RECOGNITION AND TREATMENT OF ANGULAR AND FLEXURAL LIMB DEFORMITIES - DOC

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RECOGNITION AND TREATMENT OF ANGULAR AND FLEXURAL LIMB DEFORMITIES - DOC Powered By Docstoc
					       RECOGNITION AND TREATMENT OF ANGULAR AND
               FLEXURAL LIMB DEFORMITIES

                  Susan M. Stover, DVM, PhD, DiplACVS
Angular and flexural limb deformities are among a spectrum of disorders that can result
from the abnormal distribution of biomechanical forces on rapidly growing bones and
soft connective tissues of young animals.

An angular limb deformity is a deviation of a limb in the frontal plane (seen when
viewed from the front or back of the animal). In most domestic animals a straight line
dropped perpendicular to the ground from the shoulder joint should bisect the long
bones of the limb. A varus deformity is a deviation of the distal portion of the limb from
this line toward the midline of the animal, and a valgus deformity is a deviation of the
distal portion of the limb away from the midline of the animal. The most common
condition in foals is a valgus deformity of the carpus (or distal limb), and that condition
is also referred to as “medial deviation of the carpus”.

Abnormal conformation(s) of the limb can arise from several etiologies/sources.
Although the clinical appearance is similar, it is important to determine the anatomic
source of the abnormal conformation, because the treatment and prognosis differ for
some of these conditions. In general, the angular limb deformity can be classified as
being due to one or more of the following conditions: 1) laxity of periarticular ligaments,
2) physeal/metaphyseal growth imbalance, 3) epiphyseal growth imbalance, 4) delayed
endochondral ossification/hypoplasia of a cuboidal bone/osteochondrosis of a cuboidal
bone, or 5) overt trauma.

Most of these conditions arise from an abnormal distribution of forces from the medial
to lateral aspects of the limb on the growing regions of a bone. Therefore, it is prudent
to review the regions of growth of long and cuboidal bones (of the carpus in this
example). Growth of the distal ½ of the radius, the carpus, and the proximal ½ of the
metacarpus is centered in the regions of the distal physis of the radius, the distal
epiphysis of the radius, and the cuboidal bones of the carpus. Although the basilar
layers of the articular cartilage of the metacarpus may contribute some growth, a
physis of the proximal aspect of the metacarpus is not present after birth – and
therefore, this region does not usually contribute to the development of angular limb
deformities.

The diagnosis of these conditions is largely based on the clinical appearance of the
angular limb deformity, the time of onset of the deformity, and the radiographic
appearance (geometric configuration and morphologic appearance of the bony
structures).

Careful assessment of the clinical appearance of some angular limb deformities can
localize the anatomic location serving as the major source of a deformity. For example,
a wedge shaped metaphyseal region with a deformity centered at the same location
would indicate uneven growth of the physis. The detection of joint effusion with an
angular limb deformity centered at that joint is good evidence of cuboidal bone
hypoplasia (providing the effusion is in a joint that can be detected clinically – for
example, middle carpal joint effusion can be detected clinically, but distal intertarsal
joint effusion is not detectable clinically because of the tight periarticular ligaments
surrounding this joint).
Based on the time of onset, angular limb deformities can also be classified as either 1)
present at birth (congenital) or 2) acquired within the first weeks or months of life.
Angular limb deformities present at birth are usually due to periarticular ligament laxity
or hypoplastic (incomplete ossification) cuboidal bones. Periarticular ligament laxity is
due to weak collateral ligaments (periarticular supporting structures, e.g., tendons
crossing the joints) simply because the animal has never used them outside of the
uterus, or from malpositioning of the affected joint in the uterus. Hypoplastic cuboidal
bones (in this example, carpal bones) are present in animals usually born prematurely.
The growth of cuboidal bones occurs by interstitial growth of the cartilaginous template,
which subsequently undergoes endochondral ossification. This process is occurring
rapidly around the time of birth. The cuboidal bones of animals that are born
prematurely are still largely cartilaginous (versus having a greater bony component),
and may not be strong enough to sustain the forces associated with weight bearing.
Under these conditions, growth and/or subsequent ossification of the cartilaginous
template is delayed. The cuboidal bones either do not develop normal morphologic
shapes or are actually damaged by excessive compressive forces.

The remaining types of deformities are acquired through asymmetrical growth across
the medial to lateral aspects of the limb. Animals may be born with normal limb
conformation, but over a period of weeks or months gradually acquire a deformity.
These deformities are centered at regions of rapid growth, the distal physis/metaphysis
of the radius and the distal epiphysis of the radius. In this instance, excessive
compression is associated with a retardation of growth in comparison to the side of the
limb under relative tension. Uneven growth across the region of rapid growth in the
bone results in an angular limb deformity. For example, excessive compression on the
lateral aspect of the distal physis of the radius results in a retardation of growth laterally
in relation to the rate of growth on the medial aspect of the physis. Over a period of
time this results in a valgus deformity because the medial side of the physis grows
faster than the lateral side of the physis. Although we are using the carpus of the foal
as an example, deformities can center on any portion of the limb that is undergoing
rapid growth (e.g., the distal physis of the metacarpus, the tarsus).

