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Lameness arising from knee pain is a commonly encountered
and important condition in racing Thoroughbreds. The make-up of the equine
knee is comparable to the human wrist, with the obvious exception that the
horses' knee has to endure enormous forces generated by their considerable
weight and speeds achieved during racing, approximately 500kg and 60 km/hr,
respectively. The high incidence of knee problems observed is, in part, due
to the small size of the horse's knee relative to its body weight. The
repeated concussive forces on cartilage and bone at the front of the knee
experienced with training and racing may lead to development of small bone
chips and/or arthritis.
Another commonly encountered knee condition, though less well recognised, is
pain arising from injury to ligaments that attach to the `back of the knee'
area, which more accurately represents the top of the cannon bone. In the
horse, the suspensory ligament is the major structure supporting the back of
the lower limb. This ligament attaches to the cannon bone just below the
`back of the knee' (top of the cannon bone) and extends to just above the
hoof (FIGURE 1). When Thoroughbreds exercise at galloping speeds their gait
incorporates a period of time in every stride where their entire weight is
supported by a single forelimb. This results in extreme extension of the
lower limb joints, which rely on the suspensory ligament for support. This
process places enormous tension (stretching) on the suspensory ligament at
the back of the leg and large compression forces at the front of the leg. As
the suspensory ligament has its attachment just below the back of the knee,
large forces are transmitted through the ligament to where it joins on to
the top of the cannon bone. In addition to Mother Natures' design faults,
conformation faults may exacerbate the strain experienced in this area, and
these include `offset knees' and upright pasterns and fetlocks.
Pain arising at the `back of the knee' most commonly results from tearing of
the suspensory ligament close to or as it attaches to the top of the cannon
bone. Despite this, it is now well recognised that pain in this area may
result from a number of different conditions referred to as 'subcarpal
syndrome'. In recent times, more detailed diagnostic lameness examinations
has revealed that there is a range of injuries, in addition to suspensory
ligament tearing, that includes stress fractures, incomplete fractures,
stress reaction and complete separation fractures (avulsions) of the top of
the cannon, which may or may not also have suspensory fibre tearing. This
observed spectrum of conditions is associated with a wide range of
presenting signs from very mild changes in the horses' action at fast
working speeds to very obvious lameness at the trot prohibiting training.
The anatomical location and wide range in severity of conditions seen at the
`back of the knee' can make diagnosis difficult. In the easily diagnosed
cases there is swelling, heat, obvious local pain and lameness. In the more
subtle cases, identifying the site of pain requires an injection of local
anaesthetic at the site to relieve the lameness. Unfortunately, because the
site of attachment of the suspensory ligament to the top of the cannon bone
is intimately located with parts of the knee joint, local anaesthetic may
inadvertently enter the knee joint capsule, thereby numbing both areas. In
this situation, the observed resolution of lameness following local
anaesthetic injection may be a result of relieving knee joint pain rather
than suspensory attachment pain. This lack of specific identification may be
overcome by performing alternative injections, which numb specific nerves
supplying the top of the cannon bone.
Once the site is confirmed as the source of lameness, the exact nature of
the injury can be ascertained using ultrasound, radiographs or nuclear
scintigraphy. At this stage the `back of the knee' syndrome throws up more
challenges to the veterinarian, as subtle bone and ligament injury can be
very difficult to demonstrate using ultrasound and conventional radiology.
Fortunately, recent developments in medical technology have become available
to veterinarians, which have to a large extent resolved the problems of
identifying the nature of the injury.
The recent technological advances have included digital radiography and
nuclear medicine, which have been of great benefit for early detection in
cases where the degree of bone and ligament injury is mild. In these horses
the outward signs are often nebulous, but the effect on racing performance
is costly to the trainer, owner and breeder by impeding racetrack results.
Radiographic changes seen in subcarpal or `back of the knee' syndrome range
from remodelling of bone (whitening) to obvious fracture lines. The use of
digital radiology, which has only recently become available to
veterinarians, has the advantage of providing extremely fine detail imaging
of the bone, which can be computer enhanced to specifically demonstrate
fracture lines and subtle pathology which, using conventional radiology,
were previously unrecognisable (FIGURES 2a & b).
Occasionally in some cases, despite the benefits of evaluating this site
with digital radiology and ultrasound, the bone and ligament appears
uninjured. In these situations, the use of nuclear scintigraphy, also
referred to as bone scanning, is an extremely valuable diagnostic tool. The
benefit with this imaging technique is that it provides an accurate picture
of bone activity. The ability to demonstrate the level of bone activity is
of enormous benefit, as bones are constantly remodelling with changes in
training intensity and are capable of repairing in the face of injury.
Radiography, unlike nuclear scintigraphy, does not allow assessment of the
level of bone activity. The technique of nuclear scintigraphy involves the
use of safe levels of a radioactive agent, which are injected into the
horse. The radioactive agent localises at sites of bone repair and active
remodelling.
The increased concentration of the radioactive agent at these sites is then
detected using a large radiation detector (gamma camera). The detector then
creates an image of the horses' skeleton and areas of injury are show up as
`hot spots'. (FIGURES 3A & B). This technique is extremely sensitive to any
degree of bone injury and allows detection of bone damage long before it is
visible on radiographs.
The introduction of technological advanced digital nuclear medicine units
into veterinary medicine has further enhanced our ability to more accurately
identify these types of injuries.
With more accurate diagnosis, possible with the use of such advanced
technology as digital radiology and nuclear scintigraphy, comes earlier
detection of injury and the prevention of more serious career threatening
damage.
Furthermore, such specific identification of the cause of lameness affords
owners and trainers the most appropriate, cost effective and improved
outcome when injury occurs |
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