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Lameness in racing Thoroughbreds
is a major cause of poor performance, interrupted racing schedules and
premature retirement from racing. While osteoarthritis is the most common
cause of lameness, there are several developmental bone abnormalities which
may lead to joint pain requiring treatment, particularly in the juvenile
racehorse. A common manifestation of developmental joint abnormalities is
the formation of bone cysts or subchondral cyst-like lesions.
The most common site at which bone cysts are encountered in Thoroughbred
racehorses is the stifle (the equivalent of the human knee). Specifically,
bone cysts are found in the main weight bearing aspect of the stifle (the
medial femoral condyle) and rarely at other locations within the joint
(proximal lateral tibia and lateral femoral condyle). Although commonly
referred to as bone cysts, they are not truly cystic structures by strict
definition, but the term has become ensconced in the veterinary literature;
despite this they are aptly named as they typically represent an oval or
dome-shaped defect devoid of bone on radiographs. Bone cysts are identified
on radiographs and are typically located just beneath the joint surface and
commonly communicate with the joint cavity. The size of a medial femoral
cyst varies from shallow dome-shaped defects (approximately 8mm x 3mm) to
large ovoid-shaped cysts of 40mm x 30mm.
The development of bone cysts appears to be the result of a combination of
factors, many of which contribute to other recognised developmental bone
abnormalities, such as osteochondrosis (OCD). Factors identified include
growth rate and body size, hereditability, nutrition (energy/protein/copper
levels) and exercise. The majority of stifle cysts appear to arise as a
sequel to abnormal bone development, but occasionally they may form
secondary to arthritic in older horses. The majority of cysts appear to
develop within the first eight (8) months of life but occasionally they may
arise in older horses.
Current theory on the cause of bone cysts suggests formation is a result of
abnormal skeleton development that leads to retained cartilage rather than
transformation to bone. The less resilient cartilage develops flaws and
crevices, which then allows influx of joint fluid, which results in
cavitation of the bone. Enlargement of the cyst has also been shown to
result from cells producing enzymes in the lining of developed cysts capable
of causing bone resorption.
Recently published research investigating the cause of developmental
orthopaedic disease and osteochondrosis indicates that stifle cysts may be
more prevalent in foals that have exercise restricted to stall rest for
extended periods. Further to this, research investigating stifle cysts and
other forms of developmental orthopaedic diseases has shown that joint
defects are more likely to persist, rather than spontaneously heal, in foals
that have low dietary copper.
Lameness may develop as a result of a stifle cyst at any age, but the most
typical scenario is a sudden onset of hindlimb lameness at the time of
introduction to training, whether it be during breaking-in or preliminary
training. Lameness may also be recognised prior to or during yearling
preparation for sale. Conversely, the presence of a cyst may not result in
any overt lameness, and may be an incidental finding on radiographs. In this
regard, the advent of pre-sale survey radiographic examinations has
identified a number of yearlings with
subtle changes to the shape of the femur at the typical weight-bearing site
of cyst development. Predicting whether these changes, such as shallow
concavities, develop into lameness causing bone cysts is difficult.
Radiographic evidence of active bone remodelling at the margins of the
concavity, seen as a white rim of bone and radiating decalcification, is
indicative of potential cyst enlargement. Despite the lack of reliable
prediction, anecdotally, many of the observed subtle radiographic changes do
not develop into significant bone cysts and performance limiting problems.
Treatment options for stifle bone cysts causing lameness include long term
rest, anti-arthritic and intra-articular corticosteroid therapy and surgery.
Conservative therapy, which may require nine to twelve months of paddock
rest, has been associated with resolution of lameness and return to racing.
Unfortunately the number of horses managed by conservative therapy that have
been evaluated in the veterinary literature is very limited, but success
rates are approximately 50%. Anecdotal evidence suggests conservative
therapy is associated with a more protracted convalescence and a less
predictable outcome than following surgical treatment.
Numerous surgical techniques have been used in
the treatment of bone cysts. Current recommended treatment involves
arthroscopic removal (curettage) of the cystic contents, cyst lining and the
overlying unsupported cartilage. Additional techniques to use in an attempt
to enhance healing and improve outcome have included bone drilling and
grafting, both of which are now considered to offer no
benefit.
Presently, additional techniques to arthroscopic curettage which appear
promising in improving surgical success are post-surgical intra-articular
corticosteroid therapy and joint resurfacing techniques which use laboratory
grown cartilage cells combined with growth factors to fill the bone defect.
The current success rate for return to performance in horses following
arthroscopic surgery is 50-74%. A recent review of yearlings and unraced
2-year-old Thoroughbreds undergoing arthroscopic surgery for stifle cysts
indicated that 64% of treated horses raced compared to 77% of unaffected
maternal siblings, indicating the majority of treated horses went on to
race. When these two (2) groups were further compared, horses which had
stifle cysts and underwent arthroscopic surgery earned less money per start
in their 2-year-old and 3-year-old racing years but earned equal amounts in
their 4-year-old racing season compared to the unaffected though related
group. This finding is consistent with the long convalescence associated
with this condition, i.e. 6-12 months following surgery.
A further interesting finding from this Kentucky based study was that depth
of the cyst was not as important as the diameter of the cyst opening at the
joint surface in influencing a successful outcome. Horses that had cysts
with a joint surface diameter greater than 1.Scm had a significantly poorer
outlook for racing.
Future research evaluating new joint resurfacing surgical treatments is
likely to reduce convalescence time and improve racing prospects for horses
with this common condition. Furthermore, with the advent of pre-sale
radiographic examinations, follow-up studies will allow veterinarians to
better predict which of the observed subtle radiographic bone changes are
likely to result in bone cyst development. Similarly, such studies will help
determine which bone cysts are likely to cause performance-limiting
lameness, and how they are best managed when detected. |
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