Overt trauma, either through fracture or ligamentous disruption, can occur at any time
during the pre- or post-natal life of the animal. Trauma is usually associated with the
immediate development of a deformity.

Two types of things are evaluated from radiographs of a limb affected with a deformity.
Firstly, the geometric configuration is assessed. From the craniocaudal (dorsopalmar)
radiograph, lines are drawn that bisect the longitudinal axis of the long bones.
Normally, the line bisecting the radius and the line bisecting the third metacarpal bone
should be parallel and superimposed. If an angular deformity is present, the lines will
not be parallel and they will intersect one another. The severity of the deformity can be
evaluated by measuring the angle of the intersection. This information is useful for
assessing the initial severity of the condition, but is most useful for comparison with the
angle at later times to assess progress following treatment of the condition. The
location of the intersection is an indicator of the anatomic region that is the source of
the abnormal growth. For example, the intersection will occur in the region of the
physis/metaphysis with abnormalities of growth in this region; and in the region of the
carpus with either periarticular ligamentous laxity or cuboidal bone hypoplasia.
Abnormal growth may be occurring in more than one anatomic location, in which case,
the intersection may occur in an intermediate location.

Secondly, radiographs are useful for assessing the morphology of the bones of the
limb. A wedge-shaped appearance to the metaphysis would be associated with
uneven growth across the medial to lateral aspects of the metaphysis, etc. Most
importantly, the morphology of the ossification center of the cuboidal bones can be
assessed. Abnormal shape and uneven radiodensity of the ossification center is
indicative of hypoplasia of the affected cuboidal bone(s).

Conservative treatment of these conditions is based on correcting the uneven
distribution of forces on the rapidly growing regions of affected bones. The success of
treatment is dependent on continued growth of these regions. Since growth occurs
most rapidly in the neonate and decreases exponentially until, in the case of the physis,
closure occurs; treatment is most successful when instituted as early as possible in the
life of the animal. For example, closure of the distal physis of the radius occurs a t
approximately 2-3 years of age; however, rapid growth of this region occurs only during
the first 8 months of life. Therefore, treatment should be instituted prior to 4 months of
age in order to have continued growth and be most effective.

It is important to correct uneven loading of the limbs because once an angular limb
deformity is present, the abnormal conformation accentuates the uneven distribution of
forces and may promote worsening of the condition. Alleviation of the uneven
distribution of forces can usually be achieved by corrective trimming of the feet and/or
the use of orthotics. Decreasing the magnitude of the forces can usually be achieved
by controlling the exercise of the animal in a manner that allows enough exercise to
stimulate strengthening of periarticular supporting structures, but limits excessive
exercise that would result in fatigue of periarticular supporting structures or direct
damage to developing bones. The early detection of angular limb deformities and
management by foot care and controlled exercise is associated with a good prognosis
for return to normal limb conformation and soundness.

In the young foal when the deformity is not improving with corrective hoof trimming and
exercise management, splints and/or casts can be used to obtain more immediate
straightening of the limb and ensure physiologic loading of the limbs. Splints and casts
require daily nursing care and frequent re-applications to prevent the development of
skin sores in these fast growing foals; and to minimize weakening of tendons and
ligaments that are also immobilized during treatment of the angular deformity.

The one condition associated with a guarded to poor prognosis for eventual soundness
is hypoplasia (incomplete ossification) of the cuboidal bones. With this disorder,
immediate correction of the angulation is warranted to relieve excessive compression
on the affected cuboidal bones. Early relief of the uneven distribution of forces can
result in return of normal interstitial growth of the cartilaginous template of the cuboidal
bone and subsequent endochondral ossification to form a normally sized and shaped
bone. This is often done by application of a cast that maintains the limb in a normal
conformation. If the cuboidal bone does not develop to it‟s normal conformation,
secondary degenerative joint disease will usually result.

