing signs of navic-
The majority of horses affected with navicular dis-
ease/syndrome (80%–100%) improve substantially fol- ular syndrome but with normal or mild radiographic
abnormalities of the navicular bone have documented
lowing a PD block and the lameness in the opposite that the majority of these horses’ navicular bones were
forelimb either worsens or becomes apparent if a unilat- abnormal on MRI or histological examination. 49,69,100,101
eral lameness was initially found. 99–101,115 Kinematic gait The navicular bone was considered the primary abnor-
analysis has found that the mean maximal extension of mality in only 33% of horses with a recent onset of
the fetlock during the stance phase of the stride and the lameness and in only 16% of horses with a more chronic
maximum flexion of the carpal joint during swing phase duration of lameness. 100,101
of the stride were significantly increased after PD blocks A complete radiographic evaluation of the navicular
in horses with navicular disease. 68 bone requires a minimum of lateromedial, 60° dorso-
Intrasynovial anesthesia of the DIP joint and the
navicular bursa can also be performed to potentially proximal to palmarodistal oblique, and palmaroproxi-
mal to palmarodistal oblique (skyline) high‐quality
further localize the site of pain. Historically, blocking views. 35,37,50,115 Additional radiographs such as the 60°
the DIP joint was thought to help distinguish between dorsoproximal to palmarodistal oblique view of the dis-
problems associated with the joint and the navicular tal phalanx and the weight‐bearing dorsopalmar view
region. However, several studies have demonstrated a are often included to completely evaluate all bony struc-
lack of specificity of intrasynovial anesthesia of the DIP tures in the foot. Adding a shallow angle (35°) palmaro-
joint due to diffusion of anesthetic and the location of proximal to palmarodistal oblique projection has been
sensory nerves in close approximation to the synovial recommended to improve the accuracy of identifying
outpouchings of the DIP joint. 15,16,34,55,87,107,108 Anesthesia flexor cortex lysis. This is supported by the fact that
64
of the DIP joint is known to improve the lameness in a the majority of flexor cortex lesions occur in the mid to
large percentage of horses diagnosed with navicular distal aspect of the flexor surface. 9,49 Careful attention
syndrome. 34,35,46,99,111,115 However, the amount of to foot preparation (packing the frog), limb position,
improvement in lameness following DIP joint anesthe- and centering and directing the X‐ray beam according
sia is often less than that following a PD block in most to the hoof capsule conformation is important to avoid
horses with navicular disease/syndrome. Recommended artifacts and inaccurate image interpretation. 37,50 For
methods to improve the specificity of DIP joint anesthe- example, slight obliquity to a lateral medial projection
sia are to use no more than 5–6 mL of anesthetic and to will result in a poorly defined sagittal ridge, misinterpre-
assess the response to the block within 5–10 tation of the true flexor cortex thickness and uniformity
minutes. 96,99,107,111 of opacity of the flexor cortex, poor demarcation
Anesthesia of the navicular bursa is probably the
most specific nerve block that can be used to help local- between the endosteal surface of the flexor cortex navic-
ular bone and the trabecular bone, poor representation
ize the site of pain in horses with navicular disease/ of the proximal to distal shape of the flexor aspect, and
syndrome. However, it is not routinely used as the initial poor definition of the articular margins of the navicular
diagnostic block because of the need for radiographic bone. 37
or ultrasonographic confirmation of needle placement. A There are a number of radiographic abnormalities to
positive response to intrasynovial anesthesia into the the navicular bone that have been described; some are
navicular bursa may indicate problems of the navicular considered incidental and may or may not be patho-
bursa, navicular bone, and/or navicular supporting ligaments, logic, and some correlate strongly with lameness. Those
sole, and/or toe, or distal aspect of the DDFT. 107–109,111 seen more commonly in lame horses include the
Even though diffusion of local anesthetic into the navic- following:
ular bursa occurs following DIP joint injection, the con-
verse does not occur, and analgesia of the navicular 1. Enthesophytes at the proximomedial and proximo-
bursa does not usually result in analgesia of the DIP lateral aspect of the bone. Small enthesophytes can
joint. 108,111 Pain from the DIP joint can likely be excluded be seen in sound horses, but large enthesophytes are
as a cause of lameness if analgesia of the navicular bursa usually associated with lameness.
improves the lameness within 10 minutes. 108,111 In addi- 2. Proximal or distal extension of the flexor border of
tion, a positive response to intra‐articular analgesia of the bone. This change in shape is indicative of stress
the DIP joint together with a negative response to navic- at the insertion of the CSLs and origin of the DSIL.
ular bursa analgesia incriminates pain within the DIP 3. Fragments at the distal border of the bone. These
joint as the cause of lameness. 111 fragments are usually located at the distal medial and
See Chapter 3 for further information about local lateral angles of the bone and can be associated
anesthesia. with lameness. Their presence may be due to ectopic