In older animals that have not responded to conservative therapy, surgical techniques
can be used to temporarily retard or enhance growth of selective regions of the physis.
For example, metallic implants can be placed to bridge the medial or lateral aspect of
the physis to retard growth of that aspect of the physis while they are in place
(transphyseal bridging). Alternately, the growth of an aspect of the physis can be
enhanced by transecting the periosteum in the region of the metaphysis on that side of
the metaphysis (periosteal stripping). In both of these procedures, the prognosis for
correction of the deformity is related to the amount of growth potential remaining in the
affected region of the limb. The surgical procedure itself does not correct the
deformity. The deformity is corrected through continued growth in an uneven manner
dictated by the surgical procedure. In severe cases, wedge osteotomy can be
considered.
A flexural limb deformity is a deviation of a limb in the sagittal plane (seen when
viewed from the side of the animal) usually associated with flexor tendons that prohibit
normal range of joint motion or allow excessive joint motion. These deformities are
usually manifested in hyperextension or hyperflexion of the metacarpophalangeal or
distal interphalangeal joint(s).

Congenital flexural limb deformities can be related to weak digital flexor tendons,
recognized by lowering of the fetlock toward the ground and rising of the toe off of the
ground during stance. The prognosis is often good with conservative therapy
consisting of controlled exercise and corrective shoeing. Attaching a shoe with an
extended heel to affected hooves decreases the magnitude of the joint moments
(forces that flex and extend the joints) by decreasing the length of the moment arm of
the forces associated with weight bearing around the metacarpophalangeal,
phalangeal, and digital joints. Under these conditions, the weak tendons have
sufficient strength to sustain normal joint conformation. With controlled exercise, the
tendons gradually strengthen, the heel on the shoes can be shortened until the shoe
can ultimately be removed.

Contractural flexural limb deformities are recognized by limited hyperextension
(dorsiflexion) of the metacarpophalangeal and/or distal interphalangeal joints(s). These
deformities are usually associated with „contraction of the superficial and/or deep digital
flexor tendons‟ and can be congenital or acquired. In either situation, it is important to
determine if the deformity is primary or secondary to another cause. Any apparent
nutritional imbalances should be resolved. A deformity secondary, for example to a
contralateral limb lameness, is unlikely to improve until resolution of the primary
problem.

Conservative treatment for contractural deformities includes corrective trimming and
shoeing, and splinting and casting – with or without analgesic medication. The goal is
to stretch out the „contracted tendon‟. For example, a shoe with an extended toe will
increase the extensor moment on the distal interphalangeal joint and enhance joint
extension during weight bearing. Analgesic medication may be needed to alleviate
pain associated with stretching of the deep digital flexor tendon during weight bearing.
If splints are necessary to help position the joint, it may be advantageous to place a
splint on only the dorsal surface of the limb so that the limb is not prevented from
further extension and tendon stretching. Recently, intravenous oxytetracycline (3 g in
250-500 ml of physiologic saline solution) has been advocated for congenital
contractural deformities. In theory, oxytetracycline chelates calcium making it less
available for muscle, resulting in muscle relaxation.

Surgical treatment is indicated for cases unresponsive to conservative treatment. The
goal of surgical treatment is to obtain normal joint extension by effectively lengthening
the deep and/or superficial digital flexor muscle-tendon unit. Each muscle-tendon unit
has an accessory ligament (distal and proximal check ligaments, respectively) that
protects the muscle from overstretching while preventing fetlock overextension as part
of the stay apparatus of the limb. These functions are a consequence of the stiff
components in the bone-check ligament-tendon-bone structure that imparts
considerable passive resistance to fetlock (and digital) hyperextension. Transection of
the check ligament disrupts this series of structures and their passive contribution to
limitation of extension. As the joints extend, a greater proportion of tendon load is
transferred directly to the associated muscle belly, which is less stiff than tendon and
stretches more.
Flexural deformities of the distal interphalangeal joint are associated with the distal
check ligament and deep digital flexor tendon because this is the only tendon unit that
crosses the flexor surface of the distal interphalangeal joint. Surgical transection of the
distal check ligament is indicated in cases that do not respond to conservative
treatment. Conservative treatment (corrective hoof trimming/shoeing  analgesics) is
always indicated with surgical treatment. In cases unresponsive to check ligament
transection, the deep digital flexor tendon itself may be transected, but the prognosis is
markedly worse in these cases.

Flexural deformities of the metacarpophalangeal joint may be associated with the
proximal check ligament and superficial digital flexor tendon, distal check ligament and
deep digital flexor tendon, and/or suspensory apparatus because all of these structures
cross the flexor surface of the metacarpophalangeal joint. Surgical transection of the
proximal check ligament is first indicated in cases that do not respond to conservative
treatment. Conservative treatment (corrective hoof trimming/shoeing  analgesics) is
always indicated with surgical treatment. In cases unresponsive to proximal check
ligament transection, the proximal check ligament, superficial digital flexor tendon,
deep digital flexor tendon, and/or the suspensory ligament may be transected, but the
prognosis is markedly worse in these cases.              In long standing cases, the
metacarpophalangeal joint capsule may also be contracted.